Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Patent
1996-07-02
1998-08-25
Safourek, Benedict V.
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
370342, 455305, H04J 1302
Patent
active
057990042
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT F194 00478filed Oct. 21, 1994.
BACKGROUND OF THE INVENTION
The invention relates to a method for eliminating multiple-access interference in a CDMA cellular radio system having cells each comprising at least one base station communicating with mobile stations residing in the cell and informing the mobile stations of at least one spreading code used in a neighbouring cell, the mobile stations measuring the code phase and power level of a channel of the neighbouring cell using the known spreading code.
CDMA is a multiple access method based on the spread spectrum technique, and it has been applied recently in cellular radio systems together with the earlier developed FDMA and TDMA techniques. CDMA has several advantages over the earlier developed techniques, such as higher spectral efficiency and simple frequency planning.
In CDMA, the narrow-band data signal of the user is multiplied to a relatively broad band with a spreading code having a considerably broader band than the data signal. Known test systems use bandwidths such as 1.25 MHz, 10 MHz and 25 MHz. The multiplication spreads the data signal over the entire available band. All users transmit on the same frequency band simultaneously. A spreading code is assigned to each connection between a base station and a mobile station, and the signals of different users can be distinguished from each other in the receivers on the basis of the spreading code of each user.
Correlators provided in the receivers are synchronized with the desired signal, which is recognized on the basis of the spreading code. In the receiver, the data signal is restored to the original band by multiplying it again with the same spreading code as at the transmission stage. In an ideal case, signals multiplied with some other spreading code do not correlate and are not restored to the narrow band. From the viewpoint of the desired signal, they thus appear as noise. One attempts to select the spreading codes of the system in such a way that they are orthogonal with respect to each other, i.e. do not correlate with each other.
In a typical cellular radio environment, signals between a mobile station and a base station propagate over several paths between a transmitter and a receiver. This multipath propagation is mainly due to reflections of the signal from the surrounding surfaces. Signals that have propagated over different paths arrive at the receiver at different times due to their different propagation time delays. CDMA differs from conventional FDMA and TDMA in that the multipath propagation can be utilized in signal reception. A so-called rake receiver comprising one or more rake branches or correlators is a widely used receiver solution in CDMA. Each correlator is an independent receiver unit, the function of which is to assemble and demodulate one received signal component. The implementation of a rake branch is described more closely in Modern Communications and Spread Spectrum, Chapter 12, G. Cooper, C. McGillem, McGraw-Hill, New York 1986. A CDMA receiver typically comprises a separate impulse response measuring equipment, the function of which is to search out different signal components transmitted with a desired spreading code, and detect the phases of the signal components. Each rake branch or correlator can be controlled so that it will be synchronized with a signal component propagated over a different path. In a conventional CDMA receiver, the signals of the correlators are combined in an advantageous way, thus obtaining a signal of high quality. The signal components received by the correlators may have been transmitted from one base station, or in the case of macrodiversity, from a plurality of base stations.
Generally speaking, the spreading codes are not orthogonal at all values of the delay. Accordingly, signals delayed in different ways cause interference in the signal detection. Such interference caused by different users to one another is called multiple-access interference.
Each base station
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Cooper, G. et al., "Detection of Spread-Spectrum Signals", Modern Communication and Spread Spectrum, McGraw-Hill, New York, 1986, chapter 12 .
Jolma Petri
Keskitalo Ilkka
Kiema Arto
Savusalo Jari
Nokia Telecommunications
Safourek Benedict V.
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