Receiver and repeater for spread spectrum communications

Details Receiver and repeater for spread spectrum communications Receiver and repeater for spread spectrum communications

1995-05-22

1997-07-29

Bocure, Tesfaldet

Pulse or digital communications

Repeaters

375200, 375208, 455 18, H04B 336, H04B 717, H04B 1702, H04L 2520

Patent

active

056527659

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a receiver and a repeater for spread-spectrum communications which improve diversity characteristics.


BACKGROUND ART

Generally, the spread-spectrum communications such as CDMA (Code Division Multiple Access) performs a plurality of modulations at a transmitting side, and corresponding demodulations at a receiving side, thereby communicating information symbols. More specifically, the transmitting side transmits information symbols after performing a primary modulation, such as PSK, of the information symbols, and then, spreading the primary modulation signal into a wideband frequency range by performing a secondary modulation using a spreading code such as a high-rate pseudo-random code. On the other hand, the receiving side performs despreading (secondary demodulation) using the same and synchronized spreading code with the transmitting side so that the wideband frequency received signal is inversely converted into the band of the information symbols, and then, carries out a primary demodulation corresponding to the primary modulation, thereby restoring the original information symbols.
When such a spread-spectrum communication method is employed in mobile communications, a signal transmitted from a base station or a mobile station will reflect from obstacles such as buildings or the like on its propagation paths. Accordingly, the signal is received as a multipath signal whose component waves (delayed waves) arrive at different times because individual delayed waves have different delay times on the propagation paths. If the variance of the delay times of the propagation paths is greater than one element length of a spreading code (one chip interval), fluctuations of the individual delayed wave components which are extracted at every chip interval can be handled as non-correlated quantity. In other words, the amplitude and phase of each delayed wave component can be considered to change independently. As a result, the average received level will be improved by combining the independent delayed wave components after making their phases coherent, or by selecting a delayed wave component whose amplitude is maximum. This idea is implemented as a well-known RAKE reception, wherein improvement in transmission characteristics can be expected owing to the path diversity receiving effect.
FIG. 1 is a block diagram showing a conventional spread-spectrum communication receiver carrying out the path diversity reception (RAKE reception). In FIG. 1, the reference numerals 1-1-1-N designate correlators. Each correlator 1-k (k=1-N) receives a spread-spectrum signal 100, into which a pilot signal is inserted, and performs despreading of one of delayed waves using the same spreading code. Here, the pilot signal is a signal (called a sounder) measuring a transfer function of a propagation path. The output of the correlator 1-k is supplied to a detector 2-k which detects one of the delayed waves. The output of the detector 2-k is supplied to a weighting circuit 3-k and a power detector 4-k. The power detector 4-k detects power of the delayed wave, and makes it a coefficient of the weighting circuit 3-k. Respective weighted signals are combined by a combining circuit 5. The combined signal is sent to a symbol decision circuit 6 which decides the symbol. When the weighting is performed using all the outputs of the power detectors 4-k, a maximal ratio combining is achieved, whereas when the detected signal of the maximum power is selected, a selection combining is achieved
The conventional spread-spectrum communication system has the following drawbacks. waves independently, the operation of the detectors will become unstable if the signal-to-noise power ratio (SNR) or the signal-to-interference power ratio (SIR) of the delayed waves is small. for individual information symbols, it is difficult to achieve an optimum combination even if weighting based on the received power is performed. Accordingly, only insufficient diversity effect can be obtained. locating a plurality radi

REFERENCES:
patent: 4761796 (1988-08-01), Dunn et al.
patent: 5233626 (1993-08-01), Ames
Iltis et al, "A DS Spread Spectrum RAKE Receiver With Narrowband Interference Rejection Capability: Operation in Fading Channels", MILCOM'89: Bridging the Gap 1989.
Bello, "Performance of Some RAKE Modems over the Non-Distributed Wide Band HF Channel", MILCOM'88:21st Century Military Communications--What's Possible? 1988.
Simon Haykin, "Adaptive Filter Theory" (1986), Prentice Hall, pp. 381-385.

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