GB2111732A

GB2111732A - Signal processing arrangements

Info

Publication number: GB2111732A
Application number: GB08137483A
Authority: GB
Inventors: Carl Peter Zetterberg
Original assignee: Racal Research Ltd
Current assignee: Racal Research Ltd
Priority date: 1981-12-11
Filing date: 1981-12-11
Publication date: 1983-07-06
Also published as: GB2111732B

Abstract

Coded input signals (e.g. PN sequences), arrive in phase coded form for example on a 70 MHz carrier on a line 5, there being four different coded signals in this example. By means of frequency generator 10 and a mixer 6, the phase- modulated carrier is converted into four waveforms with carrier frequencies F1 = 65.5 MHz, F2 = 68.5 MHz, F3 = 71.5 MHz and F4 = 74.5 MHz which are applied simultaneously to input 17 of a convolver 18. Frequency generator 10 also generates four control waveforms at F1, F2, F3 and F4 which are selected by filters 24 to 30 and a multiplexer 32 and each is modulated by the time-reversed replica of a respective one of the coded input signals. The modulated control waveforms are applied in succession to input 44 of the convolver which has sufficient length to enable all four input waveforms to be compared with all four control waveforms. A maximum convolved output will appear at output 46 when there is correlation alignment between the particular control waveform carrying the time-reversed replica of the coded input signal present. The frequency of this output (detected at 50 to 70) identifies the coded input signal. instead, the control waveforms can be applied simultaneously (Figure 3, not shown). <IMAGE>

Description

SPECIFICATION Improvements in and relating to signal processing arrangements The invention relates to electrical signal processing arrangements and more specifically to such arrangements for recognising and responding to predetermined electrical signals. In examples of the invention to be more specifically described, the signals to be recognised are specific ones within a group of signals.

For example, the signal processing arrangement may be part of a data communications system having a plurality of data receivers and transmitters for example, in which data to be transmitted to a particular receiving station is preceded by identification and synchronisation signals which are intended to be recognised by that receiving station only.

Various novel features of the invention will be apparent from the following description, given by way of example only, of signal processing arrangements embodying the invention, reference being made to the accompanying drawings in which: Figure 1 is a block circuit diagram of one of the arrangements; Figure 2 is a diagram for explaining the operation of the arrangement of Figure 1; and Figure 3 is a block circuit diagram of a modified form of the arrangement of Figure 1.

More specifically to be described below is a signal processing arrangement for recognising each one of a plurality of differently coded input signals, comprising means responsive to each coded input signal to produce therefrom a plurality of co-existing intermediate waveforms at respective different and predetermined frequencies and passing them simultaneously into one input of a convolver, means for generating a plurality of control waveforms each corresponding in frequency to a respective one of the intermediate frequencies and each having present therein a time-reversed replica of a respective one of the coded input signals, and means for feeding the control waveforms into the other input of the convolver whereby a maximum convolved output will be produced in response to correlation alignment between the particular one of the control waveforms having present therein the time-reversed replica of the coded input signal matching that which is present on each of the four intermediate frequencies, whereby the magnitude of the frequency of the maximum convolved output corresponds to and identifies a particular one of the coded signal inputs.

In such a case, there may be provided output means comprising a set of bandpass filters each having a passband corresponding to double the frequency of a respective one of the intermediate frequencies, and respective detection means con nected to the output of each filter, whereby the frequency of each maximum convolved output cor responds with the passband of a particular one of the filters according to the identity of the coded signal input present therein.

In such a case, the frequency changing means advantageously comprises means for generating further waveforms at predetermined frequencies which are spaced apart by the separation frequency of the intermediate waveforms, and means for mixing the input carrier waveform with the said further waveforms.

Advantageously, the means for generating the plurality of control waveforms comprises means for mixing the said further waveforms with a waveform having the same frequency as the input carrier waveform and filter means for selecting from the mixed output frequencies corresponding to those of the control waveforms, and means for modulating each of the selected frequencies with a respective one of the time-reversed replicas of the coded input signals to produce the control waveforms.

The modulating means may comprise means for sequentially modulating each of the selected frequencies with a respective one of the time-reversed replicas of the coded input signals, whereby the control waveforms are fed sequentially into the said other input of the convolver.

Instead, the modulating means may comprise means for simultaneously modulating each of the selected waveforms with a respective one of the time-reversed replicas of the coded input signals, whereby the control waveforms are fed in parallel into the said other input of the colvolver.

In a more specific sense, there will be described below a signal processing arrangement for recognising each one of a plurality of differently coded input signals occurring in succession on an input carrier, comprising means responsive to the input carrier to generate, in response to each coded input signal, a plurality of intermediate waveforms equal in number to the number of coded input signals and having respectively different and predetermined frequencies and all co-existing and having present therein the said coded input signal, a convolver having one input connected to receive all the said plurality of intermediate waveforms simultaneously, means for generating a plurality of control waveforms each having a frequency corresponding to a respective one of the intermediate waveforms and each having present therein a time-reversed replica of a respective one of the coded input signals, and means for passing all the control waveforms through the convolverfrom the other input thereof, the convolver having such length in relation to the lengths of the said waveforms that all of each plurality of intermediate waveforms are compared in the con volverwith all of the plurality of control waveforms, whereby a maximum convolved output will be produced in response to correlation alignment between the particular one of the control waveforms having present therein the time-reversed replica of the coded input signal matching that which is present on each of the intermediate frequencies, whereby the magnitude of the frequency of the maximum convolved output corresponds to and identifies a particular one of the coded signal inputs.

The foregoing are exemplary of and not exhaustive of the various novel features of the signal processing arrangements now to be more specifically described.

The signal processing arrangement to be de scribed with reference to Figure 1 forms part of a data receiver in a data communication system. In the system itself, a data transmitter transmits coded data on a communications link of any suitable type which connects with several different receivers forming part of the system. Each block of data is intended to be received by only one of the receivers (or possibly by only specific ones), and each block of data is therefore preceded by coded indentification signals which identify which receiver is intended to receive that block of data, and the coded signals may also be used by the receiver to perform a synchronisation function.Each receiver therefore includes the signal processing arrangement to be described with reference to Figure 1 and this is used, in a manner to be explained, to check the incoming data stream and to recognise those coded signals identifying that particular receiver.

In this particular example, it is assumed that each block of data is preceded by four coded signals C1, C2, C3 and C4 and these signals identify the data receiver which is intended to receive the block of data. The signal processing arrangement at each data receiver may be arranged so that it need only recognise three out of the four coded signals, for example, in order to make the decision that the block data is inteded for that receiver.

In this particular example, each coded signal is in the form of a PN code sequence. More specifically, each code sequence is a particular pseudo-random sequence of a fixed plurality of binary signals. The coded signals are transmitted in any suitable way, such as by phase shift keying, and the resultant waveforms representing the coded signals arrive serially (though not necessarily uniformly time separated) on an input line 5 (Figure 1) with a carrier frequency of 70 MHz. In the following description, references to C1, C2, C3 and C4 include references to these waveforms as well as to the actual PN code sequences themselves. Input line 5 is connected to a mixer 6 receiving a second input on a line 8 fed by a frequency generator 10. The latter produces outputs at 1.5 and 4.5 MHz on line 8 and the mixer 6 therefore mixes the 70 MHz carrier frequency on line 5 with these outputs.Mixer 6 therefore converts the input at 70 MHz into four co-existing sum and difference outputs at frequencies F1, F2, F3 and F4 where F1 = 65.5MHz, F2 = 68.5 MHz, F3 = 71.5 MHz and F4 = 74.5 MHz. These are amplified by a buffer amplifier 14 and passed through a bandpass filter 16 to the first input 17 of a convolver 18.

The frequency generator 10 also produces outputs at F1, F2, F3 and F4 on a line 20 and these are selected by respective bandpass filters 24, 26, 28 and 30.

These outputs therefore correspond in frequency to the co-existing frequencies F1, F2, F3 and F4 at the input 17 of the convolver.

The outputs of the four different frequencies F1, F2, F3 and F4 from the filters 24 to 30 are fed into respective inputs of an RF multiplexer 32. This is controlled by control logic 34 so as to select each of the four frequencies in turn and apply it to one input of a moduiator 36. The second input of the modulator 36 is fed by an output of a PN code generator 38 under control of control logic 34. The modulator 36 operates such that the particular one of the four frequencies F1, F2, F3 and F4 selected by the multi plexer 32 is modulated by the current output of the PN code generator 38 in the same manner (e.g.

phase shift keying) as the manner in which the coded input signals have been modulated onto the carrier on the line 5. The modulated output is fed through a buffer amplifier 40 and a bandpass filter 42 to the second input 44 of the convolver 18.

The code generator 38 generates in sequence codes which are the time-reversed replicas of the codes C1, C2, C3 and C4.

The convolver 18 has an output 46 which is fed through a buffer amplifier 48 to the inputs of four bandpass filters 50, 52, 54 and 56 having passbands centred at 2F1, 2F2, 2F3 and 2F4 respectively. Detecting circuits 58,60,62 and 64 monitor the outputs of the filters.

The operation of the signal processing arrangement will now be described.

If it is assumed that a particular one of the coded signals (say C1) identifying the particular receiver is being received (PSK-modulated onto the 70 MHz carrier) on the line 5, the action of the mixer 6 is to cause the coded signal C1 to be presented to the input 17 of the convolver 18 not at 70 MHz but at each of the four different frequencies F1, F2, F3 and F4 simultaneously.

The action of the control logic 34 and the multipler 32, however, is to apply signals to the second input 44 of the convolver at each of these four frequencies F1, F2, F3 and F4 in turn one after the other (that is, not simultaneously as is the case with the signals at the input 17). Each of these four signals applied in turn to the convolver input 44 is modulated by a timereversed replica of a respective one of the four coded signals C1, C2, C3 and C4 particular to that receiver; these time-reversed signals are produced in the appropriate sequence by the PN code generator 38 under control of the control logic 34.

The convolver 18 may take any suitable form having a length corresponding to a time T and performing the function 1r g(t).f(-t).dt (1) where g(t) is the signal at the input 17 ofthe convolver and f(-t) is the signal at the input 44. The resultant output in accordance with the Function (1) is obtained at the output 46. The correlator 18 may for example be of the surface acoustic wave piezoelectric type.

As the four waveforms F1, F2, F3 and F4, at 65.5 MHz, 68.5 MHz, 71.5 MHz and 74.5 MHz respectively, pass simultaneously from the input 17 oftheconvol- ver towards the input 44, the four waveforms produced by the modulator 36 will be applied in succession to the input 44 and will pass in the opposite direction toward the input 17. The length T of the convolver 18, in relation to the length of the waveforms and the code repetition rate of the multiplexer 32, is such that all the four waveforms applied to input 17 are compared in the colvolver 44 with all four waveforms applied in succession to the input 44.

This is illustrated diagrammatically in Figure 2.

Figure 2 illustrates the four waveforms at the respective carrier frequencies F1, F2, F3 and F4 which are applied sequentially to the input 44 of the convolver. Each is modulated by a different one of the four possible coded signals C1, C2, C3 and C4, as indicated by the annotations on the blocks B1, B2, B3 and B4 in Figure 2.

As the waveforms pass each other in opposite directions in the convolver, correlation is carried out in accordance with the Function (1) above. That is, the passing waveforms will be multiplied together and the products will be summed and appear at the output 46. The correlation process is such that the convolved output is a maximum when a waveform represented by one of the blocks A moves into correlation alignment with the one of the waveforms represented by one of the blocks B which it exactly matches, but at other times the convolved output will be many times smaller.

Therefore, it will be apparent that (in this example) the convolved output will have a maximum when block A1 is in exact correlation alignment with block B1. For all other alignments, the waveforms will not match because they will not be modulated by the same coded signal, and the convolved output will be low.

Because of the inherent nature of this type of convolver, there is a x2 time compression, and therefore the peak convolved output will have a frequency of 2 F1, that is, 2 x 65.5 MHz in this example. This will be passed by the filter 50 and detected by the detector 58. The other filters 52,54 and 56 will block the signal and therefore the other detectors will not be activated.

In this way, therefore, the arrival of the coded signal C1 is recognised.

If the next coded signal to arrive in the data transmission link 5 is the coded signal C2, then the blocks A in Figure 2 will again represent waveforms at F1, F2, F3 and F4 respectively but in this case they will all be modulated by the coded signal C2. The blocks B will be the same as shown in Figure 2.

Therefore, the convolver will produce a maximum convolved output when the block B2 is in exact correlation alignment with the block B2. All other alignments will produce a low convolved output.

The maximum convolved output will be at the frequency 2F2 and will thus pass through the filter 52 and be detected by detector 60.

In similar fashion, the coded signal C3 on line 5 will cause the convolverto produce a maximum convolved output at the frequency 2F3, that is, 2 x 71.5 MHz, and this will pass through the filter 54 and be detected by detector 62; and the coded signal C4 on line 5 will cause the convolver to produce a maximum convolved output at the frequency 2F4, that is, 2 x 74.5 MHz, and this will pass through the filter 56 and be detected by detector 64.

In this way, all four coded signals can be recognised using only a single convolver.

The detectors 58 to 64 operate detection logic 70 so as to produce an output on a line 72 indicating recognition when all four coded signals C1 to C4 have been received and identified (or when a particular one or ones of the four have been recognised). The detection logic 70 can also be used to produce synchronisation signals in response to recognition of the coded signals.

Figure 3 shows a modified arrangement in which items corresponding to those in Figure 1 are similarly referenced. As shown in Figure 3, there are four modulators 36A, 36B, 36C and 36D each connected to a respective one of four precode generators 38A, 38B, 38C and 38D and each modulating the output of a respective one of the filters 24,26,28 and 30.

Therefore, in contrast to the arrangement of Figure 1 where the modulated coded signals (represented by the blocks B1, B2, B3 and B4 in Figure 2) are applied in succession to the input 44 of the convolver, the modulated coded signals in Figure 3 are applied simultaneously or in parallel to the input 44.

The correlation process carried out by the convolver is the same as described with reference to Figure 2.

The arrangement of Figure 3 enables a convolver with a shorter delay length to be used than in the arrangement of Figure 1 but requires four modulators and four code generators instead of only one of each.

CLAIMS (filed on 19/11/82) 1. A signal processing arrangement for recognising each one of a plurality of differently coded input signals, comprising means responsive to a received coded input signal to produce thereform a plurality of co-existing intermediate waveforms having respectively different and predetermined carrier frequencies and passing them simultaneously into one input of a convolver, means for generating a plurality of control waveforms each having a carrier frequency equal to a respective one of the predetermined frequencies and each having present therein a time-reversed replica of a respective one of the coded input signals, and means for feeding the control waveforms into the other input of the convolver whereby a maximum convolved output will be produced in response to correlation alignment between the particular one of the control waveforms having present therein the time-reversed replica of the coded input signal matching that which is received, whereby the magnitude of the frequency of the maximum convolved output corresponds to and identifies the received coded input signal.

2. An arrangement as claimed in claim 1,further comprising output means comprising a set of bandpass filters each having a passband centred at double the frequency of a respective one of said predetermined frequencies, and respective detection means connected to the output of each filter, whereby the frequency of each maximum convolved output corresponds with the passband of a particular one of the filters according to the identity of the coded input signal present therein.

3. An arrangement as claimed in claim 1 or 2, wherein the input responsive means comprises means for generating further signals at frequencies which are spaced apart by the separation frequency of the predetermined frequencies, and means for

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. This is illustrated diagrammatically in Figure 2. Figure 2 illustrates the four waveforms at the respective carrier frequencies F1, F2, F3 and F4 which are applied sequentially to the input 44 of the convolver. Each is modulated by a different one of the four possible coded signals C1, C2, C3 and C4, as indicated by the annotations on the blocks B1, B2, B3 and B4 in Figure 2. As the waveforms pass each other in opposite directions in the convolver, correlation is carried out in accordance with the Function (1) above. That is, the passing waveforms will be multiplied together and the products will be summed and appear at the output 46. The correlation process is such that the convolved output is a maximum when a waveform represented by one of the blocks A moves into correlation alignment with the one of the waveforms represented by one of the blocks B which it exactly matches, but at other times the convolved output will be many times smaller. Therefore, it will be apparent that (in this example) the convolved output will have a maximum when block A1 is in exact correlation alignment with block B1. For all other alignments, the waveforms will not match because they will not be modulated by the same coded signal, and the convolved output will be low. Because of the inherent nature of this type of convolver, there is a x2 time compression, and therefore the peak convolved output will have a frequency of 2 F1, that is, 2 x 65.5 MHz in this example. This will be passed by the filter 50 and detected by the detector 58. The other filters 52,54 and 56 will block the signal and therefore the other detectors will not be activated. In this way, therefore, the arrival of the coded signal C1 is recognised. If the next coded signal to arrive in the data transmission link 5 is the coded signal C2, then the blocks A in Figure 2 will again represent waveforms at F1, F2, F3 and F4 respectively but in this case they will all be modulated by the coded signal C2. The blocks B will be the same as shown in Figure 2. Therefore, the convolver will produce a maximum convolved output when the block B2 is in exact correlation alignment with the block B2. All other alignments will produce a low convolved output. The maximum convolved output will be at the frequency 2F2 and will thus pass through the filter 52 and be detected by detector 60. In similar fashion, the coded signal C3 on line 5 will cause the convolverto produce a maximum convolved output at the frequency 2F3, that is, 2 x 71.5 MHz, and this will pass through the filter 54 and be detected by detector 62; and the coded signal C4 on line 5 will cause the convolver to produce a maximum convolved output at the frequency 2F4, that is, 2 x 74.5 MHz, and this will pass through the filter 56 and be detected by detector 64. In this way, all four coded signals can be recognised using only a single convolver. The detectors 58 to 64 operate detection logic 70 so as to produce an output on a line 72 indicating recognition when all four coded signals C1 to C4 have been received and identified (or when a particular one or ones of the four have been recognised). The detection logic 70 can also be used to produce synchronisation signals in response to recognition of the coded signals. Figure 3 shows a modified arrangement in which items corresponding to those in Figure 1 are similarly referenced. As shown in Figure 3, there are four modulators 36A, 36B, 36C and 36D each connected to a respective one of four precode generators 38A, 38B, 38C and 38D and each modulating the output of a respective one of the filters 24,26,28 and 30. Therefore, in contrast to the arrangement of Figure 1 where the modulated coded signals (represented by the blocks B1, B2, B3 and B4 in Figure 2) are applied in succession to the input 44 of the convolver, the modulated coded signals in Figure 3 are applied simultaneously or in parallel to the input 44. The correlation process carried out by the convolver is the same as described with reference to Figure 2. The arrangement of Figure 3 enables a convolver with a shorter delay length to be used than in the arrangement of Figure 1 but requires four modulators and four code generators instead of only one of each. CLAIMS (filed on 19/11/82)

1. A signal processing arrangement for recognising each one of a plurality of differently coded input signals, comprising means responsive to a received coded input signal to produce thereform a plurality of co-existing intermediate waveforms having respectively different and predetermined carrier frequencies and passing them simultaneously into one input of a convolver, means for generating a plurality of control waveforms each having a carrier frequency equal to a respective one of the predetermined frequencies and each having present therein a time-reversed replica of a respective one of the coded input signals, and means for feeding the control waveforms into the other input of the convolver whereby a maximum convolved output will be produced in response to correlation alignment between the particular one of the control waveforms having present therein the time-reversed replica of the coded input signal matching that which is received, whereby the magnitude of the frequency of the maximum convolved output corresponds to and identifies the received coded input signal.

mixing the received input signal modulated on an input carrier signal with the said further signals.

4. An arrangement as claimed in claim 3, wherein the means for generating the plurality of control waveforms comprises means for mixing the said further signals with a waveform having the same frequency as the input carrier signal, filter means for selecting from the mixed output frequencies corresponding to the predetermined frequencies, and means for modulating each of the predetermined frequencies with a respective one of the timereversed replicas of the coded input signals to produce the control waveforms.

5. An arrangement as claimed in claim 4, wherein the modulating means comprises means for sequentially modulating each of the predetermined frequencies with a respective one of the timereversed replicas of the coded input signals, whereby the control waveforms are fed sequentially into the said other input of the convolver.

6. An arrangement as claimed in claim 4, wherein the modulating means comprises means for simultaneously modulating each ofthe selected waveforms with a respective one of the timereversed replicas of the coded input signals, whereby the control waveforms are fed in parallel into the said other input of the convolver.

7. A signal processing arrangement for recognising each one of a plurality of differently coded input signals which occur on an input carrier, comprising means responsive to a received signal containing a coded input signal to generate a plurality of intermediate waveforms equal in number to the number of coded input signals and having respectively different and predetermined carrier frequencies and all coexisting and having present therein the said received coded input signal, a convolver having one input connected to receive all the said plurality of intermediate waveforms simultaneously, means for generating a plurality of control waveforms each having a frequency corresponding to a respective one of the carrier frequencies of the intermediate waveforms and each having present therein a time reversed replica of a respective one of the coded input signals, and means for passing all the control waveforms through the convolverfrom the other input thereof, the convolver having such length in relation to the lengths of the said waveforms that all of each plurality of intermediate waveforms are compared in the convolver with all of the plurality of control waveforms, whereby a maximum convolved output will be produced in response to correlation alignment between the particular one of the control waveforms having present therein the time-reversed replica of the coded input signal matching that which is present in each of the intermediate waveforms, whereby the magnitude of the frequency of the maximum convolved output corresponds to and identifies a particular one of the coded input signals.

8. A method of recognising which of a discrete set of coded signals is being received, comprising the steps of modulating the received coded signal onto a number of carrier signals each having a different frequency, the number of carrier signals being equal to the number of different coded signals in the set, modulating each of said carrier frequencies with a different time-reversed one of the coded signals, convolving the carrier frequencies modulated by the received coded signal with the carrier frequencies modulated by the different timereversed coded signals, and detecting the frequency of the maximum convolved output, which frequency will correspond to substantially twice the carrier frequency which was modulated with the timereversed coded signal corresponding to the received coded signal.

9. A signal processing arrangement substantially as herein described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

10. A method of recognising which of a discrete set of coded signals is being received substantially as herein described with reference to the accompanying drawings.