Performance of convolutionally coded CDMA/OFDM in a frequency-time selective fading channel and its near-far resistance

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 analyze the performance of a convolutionally-coded CDMA system combined with OFDM in a frequency/time selective fading channel, taking into account the near-far problem. This combination allows one to perform a maximum-likelihood detection (MLD), to use the available spectrum in an efficient way, to exploit frequency diversity and time diversity (provided by channel coding), and to retain many advantages of a CDMA system with a simpler hardware realization. An example for a mobile communication system using convolutionally-coded CDMA/OFDM with Walsh-Hadamard code-spreading for the downlink (base-mobile) is studied. The performance of a MLD is examined by taking into account the near-far problem. It is shown that the MLD is very robust to the near-far problem. It is also shown that by using BPSK modulation, in a 1.28 MHz bandwidth, one can transmit 64 active users at rate of about 10.34 kbit/sec