The Secretary of State for Defence in Her Britannic Majesty's Government

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The Secretary of State for Defence in Her Brittanic Majesty's Government of the United Kingdom of Great Britain and Northern Ireland

Robert G Wilkinson

A high frequency communications system comprises eight multiplexed ( ) spread spectrum channels with data in each channel transmitted using, for example, 32 semi-orthogonal codes to enable 5 bits of data to be sent for every coded symbol transmitted. The multiplexed data is connected to a phase shift key (PSK) modulator ( ) to modulate an hf carrier. At the receiver, inband interference is removed by frequency excision ( ) or weighting, making use of the facts that the interference is generally narrow-band and the spectrum of the frequency components of the received signal should be symmetrical about the carrier frequency. The detected signal is Fourier transformed by an FFT processor ( ) and then connected to an interference exciser ( ) where signal samples symmetrical about the carrier frequency are compared and any sample which exceeds the respective symmetrical sample by more than a predetermined threshold is excised or given a low weighting. After interference excision the remaining signal is connected to a bank ( ) of 32 cross-correlation detectors before connection to a maximum likelihood detector and decoder ( ).

High data rate multiplexed multichannel high frequency broadcast system

HF aerials for ship-shore communications consist of spaced dipole arrays. By appropriate adaptive phasing very high gain HF aerials are formed. On transmission a feedback signal is required from the receiver, otherwise similar algorithms are used to control the beamforming. A random phase algorithm has been devised for the phases applied to the array aerials, operating in four tranches of 100 iteration steps with progressively reduced maximum phase variation. The initial step has a phase variation in the range .+-.180.degree.. The algorithm has the advantage that there is a high probability that a relatively high gain beam will be immediately formed in the required direction and thus the system can quickly settle towards a direction where the signal is weak. When in the transmit mode the receiver returns a signal to the transmitter giving the step number of the random phases which gives the maximum received signal.

Adaptive antenna arrays for HF radio beamforming communications

High frequency multichannel diversity differential phase shift (DPSK)

An ionospheric sounding system for frequency management of a HF communications system operates between a transmitter and a remote receiver and makes use of code-modulated narrow band sounding pulses transmitted at frequencies throughout the HF band. The code has an impulsive auto-correlation function and the frequency selection is pseudo-random. The code may be a two-part complementary code. Alternatively the code may be a selected one of a family of codes possessing high auto-correlation and low cross-correlation properties, thereby enabling communications management information to be conveyed by the choice of code. Measurements of signal and noise are made at the receiver for each transmitted frequency to assist establishing a HF communications link. Synchronous detection of the received signal is used, employing correlators (103, 104) in phase quadrature.

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Portsmouth, United Kingdom

Robert G Wilkinson