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Abstract

A high frequency transmitter comprises a large number of narrowband channels spaced over a broad bandwidth transmission. In one arrangement input data to be transmitted is demultiplexed (1101) so that sections of data are transmitted in groups of channels at 50 baud (say), each 50 baud channel produces a number of diversity channels modulated using difference phase shift key (DPSK) (103) and then added (105) for transmission. In the receiver, coherent summation (803) of close-bunched channels can be used and semi-coherent channel addition (805) can be used across the complete bandwidth. The received signal is convened to digital form then processed by an FFT circuit to produce frequency bins corresponding to the transmitted diversity channel frequencies. DPSK demodulation is carried out and running averages are carried out in each channel to determine the proportion of times that the phase difference falls within allowed limits (907). A discrimination level (908) determines whether individual channels should be excised (914) because of noise corruption. The remaining channels in groups are then added together to determine each data bit received, in semi-coherent addition (905), the measured phase vectors are added vectorially and the vector sum is used to determine the data bit. In a second arrangement the channels of any one group are arranged in bunches of about 10 spread over a bandwidth of about 2 kHz (the experimentally determined coherence bandwidth) with the bunches spread over the whole transmission bandwidth. The bunch channels are added coherently and the resultants are added semi-coherently as before to determine the group data. The group data is then multiplexed to reproduce the high data rate input data. The arrangement allows high data rates to be transmitted with each of the diversity channels carrying only 50 baud.

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