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Interference Reduction in CDMA using SIC
« on: September 20, 2011, 07:43:44 am »
Author : Sukrita Swarnkar, Nipun Mishra, Mohan Awasthy
International Journal of Scientific & Engineering Research Volume 2, Issue 9, September-2011
ISSN 2229-5518
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Abstract In a code division multiple access system, multiple access interference coupled with near-far problem is one of major factors limiting system performance. To overcome these issues, various multiuser detection schemes have been proposed multiple access interference   limits the capacity of Direct Sequence Code Division Multiple Access systems. In CDMA systems MAI is considered as additive noise and a matched filter bank is employed. Traditionally, multiuser detector code-matched and a multiuser linear filter are used which increases the complexity of the system due to its nature of operation. Multiuser detection is an approach which uses both these filters for the optimization. However, the main drawback of the optimal multiuser detection is one of complexity so that suboptimal approaches are being sought. Much of the present research is aimed at finding an appropriate tradeoff between complexity and performance. These suboptimal techniques have linear and non-linear algorithms. In this work, we introduce Successive Interference Cancellation which is a nonlinear suboptimal method of MUD and is based upon successively subtracting off the strongest remaining signal. Further analysis is to be carried out and simulations to be done for better understanding of SIC.

Index Terms - Successive Interference Cancellation, Multiple Access Interference, Multiuser Detection, Code Division Multiple Access
CDMA stands for Code Division Multiple Access [1]. It is a digital cellular technology that uses spread-spectrum technique means it works by digitizing multiple conversations. It is developed by Qualcomm, Inc. and standardized by the Telecommunications Industry Association as an Interim Standard IS-95. In this several users share the share the same physical medium i.e. same frequency band at same time In CDMA every communicator will be allocated the entire spectrum all of the time. It uses codes to identify connection. A conventional DS/CDMA system treats each user separately as a signal, with other users considered as noise multiple access interference. All users interfere with all other users and the interferences add to cause performance degradation. The near/far problem is serious and tight power control, with attendant complexity is needed to combat it. All users in a CDMA system interfere with each other. Potentially significant capacity increases and near/far resistance can theoretically be achieved if the negative effect. That  each user has on others can be canceled. A solution to the shortcomings of the conventional CDMA system is Multiuser Detection in which all users are considered as signals for each other.

Interference is the major limiting factor in the performance of cellular radio system. sources of   interference include another mobile in same cell a cell in progress in a neighboring cell, other base stations operating in the same frequency band .we try to reduce the interference by various methods [2] Limitations of a Conventional CDMA System A conventional DS/CDMA system treats each user separately as a signal, with the other users considered as either interference, Multiple Access Interference, or noise[3]. The detection of the desired signal is protected against the interference due to the other users by the inherent interference suppression capability of CDMA, measured by the processing gain. The interference suppression capability is. however, not unlimited and as the number  interfering users increases, the equivalent noise results in degradation of performance, i.e., increasing bit error rate BER or frame error rate. Even if the number of users is not too large, some users may be received at such high signal levels that a lower power user may be swamped out. This is the near/far effect single user receivers treat multiple access interference (MAI), which is inherent in CDMA, as if it were additive random noise. Multi-user receivers use knowledge of the spreading sequences to exploit the structure of MAI. The major difference between single-user detector and Multi-User Detector (MUD) is that in MUD, the users are jointly detected for their mutual benefit. Multi user detection is a promising approach to overcome the limitations of single user CDMA receiver and improve system capacity. Since computational complexity of multi-user detectors like optimal receivers, De correlator, Minimum Mean Square

Error receivers are computationally complex linear MUD like interference cancellation receivers are considered. In an interference cancellation scheme, the signal is first passed through a bank of correlators and then each userís signal is re-constructed and cancelled from the received signal.

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