Research On Link Selection Technology Of Buffer-aided Relay System

Relay techniques have gained a lot of interest in the wireless communication systems. In most of the existing literature on relaying, it is assumed that relays receive a packet from the source in one time slot and forward it to the destination in the next time slot, which will be referred to as “conventional relaying”. The conventional relaying can’t achieve good performance when either of the source-relay channel state information(CSI) or the relay-destination CSI is bad.In the buffer-aided relaying we will abandon this paradigm and give relays the freedom to decide in which time slot to receive and in which time slot to transmit based on the channel state information and the appropriate selection algorithm. The buffer-aided relaying techniques open a new area.This paper studies the buffer-aided relaying transmission techniques. Depending on the differences of number of the relays there are two applications in the buffer-aided relaying. The two applications are buffer-aided relay selection in the networks with multiple relays and the buffer-aided link selection in the networks with a single relay, respectively.Firstly, for the networks with multiple relays the buffer-aided relay selection is proposed to overcome the limitations imposed by using the same relay for reception and transmission. If the relays are equipped with buffers, they can store the packets received from the source and do not have to re–transmit them immediately in the next time slot. That means the relays with the best source-relay channel and the best relay-destination channel can be selected for reception and transmission, respectively. The corresponding selection scheme is referred to as max-max relay selection(MMRS). For the practical case of finite buffer sizes, a hybrid relay selection(HRS) scheme is proposed. The analyses showed that the MMRS gets a 3dB SNR gain and the same diversity degrees compared with the conventional relay selection that only selects one relay to reception and transmission.Secondly, for the networks with a single relay. In practical systems, however, a relay must support multiple users. In this paper, for a multiuser relay network a buffer-aided relaying protocol was proposed. Different from the conventional multiuser relaying that the half-duplex relay always alternates between reception and transmission in successive time slots, the proposed protocol enables the relay to choose link to transmit based on the channel state information(CSI). The optimal link selection is derived from throughput maximization under the buffer stability constraint. We solve this optimization problem by relaxing the discrete constraints and applying the Karush-Kuhn-Tucker(KKT) conditions. The numerical results show that the buffer-aided multiuser relay network with the optimal link selection strategy achieves significant throughput gains compared with conventional multiuser relaying that use the best user selection scheme(BUS) with fixed transmission schedule. The impact of finite buffer sizes and delay are studied via simulations.Thirdly, through the analysis of the outage probability event of the suboptimal link selection, the upper bound of the outage probability is derived. The threshold of the fixed rate transmission is obtained under the buffer stability constraint. The analyses showed that buffer-aided link selection in multiuser relay networks achieves significant diversity gain. The conclusion is proofed by the simulations.