Pulse or digital communications – Repeaters – Testing
Patent
1996-06-27
1998-06-02
Chin, Stephen
Pulse or digital communications
Repeaters
Testing
375233, 375350, 370479, 3647242, H04B 169, H04B 110, H04J 1302
Patent
active
057612374
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and apparatus for reducing multiuser-interference in Code Division Multiple Access (CDMA) multi-channel communication systems, and in particular, in CDMA cellular radio communication systems. The present invention is also applicable in CDMA Infra Red (IR) networks.
BACKGROUND OF THE INVENTION
Wireless communication systems, in particular cellular radio telephone communication systems and diffused infrared (IR) systems, become more and more important because they increase mobility and offer wireless connectivity to telephone and computer users almost everywhere. While the latter ones are usually deployed indoors, e.g., for the interconnection of computers and servers, the cellular radio communication systems, e.g., the analog AMP systems in the US and the digital GSM system in Europe, facilitate mobile communication and data exchange in almost all metropolitan areas. It is expected that the emerging Personal Communications Networks (PCN) will encompass a wider range of communications capabilities than those represented by current analog or digital cellular mobile radio technology. High traffic capacity and low power consumption are two important issues in the emerging new cellular systems.
Currently, channel access in cellular radio telephone communication systems is achieved by using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA-based systems, the capacity is limited by the number of available frequency subbands, whereas the capacity of TDMA systems is limited by the number of slots per frame carrying the transmitted signals.
In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both frequency and time. Thus, all CDMA signals share the same frequency spectrum. In either time or frequency domain, the multiple access signals appear to be on top of each other. A CDMA-based communications system model is illustrated in FIG. 4. The data stream of the k.sup.th user {b.sub.k (n)}, e.g. speech or data, to be transmitted is modulated by a user specific signal s.sub.k (t). Each signal s.sub.k (t) corresponds to a unique spreading code c.sub.k. A plurality of spread spectrum signals is modulated and transmitted on a radio frequency (RF) carrier wave. At the receiver, the composite signal r(t) is demodulated and correlated with a selected spreading code c.sub.k. The correlation by the user specific spreading code isolates and decodes the corresponding data signal.
There are a number of advantages associated with the CDMA technology. The capacity of CDMA-based systems is projected to be several times higher than that of existing analog FDMA systems. In addition, CDMA is resistant to multi-path interference and fading. Furthermore, the scrambled format of CDMA signals makes it difficult and costly to eavesdrop or track calls, insuring greater privacy for users and providing greater immunity from air time fraud.
Conventional CDMA systems are multiuser-interference limited, whereas the above described TDMA and FDMA systems are primarily bandwidth limited. Thus, in practical implementations of CDMA, capacity is directly related to the signal-to-interference (S/I) ratio, which is essentially a measure of the multiuser interference, caused by other overlapping signals. The problem to be solved, therefore, is how to further increase system capacity and still be able to maintain a reasonable S/I ratio so that signal decoding can be carried out efficiently and accurately.
Conventional code-division multiple-access cellular and microcellular wireless systems use long spreading codes, i.e., sequences whose period is much longer than the data symbol duration, employ complex powerful convolutional codes to mitigate the effect of multiuser interference, and rely on power control strategies to remedy the "near-far problem". However, as the number of simultaneous transmissions in a system of fixed bandwidth increases, or as the relative power levels of the different user signals become disparate
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Evangelos Eleftheriou
Petersen Brent R.
Chin Stephen
Flynn John D.
Gluck Jeffrey W.
International Business Machines - Corporation
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