Pulse or digital communications – Repeaters – Testing
Patent
1982-09-28
1986-08-12
Safourek, Benedict V.
Pulse or digital communications
Repeaters
Testing
375102, 371 36, H04B 110
Patent
active
046060472
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to radio communication within the high frequency band of 400 Hz to 2.7 KHz and in particular to modems for transmitting and receiving radio frequency signals.
High frequency (HF) communication performance can be degraded by three principle causes. Firstly propagation conditions can vary according to the time and frequency of transmission and also according to the geographical locations of the transmitter and receiver. Secondly, frequency selective fading and intersymbol interference can occur across the HF band because of ionospheric induced variations in multipath propagation. This arises as a result of ionospheric reflection by means of which high frequency transmission beyond the line-of-sight is achieved. More than one ionospheric reflective mode can exist and this causes multipath reception. Severe distortion can be observed on the transmitted data and in-band noise or interference which would normally be disregarded becomes the predominant factor during periods of signal fading. These conditions can only be completely avoided by frequency changing but this is usually impracticable. Undesirable effects of multipath propagation can be counteracted by employing appropriate signal processing before and after transmission. This is normally done using measurement of signal (spectral) spreading. The third cause of loss of performance is interference. Most of this encountered in the HF band is narrow band interference and can be avoided by changing the transmitting frequency. This is done by changing to an alternative channel frequency although where in-band frequency agility of the signalling tones can be used channel changing is not necessary. Wideband interference will always be a nuisance since it can only be avoided by frequency changing to a clear channel. When the wideband interference is impulsive, however, it can be avoided by adopting time dispersive coding into the transmitted signal format.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a high frequency system which employs both frequency diversity and time diversity in order to counteract the adverse reception problems encountered in HF communications.
The invention provides a digital communications transmitter including message encoding means and means for transmitting each bit of an encoded message as a series of sequential signals on mutually differing frequency channels.
In another form the invention provides a receiver for receiving a digital communications message wherein each data bit of a message is transmitted as a series of sequential signals on mutually differing frequency channels, the receiver comprising a plurality of detectors each responsive to a respective frequency channel, means for scanning the detector outputs sequentially to identify individual signals and means to assign to each signal thus detected a weighting factor reflecting error probability and means to combine the weighted signals to reconstruct the communications message.
Advantageously each channel is two-tone, the first tone carrying the message and the second tone simultaneously carrying the complement of the message signal. By this arrangement the transmitter transmits only one tone or frequency at a time during transmission of a message and the transmission is at a fixed rate. By consecutively transmitting the sequential signals the power output from the transmitter can be kept constant.
Preferably the transmitter includes circuit means to generate a uniquely coded Start-of-Message (SOM) signal and the receiver includes circuit means responsive to a received SOM signal to give an accurate timing signal for the scanning of the received signal. In an advantageous form the SOM signal is a 28 digit Barker-encoded signal.
In order to scan the detector outputs the receiver includes a clock which, on receiving the SOM timing signal, generates bit timing pulses so that each tone detector output can be sampled at appropriate time intervals for the data bits. In the preferred arrangement f
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patent: 4306308 (1981-12-01), Nossen
Chin Stephen
Safourek Benedict V.
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