Improved OFDM-CDMA System coping with the Near-Far Problem

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In multi-user mobile communication system, a combination of CDMA (Code Division Multiple Access) and OFDM (Orthogonal Frequency Division Multiplexing) techniques is widely as techniques providing an efficient and robust communication system for fading channel. It is also well known that multi-user mobile communication systems need to consider the near-far problem due to signal power difference between each user and the base station. The near-far problem can occur in (1) up links (from each user to the base station) and (2) down links (from the base station to each user).The near-far problem in the up links has been well studied in the past and power control technique is widely employed to get uniform receiving power from every user at the base station. The power control by the base station for the down links, on the other hand, is recently proposed to provide homogeneous QoS (Quality of Service) to all user in the cell. In both cases, the receiver should be designed by assuming the difference of each user's signal power to provide a higher system performance.In this research, we propose a design method of a multi-user receiver which employ a fading equalizer optimizing the system performance by taking the difference of each user's signal power into account. By the proposed design method, it has been shown that a higher performance receiver can be obtained compared to the conventional methods. Moreover, if we add a couple of IML (iterative maximum likelihood) decoding further big improvement can be obtained over the conventional corresponding methods.(A journal paper including the final research results was requested to be revised by the reviewers the transaction. Now the revised version is resubmitting.)