Performance of Cooperative HARQ with Relay Selection that Exploits Inter-relay Opportunistic Listening

摘要

本稿では，自動再送要求（ARQ）を用いる選択型協調通信システムの提案を行い，そのエンド・ツー・エンドの誤り率特性に関する理論検討を行う.検討するシステムでは，目的局へと送信局が直接通信できない状況を想定し，両局間に存在するリレー局が中継を行うことによって通信リンクを確立する.そのため，送信局と目的局間には，送信局・リレー局間チャネルとリレー局・目的局間チャネルの2つのチャネルが存在することになる.リレー局・目的局間では，リレー局の選択を行うこととする.送信局からの信号を正しく受信されたリレー局の中で，目的局とのチャネル状態の最も良いものが中継を行う.その際，電波の等方性を利用して，送信局からの受信信号が誤りとなっていたリレー局も受信を行う.それにより，リレー局目的局間でARQを行う際のリレー局選択時の自由度が増大する.結果として，再送時には初回中継の中継局とは異なる中継局が選択される可能性が高まり，空間ダイバーシチ利得の獲得へとつながる.本稿では，そのような特性を用いた選択型協調通信の誤り率特性を理論的に導出し，その妥当性を計算機シミュレーションにより確認している.%In this paper, we present a practically feasible cooperative HARQ with relay selection that exploits inter-relay transmissions. We first introduced this scheme in our previous report [8]. In [8], we derived theoretical expressions for error-rate performance of the system and analytical results were verified by the simulated results. In this report, we provide further results from [8]; such as performance comparisons with the existing related schemes and performance with different relay positions. In the system under consideration, the source node is located beyond the coverage of the destination node. The relay nodes assist to establish a communication link between the source and the destination. Therefore, the end-to-end transmission channel has two independent parts: source-relay and relay-destination channel. Among the relays that receive the signal from the source correctly, the one whose relay-destination channel has the largest channel gain transmits to the destination. We employ a low overhead distributed relay selection technique. Exploiting the broadcast nature of the wireless channel, the relays with errors can opportunistically listen (OL) to the transmitted signal from a relay without error to combine with their erroneous received signals. Consequently, these relay nodes originally with erroneous received signals may be able to decode the signals successfully due to the obtained spatial diversity. This results in the increase in degree of freedom in relay selection for retransmission by ARQ in relay-destination channel. Thus the relay node selected for retransmission may be different from that selected for initial relay transmission, achieving spatial diversity gain.