GB2346764A

GB2346764A - Radio systems

Info

Publication number: GB2346764A
Application number: GB8608328A
Authority: GB
Inventors: Simon Avarne
Original assignee: Racal Research Ltd
Current assignee: Racal Research Ltd
Priority date: 1986-04-04
Filing date: 1986-04-04
Publication date: 2000-08-16
Anticipated expiration: 2006-04-04
Also published as: GB8608328D0; GB2346764B

Abstract

A frequency hopping MFSK radio system performs initial synchronisation by transmitting (2) a synchronisation sequence on known channels, at least some of which carry known symbols. The receiver 4 detects what is received during each symbol period on a selected channel, which, in one embodiment, is maintained constant over a number of symbol periods. The reciever maintains a table of probabilities, each of which pertains to a possible time offset in the synchronisation sequence from an arbitrary point.(For example, if the synchronisation sequence has a length of L symbol periods, then L probabilities are stored, one for each possible offset.) For each possible offset, the receiver determines the probability that what has been received during the current symbol period is consistent with that offset being the correct one for synchronisation. (The probability is never zero because of noise.) The table of probabilities is updated cummulatively after each symbol period using Bayes theorem. Once a particular offset attains a threshold probability close to 1, then synchronisation to within a symbol period is achieved.

Description

RADIO SYSTEMS The present invention relates to radio systems, and, more particularly, to radio systems using multiple frequency shift keying (MFSK) and frequency hopping.

A frequency hopping MFSK radio system is described in GB-B-2 129 655. In this radio system one of a number of possible fixed tones is transmitted relative to a base frequency in dependence on a symbol of data to be transmitted. The base frequency is hopped pseudo randomly amongst a number of available channels with a hop period which is equal to the symbol period. Such a radio system has very good resistance to recognition, tracking, direction finding or jamming so that, overall, there is a very low probability of intercept (LPI) for such a radio system.

A major technical problem with such a frequency hopping MFSK radio system is the achievement of synchronisation between a transmitter and receiver of the system. Both the transmitter and the receiver include a frequency synthesiser and these components must be synchronised. In GB-B-2 129 655 it is proposed that certain channels be set aside for synchronisation purposes and used to transmit predetermined sync characters when the receiver is first switched on. The characters and frequencies used are determined by a time of day clock which is maintained running continuously at both the transmitter and the receiver. If the receiver receives the correct synchronisation characters on the synchronisation channels then synchronisation has been achieved but, if the characters are not correctly received, then the time of day clock is reset by a predetermined amount in order to time-shift the operation of the key stream generator. If the first time shift does not result in correct reception of the synchronisation characters, then this is repeated until synchronisation is achieved. This is then signalled by the receiver to the transmitter so that message transmission can start.

This type of synchronisation has the disadvantage that it produces a signal which may be readily recognised because it is on a restricted number of channels and has a characteristic form. Moreover, synchronisation using this technique may take a considerable period of time, rendering the transmission more likely to be detected. Synchronisation may even be impossible if the channels set aside for synchronisation are occupied.

GB-B-2 129 655 also discloses an alternative synchronisation method. In this alternative method a key stream generator is arranged to transmit a sequence of frequencies which when compared to a base frequency represent information about the time of day. When the base frequency is sent a sync character is sent on it to identify it as a base frequency. The receiver is held tuned to the base frequency and waits for reception of the sync character. By using a fixed base frequency or one selected from a small group of channels, the sync character can easily be detected and therefore this type of synchronisation is unsatisfactory. Because the sync character can be detected the following synchronisation sequence can be jammed in order to render the system unusable.

The present invention therefore seeks to solve the technical problem of providing a synchronisation system for use in a hopping MFSK radio system which allows synchronisation to be achieved within a reasonable time while maintaining a high degree of protection against ECM and ESM systems.

Accordingly, the present invention provides a hopping MFSK radio system comprising a transmitter and a receiver, the transmitter having a frequency synthesiser and means for controlling the synthesiser to produce for each of successive symbol periods a frequency representing a channel frequency plus a tone representing a symbol to be transmitted during that symbol period, each symbol period being a period equal to a hop period for which the channel frequency is constant, the receiver comprising means for detecting and decoding symbols from the received frequency, the transmitter further comprising means for repeatedly transmitting a synchronisation sequence of tones on predetermined channels in order to synchronise the transmitter and receiver, the receiver comprising means for storing data representing a respective probability that each of a complete set of mutually exclusive hypotheses relating to the synchronisation is true, and means for updating said probabilities by repeatedly demodulating a symbol from a selected channel.

With such a synchronisation technique, it is found that synchronisation can be achieved quite rapidly without producing a synchronisation transmission which is readily identifiable. Thus the synchronisation process necessary on opening up of a radio link or after a long period of radio silence does not impair the low probability of intercept inherent in hopping MFSK systems.

For a synchronisation sequence of L tones and channels, the set of hypotheses may comprise the set of L possible time offsets in terms of symbol periods of the synchronisation sequence from an arbitrary time. In a situation where the frequency offset between base frequency standards at the transmitter and receiver may exceed the tone spacing the set of hypothesis may be expanded to L times a number of possible frequency offsets.

In one embodiment the channel selected for decoding by the receiver is maintained constant or varied periodically until one probability exceeds a predetermined threshold. In an alternative embodiment the channel selected tracks the most likely hypothesis at that time. The use of tracking facilitates rapid synchronisation. If the tracked hypothesis is wrong its probability decreases rapidly and a shift will be made to the hypothesis with the greatest probability. If the tracked hypothesis is true its probability should rapidly increase to the threshold value at which synchronisation is said to be achieved.

Preferably a minority of the symbols transmitted during the synchronisation sequence are not predetermined but are arranged to include timing information to enable a time of day generator at the receiver to be accurately synchronised to within one symbol or hop period. In a typical example where the sequence has a length of 1024, it is readily possible to transmit 16 information bits with a high degree of error protection coding so as to ensure that they are received by the receiver without error by using, say, the last 75 symbols of the synchronisation sequence.

The present invention also provides a method of synchronising a transmitter and receiver of a frequency hopping MFSK radio system which transmits one tone representing a symbol on each channel, comprising repeatedly transmitting a synchronisation sequence of symbols on predetermined channels, tuning the receiver to a selected channel for each hopping period and decoding a symbol, and using this symbol and channel information to update the probability that the synchronisation sequence is being transmitted with a time offset represented as an integral number of symbol periods from an arbitrary reference, and establishing synchronisation when the probability of one offset reaches a predetermined threshold.

A frequency hopping MFSK radio system in accordance with the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing which represents a block diagram of the system.

A frequency hopping MFSK radio system comprises a transmitter 2 and a 4.

Data is fed into a control microprocessor 6 in the transmitter 2, where it is encoded using a suitable forward error correcting code. The control microprocessor 6 controls the synthesiser 8 to produce appropriate RF tones representing the tone frequency for the encoded data symbol added to the RF base channel frequency. The RF tones are sent from the synthesiser 8 to a transmitter 10. The transmission from the transmitter 10 is received by a receiver circuit 12 of the radio receiver 4. The receiver 12 is tuned to the appropriate channel frequency under the control of a control microprocessor 14. The output of the receiver 12 is fed to a processor 16 which samples the received signal and performs a sliding discrete fourier transform on the received signal to detect which symbol tone was transmitted.

This information is passed to the control microprocessor 14 which decodes the data represented by the received symbols and produces output data on line 18. Such a frequency hopping MFSK radio system is known per se and is described in GB-A-2 129 655.

The system uses a number of tones to transmit different symbols. Each tone is transmitted for a predetermined symbol period which corresponds to the hop period. The frequency spacing and the hop or symbol period are preferably orthogonal. In a typical system sixteen tones are used at 20Hz spacings with a symbol period of 50ms. The frequency hopping transmitter will typically hop over 256 available channels spaced at at least 2kHz apart in the HF or VHF bands. Other system parameters may readily be selected but these parameters will be used for illustration purposes only in the discussion of the embodiment which follows.

In order for effective communications to take place, it is necessary that the control microprocessors 6 and 14 at the transmitter and receiver be accurately synchronised so that both transmitter and receiver tune to the same channel frequency in synchronism with each other. Both the control microprocessors, 14, maintain a time of day (TOD) clock and also include a frequency standard which may typically be accurate to plus or minus one part per million. Therefore, once the receiver and transmitter are synchronised, this synchronism will be maintained even though there may be a period of radio silence of several hours duration. However, when the radio link is first set up, it is necessary to establish synchronisation.

This process is called initial synchronisation.

In order to establish initial synchronisation, the control microprocessor 6 at the transmitter causes the synthesiser 8 to generate a synchronisation sequence of symbols on predetermined channels. All the channels to be used for synchronisation and at least the majority of symbols to be transmitted are known to the control microprocessor 14 at the receiver. The sequence may be dependent upon the key currently in operation so that the sequence is not always the same and therefore recognisable. Typically the sequence has a duration of 1024 symbol periods and is transmitted repeatedly. A small number of the symbols may be used for conveying time of day information and an indication when the last sequence is being transmitted.

Although the control microprocessor 14 at the receiver knows the sequence, it does not know at what point and time the sequence was started by the transmitter. If the sequence has a length L, that is it consists of L channels and associated tones, there are L possible starting points or cuts at which the synchronisation sequence could have started. Therefore there is a complete set of possible and mutually exclusive hypotheses for the true cut since the synchronisation sequence must have one only of the L possible cuts.

Each cut represents an offset in time of an integral number of symbol periods. Synchronisation to within one symbol period is discussed later. Therefore, in order to carry out the synchronisation, the control microprocessor 14 at the receiver stores a probability for each of these L hypotheses. Initially all the L probabilities stored are equal and set to 1/L.

The probability table stored in the control microprocessor 14 is updated after each symbol period on the basis of the results of demodulating a symbol from a selected channel. If we let the channel chosen at time t be c (t) and the demodulated symbol be s (t), then the result of the demodulation at time t will be a pair (c (t), s (t)), which may be designated d (t). The sequence of all d (t) up to time t is designated as D (t). It will be appreciated that the receiver will on many occasions merely be demodulating noise or interference. This happens if the transmitter 10 is not transmitting on the channel or the channel is occupied so that the transmission of a symbol is effectively blocked.

After each symbol period the pair d (t) can be used to update the table of probabilities by means of Bayes Theorem. The hypotheses can be designated h** where j may have any value from 1 to L. Thus at time t the probability of a given hypothesis hj being true on the basis of the accumulated sequence D (t) is as follows :

When there is no data, all hypotheses are equally likely and so have probability 1/L. This reduces the formula (1) to:

The denominator is the same for all hypotheses and is a normalizing factor which ensures the total probability is unity. In practice, it is not necessary to carry out this division after every symbol period. The numerator can be calculated by the iterative formula: P (D (t) lhj) = P (D (t-1) hj) P (d (t) lhj) (3) The starting value, P (D (O) Ihj) represents the probability of no data before any demodulation. Since this is always true, it is equal to 1. Therefore, in order to update the probabilities after each tone period, it is only necessary to multiply the existing probability by P (d (t) lhj).

Now a hopping MFSK system is likely to be operating in a radio environment where a relatively large number of the channels are occupied. Provided a sufficiently powerful forward error correcting code is used to transmit the data a hopping MFSK system can reliably transmit data with a channel occupancy level of as much as 50 per cent. It will of course be appreciated that high occupancy levels will have a significant effect on this synchronisation process. Let q be the probability that a given channel is occupied.

Therefore, if there is a 50 per cent occupancy level q = 0. 5. If at time t the selected channel c (t) disagrees with hypothesis hj, then P (d (t) lhj) = 1/T where T equals the number of tones. If c (t) agrees with hypothesis h** but the decoded symbol s(t) is not the expected value, then this can only occur if the channel c (t) is blocked. Therefore, in this event P (d (t) **h**) = q/T.

If both c (t) and s (t) agree with the hypothesis, hj, then the channel is either occupied but the symbol has by chance been demodulated successfully, or the channel is not occupied and the tone is genuine. The probability of the former is q/T and of the latter 1-q, therefore summing: P (d (t), hj) = 1-{q (T-1)/T}.

For each hypothesis, hj, P (d (t) ! hj) must be one of the above three possibilities. Conveniently, these probabilities are scaled up by T to give three alternative multiplying factors: 1, q and T-q (T-1) in dependence on which probability is appropriate for the given d (t) and hj. For T = 16 and q = 0.5 these three multiplying factors become 1, 0.5 and 8.5 respectively. This gives considerable savings in computation as, for the majority of hypothesis, c (t) disagrees with the hypothesis so that the multiplying factor is unity.

The algorithm used by the control microprocessor 14 for updating the table of probabilities may be modified in several ways in order to produce more rapid processing. For example, as previously mentioned, it is not necessary to normalise the probabilities stored in the table by dividing by the denominator in equation 2 above. Similarly, the various P (d (t) lh) can be normalised by multiplying by T, the number of tones. In this way only L multiplications are needed to update the probability table after each symbol period. Normalisation can be carried out at longer intervals and, in this way, the division can be reduced to exponent additions.

If the receiver starts synchronisation before the transmitter then the probabilities will fluctuate randomly. In such a situation it is possible that the correct hypothesis could have its probability reduced to almost zero. Because of rounding errors, this low probability might be stored as zero and this would result in failure to achieve synchronisation. In order to prevent this, a small fraction can be periodically added to each probability to prevent any probability reaching nought solely because of rounding error despite being true.

Because of the method of demodulation using a discrete fourier transform, it is preferable that the random channel selected for the demodulation of symbols be kept constant for as long as possible unless it is clearly blocked. For example, if the peak frequency of a channel is the same for three consecutive symbol periods then a channel change could be forced. During the synchronisation process the symbol clock normally used for demodulation is not available. However, during the synchronisation process, it is possible to ensure that the DFT demodulator 16 outputs a frequency and amplitude of the peak of the power 64 times in every symbol period. The largest of the 64 peaks over the whole symbol period is then taken as the decoded symbol.

The receiver carries on demodulating symbols on a selected channel or channels until one of the probabilities reaches a predetermined threshold close to 1. This hypothesis is then taken to be true and the receiver is now synchronised to within 1 symbol period of the transmitter.

In order to achieve final synchronisation, time of day information must be transmitted from the transmitter to the receiver. In order to ensure that this information is reliably transmitted, a high degree of error protection is necessary. This time of day information can be sent within the synchronisation sequence by transmitting symbols representing the TOD information in an encoded form on a small number of channels. A suitable code is a 5-way majority vote concatenated with a (15,4) Reed-Solomon inner code.

Typically, to give the required timing information within one symbol period, sixteen information bits must be transmitted. Using such a code, this would require the transmission of 75 symbols. For a pattern length L = 1024, the last 75 symbols of the synchronisation sequence could contain this information. This would mean that a control microprocessor did not know the expected symbol for 75 symbol periods of the synchronising sequence. This does not prevent operation of the probability updating process but, in this event it is only possible to distinguish between cases where c (t) agrees with the hypothesis or disagrees with the hypothesis, rather than the three situations discussed above.

In the above embodiment the channel selected to be decided is initially a predetermined channel, and this channel may be varied slowly in order to avoid occupied channels. For example a new channel may be used every 10 hop periods. However in an alternative embodiment the channel for decoding is made to track the most likely hypothesis. A given hypothesis will be tracked until the probability of its being true reaches the required threshold or another hypothesis achieves a greater probability than that of the hypothesis being tracked. This can result in even more rapid synchronisation being achieved. Using this synchronisation process on a hopping MFSK system using, for example, 256 channels, with sixteen tones, an occupancy level of 50 per cent and a pattern length L of 1024, it was found that the mean time to synchronisation was 1, 369 symbol periods. For a 50ms symbol period this represents a delay of just over one minute.

Since the frequency standards at both the receiver and transmitter are typically only accurate to 1ppm then the difference in frequency could be as much as 60Hz at 30MHz. In such a case the demodulated symbol could be three tones out even when the channel agrees with the true hypothesis. The algorithm may be modified by storing probabilities not just for the L possible cuts but also for a number of possible frequency offsets.

This results in a larger table of probabilities but the processing is essentially similar. It will be noted that the frequency offset has the effect of increasing the number of possible tones that can be decoded.

This changes the multiplying factor in the algorithm from 8.5 to 10.5 as 20 tones are now valid.

Claims

1. A hopping MFSK radio system comprising a transmitter and a receiver, the transmitter having a frequency synthesiser and means for controlling the synthesiser to produce for each of successive symbol periods a frequency representing a channel frequency plus a tone representing a symbol to be transmitted during that symbol period, each symbol period being a period equal to a hop period for which the channel frequency is constant, the receiver comprising means for detecting and decoding symbols from the received frequency, the transmitter further comprising means for repeatedly transmitting a synchronisation sequence of tones on predetermined channels in order to synchronise the transmitter and receiver, the receiver comprising means for storing data representing a respective probability that each of a complete set of mutually exclusive hypotheses relating to the synchronisation is true, and means for updating said probabilities by repeatedly demodulating a symbol from a selected channel.

2. A hopping MFSK radio system as claimed in claim 1, wherein the synchronisation sequences comprises L tones and channels and the set of hypotheses comprises the set of L possible time offsets in terms of symbol periods of the synchronisation sequence from an arbitrary time.

3. A hopping MFSK radio system as claimed in claim 2, wherein the set of hypotheses is expanded to L times a number of possible frequency offsets of each channel frequency between the transmitter and receiver.

4. A hopping MFSK radio system as claimed in any one of the preceding claims, wherein the selected channel is changed at predetermined intervals.

5. A hopping MFSK radio system as claimed in any one of claims 1-3, wherein the selected channel tracks the most likely hypothesis at the time of selection.

6. A hopping MFSK radio system as claimed in any one of the preceding claims, wherein a minority of the symbols transmitted during the synchronisation sequence are not predetermined but are arranged to include timing information to enable a time of day generator at the receiver to be accurately synchronised to within one symbol period.

7. A hopping MFSK radio system as claimed in claim 6, wherein said timing information comprises a plurality of time information bits encoded by means of a forward error correcting code.

8. A method of synchronising a transmitter and receiver of a frequency hopping MFSK radio system which transmits one tone representing a symbol on each channel, comprising repeatedly transmitting a synchronisation sequence of symbols on predetermined channels, tuning the receiver to a selected channel for each hopping period and decoding a symbol, and using this symbol and channel information to update the probability that the synchronisation sequence is being transmitted with a time offset represented as an integral number of symbol periods from an arbitrary reference, and establishing synchronisation when the probability of one offset reaches a predetermined threshold.

9. A hopping MFSK radio system, substantially as described with reference to the accompanying drawings.

10. A method of synchronising a transmitter and a receiver of a frequency hopping MFSK radio system, substantially as described with reference to the accompanying drawings.

9. A hopping MFSK radio system substantially as herein described with reference to the accompanying drawings. 10. A method of synchronising a transmitter and receiver of a frequency hopping MFSK radio system substantially as herein described with reference to the accompanying drawings.

Amendment o he citds have been fi ! cd as follows 1. A hopping MFSK radio system comprising a transmitter and a receiver; the transmitter including a frequency synthesiser and means operative to control the synthesiser to produce for each of successive symbol periods a frequency representing a channel frequency plus a tone representing a symbol to be transmitted during that symbol period, each symbol period being a period equal to a hop period for which the channel frequency is constant; the receiver including means operative to detect and decode symbols from the received frequency; the transmitter further including means operative to repeatedly transmit a synchronisation sequence of predetermined tones on predetermined channel frequencies in order to synchronise the transmitter and the receiver, each of these tones being transmitted during a respective symbol period; the receiver including synchronisation means comprising means operative to store data representing the synchronisation sequence, means storing data representing a set of hypotheses one for each symbol in the synchronisation sequence, each hypothesis representing the synchronisation sequence offset, from a time reference fixed in relation to the transmitted sequence, by a respective time offset determined by the time position of that symbol in the sequence, means storing for each hypothesis probability data representing the respective probability that that hypothesis is the true one for a current transmission of the synchronisation sequence, the stored probabilities being all equal and low before receipt of the synchronisation sequence by the receiver, means operative to decode symbols successively received by the receiver on a selected channel frequency and to compare each decoded symbol with the stored synchronisation sequence so as to identify the position or positions (if any) where such decoded symbol exists in the stored synchronisation sequence and thereby to update the probability data by raising the probabilities of the hypotheses corresponding to the identified position or positions in the synchronisation sequence relative to the probabilities of the other hypotheses, and means operative when the probability of one of the hypotheses reaches a predetermined value to accept that hypothesis as true and thereby determine the corresponding time offset and establishing synchronisation.

2. A hopping MFSK radio system according to claim 1, in which the synchronisation sequence includes L tones and channels and the set of hypotheses comprises a set of L possible hypotheses such that the stored probabilities before receipt of the synchronisation sequence by the receiver are all 1/L.

3. A hopping MFSK radio system according to claim 1 or 2, in which the stored data representing the synchronisation sequence includes data representing for each symbol in the synchronisation sequence a number of possible frequency offsets between the tone as transmitted by the transmitter and the corresponding tone as received by the receiver, and in which the set of hypotheses is increased correspondingly.

4. A hopping MFSK radio system according to any preceding claim, in which the selected channel is changed periodically.

5. A hopping MFSK radio system according to any preceding claim, including means for selecting as the selected channel the next channel as identified by the hypothesis having the highest probability at the time of selection.

6. A hopping MFSK radio system according to any preceding claim, wherein a minority of the symbols transmitted during the synchronisation sequence are not predetermined but are arranged to include timing information to enable a time of day generator at the receiver to be accurately synchronised to within one symbol period.

7. A hopping MFSK radio system according to claim 6, wherein said timing information comprises a plurality of time information bits encoded by means of a forward error correcting code.

8. A method of synchronising a transmitter and a receiver of a frequency hopping MFSK radio system which transmits one tone representing a symbol on each channel, including the steps of: repeatedly transmitting a synchronisation sequence of symbols on predetermined channels; tuning the receiver to a selected channel for each hopping period and decoding any symbol received therein; storing data representing the synchronisation sequence, comparing each decoded symbol with the stored sequence such as to determine the position or positions (if any) of the decoded symbol in the stored sequence and thereby up-dating a plurality of stored probabilities each being the probability that the time offset between a respective one of the symbol positions in the sequence and a time reference fixed in relation to the transmitted sequence is true, and accepting a particular one of the time offsets as being true when its probability reaches a predetermined value, thereby establishing synchronisation.