Direct sequence code division multiple access communication systems with multiple carriers

Abstract

In this thesis, we try to improve the spectral efficiency and the performance of DS/CDMA communication systems. First, we examine the capacity of frequency overlapped DS/CDMA (FO/CDMA) systems which share frequency bands by partially overlapping their spectra. The available wideband spectrum is divided into a number of subspectra of smaller bandwidths and each narrowband CDMA system employs spreading with a reduced processing gain relative to the conventional wideband system. We analyze the interference from the adjacent (spectrally) narrowband CDMA systems with an arbitrary amount of spectral overlap. We find the system parameters pair, the amount of spectral overlap and the number of narrowband systems, which maximizes the capacity when the overall bandwidth is fixed. The capacity is maximized for about a half overlapped case and ever increases as the number of narrowband systems increases in AWGN channels. In frequency selective Rayleigh fading channels, the optimal amount of overlap between adjacent CDMA systems also increases as the number of narrowband systems increases. However, the maximum peak capacity is obtained when the optimum number of systems are overlapped by half the null-to-null bandwidth. In frequency selective Rayleigh fading channels, it is shown that FO/CDMA is preferable in the case of relaxed bit error rate requirement and low decay rate of multipath intensity profile. Otherwise, WCDMA is superior. Second, we propose an offset multicarrier (OMC) DS/CDMA system, where two groups of multicarriers with a frequency offset between them are employed. We analyze the bit error rate in slowly-varying frequency selective Rayleigh fading channels. Our results show that the OMC DS/CDMA system outperforms the conventional multicarrier (MC) DS/CDMA system, and the performance improvement becomes more significant as the number of subcarriers increases. It is found that the OMC DS/CDMA system becomes robust to the roll-off-factor of chip wave-shaping ...