Design of adaptive filters for interference suppression in QPSK DS CDMA communication system

One of the most important objectives of telecommunications systems is to offer distinct quality of services for both voice and broadband communications that covers vast and low density of subscriber distributions. Code Division Multiple Access (CDMA) is the leading multiple access technology that possesses several advantages when compared to other techniques. However, it suffers from the effects of multipath interference, narrowband and wideband interference and Near-far problems. To overcome the effects of narrowband interference, technical solution for design of adaptive filters for interference suppression in QPSK DS CDMA Communications System is proposed. Analysis of average bit error ratio (BER) performance of a quadriphase shift keying (QPSK) modulated direct-sequence code-division multiple access (DS-CDMA) communications system with narrow-band interference (NBI) suppression complex- coefficient adaptive infinite-impulse response (HR) notch filter is presented. A closed- form expression that relates BER with complex coefficient HR notch filter parameters, received signal-to-noise ratio (SNR), number of DS-CDMA active users and processing gain are derived. The derivations are based on the Standard Gaussian Approximation (SGA) method. Accuracy of the BER expression is confirmed by computer simulation results. Keywords: Bit error rate, QPSK DS-CDMA system, Narrow-band interference (NBI), Adaptive Filtering, Direct-sequence code-division multiple access (DS-CDMA), adaptive notch filters, Signal to interference ratio (SIR), Signal to noise ratio (SNR).