Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Patent
1995-02-09
1997-04-08
Chin, Wellington
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
H04J 1300
Patent
active
056194919
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a CDMA transmission system comprising a transmitter in which a data sequence is spread by a code sequence, and a receiver in which the original data sequence is recovered by a detector.
The invention likewise relates to a receiver and a transmitter for a CDMA transmission system.
2. Description of the Related Art
In a transmission system based on Code-Division Multiple Access (CDMA), the signals of different users are simultaneously switched to a common frequency band and transmitted with a common carrier frequency. Code-division multiple access systems are based on a spread band technique, i.e., the signal to be transmitted is spread over a frequency band which is considerably broader than the least required frequency band for a signal transmission. The band spreading renders code-division multiple access systems generally highly resistant to interference.
For frequency band spreading in code-division multiple access systems each (payload) bit to be transmitted is multiplied by a codeword agreed upon by transmitter and receiver. For distinguishing from the bits of the payload data sequence to be coded, a bit of the codeword is generally referenced a chip. The use of suitable codewords precludes, in principle, mutual interference of the signals of the individual users.
For obtaining a specific bit error rate with CDMA technology, at a given interference power the signal power must be approximately proportional to the transmit payload bit rate. In CDMA systems, in which the individual users use different payload bit rates depending on the application, for example, coded speech transmission, facsimile transmission, and so on, it is desirable for reasons of economic use of frequencies to load the CDMA transmission system only with the necessary frequency bandwidth.
For example from WO 92/15164 such a CDMA transmission system is known in which a programmable clock generator is provided for obtaining a variable bandwidth, by means of which generator the chip clock rate of each selected codeword is predefined. The higher the chip clock rate is selected, the broader is the resultant bandwidth of the spread transmit signal.
For practical reasons the chip clock rates are selected to be in multiples of a basic clock rate. In the preferred embodiment of said Patent application the bandwidth is therefore 1.2 MHz or a multiple thereof.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a CDMA transmission system which makes a very great flexibility of the system possible.
This object is achieved in that a frame structure is provided and the payload information to be transmitted in a frame is compressed into signal bursts.
The transmit data occurring in a frame can thus be concentrated into a burst, while the frame length-to-burst duration ratio indicates the attained compression factor. This also permits very different and finely graded compression factors to be achieved. For very high payload data rates continuous transmission may be effected, as before, using the overall frame length as a burst. However, on the other hand, for low-rate payload data only a section of the frame is used.
A burst in a frame is characterized by frame-relative starting instant t.sub.o and an associated duration t.sub.a. These parameters can be selected to depend on the system or on the link, due to the selectable spreading, but, in addition, also be variably changed in a link. Suitable measures such as, for example, controlled predefinition of the starting instant t.sub.o by the fixed station, or also random selection of the starting instant t.sub.o for the sending station itself, may be applied so that the frames are filled as uniformly as possible to provide thus a balanced economic use of frequencies.
In a highly advantageous embodiment of the invention the spreading factor is selected in dependence on an original payload data rate. It is then possible to provide different payload bit rates with a constant output bit rate, a so-termed ch
REFERENCES:
patent: 5184347 (1993-02-01), Farwell et al.
patent: 5455822 (1995-10-01), Dixon et al.
Chin Wellington
Eason Leroy
U.S. Philips Corporation
Vu Huy D.
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