Research Of Link Adaption In Cooperation Systems

The application of Multi-Input Multi-Output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM) technologies in communication system can significantly improve transmission data rate, but the requirement of more than on antenna equipped in terminals limits the implements of MIMO. Cooperative communication technology can achieve diversity gain without requiring more than one antenna equipped in terminals, providing efficient and reliable communications. On the other side, high carrier frequency leads to a decrease in communication distance. In order to provide the same coverage, more base stations are needed, which is a large cost to operators. By forwarding signals it receives, the relay gives an alternative solution to expand coverage. At the same time, due to its forwarding, the terminals can lower the transmit power, resulting in a longer battery life. Therefore, relaying cooperation has become a key technology in LTE-Advanced system.However, additional resources are needed for relaying communication. For example, for users who rely on relays to access to the network, multi-hop tansmission means that multiple time slots or frequencies are consumed to deliver one block, making a reduction of effectiveness. By adjusting tansmission parameters adaptively according to instantaneous channel state, link adaption technologies make more effective transmission, and meanwhile, it ensures reliability.So, in this dissertation, we made a reseach on link adaption in cooperation system, aiming to make more effective transmission by making use of instantaneous channel information. Two kinds of link adaption, namely adaptive modulation and coding (AMC) and hybrid automatic retransmission request (HARQ) are studied. Schemes are designed and evaluated considering the characteristics of cooperative system.Research of AMC is mainly on how to select a modulation and coding scheme (MCS). Base on whether the direct communication link exists between the source and destination and whether the relay changes MCS, four cases are studied. The main contribution lies in:for the case of direct communication link existing between the source and destination and the relay keeping the same MCS as source, we proposed two AMC schemes which provide much better throughput than systems without AMC. The first scheme tries to ensure cooperation, while the second scheme chooses a MCS according to all the transmission links. For another case of direct communication link existing between the source and destination and the relay changing MCS, we proposed an adaptive modulation scheme. Mutual information based effective SNR mapping is adopted to predict error rate and thoughput, and MCS is then chosen.For research of HARQ, we first made a summary of HARQ mechanisms based on existing researches. They are end-to-end mechanism, hop-by-hop' mechanism and relay assisted mechanism. Then, by using weight enumerating' function and pairwise error probability analysis, we analyze the error rate of these HARQ mechanisms. First, we can get the error rate which is dependent on instantaneous signal to noise ratio, and then, by integration on distribution of signal to noise ratio, we can get the average error rate. The analysis method is verified by simulation results. It can provide reference for research work.At last, we make a joint design of AMC and HARQ in cooperation system. Based on the former research of AMC and error rate analysis, the throughput and block error rate with a number of retransmissions can be calculated. And then choose proper modulation and coding schemes according to different principles.