Performance of CDMA/OFDM for mobile communication system

The OFDM technique is an interesting approach in mobile communications in order to achieve a high spectral efficiency and to combat the frequency selectivity of the channel. Another interesting technique is a CDMA system with Rake-receiver. The main advantages of CDMA are well known, but its capacity is limited by multiuser interference. The aim of this study is to combine the principle of a CDMA system with OFDM. This combination allows one to perform a maximum-likelihood detection (MLD), to use the spectrum in an efficient way, to exploit frequency diversity, and to retain many advantages of a CDMA system. In addition, it permits simple cell-separation by using frequency hopping, and a simple hardware realization. Two examples for mobile communication system using CDMA/OFDM with Walsh-Hadamard code-spreading for the downlink (Base-Mobile) are studied. Different coherent detection algorithms such as conventional detection, MLD, and iterative detection are analyzed. The analytical performance of MLD is evaluated. It is shown that by using BPSK modulation, one can transmit 64 active users at rate of about 16.664 kbit/sec in a 1.28 MHz bandwidth, resulting in a spectral efficiency of 0.8332 bit/sec/Hz. By using QPSK modulation half of this bandwidth will be used. It is also analytically shown that the performance of a CDMA/OFDM with MLD, using frequency-diversity is comparable to an optimum CDMA system with Rake-receiver using multipath diversity. However, in practice providing diversity with OFDM is much simpler than a Rake-receiver. In addition, the OFDM technique simplifies the channel-estimation problem. Hence using CDMA/OFDM for the downlink is a promising approach since the receiver at the mobile station should be very simple.