Performance Evaluation of Least Mean Square Error (LMS) and Blind Adaptive Algorithm In Suppressing Multiple Access Interference in CDMA Receiver

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Addis Ababa University


Direct sequence code division multiple access (DS-CDMA) is a high capacity air interface as compared to time division multiple access (TDMA) and frequency division multiple access (FDMA). This system is more resistant to jamming and unintended listener. In this access technique more than one user access the same frequency band width at the same time. The main limitation of this system is that, the signal quality of the intended user degrades as the number of users increases that result in multiple access interferences (MAI). So, in this thesis adaptive algorithms are used in CDMA receiver for suppressing multiple access interference (MAI). Two families of adaptive algorithms namely, least mean square error (LMS) and constant modulus blind adaptive algorithms are used. The schematic representation of each of the two algorithm receiver is developed, and on which the simulation analysis was based. The LMS adaptive algorithm requires training sequences for its adaptation where as the later does not require any training sequences, instead needs appropriate initialization of tap weights. The performance of these adaptive algorithms in suppressing multiple access interference (MAI) was found to outperform matched filter receiver but they performs less than decorrelating detector receivers. From the simulation result the decorrelating detector achieve 10-2 BER around 10.5dB SNR whereas LMS and CMA achieve this BER around 12.5dB and 16dB SNR respectively. The matched filter cannot achieve this BER within 20dB SNR. From the result we can deduce that decorrelating detector is an optimal receiver that almost eliminates multiple access interference. In addition to this it was found that the BER performance of LMS receiver is much better than the CMA receiver. It was also shown that under perfect synchronization Walsh-Hadamard sequence can completely eliminate MAI due to their orthogonal nature. As the spreading gain of the spreading codes is higher, their ability to reduce MAI also increases which is at the cost of band width. Key Words: CDMA, LMS algorithm, CMA, decorrelating detector, matched filter receiver.



CDMA, LMS algorithm, CMA, Decorrelating detector, Matched filter receiver