Optimal time-hopping scheme for CDMA air interface in broad-bandwireless systems

摘要

Time-hopping techniques have been applied in direct sequence codendivision multiple access (DS-CDMA) to reduce the harmful effects of ansudden power surge in the received signals. The conventional approachesnmay result in either nonuniform distribution of transmitting users amongntime slots or uneven interference from the dominant interferers. Fornboth cases, the bit-error rate (BER) increases significantly for thenusers suffering the worst case condition. We propose an optimalntime-hopping scheme based on the theory of finite projective planes fornDS-CDMA to distribute interference evenly among participating users. Thenperformance evaluation is divided into four parts. We demonstrate thatnboth the average BER, probability of outage, and bandwidth efficiencyncan be improved by using the proposed time hopping scheme in comparisonnto other time-hopping schemes, such as the fixed allocation scheme andnrandom selection scheme for various modulation methods includingnfrequency shift keying-coherent demodulation (FSK-CD), differentialnphase shift keying (DPSK), binary phase shift keying (BPSK), and FSKnwith noncoherent demodulation (FSK-NCD). We compare the proposed timenhopping scheme with nontime-hopping DS-CDMA with identical signalnbandwidth. We then prove that the proposed scheme is optimal innminimizing interference