Research On The Key Techniques In Wireless Cooperative Communications

Cooperative communication is a promising diversity technique that has been attracting significant research interests in academic and industry communities, where the antennas of randomly distributed users are shared to form a virtual MIMO system and reap the benefits of spatial diversity without any requirement of additional antennas at each single-antenna users. As a potential technique for 4G, Cooperative communication can improve the channel capacity, enhance the transmission reliability of wireless channel, and enlarge the coverage. In this dissertation, some key techniques in Physical Layer are investigated.According to the signal processing method, the basic cooperative diversity protocols can be cataloged into three classes:Amplify-and-Forward (AF), Decode-and-Forward (DF), and Coded Cooperation (CC). Combine these cooperation schemes with different relay se-lection strategies, adaptive (selection) cooperation and opportunistic relaying protocols are developed. The dissertation investigates the principle and capacity performance of these cooperation schemes. Based on these results, the outage performance of OFDM/OFDMA cooperative relaying protocols are also studied. Subsequently, the relationship between the cooperative diversity and the number of relays is analyzed.In wireless communication systems, it is an important research issue that how to utilize power and frequency efficiently and appropriately. There are diverse channel and power al-location methods according to different optimization objectives. All the channel and power allocation methods are expounded in details and are cataloged into MA and RA optimization problem. The transformation between MA and RA are also analyzed. Based on these analy-sis, the subchannel and power allocation techniques in OFDM/OFDMA cooperative relaying networks are investigated thoroughly. A joint subchannel and power allocation algorithm is proposed, in which equal channel power gain is introduced and maxmin criterion is adopted as an optimization metric. The proposed algorithm properly deals with the rate fairness and system capacity. Theoretical investigation and simulation indicate that the proposed opti-mal algorithm can obtain higher system capacity while result in equal transmission rate of each user. In order to reduce the computational complexity, a suboptimal subchannel and power allocation algorithm is also proposed, in which the original optimization problem is divided into two subproblem, namely, subchannel assignment and power distribution. In the suboptimal algorithm, the available subchannels are assigned to each user under the assumption that the total transmission power is distributed equally among all subchannels. After subchannel assignment, the total transmission power is distributed among all deter-mined subchannel pairs by using multi-level water-filling algorithm.Since the cooperation partners are located randomly in wireless cooperative communi-cation networks, it is the foundation of cooperation among multiple users that how to appro-priately select one or multiple cooperation partners from multiple potential relay nodes. The dissertation studies the relay node selection algorithms in cooperative communication sys-tems. According to different achievable targets and optimization objective, the commonly used relay nodes selection strategies can be classified into the followings which are relay nodes selection strategy based on channel state information, relay nodes selection strategy based on utility function, relay nodes selection strategy based on cooperation regions, and joint relay nodes selection and power allocation, et al.. Compared with radio resource allo-cation, the objectives of relay nodes selection are improve system capacity, reduce the error probability, and save transmit power. Thus, as one of the popular issue on relay selection, relay selection can be considered jointly with subchannel and power allocation. From the point of view on optimization problem, the relay nodes selection problem in OFDMA cooper-ative relaying networks are investigate, in which power distribution, subchannel allocation, and relay nodes selection are considered jointly. An opportunistic relay nodes selection strat-egy base on instantaneous channel state information is propose in the dissertation, of which the objective is to maximize the total throughput of all users under the constraints of fair-ness on subchannel occupation of each user. Simulation results show that the capacity of the proposed algorithm approaches asymptotically to that of the optimal one without fairness constraint and the fairness on subchannel occupation by each is most. Therefore, it can be said that the proposed algorithm can do the best tradeoff between throughput and fairness.In multi-cell OFDMA networks, the subchannels used within the cell are orthogonal to each other and then intra-cell interference is little. The key factor which degrade the sys-tem performance, especially the performance of cell-edge users, is inter-cell interference (ICI). In order to depress ICI which can further improve the transmission rate of cell-edge users, many ICI reduction methods, such as Fractional Frequency Reuse (FFR), Soft Fre-quency Reuse, ICI Coordination, ICI Randomization, ICI Cancellation, have been proposed in 3GPP LTE. Another hot research topic in 3GPP LTE is Coordinated Multi-Point (CoMP) transmission technique. In the dissertation, a multi-cell cooperative transmission technique is proposed which is based on fractional frequency reuse technique. In this algorithm, each cell is partitioned into interior and exterior region, and subsequently the exterior region is divided into three sector. The frequency reuse factor is 1 in the interior region while 3 in the exterior region. Three adjacent sectors belonging to three neighboring cells is constituted to a virtual-cell. The base station controller coordinately schedules three sector antennas and served all users in in a virtual cell. Theoretical investigation show that the proposed algo-rithm improve the multiuser gain by increasing the effective users of each sector antenna. Simulation results show that the proposed algorithm improve efficiently the throughput of cell-edge users and fairness of channel occupation among all users.