Research On Transmission Schemes For Wireless Multicasting

With the enhancement of mobile terminal and the rapid spread of mobile inter-net,the growth rate of the mobile services vastly exceeds the growth rate of the air inter-face rate, so the limited spectrum bandwidth resources and the ever-growing needs of the volume of mobile services become the principal contradiction in the development of mobile communications. Wireless multicast is an efficient communication means of transmitting the same content to multiple receivers and has been applied in many pop-ular applications such as mobile TV, video conferencing and online multiplayer game. Due to the broadcast characteristic of the wireless channel, wireless multicast allowing multiple users who require the same content to form a group and share the resources, not only saves the system resources, but also improves the efficiency of utilizing the air interface. Meanwhile, the system performance is in general constrained by the worst user in the group. In addition, due to the time-varying characteristics of the wireless channel, some physical layer technologies are necessary to overcome the adverse ef-fects brought by the wireless channel. Therefore, it is of significantly importance to study the transmission schemes for wireless multicast network.In this thesis, we focuse on the transmission schemes for wireless multicasting and study the three possible ways of transmission:single-rate multicast, multi-rate multicast and mixed unicast and multicast. The primary work and main contributions of this dissertation are the following:1. Single-rate multicast:Firstly, we consider the beamforming design problem in the presence of the channel estimation errors in the Time Division Duplexing (TDD) multicast system. We aim at designing robust beamformer, which min-imizes the total transmit power subject to outage probability constraint of each user for both instantaneous Channel State Information (CSI) and long-term CSI. By transforming the non-convex form of the outage probability constraints into equivalent convex close-form expressions, we get the approximate problem which can be solved efficiently through the utilization of semidefinite relaxation (SDR). Simulation results are provided to demonstrate the satisfying performance of pro-posed designs. Secondly, we address the beamforming design problem in the presence of the quantization errors in the Frequency Division Duplexing (FDD) multicast system. We consider two different optimization criteria for both in-stantaneous CSI and long-term CSI:the total transmit power minimization and max-min Quality of Service (QoS) problem. With the aid of S-lemma, the infi-nite non-convex QoS constraints of robust downlink beamforming problem are transformed into finite linear matrix inequalities. By applying the SDR method, the robust downlink beamforming problem can be relaxed and solved efficiently. Simulation results show that the proposed designs can achieve good robustness performance but only consume litter extra power.2. Multi-rate multicast:In order to overcome the shortcomings of the single-rate transmission in which the system performance is limited by the worst user, we allow users within the same multicast group to receive multicast data stream ac-cording to their own handling ability and propose a superimposed coding-based multi-rate multicast transmission scheme. Firstly, we consider the transmit power minimization problem with fixed user set and target multicast rate and propose a SDR-based algorithm to solve it. Further, it is shown that the power minimization problem always obtains a rank-1solution for some cases. Secondly, we consider the joint beamforming design and layer admission control problems. We begin with the base layer problem, and then study the single enhancement layer case in detail, and finally extend to the multiple enhancement layers case. We propose a corresponding joint beamforming design and layer admission control algorithm to determine the maximal support layers when given fixed target rates, the user set and beamforming vector of each layer. The proposed scheme can provide dif-ferentiated multicast services for users to maintain the fairness of the users with better channel condition.3. Mixed unicast and multicast:For the traditional single-rate multicast system, the system performance is often restricted by the worst-channel user, but irrespective of the better-channel users. We addresses the problem on how to support extra unicast services for those users with better channel condition so as to achieve the user throughput differentiation based on their non-uniform handling capacities. By leveraging superposition coding, we propose a mixed unicast and multicast scheme by superposing unicast on multicast at the transmitter and decoding mul-ticast message firstly at the receivers. In particular, we first tackle the transmit power minimization problem conditioned on the fixed user set and the target rate for each user, and find out the conditions to guarantee a rank-1solution. Then, we disucss the achievable unicast/multicast rate characterization prolbem. Fi-nally, we treat the joint beamforming design and admission control problem and propose an efficient SDR-based deflation algorithm. Meanwhile, we propose a low complexity and efficient enhancement for the D-SDR algorithm. Simulation results demonstrate that the proposed algorithm works well and serves users close to the optimal exhaustive search algorithm at an affordable computational com-plexity; For the TDM-based wireless two way relay system, in order to avoid the mutual influence between the stages of unicast and multicast, we propose three basic automatic repeat-request (ARQ) protocols to improve the system through-put according to different feedback schedules. By using the method of finite-state Markov chain, we decompose the procedure of ARQ protocols into discrete states and verify the system performance by theoretical analysis and computer simula-tion. In addition, simulation results reveal that the proposed protocols can im-prove the end-to-end throughput performance, and the RT-ARQ protocol has the best performance and can best approach the upper bound under low transmission rate among all proposed protocols.