The Applications Of Handover And Differential Modulation In Relay Systems

Wireless communication brings a series of problems with its rapid development:wireless spectrum resources become increasingly scarce with the surge in the number of users; it is an urgent need to ensure the communication quality under the condition of lower transmission consumption due to the energy-constrained of wireless terminal; interference becomes anoth-er inevitable problem because of the radio characteristics of wireless communication. In order to solve the above problems effectively, relay technology gets more and more attention and research in the next generation as a key enhancement technology. For one thing, the introduction of relay can not only reduce the channel fading effectively and enlarge the cell coverage, but also per-forms significant improvement in spectral efficiency, data rate and energy efficiency. For another thing, the relay link, coupled with the direct link creats a virtual multiple antenna system, bring-ing space diversity gain for cooperative relay system. Furthermore, maximum diversity(space diversity, multipath diversity or doppler diversity) can be obtained with differential coding in cooperative relay system. In this dissertation, we mainly investigate on the handover of high-speed train, differential coding in frequency selective fading channel and doubly selective fading channel and their applications in relay system, and propose several innovative strategies. The contents of this dissertation are listed as follows.Firstly, we study an important task of high mobility wireless communication system in this dissertation-handover based on receive beamforming in multiple-antenna mobile relay system. Since the high mobility and channel feedback delay will result in unexpected information, we want to design a handover mechanism to avoid information feedback. Based on the ability of multi-antenna communication system to increase the system capacity, we exploit the multiple antenna technology and beamforming to accomplish the handover problem. This dissertation focuses on the downlink, and high-speed train running in the public area of the base station. The target Base Station (BS) is selected by maximizing Signal-to-Interference-Noise-Ratio (SINR). Firstly, a channel estimation scheme based on Minimum Mean Square Error (MMSE) is pro-posed as required. On the basis of channel estimation, the receiver form the beamforming using multiple antenna, finding the optimal beam vector for every BS by maximizing SINR, and the BS which possesses the maximal SINR is the target. Numerical simulation results show that the proposed multi-antenna relay handover scheme has good robustness.Secondly, we propose a differential space-time coding based on frequency selective fading channel and Orthogonal Frequency Division Multiplexing (OFDM) in cooperative relay system. Three node relay system with both relay link and direct link is taken into account. In such system, the two stage relaying strategy is considered, i.e., source node only transmit signals to relay node at the first stage, and then the relay node forward the received signals to destination node, source node transmitting signals to destination node simultaneously at the second stage. In high mobility communication system, channel estimation is often complicated, especially in rapid fading channel, so we develop a differential space-time scheme to get what we need with-out any channel state information (CSI) at the receiver. Moreover, we design a coding scheme combining linear constellation decimation (LCD) with subcarrier interleaving which not only has the potential of achieving full diversity, but also reduces the decoding complexity. Through the analysis of performance, we observe whatever kind of relaying strategy is exploited, the sys-tem can achieve maximum diversity. Simulation results show that the pairwise error probability performance improves significantly compared with noncooperative relay system.Finally, differential modulation is further introduced in cooperative relay system based on doubly selective fading channel, combining with the interleaver design based on the character-istics of channel parameters to combat the fading. In wireless communication systems, delay spread caused by multipath transmission and doppler spread caused by high mobility produce doubly selective fading channel. On one hand, channel estimation is often complicated and may reduce transmission efficiency with heavy reliance on pilot symbols when there is no CSI at receiver. Besides, the estimated CSI may be inaccurate, especially under rapid fading channel. Differential modulation we proposed can avoid the channel estimation. On the other hand, the transmitted symbols may experience deep fading and burst errors occur. Therefore we design the interleaver since it interweaves the error symbols affected by time and frequency fading as much as possible and makes them independent from each other, thus combating the burst errors and achieving the time and frequency diversity. Additionally, we propose a two-dimension joint time-frequency linear constellation decimation coding scheme, associated with the interleaving, which can obtain the maximum diversity gain and reduce the decoding complexity. Simulation results validate the high reliability of the proposed scheme in terms of bit error rate.