| Nowadays, portable devices such as computing devices, communication terminals, data storage, and multi-media processing equipments are getting more and more popular. A reliable, immediate and convenient wireless technology is desirable to provide links between these devices. Ultra-wideband (UWB) technology is one of the hot topics in wireless communication research area, which supports high data rates with low power consumption and low cost. The MB-OFDM UWB proposed by Wi-Media Alliance is a leading scheme which has received supports from a lot of vendors. The most attractive merit of MB-OFDM UWB is its flexibility in spectrum allocation which makes it possible to adapt to different regional specifications on spectrum usage. However, UWB signals suffer from severe multipath fading which leads to amplitude attenuation and/or waveform distortion. As a result, the performance of UWB systems degrades seriously. Cooperative communication is a new communication technology proposed in recent years. The performance of wireless communication networks can be boosted dramatically by introducing cooperation between nodes. Interestingly, almost no incremental hardware is needed in network nodes. In order to improve the transmission performance of UWB systems, this thesis focuses on partner selection algorithms and explores the combination of cooperative communication technology with ultra-wideband networks.The main contributions of this thesis can be summarized as follows:1. Aiming to make use of the ranging capability of UWB systems, this paper presents a distance-information-based opportunistic relay selection algorithm. Taking the distances between the source, idle neighbors and the destination as input, the proposed algorithm selects one of the neighbor nodes as the best relay by using the min-max optimization principle. In comparison with the classic ORS algorithm, the proposed algorithm is easy to implement and reduces the time consumption especially for multi-carrier communication systems.2. Regarding the node-intensive application scenarios, an improved algorithm, named intensive relay selection algorithm, is proposed further. This algorithm has lower time consumption than the classic ORS and meanwhile avoids the randomness of distance-information-based opportunistic relay selection algorithm. Consequently, the performance of the cooperative networks can be improved greatly when nodes are densely distributed.3. The performances of both two proposed algorithms are evaluated and compared with that of the classic ORS algorithm by simulations where single-carrier and MB-OFDM applications are considered. The simulation results suggest that:1) In both single-carrier and MB-OFDM applications, remarkable diversity gains can be achieved by using the proposed relay selection algorithms. For instance, the diversity gain is about 2 dB in single-carrier system when the bit error rate is 10-4. The diversity gain is more significant, say more than 5 dB, in the MB-OFDM system when the data rate is 160Mb/s and the bit error rate is 10-4.2) In both single-carrier and MB-OFDM systems, the diversity gains achieved by the proposed distance-information-based opportunity relay selection algorithm are almost the same as that of the classic ORS algorithm. It is worth noting that the proposed algorithm is more preferable in MB-OFDM UWB applications due to its lower complexity.3) The proposed intensive relay selection algorithm outperforms the distance-information-based opportunity relay selection algorithm in diversity gain. When the number of optional nodes is 5 and the bit error rate is 10-4, the intensive relay selection algorithm shows 1 dB and 2dB advantages over its counterpart in the single-carrier system and the MB-OFDM system with rate of 160Mb/s respectively. It is also shown that the advantage of the intensive relay selection algorithm is proportional to the number of available nodes. |
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