Pulse or digital communications – Repeaters – Testing
Patent
1993-07-09
1994-07-05
Cain, David C.
Pulse or digital communications
Repeaters
Testing
375 84, 375106, 370 951, H04K 100
Patent
active
053274555
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to satellite communication systems and in particular a Code Division Multiple Access (CDMA) communication system suitable for transmitting band-limited data.
Communication satellites are already widely used for data transmission. Now, the substantial possibilities offered by microelectronics are opening up new applications for telecommunication satellites such as digital sound broadcasting and very small aperture mobile and personal terminal (VSAT) communications. Digital sound broadcasting, in particular, is a very promising field of application and is gathering considerable interest in the field of telecommunications. The successful introduction of these new satellite based communication services, however, is driven by the performance that can be achieved, by the good use of the on-board satellite resources and by a clever ground terminal engineering. A key element determining the satellite communication system complexity and efficiency is the network access scheme.
FR-A-8910481 describes a code division multiple access communication system in which the transmission carrier is activated by the user's voice and in which synchronization is achieved by means of a master code. The key feature of this known system is the use of a different user terminal synchronizing scheme in the forward and return links. In the forward link, synchronization is achieved by broadcasting uninterruptedly a master code. In the return link towards the satellite, synchronization is achieved by an alignment procedure based on a forced carrier voice-activation and selection of a user code produced by decoding an element contained in the master code.
The multiple access scheme implemented in this known system is imposing a code period equal to the symbol duration when it is required to achieve a drastic self-noise reduction. These limitations were discussed by R. De Gaudenzi, C. Elia, R. Viola in "Performance Evaluation Of Synchronous Code Division Multiple Access (S. CDMA) For Satellite Mobile Systems", Proceedings of the IEEE Global Telecommunications Conference GLOBECOM'90, San Diego, Calif., Dec. 2-5, 1990.
This known method improves the CDMA system efficiency by reducing the acquisition time of the spread spectrum in the receive circuitries. However, two major problems were emerging.
First, it was established that the performance of the CDMA technique could be improved by combining the spreading code synchronization, as used in the aforementioned known method, with the use of synchronized Gold codes. However, using a synchronized spreading code proved to impede using an efficient error correction system because the implementation of an error correction system results in a decrease in processing capability as a result of the code period reduction and accordingly of the number of available codes.
In addition, a study performed by J. L. Massey and T. Mittelholzer (Technical Assistance For The CDMA Communication System Analysis, Final Report, Institute for Signal and Information Processing CH-8092 ETH, Zurich, September 1990) demonstrated that using preferentially phased Gold codes, i.e. Gold codes having minimum mutual cross-correlation, turns out to be an optimal choice for point-to-multipoint communication network. However, though preferentially phased Gold codes are a straight way to improve power with no reduction in the processing gain and with no bandwidth reduction in a usual asynchronous CDMA system (A-CDMA), this turned out to not be true for a synchronous quasi-orthogonal multiple access system.
As a matter of fact, the coding scheme in such a system results in a reduction in the number of available quasi-orthogonal spreading codes because the symbol rate is increasing for a given bandwidth, thus reducing the number of communication channels available.
These limitations make it difficult to use a code-synchronized CDMA system for commercial applications of a satellite communication system. For instance, for digital sound broadcasting or micro-terminal networks.
The object of the present inven
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De Gaudenzi Riccardo
Elia Carlo
Viola Roberto
Agence Spatiale Europeene
Cain David C.
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