Correlation Technology Dual-path Relay System

Relay Technology can not only improve the system capacity to extend cellcoverage, but also can effectively resist the fading effects of the shadow effect. And ithas drawn extensively attention by many researchers over past several decades dueto its novel way of achieving spatial diversity. The system consists of one source node,one destination node, and one or more relay nodes. A relay cannot transmit a data atthe same time and same frequency due to the half-duplex constraint of wirelessterminals. So the transmission of relay system can be divided into two stages. In thefirst phase, the source transmits a data to the destination node and the relay nodealso listens to this data. In the second phase, the relay node forwards the datacoming from source to destination node and destination node receives the data, atthis time, the source node remains silent. Since the source is in idle state in thesecond phase, which leads to a loss in spectral efficiency. In order to solve thisproblem, some new protocols for half-duplex relay have been proposed, whichmainly includes the Two-Way system and Two-Path system. This paper mainly studiesthe Two-Path relay system. In this system, source node continuously transmits data,and two half-duplex relays take turns to forward messages from source node todestination node. This protocol can make Two-Path system achieve the same spectralefficiency with a full-duplex relay. This paper mainly studies the frequency schedulingprotocol, downlink hybrid automatic retransmission protocol, power allocation aswell as relay selection of Two-Path successive relay system.Firstly, brief overviews about traditional relay system and device to devicesystem have been given, and then we introduced the transmission protocol ofTwo-Path successive relay system. We combined the Two-Path successive relaysystem and D2D system, and have designed a frequency scheduling protocol andhybrid automatic retransmission protocol for Two-Path D2D system. We have giventhe frequency scheduling protocol flowchart, hybrid automatic retransmissionprotocol flowchart. The designed hybrid automatic retransmission protocol accordswith the HARQ protocol of LTE system, which can make full use of the time-slots resources, reduce the burden of D2D user and improve the diversity gain of thesystem.Secondly, based on the analysis of Two-Path system outage probability, we haveproposed corresponding optimal power allocation algorithm to minimize the outageprobability for Two-Path system with inter-relay interference and without inter-relayinterference. For the case of non-interference, because the outage probability closedform solution is relatively simple, and which is also a convex function. So we can usethe Lagrange multiplication to get the optimized power allocation of each node. Forthe case of interference, due to the introduction of the interference, the outageprobability closed form solution has become more complicated. So we used aniterative algorithm to obtain an optimal solution by iterative loop. Some simulationshave been done for the two cases. Simulation results show that the proposedalgorithm can decrease the system outage probability compared with the equalpower allocation.Finally, in order to improve the capacity of Two-Path relay system, we haveproposed a relay selection strategy to maximum system capacity. Traditional relayselection strategies are generally based on the maximum and minimum criteria.Though this selection algorithm can maximize the minimum signal-to-noise ratio inthe relay system, cannot simultaneously guarantee two links quality. So this paperhas simply modified the algorithm. We set a receiving signal-to-noise ratio thresholdat the all candidate relays before the source transmits data. In the first phase, weselect the relays that receiving signal-to-noise ratio is higher than the threshold. Inthe second phase, we select the relay that has best relay-to-destination channel gainfrom the selected relays in the first phase. Simulation result shows that the proposedrelay selection strategy achieves better capacity performance than the randomselection strategy.In this paper, we assumed that there is no direct link between the source nodeand the destination node because of the fading or distance. In practice, however, thesource node and the destination node can communicate directly. This extra link canincrease the system diversity gain and improve the reliability of the system. So, in the future, we can study the power allocation and relay selection in the Two-Path relaysystem with the direct link.