Research On Adaptive Transmission Technologies For Next Generation Wireless Communication Systems

To improve the transmission efficiency and reliability, adaptive transmission technology has been widely researched and used in the next generation wireless communication systems. On the one hand, with the increasing demands of high reliability, high mobility, and low per-bit cost, researchers has paid more and more attentions to improve the performance of adaptive modulation, especially its reliability and complexity. On the other hand, the emergence of cognitive radio (CR) technology has totally broken the restriction of wireless communications by conventional spectrum authorization mechanism. Henceforth, adaptive transmission has been closely integrated with the new concepts of spectrum sharing, dynamic spectrum access (DSA) and so on.On the basis of international current research work on adaptive transmission technology, adaptive modulation for multiple-input multiple-output/orthogonal frequency division multiplexing (MIMO-OFDM) systems in high Doppler frequency environment, adaptive modulation for MIMO systems with imperfect channel state information (CSI), and orthogonal frequency division multiple access (OFDMA) resource allocation for cognitive radio (CR) systems have been investigated. The adaptive transmission algorithms have been studied on the points of efficiency, reliability, computational complexity. And some key problems have been investigated to satisfy the requirements of future wireless communications and make the adaptive transmission more practical. Main innovations of this thesis are included in the following parts.Firstly, adaptive modulation for MIMO-OFDM systems in high Doppler frequency environment has been investigated. Based on the peculiarity of equivalent channels in MIMO-OFDM systems, an adaptive modulation algorithm with Markov channel model has been proposed. Its performance and computational complexity has been analyzed in theory, and also validated by simulation. To solve the reliability problem of conventional adaptive modulation algorithms, the proposed algorithm have taken CSI imperfection into account. On the one hand, the proposed algorithm has good performance. It has high valid data rate and throughput. When the Doppler frequency is relatively high, it can satisfy the bit error rate (BER) requirement. On the other hand, its computational complexity is much lower than that of the conventional adaptive modulation algorithms using channel predictor.Secondly, robust adaptive modulation for MIMO systems with imperfect CSI has been investigated. Three novel robust adaptive modulation algorithms have been proposed. The first algorithm is robust greedy. Comparing with conventional greedy algorithm, robust greedy algorithm is more reliable. It can satisfy the BER requirement when the channel estimation error is remarkable. Then a novel algorithm with lower complexity has been proposed to make the robust adaptive modulation more practical. Subsequently, a robust adaptive modulation to maximize transmission rate has been proposed, by which the reliability and spectrial efficiency have been improved significantly.Thirdly, OFDMA resource allocation technology for CR has been investigated. Three adaptive resource allocation algorithms for downlink CR systems have been proposed. The first one is optimal resource allocation algorithm. Based on interference analysis, the problem to maximum the downlink capacity under transmission power and inference constraint is constructed as a convex optimization with linear inequality constraint and solved by optimization theory. The second one is a subcarrier, bit and power allocation algorithm under transmit power and BER constraints for downlink CR systems. It can be considered as a way to approach the capacity derived by the first resource allocation in practical systems. The third one is used to minimize the total transmit power of downlink CR systems under the transmission rate and interference constraints.