Pulse or digital communications – Repeaters – Testing
Patent
1986-02-10
1987-10-13
Griffin, Robert L.
Pulse or digital communications
Repeaters
Testing
375 58, 371 43, H04B 1500
Patent
active
047003634
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a data communication method and apparatus and in particular, but not exclusively, to a data modem for use in a satelite communication system.
BACKGROUND OF THE INVENTION
With the increasing use of satellites for business data communications there is a growing need for data modems with good efficiency of bandwidth and power.
Recent developments in high efficiency modems have tended to concentrate on power efficiency. Thus a modern data modem may take bits one or two at a time from a data stream that has been previously subject to Forward Error Control, and use these bits to control the level of phase modulation of a carrier during a corresponding modulation time interval, the resultant transmitted symbols each having two or four possible levels (this modulation process is generally referred to as two-or four-level phase shift keying, PSK). These modern modems have brought down the required satellite power by a factor in excess of 4 to 1 (6dB) with respect to the requirements of only a few years ago. However, this improved power efficiency has been at the expense of occupied bandwidth, which, allowing for adjacent channel guardbands, can be up to 3 or 4 times the data rate. At lower data rates there is sufficient bandwidth available for this to be accommodated but in the 2 or 4 Mbit/sec data rate range, bandwidth efficiency becomes much more critical.
One method of improving bandwidth efficiency is to increase the number of possible levels of each symbol to increase the number of transmitted bits/Hz of occupied bandwidth. However as the number of levels per symbol is increased, the symbol power has to be increased dramatically to prevent noise from increasing the bit error rate. The need for this dramatic increase is due to the fact that as the numbers of levels per symbol is increased, the different symbol combinations are not orthogonal to each other as in the two or four level symbol case and there is degradation similar in effect to cross-talk.
It is an object of the present invention to provide a data communication method and apparatus with good bandwidth and power efficiency.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a method of data communication including transmitting a binary data stream by a process including the steps of:
dividing the data stream into a first, base bit stream and at least one second bit stream;
subjecting the said at least one second bit stream to error control coding to produce at least one overlay bit stream, and
utilising said base and overlay bit streams to generate a succession of symbol words of binary form for controlling the level of modulation of a carrier signal during respective modulation time intervals, the most significant bit or bits of each symbol word being derived from the base bit stream while the least significant bit or bits are derived from the said at least one overlay bit stream.
It will be appreciated that the least significant bits of the symbol words are those most likely to be corrupted by noise during their transmission as modulation of the carrier (that is, as a modulation symbol) since the margin for error is least for these bits. In the present system, the fact that these least significant bits have been subjected to error control coding enables their successful recovery at a receiver at much lower signal
oise ratios than would otherwise be possible. Thus an increase in bits/transmitted symbol can be achieved without the same power level increase penalty as would be incurred with standard multi-level symbol systems.
Further according to the invention, there is provided a method of data communication including receiving data transmitted in accordance with the last but one paragraph, the receiving process including the steps of:
generating a succession of modulation-level binary words representing the level of modulation of the carrier signal during respective modulation time intervals,
deriving from the least significant bits of the modulation-level words at le
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Bramwell Jonathan R.
Tomlinson Martin
Chin Stephen
Devon County Council
Griffin Robert L.
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