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

Time-hopping techniques have been applied in direct sequence code division multiple access (DS-CDMA) to reduce the harmful effects of a sudden power surge in the received signals. The conventional approaches may result in either nonuniform distribution of transmitting users among time slots or uneven interference from the dominant interferers. For both cases, the bit-error rate (BER) increases significantly for the users suffering the worst case condition. We propose an optimal time-hopping scheme based on the theory of finite projective planes for DS-CDMA to distribute interference evenly among participating users. The performance evaluation is divided into four parts. We demonstrate that both the average BER, probability of outage, and bandwidth efficiency can be improved by using the proposed time hopping scheme in comparison to other time-hopping schemes, such as the fixed allocation scheme and random selection scheme for various modulation methods including frequency shift keying-coherent demodulation (FSK-CD), differential phase shift keying (DPSK), binary phase shift keying (BPSK), and FSK with noncoherent demodulation (FSK-NCD). We compare the proposed time hopping scheme with nontime-hopping DS-CDMA with identical signal bandwidth. We then prove that the proposed scheme is optimal in minimizing interference.