AU712700B2

AU712700B2 - Digital QAM

Info

Publication number: AU712700B2
Application number: AU64343/96A
Authority: AU
Inventors: Karlheinz Borst; Stefan Hoffmann; Uwe Lehmann
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1995-08-30
Filing date: 1996-08-29
Publication date: 1999-11-11
Anticipated expiration: 2016-08-29
Also published as: AU6434396A; EP0760567A2; EP0760567A3; CN1152209A; CN1074623C; NO963612D0; NO963612L

Description

S F Ref: 348791

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Siemens Aktiengesellschaft Wittelsbacherplatz 2 80333 Muenchen

GERMANY

Karlheinz Borst, Stefan Hoffmann and Uwe Lehmann Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia

S

S S. S S. S S SS S S.

Invention Title: Digital QAM The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845 1 Description Digital QAM The invention relates to a method for quadrature amplitude modulation and to a corresponding modulator Quadrature amplitude modulation, abbreviated below as QAM, is described in the journal "Der Fernmeldeingenieur", Verlag fir Wissenschaft und Leben Georg Heidecker GmbH Erlangen, Issues 8 and 9 of 1993, pages 51 to 54. QAM is based according to this on the use of two orthogonal carriers of the same frequency, that is to say a cosine carrier and a sine carrier which are also denoted as normal component and quadrature component. In the customary analog methods for QAM, an I baseband signal and a Q baseband signal, which can also be gener- 15 ated by multiplexing from a single baseband signal, are respectively fed to an input of a mixer. The mixer for the I baseband signal also receives the sine carrier with the phase shift of 0, while the other mixer receives the cosine carrier with the phase shift of 90. The output 20 signals of the two mixers are combined by addition to form a QAM signal which can be fed via a send filter to a transmission line, or as an IF signal to a transmit stage.

0 0

S

0000 0 0 0000 0 @00 00 *e S 00 0 0000 0000 0 00 0 0000 00 0 0 0000 00 0 t

D

It is chiefly filtered multi-stage signals which 25 ahe used -a e1 a signa^ a hieh 25 are used as baseband signals; in this case, the filter characteristic corresponds to the so-called root-Nyquist_ shaping, in order to generate bandwidth-efficient

QAM

signals. With regard to economic transmission,

QAM

signals havin 64o l and teen signals having 64 or 256 stages are being transmitted to an increasing degree. Strict requirements are made in the Processing of such signals, and it has been possible to realize them to date only with a high outlay. Thus, for example, it is required that the deviation of 900 carrier phase must be below 10, and the level difference between the two baseband signals before IF addition must be below dB. Because of the strict linearity requirements, the mixers may be operated only with a reduced output power, i V/7 O 2 that is to say, for example, with a spacing of 11 dB from the 1 dB compression point, and this results in strict isolation requirements for carrier suppression. It is also usual to arrange upstream of the mixer input lowpass filters which are realised as analog filters and which are to be subjected to equally strict requirements regarding attenuation and signal propagation time as those placed on the mixers. The realisation of the analog QAM modulators according to the prior art has so far been performed by achieving the carrier orthogonality and the synchronism of the baseband signals by factory adjustment, and achieving the carrier suppression and the linearity of the mixers by selecting suitable components, the result having been a high overall outlay.

A digital QAM modulator in which an I or Q baseband signal is generated by a modulator and output via digital filters to a module DDS for direction digital signal synthesis has already been described in "Fuba- Spiegel" 2/94, pages 11 to 14 on page 13, figure and description, last column and the whole of page 14. In addition to the comparatively very expensive solution, special problems arise in this case relating to the suppression of subsidiary waves.

The object of the invention is thus to develop a 25 less expensive QAM method and a less expensive QAM modulator. The object is achieved according to the invention by a method in accordance with Patent Claim 1, and by a QAM modulator in accordance with Claim 6.

The method according to the invention advan- 30 tageously offers the possibility of the exclusive use of digital modules and thus of the design of a QAM modulator using fully integrated technology, for example from commercially available programmable logic modules which are mostly delivered unprogrammed. Further advantages accrue by virtue of the fact that no factory adjustment is required, that the method is suitable for high QAM stage numbers, since no linearity problems occur in the mixing process, that exact orthogonality and exact synchronism of the baseband signals are ensured by signal 3 combination in one path, and also that integration with user-specific integrated circuits located upstream in the signal path is also possible for the purpose of signal processing. The method according to the invention is developed by means of the measures described in Patent Claims 2 to 5; the QAM modulator according to the invention is described in detail in Claims 6 and 8; and Claim 7 represents an alternative by means of which it is possible to eliminate a delay unit otherwise required.

The invention is to be explained in more detail below with the aid of an exemplary embodiment, represented in the drawing, of a QAM modulator.

Figure 1 shows the principle of the digital modulation and Figure 2 a QAM modulator according to the invention.

The principle of digital modulation is represented in Figure 1 for a filtered digital baseband signal having two sample values per baud period T. Represented in this case on the abscissa is a segment which shows as a typical example the signal characteristic between the 38th and the 43rd baud periods. The sinusoidal first o carrier Tl can assume the maximum amplitude values +1 and -1 and is synchronized with the baseband signal I in such a way that the sampled values of the baseband signal respectively occur at maxima or minima of the first carrier TI. In this case, the modulation is synonymous as multiplication with a sign inversion of each second value of the digital baseband signal, since the intermediate 0. values of the multiplication vanish. This principle 30 represented in Figure 1 holds for the modulation of both baseband signals I, Q, a second carrier needing to be used for the multiplication of the second baseband signal Q. Moreover, it is necessary for QAM that the carriers are offset in phase by 900 relative to one another, and so the skew of the carrier signals or of the modulated I and Q signals relative to one another must be T/4 for an intermediate frequency of fb l/T.

The digital QAM modulator represented in Figure 2 is configured starting from the above considerations.

4 Connected to the inputs EID, EQD for the I and Q baseband signals, respectively, are the inputs of a first and, respectively, second digital filter DFI, DF2, whose clock inputs receive a clock signal with a frequency corresponding to twice the desired intermediate frequency fb.

The digital filters DFl, DF2 have a root-Nyquist-characteristic with spectrum equalization; it is also possible to factor an additionally required propagation time into the filter so that then both signals can be transferred at twice the intermediate frequency. In the present case, in order to generate the additionally required propagation time, the output of the first digital filter DF1 is connected to a delay unit LZG which is clocked in the exemplary embodiment and has a propagation time corresponding to a period of a signal having four times the baud rate of the baseband signals, with the result that there is a phase shift of 900 for the output signals of the first digital filter DF1 with respect to those of the second digital filter DF2. The output signals of the 20 delay unit LZG are fed to a first input EM1 of a multiplexer MUX and, moreover, to the input of a first nverter II whose output signal is led to a second input EM2 of the multiplexer. In a corresponding way, the output signals of the second digital filter DF2 are fed 25 to a third input DM3 of the multiplexer MUX and to an input of a second inverter 12, whose output is connected to a fourth input EM4 of the multiplexer MUX, the Ssequence of sampling of the input signals being programmable and being selected for a regular spectrum in such a way that initially the second baseband signal SQ, then the first baseband signal I, then the inverted second baseband signal Q and then the inverted first baseband signal I are sampled. An inverted 0o00 spectrum requires a sequence in which the baseband P signals I, Q and the inverse baseband signals are interchanged in such a way that initially the first baseband signal I, then the second baseband signal Q, then the inverted first baseband signal and, finally, the inverted second baseband signal are sampled. Furthermore, the 5 output of a source for a clock signal having a frequency corresponding to four times the baud rate of the baseband signals is connected to a clock input TEM of the multiplexer MUX. The output of the multiplexer MUX is connected to the input of a digital equalizer DEZ which likewise receives a clock signal at four times the baud rate of the baseband signals, and has a characteristic sin 4x corresponding to an inverse -characteristic. The 4x output of the digital equalizer DEZ is connected to the input of a digital-to-analog converter DAW, which likewise receives a clock signal at four times the baud rate of the baseband signals and outputs at its output QAM signals to a lowpass filter TPF whose passband corresponds to the intermediate frequency and whose cutoff frequency corresponds to twice the intermediate frequency and whose output is connected to the output ZFA for the intermediate frequency signal which is generated.

The digital modulator according to Figure 2 has as its core a multiplexer MUX which, at twice the filter sampling rate, nests the modulated baseband signals in one another in offset fashion, and simultaneously carries S* out the 900 skew and the signal addition in the plane of the intermediate frequency. Upstream of the multiplexer, the baseband signals I, Q firstly traverse identical 25 lowpass filters which shape pulses and in which sampling is undertaken at a frequency corresponding to twice the baud rate of the baseband signals, it being the case that in the channel for the first baseband signal I, which is sampled in the downstream lowpass filter a quarter of a 30 baud period earlier, a propagation time of a quarter of a bit period is additionally realized, so that the signal delays of the two baseband signals are equal again. In the exemplary embodiment, the multiplexer MUX is fed the filtered data signals in the regular and inverted position and combined by said multiplexer at four times the baud rate to form a data stream.

Depending on the roll-off the output spectrum of the digital-to-analog converter has a useful range from 0 to times the intermediate frequency, the equalizing 6 characteristic about the intermediate frequency being, to a first approximation, linear with a slight parabolic distortion. Consequently, as linear equalizer the digital equalizer DEZ is connected downstream with an inverse sin 4x characteristic, through which it is achieved 4x that the characteristic of the output spectrum is constant up to 1.5 times the intermediate frequency; the harmonic component is suppressed by the lowpass filter TPF connected upstream of the IF output ZFA.

S

S.

S

o

Claims

1. A method for QAM modulation of two baseband signals I, Q, in which these are respectively multiplied separately by one of two carriers which are offset in phase by 900, and the resultant IF component signals are added to an IF output signal, characterized in that the baseband signals Q) are respectively separately filtered and sampled by means of a clock signal at a frequency corresponding to twice the baud rate of the baseband signals and samples are thereby generated, in that the samples of one baseband signal are delayed by a propagation time of a quarter of the period for the IF signal, in that inverse signals are formed from the delayed and the underlayed samples and said signals are added to a multiplex signal, the sampling sequence for a regular spectrum being such that initially the Q baseband signal, then the I baseband signal, then the inverted Q baseband signal and, finally, the inverted I baseband signal are sampled, and that for an inverted spectrum initially the I baseband signal, then the Q baseband 15 signal, then the inverted I baseband signal and, finally, the inverted Q baseband signal are sampled, in that the multiplex signal is equalized and, subsequently, subjected to digital- to-analog conversion, and in that the quasi-analog intermediate frequency signal thus generated is output after lowpass filtering to an IF output (ZFA). 20 2. A method as claimed in claim 1, characterized in that the baseband S signals Q) are filtered on a root-Nyquist-characteristic with combined spectrum equalization.

3. A method as claimed in claim 1, characterized in that the multiplex e sin4x 25 signal is equalized on an inverse characteristic. 4x

4. A method as claimed in claims 1 or 2, characterized in that the sampling of the baseband signals Q) is performed in conjunction with the filtering. 30 5. A method as claimed in claim 1 or 4, characterized in that the baseband signals Q) are sampled at a frequency corresponding to four times the baud rate of the baseband signals, in which case every second sample value is suppressed and the delay of the samples of one baseband signal is eliminated. I:\DayLib\LIBCC\02530.DOC -8-

6. A QAM Modulator for carrying out a method according to one or more of the preceding patent claims, characterized in that the signal input of a first digital filter (DF1) having a root-Nyquist-characteristic and spectrum equalization is connected to an input (EID) for a first baseband signal in that the second signal input (EQD) for the second baseband is connected to the signal input of a second digital filter (DF2) which has the same characteristic and function as the first digital filter (DF1), in that the clock inputs of the two digital filters (DF1, DF2) are connected to a source for a clock signal at a frequency corresponding to twice the baud rate of the baseband signals, in that the output of the first digital filter (DF1) is connected via a delay unit (LZG) having a propagation time corresponding to a period of a signal at a frequency corresponding to four times the baud rate of the baseband signals to a first signal input (EM1) of a multiplexer (MUX), and to the signal input of a first converter (II) whose signal output is connected to a second signal input (EM2) of the multiplexer (MUX), in that the output of the second digital filter (DF2) is connected to a third signal input (EM3) of the 15 multiplexer (MUX) and to a signal input of a second inverter (12) whose output terminal is connected to a fourth signal input (EM4) of the multiplexer (MUX), in that a clock signal input (TEM) of the multiplexer (MUX) is connected to a source for a clock signal *o at a frequency corresponding to four times the baud rate of the baseband signals, in that o. the output (EMA) of the multiplexer (MUX) is connected to the signal input of a digital 7 sin 4x 20 equalizer (DEZ) having an inverse equalizer characteristic whose output is 4x connected to the input of a digital-to-analog converter (DAW), of which the output is connected via a lowpass filter (TPF) to an output (ZFA) for the IF signal, and in that the clock inputs of the digital equalizer (DEZ) and of the digital-to-analog converter (DAW) are connected in parallel with the clock input (DEM) of the multiplexer (MUX).

7. A QAM modulator as claimed in claim 6, characterized in that the clock signal inputs of the first and of the second digital filter (DF1, DF2) are connected in parallel with the clock signal input (TEM) of the multiplexer (MUX), sampling is performed at four times the baud rate of the baseband signals and in this case the delay S* 30 unit (LZG) is eliminated.

8. A QAM modulator as claimed in claim 6, characterized in that the lowpass filter (TPF) has a passband corresponding to the intermediate frequency and a cut-off frequency corresponding to twice the intermediate frequency. I:\DayLib\LIBCC\02530.DOC -9-

9. A method substantially as described herein with reference to the accompanying drawings. s 10. Apparatus substantially as described herein with reference to the accompanying drawings. DATED this thirteenth Day of September, 1999 Siemens Aktiengesellschaft Patent Attorneys for the Applicant SPRUSON FERGUSON 0@ SO S S S S S S 5*55 S. 55 5 S S eSSS S .5 S S S 5055

55.5 S S S* S SS*S S S 55 0 0 S S S S 5505 S S. 0 65 S *S 1:\DayLib\LIBCC\02530.DOC