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

DETAILED DESCRIPTION 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)