Research On Outage Probability And Power Allocation Schemes Based On 2-HOP Cooperative Communication System

In recent years cooperative communications have been shown to be one of efficient methods to combat multipath fading of wireless channels and to improve wireless system capacity. Researches on cooperative communications have been focused on by academic community and among which the research on outage probability formulas and power allocation schemes is one of hotspots. In this dissertation the outage probability formulas and power allocation schemes were investigated for different protocols, such as Amplify and Forward (AAF) and Decoded and Forward (DAF), and different scenarios of 2-hop cooperative communication system.The different scenarios of 2-hop cooperative communication system in our research include:three-node cooperative model; one source node, multiple relay nodes and one destination node with one antenna model; and one source node, multiple relay nodes and one destination node with multiple antennas model. The cooperative protocols in our studied are AAF and DAF. The outage probabilities for different protocols and scenarios were derived by using the information theory and probability math tools and then the performances of cooperative system were analyzed and the power allocation schemes were studied for the cooperative system.The main researches in this dissertation are the following:1. The performances of cooperative system are investigated for both the three-node cooperative model and the multiple relay nodes with opportunistic relaying model over Nakagami-m fading channels. Every relaying node solely receives the messages from the source node and the relaying node adapts DAF strategy. The exact and closed-form outage probability expressions are derived and simulation results confirmed the presented mathematical analysis. And then based on these theoretical formulas the performances and the power allocation scheme are discussed for three-node cooperation system.2. The outage probabilities and the performances of system based on multiple relaying nodes are studied for both symmetric and asymmetric channels. For symmetric channel the affection on the performance of the system were studied with consideration of the direct link between the source and destination and the power allocation problem was analyzed by using the analytical expression of the average outage probability we derived. The conclusion shows us that the optimal power allocation ratio is subjected to the total power in networks and the networks size, and the power allocation factorα= 0.5 is sub-optimization result. For the asymmetric channel the closed-form of outage probability expression was also derived and the power allocation scheme was discussed. The conclusion we obtained is that the outage probability is affected by the power allocation factorαand pathloss. We found that when the location of relays close to the source the optimal power allocation factor a is less than 0.5 to arrive the optimal performance of the system and when the location of relays close to the destination the optimal power allocation factorαis larger than 0.5 for the optimal performance of the system.3. The performances of the cooperative communication system were studied which is extended to multiple antennas at the destination node comparing with the system model in 2. First an approximate theory closed-form of outage probability is derived. Comparing with others of approximate theory closed-form of outage probability we found that our closed-form of outage probability is optimization. Second the power allocation scheme and throughput problem were researched by using our outage probability equation for the system. For the power allocation problem, the results of analysis and simulation demonstrate that the better performance can be obtained through the virtual MIMO between multiple relay nodes and the destination node with multiple antennas when the transmitted power of the relay nodes is less. This also showed that the advantage of the MIMO. For the throughput problem, the theory expression for the optimal solution R* is obtained to reach the maximum throughput when the signal-to-noise ratio is constant and the results of simulation showed us that the increasing of the signal-to-noise ratio or the adjusting of the information rate parameter R can arrived the maximum throughput in the system.4. The power allocation optimization scheme is studied among the relay nodes under the scenario of multiple cooperative relays and one destination node with multiple receiving antennas. An optimal power allocation scheme (OPA), through singular value decomposition (SVD) of the channel matrix, is developed under a certain power constraint to achieve the maximum channel capacity. The expression of the optimal power allocation coefficient is derived. The optimal power allocation schemes for AAF and DAF and different cooperative scenarios were compared. For the AAF analysis and simulation results show that the OPA can obtain higher capacity gain and better outage probability comparing with the uniform power allocation (EPA) scheme. Moreover researches showed that the OPA with DAF has better performance than the OPA with AAF. The combining power allocation scheme for the AAF protocol was also investigated among the relay nodes and the source node. Analysis and simulation results showed that combining power allocation can obtain higher capacity, compared with the optimal power allocation only among relay nodes and in this case the power allocation factorαbetween the source node and the relay nodes is belong to [0.4,0.7].In summary, the outage probability and power allocation problem are studied in detail for different scenarios and protocols of cooperative system in this dissertation. The main contributions of this dissertation is that the derived outage probabilities formulas and power allocation schemes are developed and these achievements can be serve as productive theoretical tools for further researches and applications.