High frequency multichannel diversity differential phase shift (DPSK) communications system

Claims

I claim:

1. A high frequency multichannel communications system having at least a transmitter and a receiver:

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits:

b) the input means, connected to a plurality of separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals;

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a resultant vector signal phase for determining polarity of each transmitted data bit as a data output signal.

2. A high frequency multichannel communications system as claimed in claim 1 wherein the receiver is synchronised to the received signal.

3. A high frequency multichannel communications system as claimed in claim 1 wherein the data rate is selected such that dispersion effects during transmission from the transmitter to the receiver does not lead to intersymbol interference.

4. A high frequency multichannel communications system as claimed in claim 3 wherein said digital signal data is transmitted at a baud rate between 20 and 100 per sec.

5. A high frequency multichannel communications system as claimed in claim 4 wherein the baud rate is 50 bps.

6. A high frequency multichannel communications system as claimed in claim 1 wherein the transmitted signal has a number of phase states greater than two.

7. A high frequency multichannel communications system having at least a transmitter and a receiver:

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits;

b) the input means, connected to a plurality of separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals;

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal, wherein there is included a channel exciser for excising channels identified as noise corrupted channels.

8. A high frequency multichannel communications system having at least a transmitter and a receiver:

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits:

b) the input means, connected to a plurality of separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals;

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal wherein said DPSK modulator is an M phase DPSK modulator which produces an M phase DPSK channel signal where M is an integer and the means for identifying noise-corrupted channels includes a phase window detector having M phase windows of width <360/M deg centered on M phase states.

9. A high frequency multichannel communications system as claimed in claim 8 wherein a HIT is when a detected phase falls within a phase window and a MISS is when a detected phase falls outside a phase window, including a counter connected to each channel for providing, over a predetermined time, a first signal indicative of a running average of HITS for each channel and a second signal representing the proportion of HITs to MISSes said first and second signals connected to a discriminator for determining whether the channel is noise corrupted.

10. A high frequency multichannel communications system as claimed in claim 9 wherein each phase window is 360/2M deg and said discriminator provides an output indicating a channel to be uncorrupted by noise when said second signal indicates that the proportion of HITs to Misses is above 1.

11. A high frequency multichannel communications system as claimed in claim 10 wherein the proportion of HITs to MISSes in each channel and the number of phase windows taken to measure the proportion are selected with reference to a channel signal-to-noise (S/N) ratio and a predetermined probability of correct identification of channel corruption.

12. A high frequency multichannel communications system as claimed in claim 11, wherein the resultant vector signal phase for each phase window is connected to a PSK decoder for providing said data output signal.

13. A high frequency multichannel communications system having at least a transmitter and a receipt;

said transmitter comprising:

a) input means for receiving digital signal data for transmission Of transmitted data bits:

b) the input means, connected to a plurality of Separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits Of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals:

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal wherein the plurality of narrowband channels in the receiver are arranged into contiguous broader bandwidth groups where the channels of each group are connected to a respective coherent adder for coherently adding uncorrupted channel signals and the adder outputs are connected to the semi-coherent processing means for vectorially adding the adder outputs.

14. A high frequency multichannel communications system as claimed in claim 13 where each group has a bandwidth less than 2 kHz.

15. A high frequency multichannel communications system as claimed in claim 14, further including first and second channel excisers, wherein in said means for identifying channels corrupted by noise, the DPSK channel signal detected in each receiver channel is connected to first and second channel excisers and to an error detector; the output from said error detector being connected to said first and second channel excisers for excising noise-corrupted channels; the first exciser being connected through a semi-coherent channel vector summer and through a PSK detector to a data output and the second exciser being connected via a second semi-coherent vector summer for providing an estimated combined vector signal phase input signal to a second input of said error detector.

16. A high frequency multichannel communications system as claimed in claim 15 wherein of an excision decorrelator is connected to an input to the second exciser for preventing the number of excised channels from exceeding a pre-determined value.

17. A high frequency multichannel communications system having at least a transmitter and a receiver:

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits:

b) the input means, connected to a plurality of separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals:

c) means for identifying and of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal wherein said vectorially added DPSK detector output signals are connected to each means for identifying channels corrupted by noise.

18. A high frequency multichannel communications system as claimed in claim 17, wherein the DPSK detector output signals in each channel are connected to one input of an error detector with an estimated combined vector signal phase being connected to a second input to the error detector, an output signal from the error detector is connected to an error counter, said error counter providing a noise corrupted channel output whenever a detected error rate exceeds a predetermined threshold.

19. A high frequency multichannel communications system as claimed in claim 18 wherein said error detector only provides an output if the detected phase signal is outside a pre-determined range centered on the combined vector phase signal.

20. A high frequency multichannel communications system having at least a transmitter and a receiver:

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits:

b) the input means, connected to a plurality Of separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission: and

wherein said receiver comprising.:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals;

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted dam bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal wherein the transmitter includes a demultiplexer [(1101)]responsive to the digital signal data for providing at respective outputs demultiplexed portions of said signal data, said plurality of separate frequency channels are divided into groups of different frequency channels, each group connected to a respective demultiplexed portion of said signal data and each channel transmitting at a low data rate between 20-100 bps and the receiver includes a multiplexer to reproduce the transmitted data signal.

21. A high frequency multichannel communications system as claimed in claim 20 wherein the channels of any one multiplexed group are interleaved with the channel of every other group.

22. A high frequency multichannel communications system having at least a transmitter and a receiver:

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits:

b) the input means, connected to a plurality of separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising;

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals;

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal wherein the receiver means is connected to the receiver frequency channels via an analogue to digital converter connected to a Fast Fourier Transform (FFT) processor which has a number of frequency channels equal to the number of receiver frequency channels and the signal phase is detected in each receiver frequency channel.

23. A high frequency multichannel communications system having at least a transmitter and a receiver;

said transmitter comprising:

a) input means for receiving digital signal data for transmission of transmitted data bits:

b) the input means, connected to a plurality of Separate diversity frequency channels distributed over a broad spectral region of a high frequency band, each channel including a differential phase shift key (DPSK) modulator, for modulating a channel frequency by successive bits of the digital signal data at a low data rate to produce a DPSK channel signal; and

c) means for combining the DPSK channel signals from each of the plurality of separate diversity frequency channels for transmission; and

wherein said receiver comprising:

a) receiver means for receiving and demodulating the transmitted signals to form baseband signals;

b) said baseband signals from the receiver means being connected to a plurality of narrowband receiver frequency channels, each receiver frequency channel including a DPSK detector responsive to a respective one of the transmitted DPSK channel frequency signals;

c) means for identifying any of said receiver frequency channels corrupted by noise; and

d) semi-coherent processing means, responsive to each transmitted data bit, for vectorially adding DPSK detector output signals, excluding said channels corrupted by noise and for producing a combined vector signal phase for determining polarity of each transmitted data bit as a data output signal wherein the transmitter includes a pseudorandom number generator (PNG) connected to the means for combining the DPSK channel signals for modulating the combined channel signals with a pseudorandom code and said receiver includes a demodulator connected to the same coherent processing means for demodulating the transmitted data signal.

24. A high frequency multichannel communications system as claimed in claim 23 wherein in the transmitter the DPSK channel signals are generated at 1 bps and the pseudorandom code is generated at 50 bps and in the receiver the received signal is summed over 1 sec intervals for every 50 received bits to determine the transmitted data bit polarities.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to HF communications and in particular to a multichannel frequency diversity DPSK communications system for the HF radio band.

2. Discussion of Prior Art

The purpose of frequency diversity in a communications system is to overcome the vagaries of long range HF radio propagation and interference and thereby improve the ability to reliably detect the transmitted signal with greatly reduced errors and with increased availability.

If transmitted signals are sent using a plurality of different radio frequencies the intended receiver will be able to exploit the diversity reception to:

a. reduce the received bit error rate

b. avoid co-channel interference from other radio transmissions;

c. overcome multipath time dispersion;

d. overcome channel fading;

e. reduce the effects of time/diurnal variations in propagation;

f. exploit sporadic and transitory propagation;

g. operate with lower transmitter powers:

h. have improved performance (greater data rates); have increased availability (on-demand communications).

Diversity reception requires the provision of two or more (K) transmitted signals, each containing the same message (either simultaneously or time interleaved). On different radio frequency carriers having advantageously uncorrelated propagation characteristics: each carrier frequency defining a diversity channel.

At the receiver the diversity channels must be properly recombined in order to ideally produce an output signal which will have a much lower combined BER (bit error rate) than in any one received channel. In the simplest diversity combined the channel with the best S/N (signal-to-noise ratio) or lowest BER will be switched to the output. This type of switch `combining` only works well, however, when at least one channel is always good. When the S/N is simultaneously poor in all the channels the output will be also be poor. A more advantageous method of diversity combining is to sum the received branches after weighting each channel. The channels can be weighted according to their S/N, for example; such systems are known as Maximal Ratio Combining. Using this technique it is possible to coherently combine the wanted signals (if channel co-phasing can be used) whilst at the same time only adding the noise in each channel incoherently. This produces a combined S/N which will be 10 Log(K)dBs better than any individual diversity channel, where the S/N is the same in each. However, to be most effective at HF, the channel frequencies must be separated by more than the correlation bandwidth (the range of frequencies over which noise signals are correlated). This will ensure each channel path will be totally uncorrelated in propagation characteristics such as, fading and multipath as well as interference. Unfortunately, this also means the phase characteristics for each path will also be very different from baud to baud. This uncorrelated phasing characteristic between channels will make it very difficult to properly co-phase the wanted signals from each path particularly since the S/N will normally be poor in each. At HF therefore, diversity combining can normally only be achieved using noncoherent signal combining.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a multichannel diversity DPSK modulated communications system which overcomes the known HF propagation and interference problems and will optimally re-combine DPSK modulated signals of poor S/N and with totally uncorrelated propagation characteristics.

The invention provides a high frequency multichannel communications system having:

a transmitter comprising:

a) Input means to receive digital signal data for transmission;

b) the input means connected to a plurality of separate diversity frequency channels distributed so as to produce a signal for transmission over a broad spectral region of the hf band, each channel including a differential phase shift key (DPSK) modulator whereby each channel is modulated at a low data rate; and

c) means to combine the DPSK channel signals for transmission;

and a receiver comprising:

a) receiver means to receive the transmitted signals and convert them to baseband;

b) the receiver means being connected to a plurality of narrowband frequency channels, each channel including a DPSK detector responsive to a respective one of the transmitter diversity frequency signals;

c) means to identify channels corrupted by noise; and

d) semi-coherent processing means to vectorially combine the uncorrupted DPSK detector output signals in each baud period to determine the polarity of the transmitted data bit.

The data rate is preferably selected such that radio path time dispersion does not lead to intersymbol interference. Advantageously the transmission baud rate is between 20 and 100 per sec and is preferably 50 bps.

The modulation level (M) of the transmitted signal, i.e. the number of phase states, may be greater than two. The means to identify noise-corrupted channels is preferably a channel exciser which includes a phase window detector having M phase windows of width<360/M deg centred on the expected phase directions. Advantageously there is included a counter which takes a running average for each channel, over a pre-determined number of baud periods, of the number of times the detected phase falls within one of the phase windows (HIT). A signal representing the proportion of HITs over the pre-determined number of baud periods for each channel is connected to a discriminator to determine whether the channel is noise corrupted. Noise corrupted channels are then excised. In one arrangement each phase window is 360/2M deg and the discrimination level (HITs to Misses) is set above 1. Preferably the discrimination level applied to the ratio of HITs to MISSes in each channel and the number of baud periods taken to measure the ratio are selected with reference to the resulting signal-to-noise (S/N) ratio and the required reliability of the channel excision.

Preferably each uncorrupted channel signal is added vectorially in the semi-coherent processor and the resultant vector for each baud period is connected to a PSK decoder to determine the data output signal.

In one advantageous arrangement the receiver can be arranged to bunch the uncorrupted frequency channels into groups where the bandwidth of a group is less than the correlation bandwidth, the channels of each bunch being connected to a respective coherent processor and the coherent processor outputs being connected to the semi-coherent processor. Preferably the bunch bandwidth is less than 2 kHz.

In a preferred arrangement a combined vector signal phase output from a semi-coherent processor may be connected to the channel excisers to determine channel excision. The detected phase signal in each channel is connected to one input of an error detector with the estimated combined vector signal phase being connected to a second input to the error detector, an output signal from the error detector is connected to a channel exciser whenever the detected error rate exceeds a pre-determined threshold. The detected phase signal is considered to be good if it falls within a pre-determined range from the combined vector phase signal. Advantageously the DPSK signal detected in each receiver channel is connected to first and second channel excisers and to the error detector; the output from the error detector being connected to both excisers to excise noise-corrupted channels; the first exciser being connected via a semi-coherent channel vector summer and a PSK detector to a data output and the second exciser being connected via a second semi-coherent vector summer to provide the estimated combined vector signal phase input signal to the second input of the error detector.

Erraneous channel `capture` may be prevented by providing an excision decorrelator at the input to the second exciser used to provide the estimated group phase vector, the decorrelator being effective to prevent the number of excised channels from exceeding a pre-determined value.

In one arrangement the transmitter may include a demultiplexer whereby a high rate of input data to the demultiplexer is divided into groups of different frequency channels, each channel transmitting at a low data rate between 20-100 bps. In this arrangement the receiver includes a multiplexer to reproduce the higher data rate transmitted signal. Preferably the channels of any one multiplexed group are interleaved with the channel of every other group. By combining high modulation levels with the parallel demultiplexed data, even higher transmission rates can be achieved. In narrowband channels may be provided, spread over a 1 MHz bandwidth.

Preferably the data is differential phase shift key (DPSK) modulated in each channel. The receiver preferably includes an analogue to digital converter connected to a Fast Fourier Transform (FFT) processor which has a number of Frequency bins equal to the number of transmitted frequency channels. The signal phase is then detected in each frequency bin.

In the above arrangements the signal-to-noise of the received data signal may be improved by modulo-2 spreading the transmitted signal with a pseudorandom code (by a pseudo random number generator: PNG) and then despreading the signal in the receiver by means of a replica code. Advantageously the receiver is synchronised to the received signal by means of suitable timing signals. In the preferred arrangement data at 1 bps is spread using a PNG code of 50 bps and in the receiver the de-spread signal is summed over 1 sec or 50 received bits to determine the transmitted data bit polarities.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying Drawings of which:

FIG. 1a is a schematic block diagram of a frequency diversity transmitter employing differential phase shift key (DPSK) modulation;

FIG. 1b illustrates wideband frequency diversity;

FIG. 1c illustrates narrowband frequency diversity;

FIGS. 1d to 1f illustrate respectively BPSK (1 bit per symbol or modulation level M=2), QPSK (2 bits per symbol, M=4) and 8 PSK (3 bits per symbol, M=8);

FIG. 2 shows graphs of bit error rates (BE}R) against signal-to-noise ratios (SNR) for different modulation (M) levels;

FIG. 3 is a graph showing HF radio interference characteristics and FIG. 3b is an enlarged portion of FIG. 3a;

FIG. 4 is a graph showing the signal level probability function as a function of receiver bandwidth;

FIG. 5a shows an arrangement for converting a narrowband transmitted spectrum into parallel diversity channels and FIG. 5b in an audio baseband spectrum;

FIG. 6a is a block diagram illustrating the vector addition of detected signal phases in parallel diversity channels in a semi-coherent processor;

FIG. 6b graphically illustrates the vector addition;

FIG. 6c illustrates the data bit decision process;

FIG. 6d is the theoretical probability density function (PDF) for random noise phase shifts at the receiver input;

FIG. 6e illustrates the PDFs for BPSK signals for two different signal-to-noise ratios;

FIG. 6f shows the superimposition of phase windows on the BPSK PDF to determine a measure of channel interference;

FIG. 6g illustrates implementation of the FIG. 6f scheme for channel excision prior to semi-coherent phase vector summation;

FIG. 7 shows graphs of BER against SNR using semi-coherent channel combining for a number of different diversity channels;

FIG. 8 is a block diagram of a receiver circuit including coherent processing of channels within bunches in addition to semi-coherent processing of the channel bunches;

FIG. 9 is an alternative to the FIG. 6g arrangement in which received phase vectors are compared to estimated phase vectors and the error rate in this comparison is used to determine channel excision;

FIG. 10 is a phase diagram illustrating operation of the error detector in the FIG. 9 arrangement:

FIG. 11 is a block diagram of a demultiplexed transmitter enabling high rates of data transmission by means of 50 bps channels: and

FIG. 12 is a modification of the communications system employing spread spectrum data transmission and coherent receiver signal processing.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

FIG. 1 shows how phase modulated transmitted carrier signals for the diversity system according to the present invention may be created. Input data 101 is used to modulate K different carrier diversity frequencies 102 using Differential Phase Shift Keying (DPSK) modulators 103. The outputs 104 from the K diversity modulators are summed in an adder 105 to produce the signal 106 for transmission via a radio transmitter and aerial. The K channel frequencies may be spaced over just a few kilohertz (107) or over several megahertz (108) of transmission spectrum, however the bandwidth of each channel is much narrower than the overall bandwidth.

Widely spaced diversity channels will provide protection against long term time/diurnal variations in propagation because the receiving algorithm is capable of selecting those frequencies which can propagate from those which cannot (as the MUF and LUF changes). This level of frequency diversity will therefore also provide a means for automatic frequency management of radio circuits as well as avoiding the normal interference and fading problems.

Narrowband (eg. 3 kHz) diversity will not have the same long term propagation advantages as wideband but it can still provide substantial protection against interference and multipath particularly if the transmitted signal has 10 or more diversity channels. The number of channels which can be deployed will depend on the symbol rate used to