| The wide-band wireless digital communication technology is the key in future mobile communications. As the progress of various communication technologies and the consequent increasing needs for the market, further improvement in the use of spectrum is expected to obtain larger transmission capacity and higher transmission reliability.Based on adaptive transmission, in this paper, several key technologies are studied on the wide-band wireless digital communications. The basic idea is to decrease system spending, optimize the whole system performance, reduce computation complexity, shorten transmission delay, improve the real-time processing capability, and cut cost.The main content of the paper and the corresponding achievements are as follows:1. Two low-complexity OFDM adaptive algorithms: LMS-Bussgangare and ZF-Bussgang, were proposed. The main characteristics and advantages are: on one hand, we adopted the low-complexity Bussgang algorithm, and did blind estimation to OFDM sub-channel according to the Mean Square Error Criterion(MSE) and Peak Distortion Criterion; on the other hand, we used the differential detection, which accelerates the convergence speed and avoid the error transmission problem resulted from the Bussgang algorithm. Compared with the channel estimation algorithm based plots, these two proposed algorithms could track the changes of channel more effectively, and expressed excellent anti-fading performance, and demand less computations.2. How to optimize DAPSK constellations is analyzed and an optimization algorithm is proposed. With criterion of minimized average energy and criterion of maximized minimum euclidean distance as the basic policies, we determined the optimization mechanism of star-QAM constellation in OFDM system with fading channel and using Quasi-Coherentand demodulation. The algorithm takes advantages of the anti-fading capabilities in both OFDM and star-QAM modulation and demodulation, and provide valuable information and for further studying the potential capability in higher-order modulation-demodulation anti-fading technologies and theories.3. Two SOVA decoding algorithms in OFDM system based on BPSK modulation with fading channel were presented. One is called BCE_SOVA, which is based on adaptive blind channel recognization. It can adaptively estimate the channel state information (CSI). causes small transmission delay, but computation is complex.The other is called DSOVA, which does not need to know CSI, with low-computation complexity and small transmission delay, and is more suitable in the case when Side Information is unknown. These two algorithms effectively combine the OFDM technology and the SOVA algorithm in Rayleigh channel, resulting in an efficient SOVA decoding under frequency-selective channel. Simulation results show that the algorithms are advantageous in improving system performance, reducing redundancy, shortening transmission time-delay and cutting cost.4. A mutirate transmission protocol is proposed, which is based on the CSMA and implemented rate information exchange, duration setting, NAV update and calculating. It provides two transmission rate: variety hop and step by step, to guarantee non-interference communication between websites and eliminate the problem of duration overlong when transmission rate is increased, resulting higher bandwidth usability. This protocol can be used better in the wireless mobile network of full connection and multihop network, symmetry and dissymmetry channels, high speed mobile, variable rate, and is well compatible with current fixed-rate transmission protocol.5. A simulation and modeling mechanism is studied and proposed for multirate transmission protocol. Based on the WLAN that adopts OFDM transmission technique, we first assumed that the Small-Scall fading and multipath in OFDM sub-channel is Rayleigh, and Large-Scale path loss could be described bybi-linearity model, and then we analyzed the rationality...
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