Pulse or digital communications – Spread spectrum – Direct sequence
Patent
1997-10-15
2000-07-11
Chin, Stephen
Pulse or digital communications
Spread spectrum
Direct sequence
375144, 375147, 375152, 375346, H04B 1500
Patent
active
060883839
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to spread spectrum signal demodulators and, more particularly, to spread spectrum signal demodulators used for cellular communication systems, indoor wireless communication systems, wireless LAN (local are network) systems, etc.
BACKGROUND ART
Systems dealing with spread spectrum signals for code division multiple access (CDMA), in which a plurality of signals are transmitted in the same band, are subject to inter-signal interference that is generated in dependence on the correlation among codes assigned to individual signals. The characteristics or quality of the signals are deteriorated increasingly with increasing number of signals involved. In addition, signal level fluctuations result in increased interference of higher level signals on lower level signals, thus greatly reducing the characteristics of the lower level signals.
Some methods for improving signal characteristics by reducing such inter-signal interference have been proposed. One such method is realized by a system called decorrelation system. In this system, decorrelation is made by using known code correlations. FIG. 19 shows an example of the circuit construction which realizes this system. Referring to the figure, designated at 1001 to 100K are correlators, and at 101 is a decorrelator.
As a spectrum spreading scheme, a direct sequence (DS) system is assumed. A received signal r is expressed by formula (1) in FIG. 23.
In the formula (1), K represents the number of simultaneously transmitted signals, dk a k-th (k=1, 2, . . . , K) transmitted symbol, C.sub.k the spreading code of a k-th signal, c.sub.k a despreading operation caused by spreading code c.sub.k, n background noise introduced into signal on the transmission line, and a.sub.k a reception level.
The rule of formula (2) in the same figure is met when the despreading operation c.sub.k and the correlating operation c.sub.k are synchronized to each other.
The correlating operation c.sub.k is completed after one symbol has been transmitted, and this is expressed by the provision of a delay element z for one symbol.
With the received signal r, the k-th correlator 100K provides an output d.sub.k, which is given by formula (3) in the same figure.
Formula (4) in the same figure is a vector expression of all the signals to be demodulated in a single form. In the formula (4), C is a matrix type operator corresponding to a correlation matrix.
For the sake of brevity, it is assumed that the symbol timings of the signals are synchronized, and denoting the correlation between signals i (i=1, 2, . . . , K) and signals j (j=1, 2, . . . , K) by ci, j (ci, i=1, .vertline.ci, j.vertline..ltoreq.1) we can use formula (5) in the same figure, where the matrix C of the elements ci, j represents the correlation of signals.
The decorrelator 101 obtains an inverse matrix (C.sup.-1) to the correlation matrix C, and executes matrix multiplification on the outputs of the correlators 1001 to 100K as in formula (6) in the same figure.
Since the spreading codes of the individual signals are known, the elements ci,j of the correlation matrix C can be calculated in advance, and the inverse matrix (C.sup.-1) can be obtained in advance.
It will be seen that by substituting the formula (5) into the formula (6) the signal d.sup.(.infin.) obtained by the above decorrelation can be expressed by formula (7) in the same figure.
This means that the decorrelated signal d.sup.(.infin.) comprises the product of the reception level A of the original signal and the transmitted symbol d and noise component n that is introduced, and is not affected by the other signals that are simultaneously received. In other words, it is meant that inter-signal interference is cancelled, and that interference cancelled detected signals (or detected signals with cancelled interference) can be obtained. The interference cancelled detected signals can be demodulated through phase synchronization, for instance, and bit determination.
While the operation of the decorrelation system in the case of presenc
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Suzuki Toshinori
Takeuchi Yoshio
Chin Stephen
Kokusai Denshin Denwa Kabushiki Kaisha
Liu Shuwany
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