Research On Adaptive Transmission Technologies In Relay Cooperative System

In order to meet the increasingly growing wireless multimedia service requirements of high-speed, large capacity and multi-type, relay cooperative communication has become the new research direction for the 4th generation (4G) system following the multiple-input multiple-output (MIMO) technology. Although MIMO can obtain high diversity gain, and the bandwidth efficiency can be enhanced as well, it is difficult to be realized on portable mobile terminals for the limitations in energy, price, size and other factors. So relay cooperative communication is proposed, which enables single-antenna terminals to share their antennas and generate a virtual MIMO system that allows them to achieve space diversity, meanwhile, the cell coverage is extended and the cell edge performance is improved too. This dissertation will focus on designing the adaptive transmission technologies in relay cooperative system, and studying their respective performance through theoretical analysis and simulation as well.The preliminary work of this dissertation is to analyze the performance of traditional relay cooperative technologies. The algorithms of such traditional relay mechanisms as amplify-and-forward (AF), decode-and-forward (DF), decode-amplify-forward (DAF), incremental redundancy forward (IRF), and code cooperation (CC) are first summed up, and then the successful decoding probability formulas of AF, DF, and DAF are derived. In addition, AF, DF, selective DF (SDF) and DAF are all simulated respectively in the scenarios where the link of source node-destination node exists and not, The corresponding block error ratio (BLER) performances are compared both from simulation results and theoretical analysis. The main work of this dissertation is to design adaptive cooperation technologies for single-relay as well as multi-relay scenarios, including the adaptation in relay protocol, coordinated retransmission and distributed space-time block code (DSTBC) respectively, thus the transmission reliability and validity of the system can be improved:The proposed technologies are summarized as follows:1. An adaptive relay mechanism which is based on analog network coding (ANC) AANCF is proposed. First, the physical layer NC (PNC) is classified from these two respects:operating level and operating field, and then ANC, the most widely used branch of PNC, is redesigned with innovation:Introduction constellation rotation to ANC which breakthrough the limitation of traditional ANC, that is requirements for priori information. Based on this improved ANC, AANCF is thus proposed:the forwarding strategy is switched adaptively from four options according to the cyclic redundancy check (CRC) results of both current frame and previous frame. So the erroneously propagation is evaded, besides, the diversity gain is also maximized. The detection algorithm is designed with adaptation too, which can effectively eliminate the data interference between adjacent frames with low complexity. The superiorities of AANCF in diversity order and throughput to SDF are obvious from theoretical analysis and simulation results.2. An adaptive relay coordinated retransmission strategy which is based on hierarchical modulation (HM) is proposed. The two traditional combination schemes of 16QAM HM and retransmission are first investigated and simulated:bit rearrangement and QPSK symbol power reallocation. Then the proposed strategy is introduced:in one side, the retransmitted packet is designed to be transmitted with a new packet simultaneously by HM technology, so the additional resource consumption, which is the defect of traditional strategy, can be removed completely, in the other side, who initiates and when to start the retransmission are both decided adaptively on the channel quality indicator, so the new packet can be sent without delay and its received quality is not affected. Both the performance analysis and simulation results show that the proposed strategy can significantly lower the outage probability and improve the throughput compared with the traditional one.3. An adaptive DSTBC (ADSTBC) which is based on minimum decoding complexity quasi-orthogonal STBC (MDC-QO-STBC) is proposed. The construction method of DSTBC from MDC-QO-STBC (MDC-DSTBC) is first given, including the processing procedures at the source node and the relay node. For that MDC-DSTBC cannot realize full diversity and full rate (FDFR) simultaneously, and the complexity of the required single complex symbol maximum likelihood decoding algorithm is very high, the ADSTBC is thus proposed:A MDC-QO-STBC matrix is taken as the basic codeword and each relay node transmits one row of this matrix, while the corresponding row index for each relay is not fixed but chosen adaptively according to instantaneous channel state information, so the interference between real symbol in the obtained DSTBC codeword is minimized. ADSTBC achieves FDFR and linear decoding complexity as well, and its performance advantage, in lower BLER compared with MDC-DSTBC is evident both in the simulation results of the 4-relay and 3-relay scenarios.