GB2398464A - Apparatus and method of signal modulation - Google Patents

Abstract

Signal modulation comprising: when a first predetermined condition is met for a first signal waveform; generating one or more temporary signal state-modifications to be applied to one or more data symbols within the first waveform, to generate a second waveform (which is therefore a modification of the first waveform) such that the amplitude of at least a part of the second waveform differs from the amplitude of the corresponding part in the first waveform. The temporary modifications are introduced to counteract overshooting from inter-symbol interference (ISI) caused by constructive or destructive interference between pulses of the first waveform. The first predetermined condition is met when the amplitude of the first waveform exceeds or lies below an associated maximum or minimum amplitude threshold value. The signal may be modulated according to a Quadrature Amplitude Modulation (QAM) scheme.

Description

Apparatus and Method of Signal Modulation The present invention relates to

apparatus and methods for modulating signals for transmission. The present invention also relates particularly, though not exclusively, to apparatus and methods for generating modulated signals in the form of successive bursts of information for electromagnetic telecommunications transmission such as radio or microwave transmission.

Many telecommunications systems convey data by modulating carrier signals, such as electromagnetic waves, so as to convey a succession of data symbols or data bits. The modulation techniques employed for these purposes typically involve modulating a carrier signal waveform such that, at predetermined symbol times, periods or locations within the carrier signal, the carrier signal conforms to one of a multiplicity of different carrier signal-states. Each of a multiplicity of different data symbol (or bit) values is then mapped onto a unique one of the multiplicity of carrier signal-states.

Thus, when an appropriately modulated carrier signal is received at a given receiver, the succession of different signal-states through which the received signal is observed to change (at the predetermined periods/locations) can be directly correlated with associated symbol or bit values thereby enabling the receiver to "read" the modulated data encoded within the received signal.

A modulated signal can be modelled as a superposition of time-shifted pulses, weighted according to the data being conveyed by the modulated pulse. As an example, an idealised signal waveform S(t), comprising a train of successive data pulses each located somewhere within an associated time-slot or data "symbol period" time- shifted, may be regarded as a sum of individual time shifted pulses g(t) each having the same predetermined duration which may be at most equal to a symbol period or time-slot T. or which may be less than the symbol period T. or may be an impulse of tiny duration as follows: n='e S(t) = > dn À g(t - nT) a= Where n is an integer, do is the signal-state associated with the nth data symbol and g(t) is the "signalling pulse" or "pulse shaping function". A given nth signalling pulse g(t-nT) is, in this example, an impulse having an amplitude value of [(one) at an instant of time within the period of time (n-l) T < t < nT, and is O otherwise. In such a case, where the successive impulses are regularly spaced, the interval between impulses is equal to the "symbol period".

In practice, a signal modulation system is generally unable to generate pure impulses for such superpositions since pure impulses require an infinite spectral bandwidth in the resultant modulated signal S(t). This cannot be achieved in practice. Rather, the bandwidth available to signal modulation systems is typically limited by a band-limiting filter. The effect of a band- limiting filter is to limit the spectrum of the modulated signal waveform to a pre-defined finite frequency spectral band.

Unfortunately, restricting the bandwidth of a modulated signal waveform in the frequency domain results in a spreading, or "dispersion", of the signal waveform in the time domain. This means that the once sharp edges of a signalling pulse g(t-nT) become sloped, and not only is the signalling pulse no longer a pure impulse, it is no longer confined to a time-slot equal to or less than the symbol period (of duration T) as before, but now spreads outside its symbol period.

This spreading of signalling pulses results in one signalling pulse interfering with its neighbouring pulse, an effect which can lead to "intersymbol interference" (ISI) which can cause symbol/data errors in a modulated signal waveform. The spreading of signalling pulses can also result in signal "overshoot". Overshoot arises when a dispersed portion of a given signalling pulse, g(t-nT), being a portion that has spread outside of the symbol period associated with the given signalling pulse, constructively interferes with a neighbouring signalling pulse [e.g. g(t-(n-l)T)] so as to increase the magnitude of the total signal waveform S(t) located within the symbol period of the neighbouring signalling pulse g(t-(n-l)T). Several other signalling pulses may also constructively interfere with the signalling pulse g(t-(n-l)T) thereby compounding the signal increase within its symbol period.

If signalling pulse spread is large enough, such constructive interference may cause a modulated signal waveform to become larger than is desired. This is known as signal "overshoot" since a modulated signal waveform momentarily "overshoots" a maximum signal threshold level.

This is undesirable since signal amplifiers employed in modulators and/or transmitters employing such modulation schemes are therefore required to be capable of operating at power levels which can accommodate any power surges associated with signal over-shoot. Since such power surges will typically occur as a result of a suitably high level of constructive interference between successive symbol periods (which is typically infrequent), it is often the case that modulator and/or transmitter amplifiers spend much of their operational lifetime operating only at a fraction of their maximum permissible power levels. This is highly inefficient.

In the past, attempts have been made to overcome these over-shoots by clipping or compression of the modulated waveform, but these techniques can have highly undesirable effects on the occupied spectrum or waveform shape.

The present invention aims to overcome at least some of the deficiencies in the prior art as outlined above.

In the present context, the term "symbol" as used herein is intended to include symbols containing only one data bit as well as symbols containing more than one data bit.

The present invention is concerned with modulation schemes in which the waveform of a signal is modulated to convey a succession of data symbols each of which is associated with a respective one of a plurality of predetermined signal-states represented by, and being properties of, the signal waveform of the modulated signal within a respective symbol period thereof (or at a respective symbol time/location thereof). The present invention is also concerned with providing such a modulation scheme in which the signal waveform of the modulated signal may be controlled/modified such that, for example, "over-shoot" in the modulated signal may be controlled.

At its most general, the present invention proposes modifying the signalstate or states associated with one or more symbols in a succession of data symbols on a purely temporary basis (i.e. as and when required) whenever it is determined that modulation of a carrier signal waveform to convey successive of the one or more data symbols would or does induce unacceptable levels of carrier signal waveform over-shoot or would or does induce/produce other undesirable signal waveform shapes or properties (e.g. the waveform being too small in places, or being too susceptible to signal noise etc.).

Thus, the present invention proposes a temporary or "on the fly" modification or adjustment of the signal state(s) used, in an appropriate modulation scheme, to map onto a predetermined data symbol when and where it is determined that the unmodified signal-state (or states) would or does induce unacceptable levels of signal over- shoot if modulation were to proceed using only that unmodified signal- state(s).

Modification or adjustment of signal-states may occur in a two-step process whereby initial signal modulation is performed using a set of unmodified signal-states, and subsequently, suitable adjustment/modification of one or more of those states is calculated on the basis of observed overshoot in the initial modulated signal (corresponding to the unmodified signal-states). The suitable adjustment/modification is subsequently either applied to the original unmodified signal-states to generate a modified set of signal-states which may then be used to re-modulate a signal, or the calculated values of the adjustment/modification may be used alone to generate a separate "adjustment" waveform signal which is then combined/added with the initially modulated waveform (corresponding to the unmodified signal-states) so as to effectively modify the original signal-states and produce a final modified waveform for transmission. That is to say, the effect of the two-stage modulation is as if modulation had occurred in only one modulation step using only the modified/adjusted signal-states.

Thus, the present invention proposes to use a modulation scheme in which the predetermined set of signal-states defined thereby is a dynamic set in which although each one of the signal-states is preferably always associated with the same respective one data symbol value, each of the signal-states may change or vary from time to time depending upon the order in which successive data symbol values are modulated, and therefore upon the order in which the signal-states are employed during modulation.

The term "signal-state" is intended to refer to any predetermined physical property, or chosen combination of several different predetermined physical properties, of a signal waveform which is (are) employed to define the state or condition of the signal waveform at any given point in time. Examples of suitable predetermined physical properties are: a signal amplitude value, a signal phase value, a signal intensity value etc. Other physical properties may be employed as would be readily apparent to the skilled person. The choice of predetermined physical values employed to define signal- states generally determines a general modulation scheme, such as Quadrature Amplitude Modulation (QAM) in which the predetermined physical values are: a signal amplitude (R); and, a concurrent signal phase value (a). These two values define a coordinate (R,0) in "signal-space" within which each unique signal-state has a unique coordinate (position).

The modulation schemes in respect of which the present invention may be employed include any modulation scheme which is linearly or non-linearly filtered, such as (but not limited to) QAM (Quadrature Amplitude Modulation), PSK (Phase-Shift Keying), QPSK (Quadrature Phase-Shift Keying) or APK (Amplitude and Phase Keying). Non-linear filtering may include filtering of the phase component.

Thus, it will be appreciated that signal over-shoot is not inherently associated with any one signal state or data symbol per se, but rather, the occurrence of signal over-shoot is sensitively dependent upon the order in which data symbols are arrayed in a succession of data symbols and therefore the order in which the signal-state associated with that succession of data symbols are used during modulation. It has been appreciated by the applicant that, in some cases, the greater the difference between the signal-states associated with successive data symbols, the greater the degree of signal over-shoot occurring in the signal waveform resulting from use of those signal-states during signal modulation.

Signal-states may be said to differ if one or more of the aforementioned predetermined physical properties employed in defining a given signalstate differs from the same physical property (or properties) employed in defining another signal-state.

Accordingly, in a first of its aspects, the present invention may provide a modulation control apparatus for use in modulating signals to convey a succession of data symbols, the modulation control apparatus including: a signal-state control means arranged to receive a first signal waveform modulated according to a plurality of signal-states each signal-state being associated with a respective one of a succession of data symbols such that said first signal waveform conveys the succession of data symbols, and when a predetermined condition is met: to generate one or more temporary signal-state modification quantities to be applied to the signal-state associated with one or more data symbols within the succession of data symbols while said predetermined condition is met for use in generating a second signal waveform being a modification of said first signal waveform to convey the succession of data symbols such that the amplitude of at least a part of the second signal waveform differs from the amplitude of a corresponding part of the first signal waveform.

The signal-state modification quantities thus modify the signal states of symbols controllably by choosing which symbols to modify according to the dynamics of the signal sequence. The one or more signal-state modification quantities are preferably chosen/generated dynamically according to the succession of data symbols to be conveyed by the signal waveform. The value(s) of signal- state modification quantities are preferably determined according to the content of parts of the succession of data symbols (e.g the values of the symbols themselves and their order within the succession) since this content determines the succession of signal-states which are to be employed (in a given modulation scheme) to convey those symbols.

The signal-state control means is preferably arranged so as to separately and individually chose, determine or calculate how much to modify each signal-state (and not just between discrete signal states). Any and all signal-states employed within a given modulation scheme may be subject to application thereto of a modification quantity should the signal-state control means determine that this is required and need not be limited to a few of the signal-states employed in the modulation scheme.

Constraints may be applied to the form that the signal state modification may take. For example, modifications may be limited to changes only in the phase of a signal- state, or be limited in the extent to which a signalstate amplitude is permitted to change etc. The present invention is able to dynamically modify the signal-states of a modulation scheme in a way which adapts to the data symbols sequence to be conveyed whilst still substantially retaining the desired spectral characteristics of the modulated signal.

The modulation control apparatus may also include modulation means arranged to receive the succession of data symbols, to associate each received data symbol with a respective one of the received signal states, to generate the first signal waveform accordingly, to receive said one or more signal-state modification quantities, and to generate the second signal waveform employing the one or more signal-state modification quantities when the predetermined condition is met.

The signal-state control means may be arranged such that the amplitude of said at least a part of said second signal waveform is less than the amplitude of said corresponding part of said first signal waveform where signal overshoot occurs.

Thus, the present invention in its first aspect provides dynamic adjustment, variation or modification of the signal-states employed in a given modulation scheme. The predetermined condition is preferably any suitable property condition of the signal waveform which is indicative of the presence of signal over-shoot in that waveform. Signal-states are consequently modified in order to reduce the degree of signal over-shoot that may occur as a result of, for example, constructive interference between signalling pulses in successive data symbol periods of a modulated carrier signal waveform.

The signal-state control means provides a dynamic control of the signalstates of the given modulation scheme in the sense that signal-state modification quantities are only applied when it is deemed necessary to do so. In turn, the presence and extent of any overshoot will depend upon the order in which data symbols are arranged in a succession of data symbols being modulated, and therefore the order in which the predetermined signal- states associated with those data symbols are used during modulation. In this sense, the signal-state control means is arranged to temporarily modify signal-states.

The signal-states represent one or more predetermined physical properties of a signal waveform modulated according thereto, which predetermined physical properties preferably include at least a signal amplitude value and wherein the signal-state control means is arranged to generate one or more temporary signal-state modification quantities to be applied to the signal states associated with the one or more data symbols so as to modify (e.g. reduce or increase) at least the signal amplitude values thereof.

Thus, the signal-state control means may reduce or suppress signal overshoot within a given symbol period simply by applying (or generating for application by a modulating means) the signal-state modification quantity associated with the data symbol being conveyed within symbol periods at, near to, or surrounding these parts of ]6 the signal waveform suffering overshoot. For example, the signal-states associated with at least one of (or both of) the data symbols immediately succeeding and preceding the region of overshoot may be preferentially modified in this way, being nearest to that overshoot region.

Alternatively, or additionally, the signal-state control means may generate and/or apply signal-states modification quantities to any number of the aforementioned one or more data symbols within the succession of data symbols.

Preferably, the one or more predetermined physical properties which define each of the plurality of signal- states represent a coordinate within a common signal- space conforming to a predetermined modulation scheme in which data symbols conveyed by modulated signals correspond with respective coordinates in the common signal-space, and the signal-state control means is arranged to generate the signal-state modification quantities of the one or more data symbols such that, when applied, a given modification quantity will modify the signal-space coordinates thereof. For example, the predetermined modulation scheme may be a Quadrature Amplitude Modulation (QAM) modulation scheme or a QPSK scheme or the like.

Thus, when QAM or QPSK is employed in modulating signals, the plurality of predetermined signal-states defined by the modulation scheme form a constellation of signal- state points in signal-space. These signal-states are employed to map symbol values onto the phase and amplitude of the carrier signal waveform. The position of each signal-state point in the QAM or QPSK constellation represents both a phase difference between the Quadrature (Q) and In- Phase (I) components of the carrier signal waveform, and an amplitude thereof.

Consequently, control of the modulation of the portions of a signal waveform associated with selected data symbols (e.g. the signalling pulse of given symbols) can be achieved, according to the present invention, by actually or effectively moving a signal-state coordinates of the selected data symbol(s) relative to the rest of the QAM constellation. This signalstate coordinates movement is achieved by applying a signal state modification quantity thereto, the modification quantity being (or being equivalent to) a displacement in QAM space or in the signal space of any other suitable modulation scheme other than QAM such as Quadrature Phase Shift Keying (QPSK) or any other linear modulation scheme, the invention being not limited to QAM.

Additionally, or alternatively, such modulation control may be achieved by moving the signal-state coordinates of data symbols other than the selected data symbol(s) relative to the signal-state coordinate of selected data symbol(s). Consequently, the phase and/or the amplitudes of the Quadrature (Q) and In-Phase (I) components of the modulated carrier signal waveform may be suitably modified so as to affect a modulation of the given waveform that reduces over-shoot in that waveform.

The signal-state control means may be arranged to temporarily modify the signal-states (via application of modification quantities thereto) associated with the one or more data symbols such that the waveform(s) thereof (e.g. signalling pulses) within, or contributing to, the second signal waveform destructively interfere with the waveform(s) (e.g. signalling pulses) of at least one other data symbol within, or contributing to, the second signal waveform so as to modify (e.g. reduce) the amplitude of at least a part of the second signal waveform relative to the corresponding part(s) in the first signal waveform.

This arrangement is born of the realization that the component of the waveform of the modulated carrier signal associated any one symbol (or symbol period), such as a signalling pulse, is often able to influence the waveform of the carrier signal neighbouring that symbol period, including the waveform contained within at least one or more other symbol periods (e.g. immediately neighbouring symbol periods). Thus, one may indirectly influence the extent of over-shoot present within a signal waveform located in a given symbol period by directly modifying the signal waveform located in, or associated with, neighbouring symbols and symbol periods (e.g. modifying the associated signalling pulses). The signal-state control means is preferably arranged to implement such indirect modulation control.

The signal-state control means may be preferably arranged to generate temporary signal-state modification quantities to temporarily modify each of the signal- states associated with the one or more data symbols according to the contribution made by the signalling pulse associated with the respective data symbols to the said corresponding part of the first signal waveform (e.g. where overshoot occurs in the first signal waveform). Thus, for example, the degree of destructive interference (or reduction in constructive interference) required to be affected by the temporary modification of signalling pulses will depend upon the size/nature of the waveform to be interfered with.

Preferably, the predetermined condition is met when the magnitude or amplitude of the first signal waveform exceeds a predetermined amplitude threshold value. But other predetermined conditions may be employed.

In a variation of the invention according to its first aspect the signalstate control means is arranged, when a predetermined condition is met: to generate one or more temporary signal-state modification quantities to be applied to the signal-state associated with one or more data symbols while said predetermined condition is met for use in generating a second signal waveform being a modification of said first signal waveform to convey the succession of data symbols such that the amplitude of at least a part of the second signal waveform is greater than the amplitude of the corresponding part of the first signal waveform.

Thus, whilst the invention may be used to reduce the amplitude of peaks in a signal waveform to avoid or reduce signal overshoot, a variation of the invention is to reduce the amplitude (i.e. depth) of troughs in a signal waveform, or to increase the amplitude of regions of a signal waveform which have amplitude peaks of undesirably small magnitude. The effect is to increase or amplify the signal power in specified regions of the signal waveform where it is desirable to do so.

Consequently, a suitable predetermined condition which may be employed in the above variation may be any suitable property or condition of the signal waveform which is indicative of the signal magnitude/power being too low either in absolute terms (i.e. relative to a minimum threshold value) or in relative terms (i.e. relative to surrounding/neighbouring levels of signal magnitude/power).

In yet a further variant of the invention in its first aspect, the predetermined condition may be one which indicates the presence of (or the likely susceptibility to subsequently incur) unacceptable levels of noise in the first signal waveform, and the signal-state control means may be arranged, according to this further variant, to generate temporary signal-state modification quantities to be applied to the signal-state associated with one or more data symbols while said predetermined condition is met for use in generating a second signal waveform being a modification of the first signal waveform to convey the succession of data symbols such that the signal-states associated with at least a part of the second signal waveform are less similar to each other (e.g. have a greater separation in signal-state space) than are the signal-states associated with the corresponding part of the first signal waveform.

Thus, by increasing the dissimilarity between signal states of some or all of the signal-states used to modulate the second waveform, as compared to the corresponding levels of similarity between signal-states used to modulate the first waveform, one may improve the immunity of the second signal waveform to noise. The modification quantities may be chosen so as to increase the "separation" between signal states used to modulate the second waveform. For example, the signal-state coordinates in the modulation scheme employed (e.g. QAM) S may become more dispersed or separated in the associated modulation space (e.g. QAM space).

A suitable predetermined condition for use in this further variation could be the presence of noise or symbol errors (e.g. at the receiver receiving the second signal waveform) exceed a given threshold or are otherwise deemed to be excessive.

It is to be understood that the above variation and the above further variation to the invention in its first aspect may also comprise all, some or none of the preferable features and variants described above in relation to the first aspect of the invention. Of course, any suitable modifications to those preferables and variants such as would be required so as to properly conform to the aims, purpose and function of the above variation or the above further variation, are encompassed within the scope of the present invention.

Preferably, the one or more data symbols include the data symbols associated with regions (e.g. symbol periods) of the first signal waveform immediately succeeding and immediately preceding the regions of the first signal waveform in which the predetermined condition is met.

For example, the one or more data symbols may form a predetermined range of successive data symbols.

Preferably, the signal-state control means is arranged to generate said temporary signal-state modification quantities to constrain symbol error associated with the one or more data symbols resulting from the temporary modification of the signal-states thereof.

Thus, as will be readily appreciated, in modifying the signal-state assigned to a given data symbol, one may inadvertently incur symbol error at a receiver receiving a signal modulated according to a signal-state so modified. This is because, due to the dynamic and temporary nature of the modulation scheme according to the present invention, a receiver may be "unaware" of any such signal-state modifications and, consequently, may be unable to map the signal-state of a received (modified) signal to any of the plurality of predetermined signal states which it "expects" to see, or may map the received signal-state (modified) to the wrong symbol.

Such symbol errors may be constrained, while still allowing signal-state modification to occur in accordance with the present invention, by estimating the extent of symbol error likely to occur at the receiver in respect of a given number of symbols to be transmitted. If the estimated value of symbol error exceeds a predetermined threshold value, the signalstate control means may be arranged to re-modify one or more signalstates and Demodulate the signal to be transmitted in such a way as to lower the expected level of symbol error to a value below the threshold value.

Various Least-Mean-Square minimization algorithms may be employed to achieve this end as described herein.

Preferably, each of the plurality of signal-states is at least partly defined by a signal amplitude value associated with the signal-state.

For example, each of the plurality of signal-states may be a QuadratureAmplitude Modulation (QAM) signal-state.

Each CAM signal state is preferably defined by an amplitude value of the signal-state and a phase- difference value associated with the phase difference between the In- phase (I) and Quadrature (Q) components of the signal-state.

The invention encompasses a signal modulator or a signal transmitter comprising a modulation control apparatus according to the present invention in its first aspect for example whether or not including any or all of the aforementioned variants or preferable features thereof.

The signal modulator preferably includes modulation means arranged to receive the succession of data symbols, to IS associate each received data symbol with a respective one of said plurality of signal states, to generate the first signal waveform accordingly, to receive said one or more signal-state modification quantities, and to generate the second signal waveform employing the one or more signal state modification quantities when said predetermined condition is met.

The modulation means may be arranged to generate a modification waveform being a signal waveform modulated according to said signal-state modification quantities, and to subsequently combine said modification waveform with said first signal waveform thereby to apply said signalstate modification quantities conveyed by said modification waveform to said signal-state conveyed by said first signal waveform, and thereby to generate said second signal waveform.

The signal modulator may further include a variable signal delay means operable to apply a variable signal delay to said first signal waveform prior to the combination thereof with said modification waveform such that the modification waveform is adjustably time-shifted with respect to said first signal waveform when combined therewith thereby to adjustably control the second signal waveform.

It will be readily apparent that the signal modulator according to the first aspect of the present invention implements a method of modulation control. Accordingly, in a second of its aspects, the present invention may provide a method of modulation control for use in modulating signals to convey a succession of data symbols the method of modulation control including: receiving a first signal waveform modulated according to a plurality of signal-states each signal- state being associated with a respective one of a succession of data symbols such that said first signal waveform conveys the succession of data symbols, and when a predetermined condition is met: generating one or more temporary signal-state modification quantities to be applied to the signal-state associated with one or more data symbols within the succession of data symbols while said predetermined condition is met for use in generating a second signal waveform being a modification of said first signal waveform to convey the succession of data symbols such that the amplitude of at least a part of the second signal waveform differs from the amplitude of a corresponding part of the first signal waveform.

The amplitude of said at least a part of the second signal waveform may be less than the amplitude of the said corresponding part of the first signal waveform where signal overshoot occurs, or may be greater than the amplitude at said corresponding part (e.g. if it is desired to increase a weak signal).

The method preferably includes generating said second signal waveform employing one or more signal-state modification quantities when the predetermined condition is met, and may also include the step of initially generating said first signal waveform.

Preferably the signal-states represent one or more predetermined physical properties of a signal waveform modulated according thereto, which predetermined physical properties include at least a signal amplitude value and wherein the method includes generating one or more temporary signal-state modification quantities to be applied to the signal states of the one or more data symbols to modify (e.g. reduce or increase) the signal amplitude values thereof.

Preferably the one or more predetermined physical properties which define each of the plurality of signal- states represent a coordinate within a common signal space conforming to a predetermined modulation scheme in which data symbols conveyed by modulated signals correspond with respective coordinates in the common signal-space, and the method includes generating the signal-state modification quantities of the one or more data symbols such that, when applied, a given modification quantity will modify the signal-space coordinates thereof. The predetermined modulation scheme may be a Quadrature Amplitude Modulation (QAM) modulation scheme or any other linear modulation scheme such as Quadrature Phase Shift Keying (QPSK).

The method may include temporarily modifying the signal- states associated with the one or more data symbols such that the waveform(s) thereof (i.e. the signalling pulse(s) associated therewith) within the second signal waveform destructively interfere with the waveform(s) (i.e. the signalling pulse(s) associated therewith) of at least one other data symbol within the second signal waveform so as to modify (e.g. reduce) the amplitude of at least a part of the second signal waveform relative to the corresponding part(s) in the first signal waveform.

For example, the method may include generating said temporary signalstate modification quantities to temporarily modify each of the signalstates associated with the one or more data symbols according to the contribution made by the signalling pulse associated with the respective data symbols to the said corresponding part of the first signal waveform.

The method may be such that the predetermined condition is met when the amplitude of the first signal waveform exceeds a predetermined amplitude threshold value.

Preferably the one or more data symbols include the data symbols associated with regions of the first signal waveform immediately succeeding and immediately preceding the regions of the first signal waveform in which the predetermined condition is met.

The one or more data symbols may form a predetermined range of successive data symbols.

The method preferably also includes generating said temporary signalstate modification quantities to constrain symbol error associated with the one or more data symbols resulting from the temporary modification of the signal-states thereof.

The present invention, in its second aspect, may provide a method of signal modulation comprising a method of modulation control as aforementioned. The present invention may also provide a method of signal transmission comprising such a method of signal modulation control.

The method of signal modulation may include: receiving said succession of data symbols, associating each received data symbol with a respective one of said plurality of signal states, generating a first signal waveform accordingly, receiving said one or more signal-state modification quantities, generating the second signal waveform employing the one or more signal- state modification quantities when said predetermined condition is met.

The method may further include generating a modification waveform being a signal waveform modulated according to said signal-state modification quantities subsequently combining said modification waveform with said first signal waveform thereby applying said signal-state modification quantities conveyed by said modification waveform to said signal-states conveyed by said first- signal waveform, and thereby to generate said second waveform.

The method may also include applying a variable delay to said first signal waveform prior to the combination thereof with said modification waveform such that said modification waveform is adjustably time-shifted with respect to said first signal waveform when combined therewith thereby to adjustably control the second signal waveform.

For example, each of the plurality of signal-states may be a Quadrature Amplitude Modulation (QAM) signal-state.

The present invention, in any of its aspects, may be implemented using software means, and the present invention may therefore provide a computer program or software product containing computer program codes which implement the method of modulation control when appropriately executed via a computer.

It will be readily appreciated that the present invention encompasses a method of signal modulation and/or of signal transmission comprising the method of signal modulation control according to the present invention in its second aspect.

Examples of the invention shall now be illustrated in the following nonlimiting embodiments with reference to the accompanying drawings of which: Figure 1 schematically illustrates part of a QAM S signal-space containing a constellation of 16 distinct QAM signal-states; Figures 2 and 3 illustrate a succession of four signal-state amplitude values, and the associated modulated carrier signal waveform before modification of signal-states; Figure 4 illustrates the QAM signal-space illustrated in Figure 1, in which modification of two signal-states is illustrated; Figure 5 illustrates a succession of four signal state amplitude values, and the associated modulated carrier signal waveform after the modification of two signal-states; Figure 6 illustrates the dynamic QAM signal-space illustrated in Figure 1 over a period of time and subject to many modifications to each signal-state; Figure 7 illustrates the static QAM signal space illustrated in Figure 1 together with the trajectory, through signal space, of the modulated carrier waveform over a period of time; Figure 8 illustrates the dynamic QAM signal space illustrated in Figure 6 together with the trajectory, through signal space, of the modulated carrier waveform over a period of time; Figure 9 illustrates the static QAM signal space containing a circular constellation of eight distinct QAM signal-states together with the trajectory, through signal space, of the modulated carrier waveform over a period of time; Figure 10 illustrates the dynamic QAM signal-space comprising the circular array of eight distinct QAM signal-states subject to dynamic variation over a period of time together with the trajectory, through signal space, of the modulated carrier waveform over the same period of time; Figure 11 illustrates both the static circular constellation of signal-states illustrated in Figure 9 and the dynamic signal-states illustrated in Figure 10 over the same period of time during the modulation of a signal carrier waveform; Figure 12 schematically illustrates the impulse response of a band-limiting signal filter; Figure 13 illustrates a signal transmission system comprising two separate band-limiting transmitters; Figure 14 illustrates a transmission system comprising the single band limiting transmitter filter In the Figures, like items have been assigned like reference numerals for consistency.

Referring to Figure 1 there is illustrated a Quadrature Amplitude Modulation (CAM) signal space containing a QAM I signal-state constellation of 16 signal-states each one - of which is defined a unique pair of polar coordinates (R. 0). The radial coordinate R represents the amplitude of a signal and the angular coordinate O represents the phase difference between the In- Phase component (I) and the Quadrature component (Q) of the signal. The radial coordinate R is related to the In-phase (I) components and Quadrature-phase component (Q) by the relation: R2=I2+Q2. The I and Q components are modulated individually and separately.

Each one of the 16 unique signal-states in the signal- state constellation illustrated in Figure 1 is assigned a unique 4-bit data symbol from the range of data symbols starting from the 4-bit symbol "0000" up to and including the 16th 4-bit data symbol "1111", and including all intermediate 4-bit symbol values. For example, the first quadrant of the QAM signal-space contains the four signal-states A, B. C and D which are defined by their signal-space coordinates A (RA, OA) , B (RB, OB) , C (RC, OC) and D (RD' OD) respectively, and the third quadrant contains state E (RE, OK) . The signal-state A is assigned or associated with a first 4-bit data symbol "0000", the signal-state B is assigned a different 4-bit signal-state being "0001", the signal-state C is assigned a 4-bit data symbol "0010", the signal-state D is assigned the symbol value of "0011", and the signal-state E is assigned the symbol value "1011".

Thus, in order to modulate a carrier signal to convey the 4-symbol succession of 4-bit data symbols: [0000 0000 1011 0001], a modulator would be required to modulate a carrier waveform in accordance with the following four QAM signal-states in the following order: [A A E B].

Figure 2 illustrates the In-Phase component (I) of a signal waveform modulated according to a QAM modulation scheme. The portion 20 of the waveform illustrated has been QAM modulated so as to convey the aforementioned 4 symbol succession of 4-bit data symbols represented by the following order of QAM signal-states: [A A E B].

The filtering applied to the signal waveform in the present invention is preferably designed so that there is little or no inter-symbol interference. This is preferably achieved using a filter of a particular shape (a Nyquist filter). In the present invention a part of this filtering may be applied at the transmitter to the signal waveform to be transmitted prior to its transmission, and part of the filtering may then be applied to the received signal at the receiver (e.g. see the description relating to Figures 6 to ll below). As such, the transmit filter may not meet the zero (inter symbol interference) ISI criterion on its own, and consequently the QAM constellation points will tend not to coincide exactly with the transmitted waveform (although they will be close), but will coincide with the waveform after receive filtering. An exception to this (also applicable in the present invention) is known in the art as a "brick wall" filter with bandwidth matching substantially exactly the bandwidth of the symbol rate, and with infinitely sharp transition at the bandwidth edges. Once this type of filter has been applied to a signal waveform at the transmitter the QAM points will coincide with the transmitted waveform.

Application of another "brick wall" filter at the receiver will have no further effect on the relevant part of the received signal waveform.

For the purposes of clarity of illustration, a brick wall (no filter roll-off) filter has been applied in producing the waveforms illustrated in Figures 2, 3, and 5 such that the modulated QAM points do coincide with the state of the waveform (e.g. signal magnitude) at modulation points therein. However, it is to be noted that the present invention is generally applicable to any roll-off where the QAM points do not exactly coincide with the transmitted waveform (but do coincide when filtering is subsequently applied at the receiver as discussed above).

Thus, in the present examples illustrated in Figures 2, 3 and 5, the modulated signal waveform 20 is modulated such that the signal magnitude value thereof coincides with the amplitude value associated with a predetermined QAM signal-state at any time equal to an integer multiple of the symbol period of data symbols conveyed by the modulated waveform. That is to say, at time t=T the magnitude value 31 of the signal waveform 20 has a value Ik, while at time t=2T the magnitude value 32 of the modulated signal waveform once more has a value equal to IA. Similarly, at the succeeding instant of time t=3T, the magnitude value 33 of the modulated signal waveform 20 is equal to IS, and then at time t=4T has the value IB Thus, at successive predetermined instants in time, separated by a symbol period, the modulated single waveform 20 is shaped so as to acquire predetermined values at those instants of time. The predetermined values are the magnitude values associated with the In- Phase (I) component of the predetermined signal-states which are associated with the data symbols being conveyed by the modulated waveform 20 (together with the Quadrature-Phase (Q) component of the complete modulated signal). The predetermined times in question are at the ends of symbol periods of duration T seconds.

It will be seen from Figure 2 that certain regions of the modulated signal waveform 20 exceed a predetermined threshold signal value "Th" and consequently result in signal overshoot "o/s". The signal threshold value "Th" is a predetermined user-defined maximum signal value for the InPhase (I) component of the modulated signal waveform 20, and represents the signal value above which it is undesirable for amplifiers within a signal modulator to operate. The cause of such overshoot "o/s" can be appreciated with reference to Figure 3. Figure 3 illustrates the modulated In-Phase (I) component 20 of the modulated signal waveform, together with its associated magnitude values IA and IE at signal times T. 2T and 3T (i.e. items 31, 32 and 33 respectively).

Also shown in Figure 3 are the signalling pulses associated with each of the three data symbols represented by the modulated signal waveform 20 at the three aforementioned times. It is principally these three signalling pulses which, in superposition, define the shape and form of the modulated signal 20 as shall now be explained.

A first signalling pulse 21 is associated with the data symbol "0000" and therefore with the DAM signal-state A as illustrated in Figure 1. The first signalling pulse 21 is also associated with the symbol period which extends from t=0 to t=T. In the absence of any signal spectrum band-limiting, the first signalling pulse 21 would be of an impulse form fully confined within its associated symbol period and located at time t=T.

However, due to the dispersion or spreading inherent in band-limiting, the first signalling pulse 21 is no longer confined to its associated symbol period and spreads into neighbouring and successive symbol periods. Indeed, with this basing of the time axis the maximum magnitude value of the first signalling pulse 21 occurs at the very end of its own symbol period (i.e. at t=T) as indicated by the magnitude value 31 of Figure 3.

A second signalling pulse 22 is associated with the successive single period extending from t=T to t=2T and is intended to convey the data symbol "0000" associated with the QAM signal-state A. Once more, the dispersive effect of spectral band-limiting has caused the impulse (at t=2T)of the second signalling pulse 22 to spread beyond the bounds of its symbol period. Thus, the waveform of the second signalling pulse 22 enters into the symbol period associated with the first signalling pulse 21 and destructively interferes with the portions of the first signalling pulse located therein.

Conversely, the portions of the first signalling pulse 21 which extend into the symbol period associated with the second signalling pulse 22 constructively interfere with the second signalling pulse within that symbol period. Once more, the maximum magnitude value of the second signalling pulse 22 resides at the end of the symbol period associated with that signalling pulse (i.e. at t=2T) and has a value of IA at that point in time.

A third signalling pulse 23 associated with the successive symbol period extending from t=2T to t=3T, is intended to convey the data symbol "1011" and is therefore associated with the QAM signal-state E as illustrated in Figure 1. Yet again, dispersion of the impulse (at t=3T) of the third signalling pulse 23 has caused the waveform of that pulse to spread into the symbol period associated with both the second signalling pulse 22 and the first signalling pulse 21. Similarly the signalling pulse waveforms of both the first signalling pulse 21 and the second signalling pulse 22 each extend into the symbol period associated with the third signalling pulse 23. Consequently, constructive and destructive interference occurs between the waveforms associated with each of the three signalling pulses 21, 22 and 23 within the respective symbol periods associated therewith.

In particular it is to be noted that the portions of the first signalling pulse 21, the second signalling pulse 22 and the third signalling pulse 23 which are located within the symbol period of the second signalling pulse (i.e. between t=T and t=2T) all constructively interfere.

This means that those portions of the three signalling pulses superpose constructively and result in a relatively large resultant signal waveform 20 within the mid-regions of the second symbol period. As can be seen from Figures 2 and 3, this constructive interference causes the resultant signal waveform 20 to exceed the signal threshold "Th" thereby resulting in "signal overshoot".

Thus, signal overshoot may result from the dispersion of signal impulses inherent in the band limiting of a modulated waveform. The same general dispersion would be seen if the signalling pulses were rectangular in their undispersed form rather than pure impulses.

Careful choice of the band-limiting filter used to effect such bandlimiting may reduce or substantially eliminate intersymbol interference (ISI). It is to be noted that a signal receiver may be arranged to sample received modulated signals at sampling times which are an integer multiple of the symbol period (T) of a received and modulated signal. By ensuring that all filtered signalling pulses (e.g. signalling pulses 21, 22 and 23.) within a modulated waveform 20 are shaped so that any given signalling pulse will pass through O (i.e. have O pulse value) at all receiver sampling times other than the sampling time of the given pulse, it is possible to suppress ISI. Such an arrangement is employed in signal! modulation according to the present invention as illustrated in Figure 3 wherein the waveform of any given signal impulse passes through a O pulse value whenever the magnitude of another signalling pulse reaches its maximum value.

For example, the first signalling pulse 21 reaches a peak at time t=T, and each of the second and third signalling pulses, 22 and 23 respectively, have a value of 0 at t=T.

Similarly, while the second signalling pulse 22 has a maximum value at t=2T, the neighbouring first and third signal impulses, 21 and 23 respectively, each have a value of O at t=2T (note, a brick-wall filtering is assumed here as discussed above). I Another consequence of this arrangement is that the magnitude value of the resultant modulated waveform 20 coincides with the maximum value of a signalling pulse at times which are an integer multiple of the symbol period (T). Thus, control of the maximum value of a given signalling pulse directly controls the signal-state of that signal at that time, and thereby controls the data modulation of the resultant waveform 20.

In order to reduce the degree of overshoot "o/s" occurring within the signal waveform 20 during the symbol period associated with the second signalling pulse 22, the modulation control method and apparatus of the present invention modifies one or more signal impulses in order to reduce the magnitude of the resultant waveform arising from the superposition of contributing portions of the component signalling pulses (e.g. 21, 22 & 23) of the resultant waveform.

For example, regarding the symbol period associated with the second signalling pulse 22 (i.e. between t=T and t=2T), the three principle contributions to the resultant signal waveform 20 arise from the first, second and third signal impulses, 21, 22 and 23. The contributions from each of these three signalling pulses superpose constructively within this symbol period. According to the present invention, the result of such superposition may be reduced by reducing the maximum magnitude value of one or more of any of the three signalling pulses 21, 22 or 23. For example, by reducing the maximum magnitude value 31 associated with the first signalling pulse 21, the contribution of that signalling pulse to signal overshoot is reduced. Similarly, by reducing the maximum magnitude value 32 associated with the second signalling pulse 22, the size of the contribution of that signalling pulse to signal overshoot is also reduced. Again, by reducing the maximum magnitude value 33 associated with the third signalling pulse 23, one may reduce the contribution provided by that signalling pulse to the aforementioned overshoot.

Of course, the principle contributing signalling pulses are the first and the second signalling pulse, 21 and 22 respectively. By reducing the maximum magnitude values, 31 and 32, of both of the first and second signalling pulses 21 and 22 one may more effectively suppress signal overshoot or, put another way, by reducing the magnitude values 31 and 32 of the two signalling pulses 21 and 22 immediately neighbouring either side of the region of signal overshoot, such signal overshoot may be suppressed by smaller magnitude reductions than would be the case were only one of the two signalling pulses 21 and 22 modified.

It will be readily appreciated that the nature and extent of modification of a signalling pulse, in order to achieve a suppression of signal overshoot, will depend to sensitively upon both the nature of and the order in which data symbols are to be modulated upon a signal waveform 20 since the shape and magnitude of the resultant waveform 20 will, at any instant in time, be sensitively dependent upon the magnitudes and polarities of each of the contributing portions of the many different signalling pulses which superpose to form the resultant waveform 20 at that instant in time.

For example, Figures 2 and 3 illustrate a resultant waveform 20 modulated so as to convey the following succession of four-bit data symbols: [0000 0000 1011 0001]. That is to say, the signal waveform 20 is modulated such that at the four successive signal sampling times t=T, 2T, 3T and 4T, the waveform 20 possesses magnitude values of IA, IA, IE and IB, each such value being associated with the signal-states A, A, E and B respectively in succession. It will be readily appreciated that the In-Phase (I) component of signal state E is equal in magnitude but opposite in polarity to the In-Phase (I) component of the signal-state A. Thus, IA = - IE.

However, now consider a slightly different signal waveform which is very similar to waveform 20 but modulated so as to convey the following succession of data symbols: [0000 0000 0000 0001], this would require the same first, second and fourth signalling pulses as used in waveform 20, but would require that the polarity IS of the third signalling pulse 23 of Figure 3 to be reversed. It would be substantially identical to both the first and the second signalling pulses, but with its maximum magnitude value located at t=3T.

Thus, if the data symbol associated with the third signalling pulse were to be the value "0000" (signal- state A) rather than the value "1011" (signal-state E), then the polarity of the portion of the third signalling pulse which resides within the symbol period associated with the second signalling pulse 22, (i.e. between times t=T and t=2T) would be negative rather than positive.

That negative portion would consequently destructively (rather than constructively) interfere with the concurrent positive portions of the first and second signalling pulses 21 and 22 thereby substantially reducing the resultant superposition, and therefore the magnitude of the signalling waveform 22 residing within the aforementioned symbol period.

Should the destructive interference provided by this portion of the third signalling pulse be sufficiently large, signal overshoot in the resultant signal waveform may be suppressed.

The present invention proposes, in such situations, increasing the degree of destructive interference (or reducing the degree of constructive interference) provided by a contributing portion of a signalling pulse within a given symbol period so as to suppress signal overshoot within that symbol period. In the present example, concerning the train of states[A A A B], such an increase of destructive interference could be achieved by applying a signal-state modification to the signal state A of the third signalling pulse temporarily such that, during time t=2T to t=3T, A(RA,OA) -A A'' (RA'',OA'') as illustrated in Figure 4, so as to increase the magnitude of that signalling pulse. The result would be to increase the negative contribution to the composite waveform 20 provided by the third signalling pulse (pulse 23 inverted) in the region t=T to t=2T.

Alternatively, or additionally, one may reduce the degree of constructive interference occurring in the region t=T to t=2T by temporarily applying a modification to the signal-state A during the period t=0 to t=T and t=T to t=2T such that A -A A' and/or temporarily modifying signal-state E during t=2T to t=3T such that E -a E'.

This would reduce the magnitudes of the first, second and/or third signalling pulses, and thereby also reduce their constructive contributions to the overall waveform in the period t=T to t=2T.

Figure 4 illustrates the QAM signal-space illustrated in Figure 1. Additional signal-states are illustrated in the form of modified signalstate A'(RA', DA'), A'' (RA'', DA'') and the modified signal-state E' (RE', SE')- The modified signal-state A' illustrates the QAM signal-space coordinate position to which the original signal-state A may be moved temporarily in order to affect the modification in a signalling pulse employed to convey the data symbol "0000" in accordance with the signalstate A'. This is achieved by applying a signal-state modification quantity A (R. O) to the signal state A at least during the time t=T to t=2T such that: A (RA, (3A) A (RA, DA) + = A (RA', DA' ) at least during that time period and possibly longer, the to state A' may then be returned to its original value A thereafter. Thus, the first and second signalling pulses 21 and 22 of Figure 3 are such that their maximum magnitude values, 31 and 32, correspond with the In-Phase component (I) of the signal-state A of Figure 4.

However, by temporarily modifying the signal-state A, by changing it into signal-state A', one correspondingly modifies the maximum amplitude values associated with the first and second signalling pulses 21 and 22. In the example shown in Figure 4, the maximum magnitude values, 31 and 32, of the first and second signalling pulses 21 and 22 would be reduced thereby assisting in a suppression of signal overshoot "o/s" as discussed above.

Similarly, Figure 4 illustrates a modified DAM signal space position for the signal-state E, used to convey the data symbol "1011". The modified signal-state being E' (RE,,OE,) which, once more, has a smaller In-Phase (I) component than does the signal-state E. This is achieved by applying a signal-state modification quantity A (R,8) to the signal state E at least during the time t=2T to t=3T such that: E (RE,EE) E (RE,EE) + = E (RE',EE, ) during that time period at least, and then returning E' to its original state E thereafter.

Consequently, by temporarily modifying the third signalling pulse 23 such that its maximum magnitude value 33 conforms to the In-Phase (I) component of the temporarily modified signal-state E', one may reduce the contribution of the third signalling pulse to signal overshoot as discussed above.

Figure 5 illustrates the consequence of temporarily modifying the signalstates A'A' and E'E' during modulation of a signal waveform using the aforementioned first, second and third signalling pulses 21, 22 and 23 correspondingly temporarily modified. The maximum magnitude values, 42 and 43, associated with the first and second modified signalling pulses are both reduced from a value IA to a value IA', while the maximum magnitude value 44 associated with the third modified signalling pulse is reduced from a value IS to a value IE' Reducing the maximum value of any given signalling pulse has the effect of reducing all other parts of that signalling pulse. Consequently, those parts of the first, second and third modified signalling pulses which once contributed to signal overshoot "o/s" within the symbol period extending from t=T to t=2T have been substantially reduced by applying suitable signal-state modification quantities to appropriate signal-states.

The result is that the resultant modulated signal waveform 20' is reduced within this symbol period as indicated by arrow 40 in Figure 5. Other peak regions of the resultant waveform may be similarly reduced (in the sense indicated by arrow 41 of Figure 5). It is to be noted that though the present example is illustrated in terms of the In-phase component (I) of a modulated signal waveform, the invention in any of its aspects is of course equally applicable to the Quadrature-phase (Q) component of the signal waveform either concurrently or separately, or to maintain the same phase or constrain some other characteristic of I or Q or their modification or a function thereof.

Figure 6 illustrates a 16-state QAM constellation having been subject to repeated application of signal-state modification quantities (I) over an extended period of time during signal modulation. Repeated application may result in the values being changed at each repeat. At any given instant in time the QAM constellation comprises lO only the 16 distinct signal-states (i.e. points in the constellation) illustrated in Figure 1. However, each signal-state is subject to many different position modifications (I) over time and each of these different positions is illustrated in Figure 6.

The result of this recordal of signal-state modification is the generation of 16 distinct signal-state point clusters, such as cluster 60, which records the different positions occupied by one (changing) signal state, in this case signal state E(RE,EE).

Figure 7 illustrates a static 16-state QAM constellation (as illustrated in Figure 1) in use over an extended period of time during modulation of a carrier signal waveform. The meandering line 73 represents the trajectory in DAM space of the carrier signal being modulated using the static 16-state constellation 70.

As can be seen, the trajectory 73 of the carrier signal waveform hops from one signal-state to another signal- state as the carrier signal waveform is modulated according to those signal-states.

However, from time-to-time the trajectory 73 of the carrier signal waveform overshoots a signal-state and passes beyond the outermost signalstates of the constellation thereby entering positions in QAM space having coordinates which represent high carrier signal amplitudes and therefore high signal power levels.

It is desirable that such high signal amplitudes do not exceed a predetermined amplitude threshold 72 which corresponds to a maximum safe signal power level beyond which it is detrimental/unsafe to operate the modulator/transmitter equipment which performs the modulation/transmission of the resulting modulated signal waveforms.

In order to accommodate signal trajectory overshoot, the QAM constellation 70 is compressed so as to occupy a relatively small inner region 71 of QAM space well within the region of DAN space defined by the threshold 72.

This ensures that while signal trajectories 73 might overshoot the boundary 71 of the inner region, they will not overshoot the threshold 72. However, since overshoot occurs relatively infrequently, this means that the signal trajectory spends most of its time confined to the inner region 71 and relatively little time in the outer region between the boundary 71 of the inner region and the threshold 72 of QAM space.

Thus, the carrier signal waveform is forced to possess relatively small power levels most of the time, only gaining high power levels when overshoot briefly occurs and never when it acquires a QAM signal-state value.

Figure 8 illustrates a solution to this waste of potential carrier signal power in accordance with an embodiment of the present invention. Referring to Figure 8, the 16 QAM signal-state constellation 80 is expanded in QAM space such that the separation between QAN signal- states is increased and the outermost signal-states occupy the outer region of QAM space between the inner region 71 and the outer threshold 72. This permits the modulated carrier signal waveform to possess a higher average power as can be seen from the expanded trajectory 81 thereof.

In addition, the expanded signal state constellation 80 is rendered a dynamic constellation in accordance with the present invention so as to suppress any overshoot in the expanded carrier signal trajectory 81 which might otherwise have caused that trajectory to exceed the outer threshold 72 of QAM space.

It will be noted that the QAM constellation of Figure 8 illustrates 16 distinct signal-state position clusters which illustrate a record of the various positions occupied by each one of the 16 dynamic signal-states over time as has already been discussed with reference to Figure 6.

Thus, higher average power output is possible in the carrier signal waveform without suffering the damage/detriment of signal overshoot. Additionally, by expanding the QAM constellation one increases the separation between adjacent signal-states in the QAM constellation thereby making those signal states more different/distinguishable from each other and so increasing their immunity to noise etc. when a carrier signal modulated according to those separated signal- states is subsequently decoded.

Figure 9 illustrates a static QAM constellation comprising 8 static signal-states 90 in a circular array representing an inner region within which the trajectory 9l of a carrier signal waveform, modulated according to the static signal- states 90, is mostly confined save for brief periods of overshoot into the outer region adjacent the threshold 72.

Figure lo illustrates the effect of expanding the QAM constellation of Figure 9 and rendering that constellation dynamic according to the present invention.

The new carrier signal trajectory 92 occupies the high- power outer region as a result.

In this example the signal-state modifications (I) are such that modification of a signal-state occurs only along a line joining the signal-state to the origin of QAM space. This has the advantage of reducing phase errors during demodulation of the modulated carrier signal.

Figure 11 illustrates the clusters 93 of positions which have been occupied by dynamic QAM signal-states of Figure together with the positions of the static signal- states of Figure 9.

]O In Figures 6 to 11 the dynamic QAM points shown are always those after a receiver filter has acted upon a received modulated waveform (i.e. other than a brick-wall filtering process has been used in which filtering is split between the transmitter and the receiver). The trajectory lines (73, 81, 91 and 92) correspond with the state of the modulated signal waveform after the transmit filtering and before the receive filtering (i.e. at the transmitter amplifier where we are interested in overshoot).

This signal-state modification quantities (a) may be determined according to any number of methodologies, algorithms or procedures and one such suitable methodology is a Least Mean Square (LMS) method as follows.

Least Mean Square (LMS): Let a linear filtering function have an impulse response of GO(t), where t=0 is at the peak of the response for the purposes of example and, without any loss of generality, has a value of 1 at its peak. Let Gl(t), be another filtering function, which may be the same as GO(t) or may be different from that impulse response.

Referring to Figure 12 there is schematically illustrated a representation of the impulse response corresponding to a typical signalling pulse generated by either or both of the Finite Impulse Response filters employed in band- limiting the frequency spectrum of impulse-like signalling pulses. Preferably, the filtering functions will be Finite Impulse Response filters. Alternatively, other filtering forms such as Infinite Impulse Response filters are used.

Consider that a modulated signal waveform is produced by a succession of signalling pulses such as the one illustrated in Figure 12 and let T be the interval between successive signalling pulses. If the epoch of the peak of the overshoot through filter GO(t) is centred at a point in time L measured relative to the time position of the preceding signalling pulse, and assuming T to be the interval between successive signalling pulses, then the contribution C arising from each neighbouring signalling pulse k (where k= -N,-N+l,...,N) near to the epoch of overshoot may be determined according to the equation: Ck = Gl(L-kT) The contribution Ck from the kth neighbouring signalling - pulses are summed and then scaled (normalized) such that the sum is equal to one (l). This is achieved simply by multiplying each contribution Ck by the reciprocal of the sum of the contributions.

Figure 12 illustrates how each unmodified contribution at the point of overshoot may be determined. The contribution to the overshoot arising from each of the successive neighbouring unmodified signalling states (Sk) is determined from the value of the impulse response function GO(t), at those points on the time axis t=L-kT where the centre of each of the neighbouring signalling pulses is located, the overshoot sample being the sum of the products Sk GO(L-kT).

Let the value of the overshoot to be reduced be X, then the signal-state modification quantity Ok of each signalling pulse k is the product -X.Ck using the rescaled/normalised values of Ck. Without any loss of generality, all of these numbers may be complex-valued numbers. In certain instances it may be desirable to increase or decrease the value of a signal-state modification quantity by multiplying the value so derived by a multiplier which is greater or less than 1. For example a multiplier slightly greater than 1 may be applied such that Ok = -X. Ck. 1. 05 in order to reduce overshoot more than would be the case in the absence of the multiplier.

The signal-state modification quantities Ok therefore represent a displacement vector defined in signal-space (e.g. QAM space) having coordinates in that space (e.g. on In-phase component (I) and a Quadrature-phase (Q.) component in QAM space) which, when added to an existing signal-state, cause that signal-state to be displaced in signal space.

Thus, with Gl(t)=GO(t), if an original signal-state is given by Sk, then the modified signal-state S' k is given by S' k = Sk + Ilk. The modified signal-state is used to define a modified signalling pulse which is then subsequently band-limited using a filter having an impulse response GO(t) to provide the signal waveform to be transmitted. It should be noted that since the signalling states are filtered, the spectral characteristics of the transmitted waveform are retained whether S' k or Sk are used.

It is to be noted that a single band-limiting filter may be used for the purposes of filtering each of: the unmodified signal-states Sk; the signal-state modification quantities Ilk; and, the modified signal-state values S' k.

However, separate filters may be used whereby the signal states Sk are passed to the filter with impulse response GO(t) and the signal-state modification quantities Ilk are passed to a separate filter having an impulse response Gl(t) which may be identical or different to GO(t). In this latter case the output of both filters may subsequently be summed together to provide a signal waveform to be transmitted. The spectral characteristics of the transmitted waveform will now also depend on the filter Gl(t), which can be chosen as a compromise between spectral performance and achieving the required signal modification.

Referring to Figure 13 there is illustrated a signal telecommunications system employing a signal modulator in which a modulation control apparatus (and method) is employed according to the present invention.

The telecommunications system includes a CAM signal modulator unit 108 which is operable to receive via a data input port thereof, and to store therein, a succession of data symbols 107 which are subsequently to be conveyed by a modulated signal waveform. The QAN modulator unit assigns to each one of the received data symbols (e.g. the kth received data symbol) a corresponding QAM signal-state Sk. Thus, the string of data symbols 107 received by the QAN signal modulator unit is converted thereby into a string of signal-states Sk for output thereby to an input of a first Finite Impulse-Response (FIR) transmitter filter 109 having an impulse response GO(t). Alternatively 109 may be an Infinite Impulse Response (IIR) Filter.

The first band-limiting filter 109 is operable to generate a succession of signalling pulses GO(t-nT) in the form of a train of dispersed signalling pulses such as is illustrated in Figure 3 and Figure 12, being centred at times t=nT (n=1,2,3. . .) where T is the interval between successive signalling pulses.

A "raised-cosine" type filter or a root raised-cosine" filter such as would be readily appreciated by the skilled person, is often employed for this purpose. The effect of the first band-limiting filter 109 is to limit the spectrum of the modulated signal waveform to a predefined finite frequency spectral band, the boundaries of the spectral band having a smooth downward "roll-off" region (i.e. where the spectrum edge is) that is in the shape of part of a raised cosine function. A "root raised-cosine" filter results in a spectral band having a shape equal to the square root of a raised-cosine type spectrum.

Band-limiting such as this results in each of the signalling pulses being an "impulse" that has been dispersed in time as discussed above.

The modulated signal waveform generated by the first band-limiting filter 109 is output thereby on a bifurcating transmission line 102 simultaneously to a variable signal delay unit 105 and to a modulation control unit 100. The modulation control unit 100 is operable to monitor the modulated signal waveform output by the first band-limiting filter for the presence of overshoot therein. Should the presence of overshoot be detected, the modulation control unit 100 is operable to calculate the appropriate signal-state modification quantity ink according to the Least Mean Square (LMS) method described above employing a predetermined range (i.e. k = -N to N) of signalling pulses adjacent the region of overshoot within the modulated signal waveform.

The signal-state modification quantities ink are then output from the modulation control unit 100 to an input of a second Finite Impulse Response (FIR) band-limiting filter 110. The second band-limiting filter has an impulse response of Gl(t) which may be different to the impulse response GO(t), but in this embodiment is equal to the latter (i.e. Gl(t) = GO(t)). The second band limiting filter generates a modification waveform which corresponds to a series of signalling pulses each generated in accordance with QAM signal-space coordinates corresponding to the signal-state modification quantities ink generated by the modulation control unit 100.

The modification waveform generated by the second band- limiting filter is output upon a transmission line 104 lO and input to an adder unit 106 simultaneously with the original (unmodified) modulated signal waveform emanating from the variable signal delay unit. The signal delay imposed by the delay unit 105 is such as to delay the signals input thereto by a period of time equal to that required to generate an output from the second band- limiting filter in respect of the same signal. The modulated signal waveform modulated in accordance with static DAM signal-states is output by the delay unit 105 upon a transmission line 103 to be combined at an adder unit 106 with the dynamic (i.e. variable over time) modification waveform.

The effect of adding the modification waveform to the original modulated signal waveform is to cause the former to effectively modulate the latter so as to effectively apply the signal-state modification quantities thereto.

The resulting modified modulated signal waveform is output by the adder unit 106 on an output line 111 for a subsequent transmission by an antenna 112.

It is to be noted that the effect of applying the modification waveform from the second band-limiting filter 110 to the modulated signal generated by the first band-limiting filter 109 is to indirectly modify the QAM signal-state constellation used to modulate the final signal waveform to be transmitted. That is to say, the signal-state of the final modulated waveform at points in time t=nT (n=1,2,3,...), where T is the interval between signalling pulses, is a signal-state which has been modified according to the signal-state modification quantities generated by the control unit 100. Thus, the adder unit 106 together with the second band-limiting filter 110 act together as a means of modulation of the signal waveform in accordance with the present invention.

Figure 19 illustrates an alternative embodiment of the present invention in which the second band-limiting filter 110, the signal delay unit 105 and the adder unit 106 are no longer present. Instead, the output Ok of the modulation control unit 100 is input to a single band limiting filter 120, and that filter alone is operable to receive both the original signal-states Sk and also the signal-state modification quantities ink generated by the modulation control unit 100. The single band-limiting filter 120 is further operable alone to generate the final modulated signal waveform S' k = Sk + ink for subsequent transmission via antenna 112. To achieve this, the single band-limiting filter initially generates the unmodified signal-states Sk, inputs those to the modulation control unit 100, receives any signal-state modification quantities ink therefrom and adds those modification quantities to the original signal-states so as to generate the modified signal-states S' k, and replaced the original signal states Sk with the modified signal-states S' k, then outputs the result for subsequent transmission. This process may be iterated several times in order to further reduce signal overshoot if one cycle of the procedure is not sufficient.

Similarly, where two separate band-limiting filters are employed such as illustrated in Figure 13, an iterative or repeating procedure may be performed whereby the modified waveform output by the adder unit 106 of the transmitter is directed to the input of the first band limiting filter 109 and is subsequently thereby subjected to a further round of signal overshoot reduction.

A number of variants to the basic Least Mean Square (LMS) algorithm above are described below.

In a first variant, herein referred to as Constant Phase LMS, the signalstate modification quantities are calculated such as to displace signalstates only along a line through the origin of the modulation constellation in QAM space (or in the appropriate space of some other suitable modulation procedure such as Phase Shift Keying). This results in substantially no phase error in the modified signal-states and is particularly suited to phase modulation schemes such as Phase Shift Keying. The perturbations of each signal-state are readily calculated, these being the same as those for the above LMS method, but of angle along the first line below and with amplitude (prior to scaling to a magnitude of 1) multiplied by the sine of the angle between the following two lines in signal space: the first line is the line between the origin of signal space and the position of the signal-state to be modified; and the second line is the line connecting the origin of signal space and the position in signal space of that part of the trajectory of the modulated signal waveform which displays signal overshoot.

When all of the LMS contributions have been modified by this multiplication, they are subsequently summed. The contributions are then each normalised/rescaled (i.e. multiplied by the reciprocal of their sum) to ensure that their sum is equal to a value of 1. These new contributions are then multiplied by the overshoot value X which it is desired to reduce so as to yield the signal-state modification quantity Ilk for each individual signal-state K in the same manner as discussed above in respect of the basic LMS method.

In a further variant, the number of symbols k to which a signal-state modification quantity Ilk iS applied, according to any of the above algorithms, is limited to those symbols near to the region of overshoot within the modulated signal waveform. For example two of these signal impulses before and two of the signal impulses after the region of overshoot may be the only signal impulses subjected to signal-state modification in order to reduce or remove that particular region of signal overshoot.

In a further variant of the LMS algorithm above, the magnitude of the signal-state modification quantities ink are constrained in magnitude such that they always lie within a circle of chosen radius originating from the original signal-state constellation point to which they are applied. The limitation is applied such that the phase angle with the constellation of a signal-state modification quantity remains predominantly unchanged.

In yet a further variant of the above LMS algorithm, the magnitude of some or each signal-state modification quantity is constrained such that the component coordinate values of the modification quantity are independently limited to lie within a predetermined range of values. This means that, for example, in QAM signal space where the In-phase component (I) and the Quadrature-phase component (Q) axes are at right angles to each other, each modified signal-state is limited to reside within a square of signal space centred upon the original unmodified signal-state. This variant is particularly useful in preventing errors in QAM modulation by limiting the modification of a given signal-state to half the distance (or less) between S neighbouring signal-states within a given constellation.

With reference to the transmission system illustrated in Figure 13, it is to be noted that the signal delay unit may be suitably controlled such that the signal delay to applied thereby is variably different from the delay incurred by operation of the modulation control unit 100 and the second band-limiting filter 110. The consequence of this variable delay is to vary the time-position of the original modulated waveform relative to the time IS position of the modification waveform as the two are added together at the adder unit 106 of the transmitter.

Time-shifting of the modification waveform relative to the original modulated in this way offers greater versatility and flexibility in adjusting or refining the precise effects of the modulation of the original modulated signal waveform which results from the application thereto of the modification waveform.

The modulation control apparatus in employing the above LMS method (and any of its variants) inherently employs a symbol-error minimization algorithm in which the signal state of a predetermined range of successive data symbols are modified collectively and in dependence on each other in such a way as to reduce the occurrence of signal over- shoot within the modulated carrier waveform associated with that predetermined range of data symbols, whilst constraining or minimising the extent of symbol error to associated with that predetermined range of symbols as conveyed by the modulated carrier waveform.

The signal receiver of the signal transmission system of Figure 13, and Figure 14 comprises a receiver antenna 114, a signal demodulator 118 and a FIR or IIR signal filter 116. Signal transmissions 113 received by the antenna 114 of the receiver system are passed to an input of the signal filter 116 for filtering thereby.

Filtered signals are output 117 from the signal filter 116 for input to a DAM demodulator for signal demodulation thereby. The filter 116 is preferably a root-raised-cosine type filter such as would be readily apparent to the skilled person, and preferably will have a lowinter-symbol interference characteristic in combination when following filter 109 or filter 120 for Figure 13 and Figure 14 respectively.

Demodulated and filtered signals 119 are subsequently output as received data for subsequent data processing.

Such data processing may include error correction processing which may be required in order to recover from bit or symbol errors which may have been induced through any temporary CAM signal-state modification enforced by the modulation control unit 100 of the signal transmitter.

The first and second (when present)finite-impulse response filters of the transmitter 109/110 and the FIR filter 116 of the receiver are preferably "matched" filters of a type such as would be readily apparent to the skilled person for the purposes of reducing noise and symbol errors in the de-modulated data signal 119 generated by the signal receiver.

The combination of the transmitter filter 109 (or 109 and if different) and the receiver filter 116 is preferably to provide a zero inter-symbol interference (ISI) filter. Thus, half of the spectral filtering may be applied at the transmitter filter 109 (or 109 and 110) and half at the receiver filter 116. This is optimum in terms of reducing signal noise at the demodulator 118.

The carrier signal waveform employed in the present invention may comprise an electrical signal, an electromagnetic signal e.g. an optical signal.

The invention may include any variations, modifications and alternative applications of the above embodiments, as would be readily apparent to the skilled person, without departing from the scope of the present invention in any of its aspects.

Claims (38)

1. Claims: 1. A modulation control apparatus for use in modulating signals to

   convey a succession of data symbols, the modulation control apparatus including: a signal-state control means arranged to receive a first signal waveform modulated according to a plurality of signal-states each signal-state being associated with a respective one of a succession of data symbols such that said first signal waveform conveys the succession of data symbols, and when a predetermined condition is met: to generate one or more temporary signal-state modification quantities to be applied to the signal-state associated with one or more data symbols within the succession of data symbols while said predetermined condition is met for use in generating a second signal waveform being a modification of said first signal waveform to convey the succession of data symbols such that the amplitude of at least a part of the second signal waveform differs from the amplitude of a corresponding part of the first signal waveform.
2. 2. A modulation control apparatus according to Claim 1 in which said signal-state control means is arranged such that the amplitude of said at least a part of said second signal waveform is less than the amplitude of said corresponding part of said first signal waveform where signal overshoot occurs.
3. 3. A modulation control apparatus according to Claim 1 in which said signal-state control means is arranged such that the amplitude of said at least a part of said second single waveform is greater than the amplitude of said corresponding part of said first signal waveform.
4. 4. A modulation control means according to any preceding claim wherein the signal-states represent one or more predetermined physical properties of a signal waveform modulated according thereto, which predetermined physical properties include at least a signal amplitude value and wherein the signal-state control means is arranged to generate temporary signal- state modification quantities to modify the signal states associated with the one or more data symbols by modifying at least signal amplitude values thereof.
5. 5. A modulation control means according to Claim 4 wherein the one or more predetermined physical properties which define each of the plurality of signal-states represent a coordinate within a common signal-space conforming to a predetermined modulation scheme in which data symbols conveyed by modulated signals correspond with respective coordinates in the common signal-space, and the signal-state control means is arranged to generate temporary signal-state modification quantities which when applied modify the signal-state of the one or more data symbols by modifying the signal-space coordinates thereof.
6. 6. A modulation control means according to Claim 5 IS wherein the predetermined modulation scheme is a Quadrature Amplitude Modulation (QAM) modulation scheme.
7. 7. A modulation control means according to any preceding claim wherein the temporary signal-state modification quantities temporarily modify the signal-states associated with the one or more data symbols such that the signalling pulse(s) associated therewith within the second signal waveform destructively interfere with the signalling pulse(s) associated with at least one other data symbol within the second signal waveform so as to modify the amplitude of at least a part of the second signal waveform relative to the corresponding part(s) in the first signal waveform.
8. 8. A modulation control apparatus according to any preceding claim wherein the signal-state control means is arranged to generate said temporary signal-state modification quantities to temporarily modify each of the signal-states associated with the one or more data symbols according to the contribution made by the signalling pulse associated with the respective data symbols to the said corresponding part of the first signal waveform.
9. 9. A modulation control apparatus according to any preceding claim wherein the predetermined condition is met when the amplitude of the first signal waveform exceeds a predetermined amplitude threshold value.
10. 10. A modulation control apparatus according to any preceding claim wherein the one or more data symbols include the data symbols associated with regions of the first signal waveform succeeding and preceding the regions of the first signal waveform in which the predetermined condition is met.
11. 11. A modulation control apparatus according to any preceding claim wherein said one or more data symbols form a predetermined range of successive data symbols.
12. 12. A modulation control apparatus according to any preceding claim wherein the signal-state control means is arranged to generate said temporary signal-state modification quantities to constrain symbol error associated with the one or more data symbols resulting from the temporary modification of the signal-states thereof.
13. 13. A signal modulator comprising a modulation control apparatus according to any preceding claim.
14. 14. A signal modulator according to Claim 13 including modulation means arranged to receive the succession of data symbols, to associate each received data symbol with a respective one of said plurality of signal states, to generate the first signal waveform accordingly, to receive said one or more signal-state modification quantities, and to generate the second signal waveform employing the one or more signal-state modification quantities when said predetermined condition is met.
15. 15. A signal modulator according to Claim 19 wherein said modulation means is arranged to generate a modification waveform being a signal waveform modulated according to said signal-state modification quantities, and to subsequently combine said modification waveform with said first signal waveform thereby to apply said signal-state modification quantities conveyed by said modification waveform to said signal-state conveyed by said first signal waveform, and thereby to generate said second signal waveform.
16. 16. A signal modulator according to Claim 15 including a variable signal delay means operable to apply a variable signal delay to said first signal waveform prior to the combination thereof with said modification waveform such that the modification waveform is adjustably time-shifted with respect to said first signal waveform when combined therewith thereby to adjustably control the second signal waveform.
17. 17. A signal transmitter comprising a signal modulation control apparatus according to any preceding claim.
18. 18. A method of modulation control for use in modulating signals to convey a succession of data symbols, the method of modulation control including: receiving a first signal waveform modulated according to a plurality of signal-states each signal- state being associated with a respective one of a succession of data symbols such that said first signal waveform conveys the succession of data symbols, and when a predetermined condition is met: generating one or more temporary signal-state modification quantities to be applied to the signal-state IS associated with one or more data symbols within the received succession of data symbols while said predetermined condition is met for use in generating a second signal waveform being a modification of said first signal waveform to convey the succession of data symbols such that the amplitude of at least a part of the second signal waveform differs from the amplitude of a corresponding part of the first signal waveform.
19. 19. A method according to Claim 18 in which said amplitude of said at least a part of said second signal waveform is less than the amplitude of said corresponding part of said first signal waveform where signal overshoot occurs.
20. 20. A method according to Claim 18 in which said amplitude of said at least a part of said second signal waveform is greater than the amplitude of said corresponding part of said first signal waveform.
21. 21. A method of modulation control according to claims 18 to 20 wherein the signal-states represent one or more predetermined physical properties of a signal waveform modulated according thereto, which predetermined physical properties include at least a signal amplitude value and wherein the method includes generating temporary signal- state modification quantities to modify the signal states associated with the one or more data symbols to modify at least the signal amplitude values thereof.
22. 22. A method of modulation control according to Claim 21 wherein the one or more predetermined physical properties which define each of the plurality of signal-states represent a coordinate within a common signalspace conforming to a predetermined modulation scheme in which data symbols conveyed by modulated signals correspond with respective coordinates in the common signal-space, and the method includes generating temporary signal state modification quantities which when applied modify the signal-state of the one or more data symbols by modifying the signal-space coordinates thereof.
23. 23. A method of modulation control according to Claim 22 wherein the predetermined modulation scheme is a Quadrature Amplitude Modulation (QAM) modulation scheme.
24. 24. A method of modulation control according to any of preceding claims 18 to 23 wherein the method includes temporarily modifying the signal- states associated with the one or more data symbols such that the signalling pulse(s) associated therewith within the second signal waveform destructively interfere with the signalling pulse(s) associated with at least one other data symbol within the second signal waveform so as to modify the amplitude of at least a part of the second signal waveform relative to the corresponding part(s) in the first signal waveform.
25. 25. A method of modulation control according to any of claims 18 to 24 including generating said temporary signal-state modification quantities to temporarily modify each of the signal-states associated with the one or more data symbols according to the contribution made by the signalling pulse associated with the respective data symbols to the said corresponding part of said first signal waveform.
26. 26. A method of modulation control according to any of preceding claims 18 to 25 wherein the predetermined condition is met when the amplitude of the first signal waveform exceeds a predetermined amplitude threshold value.
27. 27. A method of modulation control according to any of preceding claims 18 to 26 wherein the one or more data symbols include the data symbols associated with regions of the first signal waveform immediately succeeding and immediately preceding the regions of the first signal waveform in which the predetermined condition is met.
28. 28. A method of modulation control according to any of preceding claims 18 to 27 wherein said one or more data symbols form a predetermined range of successive data symbols.
29. 29. A method of modulation control according to any of preceding claims 18 to 28 wherein the method includes generating said temporary signal- state modification quantities to constrain symbol error associated with the one or more data symbols resulting from the temporary modification of the signal-states thereof.
30. 30. A method of signal modulation comprising a method of modulation control according to any of preceding claims 18 to 29.
31. 31. A method of signal modulation according to Claim 30 including: receiving said succession of data symbols, associating each received data symbol with a respective one of said plurality of signal states, generating a first signal waveform accordingly) receiving said one or more signal-state modification quantities, generating the second signal waveform employing the one or more signal-state modification quantities when said predetermined condition is met.
32. 32. A method according to Claim 31 including: generating a modification waveform being a signal waveform modulated according to said signal-state modification quantities, substantially combining said modification waveform with said first signal waveform thereby applying said signalstate modification quantities conveyed by said modification waveform to said signal-states conveyed by said first signal waveform, and thereby to generate said second waveform.
33. 33. A method according to Claim 32 including applying a variably delay to said first signal waveform prior to the combination thereof with said modification waveform such that said modification waveform is adjustably time- shifted with respect to said first signal waveform when combined therewith thereby to adjustably control the second signal waveform.
34. 34. A method of signal transmission comprising a method of signal modulation control according to any of preceding claims 18 to 29. Do
35. 35. A computer program product comprising computer codes which implement the method according to any of claims 18 to 34 when executed on a computer.

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36. 36. A computer when programmed with computer codes which implement the method according to any of claims 18 to 34 when executed.
37. 37. A modulation control apparatus substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings.
38. 38. A method of modulation control substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings.