GB2134755A - Satellite transmission systems - Google Patents

Abstract

A random multiple access communication system comprising a transmitting means 23, 24, 25, 26, 27 situated at a transmitting station and radiating a chirp modulated signal provided by a pulsed surface acoustic wave dispersive delay line 22a binary coded to carry the required data with either a positive or negative frequency slope by encoders 11, 12 and flip-flop 16 a responder means 9 carried by an orbital satellite and including frequency shift means 4, 5, 6 operating on the signal received at the antenna 2 before re-radiating such signal with chirp modulation retained from antenna 8, receiving means comprising antenna 28 and a surface acoustic wave dispersive delay line 31 for compressing the chirp pulse, the compressed pulse outputs being sorted into a single pulse series by flip-flop 36 fed to decoders 42, 43 to recover the originally encoded data. The communication system may be applied to slot acquisition in a time division multiple access satellite communication system, provision of a talk-back facility in a television system, to emergency position indicating radio beacons and to data collection from distributed platforms. <IMAGE>

Description

SPECIFICATION Method and apparatus for transmitting data via a satellite This invention relates to a method of and system (apparatus) for transmitting data between a transmitting station, a transponder station and a receiving station.

In recent times systems for data communication which make use of a transponder on an orbital satellite (including satillites in geostationary orbits) have come more extensively into use but many problems in connection with such systems remain to be solved.

Satellite-carried transpondser involve substantial cost to construct and to put into orbit.

Such cost may be justified when the transponder can be used to provide a larger number of communication channels, but this requires that any given pair of transmitting and receiving stations shall be capable, so far as possible, of providing communication without prejudice to the proper operation fo any other of the pairs.

Broadly speaking, systems where a plurality of transmitting and a plurality of receiving stations share a common transponder station and which are termined multiple access systems, comprise the following, namely FDMA (frequency division multiple access), TDMA (time division multiple access), both of which preferably operate with coordinated access of each transmitting station to the transponder. A further system is SSMA (spread spectrum multiple access) which admits of uncoordinated, i.e. random, access by the transmitting stations (RAMA-random access multiple access). All entail certain limitations or drawbacks.

FDMA necessitates that the satellite tansponder power amplifier be operated below its saturated output power to control the amount of interference between carriers pertaining to respective pairs of transmitting and receiving stations and this reduces the capacity of the satellite.

TDMA requires complex design of the groundbase stations to ensure accurate synchronisation requirements, and also requires a high degree of coordination among users and the employment of ultra high stability osciliators.

RAMA, whether utilised in FDMA, TDMA or SSMA systems, is applicable primarily where the communication traffic is bursty, i.e. occurs in bursts rather than continuously.

If there is interference, in the sense of two or more emissions reaching the transponder simultaneously, the data may be lost and, therefore, detection of interference is necessary to enable such lost data to be retransmitted, preferably with random delays to avoid risk of repeated interference. The protocol is known as ALOHA and is described in a paper by Abramson M entitled "The throughput of packet broadcasting channels" I.E.E.E. Transactions on communication Vol. COM-25, June 1977, pages 117-128.

In SSMA systems the bandwidth to accommodate the modulation of the carrier is a great deal wider than is required for transmitting the information. The information is combined with a code (which may be pseudo-random) and the combined signal may be used to modulate the carrier in a mode termed direct sequence (providing phase shifting of the carrier, frequency hopping (change of carrier frequency between discrete values in accordance with the code within the bandwidth), or time hopping (change of transmission times in accordance with the code).

Since the received signal may be buried in interference and noise, recovery requires de spreading of the spectrum and thus entails provision of complex equipment at the receiving station to generate a replica of the code and recover the information from the received signal.

The present invention has for one of its objects the provision of a simplified low cost SSMA system operable in a RAMA mode but in which the probability of data loss due to interference between transmitted signals from different transmitting means of the system is reduced, and which may be operated without, or with reduced need for, use of the ALOHA protocol or similar procedures.

Thus, according to a first aspect of the invention, there is provided a communication system comprising (a) a transmitting means for operating at a transmitting station and including (i) means for transmitting a signal (the transmitted signal) comprising a succession of pulses of carrier wave energy, (ii) modulator means for frequency modulating the carrier within the pulse interval in a plurality of different modes, (iii) encoder means for selecting the mode of frequency modulation in response to respective different bits of data, (b) transducer means for operation at a transponder station remote from the transmitting station and including means for receiving the transmitted signal and re-radiating a corresponding but frequency shifted signal.

(c) receiving means for operating at a receiving station remote from both of the foregoing stations and including (i) a plurality of receiving elements each responsive to a respective one of. the different modes of frequency modulation to produce an output signal (ii) decoder means for receiving the output signals and recovering the data originally encoded by the encoder means.

Whilst there is flexibility as to the different modes of frequency modulation which may be employed to carry data, codes produced from the data can be carried simply and conveniently using frequency modulation which provides selectively for increase, or decrease, of the carrier frequency to represent the code symbols in the transmitted signal. For example, a binary '1 ' may be represented by an increasing frequency and an 'O' by a decreasing frequency.

The receiving slements each advantageously presents frequency dispersive characteristics matching the particular form of frequency modulation of the received signal to which each is required to respond, so that the output signal produced is a (time) compressed signal and is developed uniquely in response to that particular form of frequency modulation. Such elements are commonly termed frequency dispersive filters or dispersive delay lines.

In the receiving elements, successive portions or slices of the signal (notionally each radiated at a respective different frequency) are subjected to delays of respective magnitudes so correlated to the frequency subsisting during said portion or slice as to provide an output in which the portions or slices are additive and thus present signal gain.

The receiving elements may comprise surface acoustic wave filters. The modulator means may provide for a linear increase or linear decrease of the carrier frequency and the decoder means included in the receiving means may comprise surface acoustic wave filter means having linear frequency dispersive characteristics providing outputs in response to the particular rate of carrier frequency increase and decrease produced by the modulator means. Non-linear frequency modulation could be used and the surface acoustic wave filters would then have non-linear dispersive characteristics matching the respective modulations of the signals to which they are required to respond.

From a second aspect the invention resides in the provision of a method of communicating data between a transmitting station, a transponder station, and a receiving station, said method comprising: (a) transmitting from a transmitting station a succession of pulses of carrier wave energy each being frequency modulated within the pulse duration in accordance with one of a plurality of different modes of frequency modulation and representing respective bits of coded data, (b) receiving the transmitted signal at the transponder station and re-transmitting a corresponding but frequency shifted signal, (c) receiving the transponder signal at the receiving station and feeding it to a plurality of receiving elements responsive respectively to different ones of said modes of frequency modulation to produce an output signal, (d) decoding the output signals to recover the data.

The system in accordance with the first aspect of the invention and the method in accordance with the second aspect of the invention, both as herein defined, provide greatly reduced risk of interference (and loss of data) between signals transmitted from different transmitting means or stations on the same carrier frequency (mid frequency) and chirp modulated in the same manner even if partially (time) overlapping at the time of transmission. This occurs because after (time) compression at the receiving station(s) the compressed output signals are very much shorter (having a duration approximately equal to the reciprocal of the difference between the lowest and highest values of frequency of the chirp signal and therefore non-overlapping in time at the receiver output.

Further, with this system and method, interference can, if obscured, be avoided altogether between pairs of stations by utilising different frequency modulations for their encoded data. Time coordination among such pairs of transmitting and receiving stations is not required since the receiving elements are each responsive only to the respective different modes of frequency modulation produced by its associated transmitter. It will, of course, be evident that a given transmitter may transmit data to more than one receiving station without difficulty.

The inventoin may be applied advantageously, for slot acquisition, to a TDMA system of communication comprising a plurality of transmitting means for operation at respective transmitting stations, a transponder means for operation at a transponder station remote from the transmitting stations, one or more receiving means for operating at a receiving station or a plurality of receiving stations remote from the transponder and transmitting stations, and time division means for establishing coordinated operation of each transmitting means in a respective period (time slot) such that all of the transmitted data from each of the transmitting stations is incident at the transponder only during a respective time period (slot) allocated to that transmitting station in each of a succession of longer time periods (frames) afforded by the transponder means.

One of the problems which exists in TDMA systems is the acquisition by each transmitting station of its allocated time slot. Existing acquistion means are in some cases complex and in other cases relatively slow in operation in terms of the time available.

Thus, according to a third aspect of the present invention, a TDMA communication system includes a means for establishing acquisition of its proper time slot in the frame of the transponder means by any one of the transmitting means comprising .

(a) at the transmitting station concerned, means for transmitting a signal (the acquisition signal) comprising a succession of pulses of carrier wave energy, (b) modulator means for frequency modulating the carrier within the pulse interval in a predetermined mode, (c) in association with the receiving means, (i) a receiving element responsive to the predetermined mode of frequency modulation of acquisition signal pulses retransmitted by the transponder means with a shift of frequency but otherwise correspondingly frequency modulated, to provide, by way of the receiving element, a succession of output signal pulses, (ii) means responsive to the time relation between the transmitted acquisition signal pulses, the output signal pulses, and the time slots allocated to the transmitting station concerned to cause the transmitting means to transmit communication signals at times such that these will be incident at the transponder means within the proper time slots.

The means for transmitting the acquisition signal may be the transmitting means which normally transmits the time divided communication signal, but including means for reducing the power level of the acquisition signal relative to that of the communication signal.

From a fourth aspect the invention resides in the method of operating a TDMA communications system to acquire for any one of the transmitting means its proper slot in the frame of the transponder means, said method comprising the steps of: (a) transmitting from any one of the transmitting stations to the transponder, a signal (the acquisition signal) comprising a succession of pulses of carrier wave energy frequency modulated within each pulse duration in a predetermined mode, (b) receiving the acquisition signal pulses retransmitted from the transponder means with a shift of frequency but otherwise correspondingly frequency modulated and feeding the received signal to a receiving element responsive to the predetermined mode of frequency modulation to provide an output signal comprising a succession of pulses, (c) detecting time correlation between the incidence of the transmitted acquisition signal pulses, the output signal pulses, and the time slots allocated to the particular transmitting station from which the acquisition signals are transmitted, (d) and controlling the transmission times of communication signals transmitted from transmitting means at that station containing the data required to be communicated to the receiving station or one of same as a function of said time correlation.

Further, the invention can be applied to provide a talk-back facility in a communication system including video and audio channels such as a television transmitting and receiving system.

Thus, according to a fifth aspect of the invention, a communication system comprising a transmitting means for operation at a transmitting station and a plurality of receiving means for receiving signals transmitted by the transmitting means comprising a carrier wave modulated to transmit video and audio data, includes "talkback" means comprising: (a) at selected receiving stations (i) talk-back transmitting means for transmitting a talk-back signal comprising a succession of pulses of carrier wave energy, (ii) modulator means for frequency modulating the carrier within the pulse duration in a plurality of different modes, (iii) encoder means for encoding the talk-back information into a code in terms of the mode of frequency modulation, (b) talk-back receiving means at the transmitting station and including:: (i) a plurality of receiving elements each responsive to a respective one of the different modes of frequency modulation to produce an output signal, (ii) decoder means for receiving the output signal and recovering the talk-back data originally encoded by the encoder means.

A sixth aspect of the invention resides in a method of effecting talk-back in a communication system comprising a transmitting means for operation at a transmitting station, and a plurality of receiving means for receiving a signal transmitted by the transmitting means, such signal comprising a carrier wave modulated to transmit video and audio data, such method comprising the steps of:: (a) transmitting from any selected one or more of the receiving stations a talk-back signal comprising a succession of pulses of carrier wave energy, each being frequency modulated within the pulse duration in accordance with one of a plurality of different modes of frequency modulation, (b) encoding the talk-back data in terms of the different modes of frequency modulation, (c) applying the code to determine the particular one of the modes of frequency modulation applied to the signal, (d) receiving the talk-back signal at the transmitting station and feeding it to a plurality of receiving elements responsive respectively to different ones of said modes of frequency modulation to produce respective output signals, (e) decoding the talk-back information from the output signals.

In the second, third, fourth, fifth and sixth aspect of the invention the forms of frequency modulation, and the receiving elements responsive thereto, may be of the form, or may operate in the manner, as the case may be, already mentioned in connection with the first aspect of the invention.

The invention will be described by way of example with reference to the accompanying drawings wherein.

Figure 1 illustrates a typical "chirp" waveform which may be employed as the transmitted signal in a system according to the invention operationg in accordance with the method thereof; Figure 2 illustrates transmitted and receiving wave forms and the use of a dispersive delay line in the receiving means for effecting compression of the received "chirp" signal to produce an output pulse presenting signal gain; Figure 3 is a block schematic circuit diagram of one embodiment of transmitting means in accordance with the present invention for carrying out the method thereof; Figure 4 is a block schematic circuit diagram of one embodiment of receiving means in accordance \with the present invention for carrying out the met 7d thereof;; Figure 5 is a diagram plotting received signals and gate signals versus time and illustrating synchronisation of the received and gate signals preparatory to processing the required data; Figure 6 is a diagram plotting received signals and gate signals versus time for three series of similarly frequency modulated received signals and illustrating separation thereof; Figure 7 is e graphical representation plotting relative power level against frequency in respect of the spectral occupancy of a main communication signal in a conventional TDMA communication sytstem showing the main carrier and the carrier of an acquisition signal; Figure 8 is similar to Figure 7 but showing the spectral occupancy utilising a chirp modulated acquisition carrier;; Figure 9 is a block schematic circuit diagram of a TDMA system showing one embodiment of the invention applied to slot acquisition in such a system using a chirp modulated acquisition carrier such as that of Figure 8; Figure 10 is a block schematic circuit diagram of a television receiving system illustrating one embodiment of part of a system in accordance with the present invention providing a talk-back facility to the transmitting station; Figure 11 is a block schematic circuit diagram of a television transmitting system illustrating one embodiment of a further part of the talk-back facility of Figure 10 provided in this case at the television transmitting station; and Figure 12 illustrates application of the invention to data collection or search and rescue systems.

Referring to Figure 1, in each section of which amplitude A is plotted against time t the transmitted "chirp" signal results from modulation of a carrier in a pulse mode, each qlse having a duration T, (a) Figure 1 and in a frequency mode, (b) Figure 1 ,for communication between transmitting and receiving stations utilising an orbital satellite. Since initial development has been considered relative to an orbital test satellite (OTS) reference will be made to parameters applicable to this. In this case, the mean carrier frequency f0 may typically be 14GHz and the duration T of each pulse of the modulation, may typically be from 10 microseconds to as much as several hundred microseconds.

The frequency sweep f2-f1 utilised, as illustrated in section (b) of Figure 1, may typically be 5MHz.

The signal/noise ratio achieved on reception and processing (gain) of the signal increases as a function of the product of time (duration of pulse) and range of frequency sweep. Both of the factors, time, and range of frequency sweep, introduce practical limitations arising from respective rate of data transmission required, and cost and availability of apparatus.

Section (c) of Figure 1 is a simplified representation of the carrier wave when subjected to the "chirp" modulation (by way of a linear up sweep of frequency).

In Figure 2, amplitude A is plotted against time t in sections (a), (b), (c), (d) and (f). A "chirp" signal illustrated in section (c) of Figure 2 may be produced from a voltage controlled oscillator VCO shown in section (b) fed with a saw tooth control signal as seen in section (a). A preferred method however is to impulse a dispersive delay line, e.g.

a surface acoustic wave filter with a ver short dt ation pulse.

It will be noted that the "chirp" signals illustrated in section (c) of Figure 1 and of Figure 2 have respectively increasing and decreasing frequency sweep and may thus be utilised to represent "0" and "1" bits of a binary code.

In section (e) of Figure 2 the received "chirp" signal is applied at one terminal t1 of a dispersive delay line or chirp matched filter and this has the effect of compressing the frequency sweep which is conveniently linear so that, in effect, successive time portions or slices of the received signal are subjected to different delays and become additive at terminal t2 and form in affect a compressed pulse as represented in section (f) of Figure 2. This will take place only if the delay characteristics of the dispersive delay line are matched to the frequency sweep characteristics of the received signal. Frequency slopes of different value or other shapes of frequency modulation envelope would fail to produce a pulsed output.

Referring to Figure 3, data which may be generated from a keyboard 10 is fed to two convolutional encoders 11 and 12 effectively in series with each other, a typical data rate being 110 Bauds. The encoders have a rate 7/8, that is for every seven input bits each coder generates eight output bits.

It is, of course, contemplated that data in an appropriate form, e.g. binary digital, may be generated in other ways. Thus, analogue data may be converted automatically to binary digital data in a converter providing output at the required rate for feeding to the encoders.

To enable the transmitted signal to be transmitted in a burst-mode, the encoded data is stored in buffers 13 and 14, one of which is operated in a READ mode while the other is operated in a WRITE mode. A selector means 1 5 selects the READ mode of buffer and feeds a Dflip flop 1 6 which is also fed at one input with a clock signal from clock 1 7 to provide "1" outputs at output Q and "0" outputs at Q.

An OR circuit 18 feeds the "is" and the "Os" in the sequence in which they are read from the appropriate buffer to the dispersive delay line driver 1 9 which in turn feeds opposite ends of a dispersive delay line (of the surface acoustic wave type) through AND circuits 20 and 21.

The dispersive delay line 22a transmits an output to line 22b comprising a signal of which successive elements have the frequency versus time characteristics of a saw tooth shape (rising for the "1" bits and falling for the "0" bits). The delay line 22a generates an intermediate frequency chirp modulated signal which may have a centre frequency of the order of ten times the frequency of the sweep band width, by the transmitter, typically the intermediate (centre) frequency being of the order of 50 MHz. This is amplified in amplifier 22 and fed to a modulator 23 also fed from a local oscillator 24 and serves to perform an upward frequency conversion to provide both sum and difference frequency signals.

A band pass amplifier 25 transmits only the required frequency signals comprising the local oscillator frequency typically 14GHz) and the frequency modulation (typically up to 2.5 MHz either side of the carrier frequency).

The resultant signal is fed to a power amplifier 26 and thence to transmitting antenna 27.

At the transponder 9, the chirp modulation is retained but the carrier frequency is changed typically to 11 GHz. The transponder (shown for convenience as part of Figure 3) comprises a receiving antenna 2, amplifier 3, modulator 4 and local oscillator 5 for effecting the frequency change, band pass filter 6 passing the modulated but frequency shifted carrier, power amplifier 7, and transmitting antenna 8.

Referring to Figure 4 which illustrates the receiving means, the reradiated transponder output signal is received by a receiving antenna 28 and, after amplification in a low noise amplifier 29, is subjected to down conversion in a frequency changer comprising modulator 30a and local oscillator 30b. The local oscillator frequency is selected to provide a suitable intermediate frequency carrier at the output terminal modulator 30a, the chirp modulation being retained at this point. This signal is fed to the central connection of a further dispersive delay line (surface acoustic wave type) 31.

Output in the form of a short pulse is developed either at the up chirp terminal 32 if the received pulse has a rising frequency modulation, or at the down chirp terminal 33 if the required pulse has a falling frequency modulation, these signals being fed to full wave biased detector 35 before being applied by way of lines 37, 38 to inputs J, K of a J-K flip-flop 36 clocked by an aperture gate signal generated in a synchronisation generator 39 and fed to the flipflop 36 by way of line 40.

The regenerated pulses at the Q terminal of the flip-flop are fed by line 41 to threshold decoders 42 and 43, which respectively de-code the codes applied by encoders 12 and 11, the output then comprising the same sequence of "1" and "0" signals as was originally encoded in the encoders 11 and 12 of the transmitting means.

Figures 5 and 6 illustrate reception and treatment of chirp modulated signals at the receiver provided with a frequency dispersive filter such as a surface acoustic wave filter 31 matched to the particular modulation characteristics of the transmitted chirp pulse as in the schematic diagram of Figure 4.

The treatment by the receiver is broadly illustrated in Figure 5 and is as follows 1. It looks for compressed pulses a1, a2, a3 etc.

as shown in section (a) Figure 5 from the SAW matched filter 31 by way of a relatively long aperture initiation signal b, shown in section (b).

2. Upon finding any one pulse, it initiates the generation of narrower aperture gate signals b1g, b2g, b3g etc. of a specified width centred about the expected time of receipt of the next compressed pulse, i.e.

one expanded-pulse-duration (T seconds) later.

3. If detection is found at the gate output (i.e.

in the flip-flop unit 36), another aperture gate such as b2g is generated T seconds later. This process continues till a specified number of consecutive detections are made.

4. After the specified number of consecutive detections, which event is signalled via line 44 to unit 39, the aperture gate signal is generated periodically every T seconds until the end of the burst.

5. If, as shown in section (c) Figure 5, the gating signal were initially triggered by detection of a false pulse c1f in the wide aperture gate signal c1, (as shown in section (d)) there would be no detection (with a very high probability) at the output of the gate of unit 36 upon generation of the first (narrower) aperture gate signal c1g.

Detections would then not be looked for in any subsequent specific time slot. Steps 1, through to 4. would then be repeated. A further (long) aperture gate signal C1g would gate pulse C2 (section C) and a (narrower) aperture initiation signal ci2 would be generated by the genuine compressed pulse c2 (c1 being missed) resulting in the subsequent gating of the genuine pulses C3, C4, c5 etc.

6. If during the first set of consecutive detections, any detection is found missing, the gating pulse generation would enter a "search" mode and performs steps 1, through to 4.

Provision can be made for receiving many start-time separated identical signals emitted from different stations. A possible timing diagram of the aperture generation logic for receiving a number of identical signals is shown in Figure 6.

Sections (a) and (b) of Figure 6 show genuine received compressed pulse series designated a1, a2, a3, aperture initiation and gate pulses bj, b1g, b2g, b3g etc. for reception of signals from a transmitting station No. 1, sections (c) and (d) show the time relationships for genuine compressed pulses c1, c2, c3 etc. from a station 2 and aperture initiation and gate pulses dt, d1g, d2g, etc. for reception from a transmitting station 2. Sections (e) and (f) similarly illustrate operation for a transmitting station 3.

It will be noted that the corresponding (nearest in time) received compressed pulses of the three series from stations 1, 2 and 3, although possibly produced from transmitting stations using the same carrier frequency and chirp modulation parameters, and possibly partially overlapping in their initial transmitted (non-compressed) forms, at time spaced after receipt and compression and do not interfere at the transponder. This provision assumes, of course, that the power sharing conditions of the transponder output power are not violated.

Reference is now made to the application of the invention to a TDMA communication system, wherein the invention is applied to slot acquisition.

Figure 7 illustrates a conventional system in which the main TDMA carrier is centered at 70 MHz and for a data rate of 60 M Bits per second has a spread of 18 MHz each side of the carrier frequency, as indicated at Cm. An acquisition signal is radiated at a peak level 25dB below the main carrier and is shown at Cs. In Figure 8 there is illustrated the corresponding spectral occupancy of a main TDMA carrier Cm and a chirp modulated subcarrier c3 utilised in accordance with the invention for initial acquisition of an allocated slot in the TDMA system.

This can be utilised both for transmitting stations able to receive their own re-transmitted bursts from the transponder and also transmitter which are not so able. The chirp modulated signal can remain at sufficiently low level, for example 30dB below the main carrier, and thus provides an improvement over the conventional system as regards interference with the main signal. A chirp signal entailing linear frequency modulation has a flat spectrum and resembles white noise-like spectral characteristic. Conventional systems have a sin x x characteristic which may be more damaging to the TDMA communication signal.

A block schematic circuit diagram showing one embodiment of this application of the invention to a TDMA communication system is shown in Figure 9. An otherwise conventional TDMA transmit and receive unit 45 provides for transmission of communication data through line 46 to an antenna 47 and for reception by the antenna of an incoming signal fed thereafter along line 48. In the transmission cycle a three second slot is allocated to the transmission of acquisition signals in a 100 second superframe (the end of a superframe 2'7 frames-one frame being 750 microsecs in duration). The acquisition slot would be 212 frames.Unit 45 is either a master unit having clock means for providing a real time synchronising signal to other units 45 (not being master units) in the system, or is itself a non-master unit receiving a real time signal from the master unit of the system to mark the start of each cycle. Unit 49 receives a derived synchronising signal along line 50 form unit 45 to mark the beginning and end of the three second slot. Thus, the unit 49 controls the starting and stopping times of an impulse generator 51 through start and stop lines 52, 53 to confine the acquisition pulse series to the three second slot.

The impulse generator 51 acts as a driver for dispersive (expander) delay line unit 54, e.g. a surface acoustic wave filter as already mentioned, drive being effected through a 55. Output is applied through line 56 to an amplifier 57 which determines the level of the output transmitted signal and confines this to a predetermined value, for example 30 dB below the level of the main transmission. Line 58 feeds the output of amplifier 57 to the unit 45 to bring about transmission of the chirp modulated acquisition signal.

At the transponder shown conveniently as part of Figure 9, the frequency modulation, i.e. chirp modulation, is maintain but with a frequency shift.

The transponder can be as already described with reference to Figure 3 and corresponding components are designated in Figure 9 by like reference. The preceding description is to be deemed to apply with appropriate parameter changes as regards the incoming and outgoing signals. The frequency shifted received signal from the transponder and incident at the antenna 47 is transmitted through line 48 and unit 45 through line 59 to a compressive (dispersive) delay line 60 such as a further surface acoustic wave filter.

The compressed output (pulse) from unit 60 is fed to amplifier 61 through line 62.

Inputs to delay measurement unit 63 are taken from line 55 through line 64, and from amplifier 61 through line 65, the delay between the two signals on these lines being representative of the range, i.e. distance between the transmitting station and the transponder.

The range measurement output on line 66 is fed to unit 45 which includes means for responding to the range data and controlling the time relative to the real time reference at which communication transmission is affected to ensure arrival thereof at the transducer in accordance with the allocated slot for the particular transmitting station.

Typical system parameters for this application of the invention would be as follows:- TDMA Main Carrier Uplink transmit frequency 6 GHz band Downlink transmit frequency 4 GHz Chirp signal Centre frequency 16.25 MHz (offset from l.F).

Bandwidth 2.5 MHz Power level 30 dB below main carrier A further application of the invention is to the provision of a talk-back facility by way of an audio link in a television transmission-reception system, and has especial but not exclusive value for educational transmissions.

The system is illustrated in block schematic form in Figures 10 and 11. Referring firstly to the system broadly, the video and audio programme signal is transmitted as a frequency modulation typically within +13.5MHz of the main carrier (14 GHz) and the talk back audio data is carried by a chirp signal contained in the last 5 MHz of one of the (13.5 MHz) side bands. The receiving channel 67 comprises a receiving antenna 67a, a low noise amplifier 67b, a down converter 67c and a modified T.V. receiver 67d including a discriminator to recover the incoming video and audio programme signals. The antenna 67a is also used as the transmitting antenna for the audio talk-back facility.

The components of the system providing the talk-back facility comprise a transducer 68 such as a microphone feeding a delta modulator unit 69 providing a binary digital output comprising "1s" on line 70 and "Os" on line 71 to respective impulse generators 72, 73 feeding opposite ends of a dispersive delay line (e.g. a surface acoustic wave filter) 74 through lines 75, 76. The expanded pulses are fed through line 77 to 78, and then through line 79, to an up-converter unit 80 which transmits a sub-carrier having, typically, a band width of +2.5 MHz located as mentioned at one end of the video signal band of the main television transmission system.

The output fed through line 81 to an amplifier 82 has typically an output of 10 watts and feeds the transmitting antenna through line 83.

Figure 11 is a block schematic circuit of the receiver means provided at the television transmitter to complete the talk-back facility.

The antenna 84 which normally radiates the television transmission signal from unit 85 connected to the antenna through line 86 is utilised also for reception of the chirp modulated "talk-back" signal.

This is fed through line 86 to a low noise amplifier 87 and then through line 88 to a down converter unit 89 in which the chirp modulation signal is recovered and fed through line 90 to a (compressive) dispersive delay line (e.g. surface acoustic wave filter). Outputs are taken on lines 91, 92 from opposite ends of the delay line through amplifiers 93, 94 and lines 95, 96 to envelope detectors 97, 98.

The outputs from units 97 and 98 are fed on lines 99 and 100 via a comparator 111 to line 101 connected to one input of a flip-flop unit 102.

Also the outputs on lines 99 and 100 are fed on lines 103 and 104 to synchronisation generator 105 controlling turn-over of the flipflop by an output on line 106.

Output on line 107 from the flip-flop is integrated in unit 108, the output of which is the audio talk-back signal and is fed via line 109 to output terminal 110 for connection to any suitable transducer such as headphones or a loudspeaker.

Still further applications of the system and method of the invention include: (i) Data collection: In this case there are n any platforms that periodically need to transmit information that has been obtained by external sensors. The devices may have the capability to store or buffer data or it may be necessary to transmit as data is being gathered. These systems could be used in such diverse application areas as natural resource allocation, railroad freight inventory management, weather forecasting, air pollution control etc.

ii) Surveillance: In a dependent surveillance system a mobile vehicle has a self contained position determination device.

The output of this device as well as a vehicle identity code can be transmitted via a satellite to the Head Quarters. These systems have possible applications in the areas of oil tanker and fishing ship surveillance. With the advent of domestic mobile systems, they may be used for police surveillance and immigration controls.

iii) Search and Rescue (SAR): When ships and aircrafts are in distress a signal containing information regarding the position of the vehicle should be emitted so that search and rescue vehicles can be dispatched.

Such systems have to be designed for low blocking in the event of multiple SAR signals. Several experimental proposals for this application already exist and are popularly known as Emergency Position Indicating Radio Beacons (EPIRB).

Figure 12 illustrates an arrangement applicable to the above mentioned applications (1) (Data collection) and (iii) (EPIRB).

Stations are situated at a number of sites typically shown as sites 1 to 5 at positions distributed over the surface of the earth. The equipment at these sites would be as already described with reference to Figures 1 to 4, the encoder means 11, 12 would be fed from a unit such as 10 modified to respond to the particular form of data required to be collected.

For the EPIRB application, the sites 1 to 5 would be likely to be in sea or ocean areas or relatively unpopulated large areas of land. For the data collection application, the sites 1 to 5 would be situated at places appropriate to the nature of the data to be collected. The encoder means 11, 12 would be fed from a unit such as 10 pre-set to provide distress data, e.g. internal language or morse code signals, together with other appacpriate d Aa f required giving position and status of the event giving rise to the distress condition.

In both applications, the transponder would be carried by a satellite 6 having transmitting and receiving antennae 6a, 6b and solar energy collecting panels 6c.

Claims (20)

Claims

1.A A random. access, multiple access, communication system comprising (a) a transmitting means for operating at a transmitting station and including (i) means for transmitting a signal (the transmitted signal) comprising a succession of pulses of carrier wave energy, (ii) modulator means for frequency modulating the carrier within the pulse interval in a plurality of different modes, (iii) encoder means for selecting the mode of frequency modulation in response to respective different bits of data, (b) transponder means for operation at a transponder station remote from the transmitting station and including means for receiving the transmitted signal and re-radiating a corresponding but frequency shifted signal, (c) receiving means for operating at a receiving station remote from both of the foregoing stations and including (i) a plurality of receiving elements each responsive to a respective one of the different modes of frequency modulation to produce an output signal, (ii) decoder means for receiving the output signals and recovering the data originally encoded by the encoder means.

2. A system according to claim 1 wherein (a) the modulator means for frequncy modulating the carrier provides selectively for increase or decrease of the carrier frequency to represent binary coding bits in the transmitted signal, (b) the encoder means provides first and second output signals to select one or the other of said bits.

3. A system according to either of claims 1 and 2 wherein the receiving elements each presents frequency dispersive characteristics matching the particular form of frequency modulation of the received signal to which each is required to respond, so that the output signal produced is a (time) compressed signal and is developed uniquely in response to that particular form of frequency modulation.

4. A system according to claim 3 wherein the receiving elements comprise surface acoustic wave filters.

5. A random access, multiple access method communicating data between a transmitting station, a transponder station, and a receiving station, said method comprising: (a) transmitting from a transmitting station a succession of pulses of carrier wave energy each being frequency modulated within the pulse do ration ir accordance with one of a plurality of different modes of frequency modulation and representing respective bits of coded data, (b) receiving the transmitted signal at the transponder station and re-transmitting it with corresponding but frequency shifted signal, (c) receiving the transponder signal at the receiving station and feeding it to a plurality of receiving elements responsive respectively to different ones of said modes of frequn~y modulation to produce an output signal, (d) decoding the output signals to recover the data.

6. A method according to claim 5 wherein one mode of frequency modulation provides for an increase in the frequency and another provides for a decrease, such increase and decrease forming the bits of the code which is binary.

7. A method according to either of claims 5 or 5 wherein, in the receiving elements, successive portions or slices of the signal are subjected to delays of respective magnitudes so correlated to the frequency subsisting during said portion or slice as to provide an output in which the portions or slices are additive and thus present signal gain.

8. A time division, multiple access communication system including means for establishing acquisition of its proper time slot in the frame of the transponder means by any one of the transmitting means comprising: (a) at the transmitting station concerned, means for transmitting a signal (the acquisition signal) comprising a succession of pulses of carrier wave energy, (b) modulator means for frequency modulating the carrier within the pulse interval in a predetermined mode, (c) in association with the receiving means, (i) a receiving element responsive to the predetermined mode of frequency by the transponder of acquisition signal pulses re transmitted by the transponder means with a shift of frequency but otherwise correspondingly frequency modulated, to provide, by way of the receiving element, a succession of output signal pulses, (ii) means responsive to the time relation between the transmitted acquisition signal pulses, the output signal pulses, and the time slots allocated to the transmitting station concerned to cause the transmitting means to transmit communication signals at time such that these will be incident at the transponder means within the proper time slots.

9. A system according to claim 8 wherein the receiving element at the receiving means is as claimed in either of claims 3 and 4.

10. A method of operating a time division multiple access communication system to acquire for any one of the transmitting means its proper slot in the frame of the transponder means, said method comprising the steps of: (a) transmitting from any one of the transmitting stations to the transponder, a signal (the acquistion signal) comprising a succession of pulses of carrier wave energy frequency modulated within each pulse duration in a predetermined mode, (b) receiving the acquisition signal uplses retransmitted from the transponder means with a shift of frequency but otherwise correspondingly frequency modulated and feeding the received signal to a receiving element responsive to the predetermined mode of frequency modulation to provide an output signal comprising a succession of pulses, (c) detecting time correlation between the incidence of the transmitted acquisition signal pulses, the output signal, and the time slots allocated to the particular transmitting station from which the acquisition signals are transmitted, (d) and controlling the transmission times of communication signals transmitted from transmitting means at that station containing the data required to be communicated to the receiving station or one of same as a function of said time correlation.

11. A method according to claim 10 wherein, in the receiving element, successive portions or slices of each of the received acquisition signal pulses are subjected to delays of respective magnitudes so correlated to the frequency subsisting during said portion or slice as to provide an output signal pulse in which the portions or slices are additive and thus present signal gain.

1 2. A communication system comprising a transmitting means for operation at a transmitting station and a plurality of receiving means for receiving a signal transmitted by the transmitting means, such signal comprising a carrier wave modulated to transmit video and audio data, said system including "talk-back" means comprising: (a) at selected receiving stations (i) talk-back transmitting means for transmitting a talk-back signal comprising a succession of pulses of carrier wave energy (ii) modulator means for frequency modulating the carrier within the pulse duration in a plurality of different modes, (iii) encoder means for encoding the talk-back information into a code in terms of the mode of frequency modulation, (b) talk-back receiving means at the transmitting station and including::- (i) a plurality of receiving elements each responsive to a respective one of the different modes of frequency modulation to produce an output signal, (ii) decoder means for receiving the output signal and recovering the talk-back data originally encoded by the encoder means.

13. A system according to claim 12 wherein (a) the modulator means for frequency modulating the carrier provides selectively for increase or decrease of the carrier frequency to represent binary coding bits in the transmitted signal, (b) the encoder means provides first and second output signals to select one or the other of said bits, (c) the decoder means at the transmitting station includes means for decoding the binary code into an audio frequency signal presenting the talk-back information.

14. A system according to either of claims 12 and 1 3 wherein the receiving elements at the transmitting station each presents frequency dispersive characteristics matching the particular form of frequency modulation of the received signal to which each is required to respond, so that the output signal produced is a (time) compressed signal and is developed uniquely in response to that particular form of frequency modulation.

1 5. A system according to claim 14 wherein the receiving elements comprise surface acoustic wave filters.

1 6. A method of effecting talk-back in a communication system comrpising a transmitting means for operation at a transmitting station, and a plurality of receiving means for receiving a signal transmitted by the transmitting means, such signal comprising a carrier wave modulated to transmit video and audio data, the method comprising the steps of:: (a) transmitting from any selected one or more of the receiving stations a talk-back signal comprising a succession of pulses of carrier wave energy, each being frequency modulated within the pulse duration in accordance with one of a plurality of different modes of frequency modulation, (b) encoding the talk-back data in terms of the different modes of frequency modulation, (c) applying the code to determine the particular one of the modes of frequency modulation applied to the signal, (d) receiving the talk-back signal at the transmitting station and feeding it to a plurality of receiving elements responsive respectively to different ones of said modes of frequency modulation to produce respective output signals, (e) decoding the talk-back information from the output signals.

1 7. A method according to claim 1 6 wherein, in the receiving elements, successive portions or slices of the signal are subjected to delays of respective magnitudes so correlated to the frequency subsisting during said portion or slice as to provide an output in which the portions or slices are additive and thus present signal gain.

1 8. A communication system substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.

1 9. A method of communicating data substantially as hereinbefore described with reference to and illustrated by the accompanying drawings.

20. Any novel feature or novel combination of features disclosed herein and/or shown in the accompanying drawings.