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 s urge in the received signals. The conventional approaches may result in eit her nonuniform distribution of transmitting users among time slots or uneve n interference from the dominant interferers. For both cases, the bit-error rate (BER) increases significantly for the users suffering the worst case condition. In this paper, we propose an optimal time-hopping scheme based o n the theory of finite projective planes for direct sequence code division multiple access to distribute interference evenly among participating users . The performance evaluation is divided into four parts. We demonstrate tha t both the average BER, probability of outage, and bandwidth efficiency can be improved by using the proposed time hopping scheme in comparison to oth er time-hopping schemes, such as the fixed allocation scheme and random sel ection scheme for various modulation methods including frequency shift keyi ng-coherent demodulation (FSK-CD), differential phase shift keying (DPSK), binary phase shift keying (BPSK), and FSK with noncoherent demodulation (FS K-NCD), We compare the proposed time hopping scheme with nontime-hopping di rect sequence code division multiple access (DS-CDMA) with identical signal bandwidth. We then prove that the proposed scheme is optimal in minimizing interference.