| As an emerging wireless communication technology for short-range links, Ultra-wideband (UWB) has the merits of supporting high data rates (up to 100Mbps-1Gbps) with strong Anti-multipath capability, high penetration and large Channel capacity. Therefore, It is viewed as the most ideal implementation plan for short-range wireless communications, and with far-reaching significance. However, UWB signals suffer from severe frequency selective fading, leading to energy attenuation and waveform distortion. As a result, the performance of UWB systems degrades seriously. The performance of UWB networks can be boosted dramatically by introducing Adaptive technique. Due to the inherent advantages of adaptive technology, such as high spectrum efficiency and robust resistance to interference, which can improve the noise immunity of UWB systems through adjusting system modulation mode, varying transmission power and coding schemes or other system parameters dynamically. Interestingly, almost no incremental hardware is needed in UWB network. Thus, this thesis focuses on the combination of adaptive transmission strategy with ultra-wideband networks, aiming to enhance the performance of UWB systems.The main contributions of this thesis can be summarized as follows:1. In this paper, based on the Contribution of H. Rohling, and C. Fellenberg, (Successive Bit Loading, SBL), a new algorithm named as ISBL algorithm, which has much lower complexity and is still based on the concept of remaining signal to noise ratio, is proposed to minimize the bit error rate of the system. Since the algorithm introduces the idea of group sub-bits, the complexity of ISBL algorithm will be related to the maximum number of bits each sub-carrier allowed. Simulation results verify that comparing with the SBL algorithm, ISBL algorithm has lower complexity, faster convergence, and no loss in performance.2. The performance of several typical fixed power bit loading algorithms is analyzed here, and taking the UWB circumstance and the stringent limit on transmission power specified by FCC into consideration, the noise immunity performance of the proposed ISBL algorithm in multi-carrier OFDM system and typical MB-OFDM-UWB scenarios is analyzed and compared with the other two typical equal power bit allocation algorithm. Then we explore the application of ISBL algorithm in UWB Network Coverage Expansion. The simulation results suggest that:1) Comparing with the WiMedia communication system which using fixed modulation mode, remarkable performance gains can be achieved by introducing adaptive bit loading algorithms. For instance, the SNR gain is about 9.8 dB in OFDM system when the bit error rate is 10-2. Also, there is 1.5dB available SNR benefits in adaptive systems under CM1 channel environment in the MB-OFDM application when the data rate is 106.7Mb/s and the bit error rate is 10-3. Similarly, the performance gain is more significant, say more than 2dB, under CM3 circumstance. And the performance improvement is increasing greatly with the growing rate.2) The performance simulation results of SBL and ISBL algorithms suggest that:In the Rayleigh channel and the UWB channel simulation environments, both of them have exactly the same error performance. Thereby, comparison confirmed that although improved ISBL algorithm has reduced complexity, there is no consequent performance loss; and the two algorithms in UWB environments perform better than Rayleigh channel environment.3) The improved ISBL algorithm based on MQAM modulation outperforms the BER minimization algorithm based on MPSK in improving the coverage of UWB. For example, in the MB-OFDM system under the CM1 channel environment, when the transfer rate is 106.7Mb/s and PER=8%, the WiMedia system can transmit 12.2m, adaptive system using Cao's algorithm can transmit 14.8m, and system with ISBL algorithm can transmit 15.1m; Similarly, transmission distance of these three systems can reach up to 11.8m,14.6m and 14.75m respectively in CM3 channel environment. |
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