GB2396493A

GB2396493A - Processing a signal to adjust for the effect of a changing gain

Info

Publication number: GB2396493A
Application number: GB0229436A
Authority: GB
Inventors: Nigel Hoult; Julian Whiffen
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2002-12-17
Filing date: 2002-12-17
Publication date: 2004-06-23
Anticipated expiration: 2022-12-17
Also published as: GB0229436D0; GB2396493B

Abstract

The invention relates to communications systems and in particular to hf communications systems which carry high rate modem data incorporating training sequences or preambles. The invention is particularly concerned with receiving systems 10 with automatic gain control (agc) circuits 12, 13, and seeks to integrate the behaviour of the agc circuit 12, 13 with that of the modem 20. The invention provides a method of processing a signal including the steps of applying gain to a received signal 100 to create a modified signal 110, communicating the modified signal 110 to a decoder such as a modem 20, and processing 22 the modified signal 110 to adjust for the effect of the gain applied by the agc system earlier in the processing. There may be communication of information 111 between the receiver/agc and the decoder. Alternatively, the decoder may be pre-programmed with general information about the characteristics of the agc system. The agc system may act to apply gain changes to the received signal only at predetermined points in the signal, and the predetermined points may be determined by either the receiver or by the decoding device.

Description

COMMUNICATIONS SYSTEM

The present invention relates to a communications system and methods for operating a communications system.

5 It is particularly, but not exclusively, concerned with receiving systems which have automatic gain control (AGC) circuits. Automatic gain control (AGC) circuits are commonly used in signal processing apparatus which processes data 10 before it is passed to a modem or other decoding device.

Such AGC circuits introduce non-linear behaviour, such as "fast-attack" or "slow-decay" of the amplitude of the received signal, and this degrades the performance of equaliser(s) which may be used in subsequent modems, and 15 therefore of the receiving system as a whole.

High rate modems use equalisers to estimate the channel characteristics and correct the received signal accordingly. This is usually achieved using "training sequences" (sometimes called "preambles") which comprise 20 a sequence of one or more known data symbols repeated at regular intervals. The data between the training sequences is processed as a block, based on the estimates of the channel made using the training sequences which may be updated during the processing. Some modems use 25 data from more than one training sequence, each of which may be transmitted before or after the data sequence

! question, to obtain better estimates of the channel characteristics. Generally the signal processing apparatus (e.g. a radio receiver) and the modem or other decoding device 5 which decodes the received data have been separate devices, and so it has not been possible to integrate the behaviour of the AGC circuit with the modem. To reduce the impact of the application of AGC on the transmissions, very slow acting AGC has been used, so 10 that there is little change in gain between successive preambles. At its broadest the present invention provides a method of processing a modified communications signal including processing the signal to adjust for the effect 15 of changing gain, particularly AGC, applied earlier in the processing.

The method may also include the steps of applying gain determined by AGC to a received signal to produce the modified signal.

20 The processing of the modified signal may be carried out in a decoder or modem, and is preferably done on the basis of information about the gain which was applied to produce the modified signal.

The information about the gain applied may take any 25 of a number of forms. In particular embodiments, this information may include one or more of: information about

the level of gain applied at a particular point in the modified signal; information about the level of gain applied to a particular level of received signal, or to create a particular level of modified signal; and/or 5 information about how the level of gain applied changes with time.

The method of processing may also include communicating information about the gain applied to the received signal by AGO to a modem or other decoding 10 device, the step of adjusting the modified signal according to that data being carried out in the decoding device. The present invention also provides communications apparatuses) for carrying out these methods.

15 Such apparatus may have a receiver and a decoder, the receiver including: means for receiving a data signal; means for determining the gain to be applied to that signal at a particular time; means for applying said gain to the data signal to produce a modified 20 signal; and means for communication the modified signal to the decoder, the decoder including: means for receiving the modified signal; and means for adjusting the modified signal to correct for the effect of the gain applied to that modified signal.

25 The means for adjusting may use information about the gain applied to create said modified signal.

According to an aspect of the present invention, there is provided a method of processing a communications signal including the steps of: measuring the level of a received signal; applying gain to that signal, using an 5 automatic gain control algorithm, to produce a modified signal) communicating that modified signal and information about the gain applied to the received signal; receiving the modified signal and said information in a decoding device) and processing the 10 modified signal to correct for the effect of the gain applied to the signal according to the information received. The information about the gain applied may be the direct output of the AGC algorithm used to determine the 15 gain to be applied, or may be a representation of the gain applied to the data signal as a result of the AGC algorithm. According to another aspect of the present invention there is provided a communications apparatus having a 20 receiver and a decoder, the receiver including: means for receiving a data signal) means for determining the gain to be applied to that signal at a particular timed means for applying said gain to the data signal to produce a modified signal; and means for communicating said 25 modified signal and information about the gain applied to create said modified signal, the decoder including: means

for receiving a modified signal; means for receiving information about the gain applied to create said modified signal) and means for adjusting the modified signal to correct for the effect of the gain applied to 5 that modified signal according to the information received. The above apparatus may also include means for controlling when the information about the gain applied is communicated to the decoder. This may allow the 10 information to only be communicated at predetermined intervals. If these intervals are chosen to be less frequent than the normal sampling rate of the data signal, this reduces the amount of information being communicated to the decoder.

15 If the information is only being communicated at predetermined intervals, in between each communication the decoder may use the last information communicated to adjust the modified signal, or may change the adjustment during the intervals, for example by extrapolation of the 20 information last communicated, or by interpolation between two or more sets of communicated information.

In a further aspect of the present invention, the method of processing includes the steps of: measuring the level of a received signal; applying gain to that signal 25 using automatic gain control to produce a modified signal; communicating that modified signal and

information about the gain applied to the received signal; receiving the modified signal and said information in a decoding device; and processing the modified signal to correct for the effect of the gain 5 applied to the signal according to the information received, wherein the gain applied to the received signal is only changed at predetermined points in the received signal. Preferably the information about the gain applied is 10 only communicated to the decoder at those predetermined points, or at a set interval after each of those points.

In this aspect, the predetermined points may be determined by the decoding device, in which case their position(s) will need to be communicated to the receiver.

15 Similarly, if the predetermined points are determined by the receiver, their position(s) will need to be communicated to the decoding device. This requires more communication between the receiver and the decoder, but since this communication is less frequent, the overall 20 amount of information that is communicated can be made less than in the aspects above.

This aspect of the present invention may be implemented in a development of the communications device described above, in which the receiver may further 25 include means for selecting whether or not the gain computed by the AGC is applied to the received signal at

a particular moment in time. The same or alternative means for selecting may also determine when information about the level of AGO applied is communicated to the decoder. 5 The predetermined points preferably fall within and form some or all of the training sequences of the received signal. These training sequences are normally determined by the decoding device, in which case their position(s) can be communicated to the receiver. Since 10 the training sequences generally will occur regularly in the received signal, it may only be necessary for these position(s) to be communicated infrequently, for example when a new signal is received/detected.

In a further aspect of the present invention, there 15 is provided a method of processing a communications signal including the steps of: measuring the level of a received signal; applying gain to that signal using automatic gain control to produce a modified signal; communicating that modified signal; receiving the 20 modified signal in a decoding device; and processing the modified signal to correct for the effect of AGC on the signal according to the information received, wherein no specific information regarding the gain applied to the received signal is communicated to the decoding device, 25 and the decoding device uses general information

l regarding the performance of the automatic gain control to process the modified signal.

The method may also include the step of calculating, or otherwise determining, the general information 5 regarding the performance of the automatic gain control, for example by measuring and/or analysing the modified signals received by the decoding device over a sufficiently long time frame.

Alternatively, the general information referred to 10 may be preprogrammed into the decoder, or the decoder may be adjustably programmed with such information.

The general information referred to may be, for example, one or more time constants of the AGC, or some or all of the AGC algorithm used in the receiver.

15 Preferably this method is used on data signals in which the highest amplitude parts of the signal are contained in the training sequences, and the decoding device may then identify those training sequences in the modified signal prior to processing the modified signal.

20 This results in the gain applied by AGC in the data block between the training sequences being independent of the content of the data block, and therefore predictable by the decoder on the basis of the general information available to the decoder.

25 In a further aspect, the present invention provides a communications apparatus having a receiver and a

decoder, the receiver including: means for receiving a data signal) means for determining the gain to be applied to that signal at a particular time; means for applying said gain to produce a modified signal; and means for 5 communicating the modified signal, the decoder including means for receiving a modified signal, means for adjusting the modified signal to correct for the effect of the gain applied to that modified signal; means for identifying predetermined sequences in the modified 10 signal; and means for storing general information about the operation of the means for applying gain to the data signal, the means for storing being connected to the means for adjusting the modified signal.

The apparatus may also include means for determining 15 the general information connected to the means for storing said information.

The AGC applied in all the above aspects may be feed-forward or feed-back AGC.

The receivers which form part of the present 20 invention may also have other standard features such as filters, mixers and fixed gain devices.

Similarly, the decoding devices may have other standard features such as equalisers, correlators, demodulators and timing recovery.

25 A particular implementation of the above aspects of the present invention is envisaged in high-frequency (HF)

communication systems. Applications in other communication bands are envisaged, and fall within the scope of the present invention.

S Embodiments of the present invention will now be described in relation to the accompanying drawings, in which: Figure 1 is a schematic diagram of a communications system according to a first embodiment of the present 10 invention; Figure 2 is a schematic diagram of a communications system according to a second embodiment of the present invention; Figure 3 is a schematic diagram of a communications 15 system according to a third embodiment of the present invention; Figure 4 shows the effect of AGC as applied according to a first embodiment of the present invention; Figure 5 shows the effect of AGC as applied 20 according to a second embodiment of the present invention; Figure 6 shows the effect of AGC as applied according to a third embodiment of the present invention; and

Figure 1 is a schematic block diagram of a communications system according to a first embodiment of the present invention. The system comprises a receiver 10 and a decoder 20, which in this embodiment is a modem.

S The receiver 10 receives an input data signal 100 which is eventually produced as a decoded data signal 120 from the decoder 20.

The receiver front end 11 includes all the filtering, mixing and fixed gain circuits normally found 10 in such receivers and produces a received signal 101.

A measured signal level 102 of the input data signal 100 is passed to AGC algorithm 12 which computes the level of variable gain to be applied to the signal, and the way in which it is to be applied. For example, the 15 algorithm may have a fast "attack" time constant when the measured signal level is rising and a slow "decay" time constant when the measured signal level is falling.

The necessary gain as calculated by the AGC algorithm 12 is applied to the received signal 101 by 20 processing block 13. This represents all the variable gain stages in the receiver 10 which are controlled by the AGC. For simplification, they are shown as one processing block 13 here, but will normally be distributed throughout the receiver 10.

25 The arrangement shown implies that the AGC circuit is a feed-forward circuit, but this is for convenience of

representation. AGC feed-back circuits can also be used in the present invention, in which case the signal level is measured at modified signal 110, rather than input data signal 100.

5 Control switch 14 controls whether or not the output 103 of AGC algorithm 12 is fed to the decoder 20 at a particular time and is optional in this embodiment. If not present, then signals 103 and 111 are always the same. 10 Both the modified signal 110 to which the AGC gain has been applied, and information about the gain applied 111 are communicated to the decoder 20.

The information 111 is received by a gain processing algorithm 23 which determines the effect of the applied 15 gain and controls the adjusting circuit(s) generally denoted by 22. Like processing block 13 which applies the gain in the receiver 10, adjusting circuit(s) 22 may be located or distributed throughout the other processing units contained in decoder 20, but are shown here in a 20 single block for simplification.

The adjustments made by the adjusting circuit(s) 22 use conventional techniques and methods which are known in this field.

The adjusted signal is decoded by conventional 25 decoding algorithms generally denoted by 21 to produce

decoded data signal 120. The decoding algorithms may include an equaliser and/or other standard features.

Figure 2 shows a schematic block diagram of a 5 communications apparatus according to a second embodiment of the present invention. The structure is generally similar to that of Figure 1 and identical components and signals carry the same reference numerals.

In addition to the features of the first embodiment, 10 Figure 2 shows a feedback signal 112 which controls control switch 14. In this embodiment, both the gain applied by block 13 and the information 111 fed to the decoder 20 are controlled by this switch 14; when it is open, the gain applied by block 13 remains constant at 15 its previous value.

In this embodiment, the feedback signal 112 is generated by the decoder 20. For example feedback signal 112 may be produced by an equaliser in decoder algorithms 21 and may control the control switch 14 such that AGC is 20 only applied to the received signal 101 at a particular point in the training sequences identified by the equaliser. As an alternative, the feedback signal 112 could be generated in the receiver 10, and fed from the receiver to the decoder 20.

25 In addition to controlling the control switch 14, the feedback signal 112 is also passed to gain processing

algorithm 23 so that that algorithm is aware when a change in gain is applied.

The information about the gain applied 111 is generally only communicated to the decoder 20 when 5 control switch 14 is activated and a new level of gain applied by AGC.

Although the apparatus and method of this second embodiment of the present invention requires two-way communication between receiver 10 and decoder 20, this 10 communication is only necessary once in every training sequence, and so is at a much lower rate than that of the received data 110. In the alternative situation, where the feedback signal 112 is generated in the receiver 10, only one-way communication is required but there are 15 three signals to communicate. However, in this situation, two of these (information 111 and feedback signal 112) can be communicated at a lower rate than the modified data signal 110.

20 Figure 3 shows a schematic block diagram of a communications apparatus according to a third embodiment of the present invention. The structure is generally similar to that of Figures 1 and 2 and identical components and signals carry the same reference numerals.

25 However, the communications apparatus according to the third embodiment has no further communication between

receiver 10 and decoder 20 (e.g. information about gain applied 111 or feedback signal 112) apart from the modified signal 110.

In the third embodiment, input signal 100 is 5 preferably such that its signal level 102 is always lower during the data blocks than it is during the surrounding training sequences. This results in AGC algorithm 12 only decreasing the gain applied by block 13 during the training sequences, and the gain applied during the data 10 blocks being a steady rise that is determined by characteristics of the AGC algorithm 12.

Gain processing algorithm 23 stores general information relating to the characteristics of AGC algorithm 12. This general information may be programmed 15 into gain processing algorithm 23, or may be determined by gain processing algorithm 23 by analysis of the modified signals output by receiver 10 over a long period of time.

20 Figures 4 to 6 show the determination of AGC to be applied to a received signal, and the gain applied to that signal for each of the first to third embodiments described above in relation to Figures 1 to 3 respectively. The horizontal axis in each case 25 represents time.

The upper part of each of Figures 4 to 6 shows the measured signal amplitude 102 as the thin line, and the output 103 of the AGC algorithm 12 as the thick line, both on a logarithmic scale.

5 The lower part of each of Figures 4 to 6 shows the gain applied 130 to the received signal by block 13 to produce the modified signal 110, again on a logarithmic scale. This is the AGC output 103 reflected about a horizontal axis.

To The training sequences of the signal are shown as shaded portions 131 and 132.

In Figure 4, the AGC algorithm is operational and applied to the received signal 101 continuously. The information communicated to the decoder 20 is identical 15 to the output of the AGC algorithm 12 and is communicated continuously as well. In a development of the first aspect, the output of the AGC algorithm may be sampled at a predetermined rate to reduce the amount of information that needs to be communication to the decoder 20. In 20 this case, the processing algorithm may simply use the information supplied about the gain applied until the next sampling point, or may apply some form of extrapolation or interpolation between sample points.

In Figure 5 the actual output 103 of the AGC 25 algorithm 12 is shown as the lighter coloured thick line, whilst the information 111 about the gain applied to the

received signal 101 and communicated to the decoder 20 is shown as the darker coloured thick line.

AGC 12 is continuously operational, but the gain is only applied to the received signal 101 during training 5 sequences 131 and 132, as shown by the lower part of Figure 5.

Consequently the information supplied about the gain to the decoder only changes in the training sequences, and so only needs to be communicated at or after those 10 points.

In the embodiment shown, the AGC applied is changed at the start of each training sequence. In other embodiments, the applied AGC may be changed at different points in the training sequence.

15 In Figure 6 the situation is similar to that shown in Figure 4, as the AGC is applied continuously to received signal 101. However, no information is communicated from the receiver 10 to the decoder 20.

In this embodiment it is important that the level of 20 the received signal 102 is well structured such that the signal level during the data block is always less than that during the training sequences, as shown. This results in the gain applied during the data block showing a steady rise independent of the data content. Since the 25 processing algorithm in the decoder is aware of the location of the training sequences, and also of the

waveform of those sequences, the modified signal can be adjusted either continuously or periodically on the basis of the known performance characteristics of the AGC algorithm. s The above embodiments are intended to be examples of the present invention and variants and modifications of those embodiments, such as would be readily apparent to the skilled person, are envisaged and may be made without 10 departing from the scope of the present invention.

Claims

1. A method of processing a communications signal including the steps of: 5 applying gain to a received signal by automatic gain control to create a modified signal; communicating the modified signal to a decoding device; and processing the modified signal to adjust for the 10 effect of the gain applied.

2. A method according to claim l wherein the step of processing the modified signal is carried out on the basis of information about the gain applied to produce 15 the modified signal.

3. A method according to claim 2 wherein the information about the gain applied includes information about the level of gain applied at a particular point in 20 the modified signal.

4. A method according to claim 2 or claim 3 wherein the information about the gain applied includes information about the level of gain applied to a particular level of 25 received signal.

5. A method according to any one of the claims 2 to 4 further including the step of communicating to the decoding device the information about the gain applied to the received signal.

s 6. A method according to claim 5 further including the steps of: measuring a level of the received signal prior to applying automatic gain control to that signal; 10 receiving the modified signal and the information in the decoding device.

7. A method according to claim 5 further including the steps of: 15 measuring a level of the received signal after applying automatic gain control to that signal; receiving the modified signal and the information in the decoding device.

20 8. A method according to any one of claims 5 to 7 wherein the information is continuously communicated to the decoding device.

9. A method according to any one of claims 5 to 7 25 wherein the information is communicated to the decoding device at first predetermined points.

10. A method according to claim 9 wherein the decoding device determines the adjustment necessary at points between the first predetermined points and applies that 5 adjustment.

11. A method according to claim 9 or claim 10 wherein the first predetermined points fall within training sequences of the received signal.

12. A method according to any one of the preceding claims wherein the gain applied to the received signal is only changed at second predetermined points.

15 13. A method according to claim 12 as dependent on any one of claims 9 to 11 wherein the first and second predetermined points are the same.

14. A method according to claim 12 or claim 13 wherein 20 the second predetermined points are set by the decoding device. 15. A method according to any one of claims 12 to 14 wherein the second predetermined points fall within 25 training sequences of the received signal.

16. A method according to any one of claims 2 to 4 wherein no specific information regarding the gain applied is communicated to the decoding device, and further including the step of storing, in the decoding 5 device, general information regarding the performance of the automatic gain control, which general information is used to process the modified signal.

17. A method according to claim 16 wherein the decoding 10 device is preprogrammed with said general information.

18. A method according to claim 16 wherein the decoding device calculates said general information from long-term measurement and analysis of the modified signals received 15 by the decoding device.

19. A method according to any one of claims 16 to 18 wherein the received signal is such that highest amplitude parts of that signal are contained in the 20 training sequences, and the decoding device identifies those training sequences in the modified signal prior to processing the modified signal.

20. A method according to any one of claims 16 to 19 25 wherein said general information includes at least one time constant of the automatic gain control process.

21. A method according to any one of claims 16 to 20 wherein said general information includes the algorithm used in the automatic gain control process.

s 22. A communications apparatus having a receiver and a decoder, the receiver including: means for receiving a data signal; means for determining the gain to be applied to that 10 signal at a particular time; means for applying said gain to the data signal to produce a modified signal; and means for communicating said modified signal to the decoder, the decoder including: 15 means for receiving the modified signal; and means for adjusting the modified signal to correct for the effect of the gain applied to that modified signal. 20 23. A communications apparatus according to claim 22 wherein the means for adjusting the modified signal uses information about the gain applied to produce the modified signal.

25 24. A communications apparatus according to claim 23 wherein the information about the gain applied includes

information about the level of gain applied at a particular point in the modified signal.

25. A method according to claim 23 or claim 24 wherein 5 the information about the gain applied includes information about the level of gain applied to a particular level of received signal.

26. A communications apparatus according to any one of 10 claims 23 to 25 wherein the receiver further includes means for communicating information about the gain applied to create said modified signal, and the decoder further includes means for receiving that information.

15 27. A communications apparatus according to any one of claims 22 to 26 wherein the receiver further includes means for selecting whether or not the gain applied to the received signal is to be changed at a particular time. 28. A communications apparatus according to claim 27 wherein the means for selecting is controlled by an input from the decoder.

25 29. A communications apparatus according to claim 28 wherein the decoder further includes means for

identifying predetermined sequences in the modified signal. 30. A communications apparatus according to claim 29 5 wherein the means for selecting is activated by the decoder only during said predetermined sequences.

31. A communications apparatus according to any one of claims 22 to 28 wherein the decoder further includes 10 means for identifying predetermined sequences in the modified signal, and means for storing general information about the operation of the means for applying automatic gain control, wherein said means for storing is connected to the means for adjusting the modified signal.

32. A communications apparatus according to claim 31 further including means for determining said general information connected to the means for storing said general information.

33. A method of processing a communications signal substantially as herein described with reference to the accompanying drawings.

34. A communications apparatus substantially as herein described with reference to, or as illustrated in, the accompanying drawings.