This letter describes two low complexity receiver structures over a multi-broadcast channel of an orthogonal frequency division multiple access (OFDMA) multi-user system. The first is a one-group occupied receiver structure, whose complexity is much lower than that of a conventional OFDMA receiver structure. The other one, a multi-group occupied receiver structure, exploits multiple groups for one user, by which users' down-link data rate can be adaptively controlled by a base station (BS). Unlike unchangeable complexity of an OFDMA receiver structure that performs full-size of a fast Fourier transform (FFT) operation although only a few subcarriers are taken, its complexity linearly increases with the number of occupied subcarrier groups. The proposed receiver structures can meet the possible high-rate demand in the down-link and will become one of the strong candidates in next generation mobile communication systems.

For down-link communication in a DS/CDMA cellular mobile radio system, the transmitted power of the base station must be effectively adjusted in real-time fashion to achieve good communication quality because the system exists multipath short-term fading and near-far problem. In this paper, we propose two down-link power control mechanisms: one is the centralized SIR-balancing method and the other is the distributed SIR-based method. We investigate how the outage probability is affected by system parameters and optimally design the parameters for these two down-link power control mechanisms. Also, the two down-link power control mechanisms are compared and discussed.