Research On Adaptive Transmission Techniques For MIMO-OFDM Systems

Recent research has shown that multiple-input multiple-output (MIMO) technique can be used to improve the system capacity and spectral efficiency significant. Meanwhile, orthogonal frequency division multiplexing (OFDM) technique has attracted much attention for its high tolerance to multi-path fading. MIMO-OFDM technique, widely regard as a most promising solution, is efficient for wideband multimedia transmission because of its high ability of anti-fading, high channel capacity and high bit-rate data. In frequency selective channels, adaptive MIMO-OFDM systems are able to support high reliable and high spectral efficiency communication with low complexity, while it can adjust various parameters dynamically so that the system resources adapt to the variations of wireless environment. Our work is supported by the National High Technology Research and Development Program of China under Grant No. 2001AA123014 and National Science Foundation of China under Grant No. 60496310.Firstly, we propose adaptive bit and power allocation algorithms for subchannels in OFDM systems, which can be used over multipath frequency selective fading channels. According to instantaneous channel state information, the adaptive algorithms assign the bits and power to the subchannels. The margin adaptive optimization algorithm and the rate adaptive optimization algorithm are proposed. An adaptive transmit strategy is addressed for MIMO-OFDM systems with instantaneous channel state information at transmitter. Based on optimal precoding design at every subcarrier and water-filling distribution of power over the eigen subchannels, an algorithm is proposed for bit allocation, which can maximize the transmit data rate subject to average transmit power const rain t s and the given BER requirement.And then, we investigate adaptive resource allocation strategy for MIMO-OFDMA systems, and propose a beamforming-based transmit scheme as a concatenation of dynamic subcarriers, adaptive modulation and adaptive bit loading. The optimal goal is margin adaptive optimization, which minimize the total transmission power subject to a target BER performance constraint on each sub-carrier and constant bit rate. Simulation results have shown that, compared with fixed allocation schemes, the presented criteria has improved the performance significantly by multiuse diversity and antennae array gain and diversity gain.An adaptive subcarrier allocation algorithm for multiuser OSTBC-OFDM system is discussed in this paper. Firstly, with the goal of minimizing the overall transmit power subjected to meet the target BER, the adaptive subcarrier general allocation algorithm with efficiency-priority is put forward. On second thoughts, targeting a desirable tradeoff between efficiency and fairness, all users are endowed with the transmission rate guaranteed, and the amendatory four-step algorithm with fairness constraint is described. Simulation results reveal that the proposed algorithms improve the BER performance of the OSTBC-OFDM system.Adaptive cross layer design is becoming a popular design methodology for the computer and communication systems. A cross-layer adaptive transmission algorithm for downlink OFDM system is introduced in this paper. This algorithm takes into consideration not only the channel state information and power limitation in the physical layer, but also traffic scheduling and user fairness constraint at the data link layer. Simulation results show that the proposed cross-layer scheme outperforms the mere physical layer adaptive markedly. The novel algorithm is able to improve performance of the packet success rate and average packet delay, and support added active users.Antenna subset selection keeping many advantages of MIMO system is an efficient way to reduce cost and complexity. we discusse the problem of antenna subset selection in Vertical-Bell Laboratories Layer Space-Time (V-BLAST) systems with Ordered Successive Interference Cancellation (OSIC) detection. Taking into account the impacts of ordering and cancellation, by use of permutation and QR decomposition, this paper put forward a terse mathematic expression, and analyzes its performance. Simulations show that the proposed selection criterion achieves near-optimal outage capacity and outperforms several existing ones in performance with reasonable complexity.