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

在多用户移动通信系统中，码分多址(码分多址)和正交频分复用(Ofdm)技术的结合作为为衰落信道提供高效和健壮的通信系统的技术而得到广泛的应用。众所周知，由于每个用户和基站之间的信号功率差异，多用户移动通信系统需要考虑远近问题。远近效应可以出现在(1)上行链路(从每个用户到基站)和(2)下行链路(从基站到每个用户)中，过去人们已经很好地研究了上行链路中的远近问题，并且广泛地采用功率控制技术来从基站上的每个用户获得统一的接收功率。另一方面，最近提出了由基站对下行链路进行功率控制，以向小区中的所有用户提供均匀的服务质量(Quality of Service，QOS)。在这两种情况下，接收机的设计都需要假设每个用户信号功率的不同来提供更高的系统性能。在本研究中，我们提出了一种采用衰落均衡器的多用户接收机的设计方法，该方法通过考虑每个用户信号功率的差异来优化系统性能。仿真结果表明，与传统的设计方法相比，该设计方法可以获得更高的性能。此外，如果我们增加几个迭代最大似然(IML)译码，可以获得比传统的相应方法更大的改进。(审阅者要求修改一篇包含最终研究结果的期刊论文。现在，修订版正在重新提交。)

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.)