Nested符号を用いた送信ダイバーシティの設計とその解析

摘要

本稿では，Nested符号を用いた送信ダイバーシティを提案し，その基本設計と理論解析を行う．一般にダイバーシティ利得を得るためには，アンテナや周波数，時間等の新たなリソースが必要となる．しかし，本提案方式は新たなリソースを必要としないことを特徴とする．具体的には，複数の情報を独立な符号器で符号化し，複数の時点にXOR演算を用いて重畳させる．複数の情報が重畳されている信号を，時間変動のある通信路において複数時点用いて送信する．これにより，時間ダイバーシティ効果を得る．また，前時点において復号に成功した場合は，重畳されている情報を既知の信号と見なすことができるため，XOR演算のみで現時点の受信信号から重畳されている情報を減算できる．XOR演算のため，減算しても全く電力ロスはない．逆に復号に失敗した場合は，失敗した復号結果を事前確率として，失敗した情報が重畳されている他の時点の復号に用いる．提案方式の性能評価にアウテージ確率を用いて解析した．その結果，提案方式はダイバーシティ利得を得ることができ，アウテージ確率を大幅に改善することを報告する．%We propose a novel transmit diversity technique with a nested code. In general, to achieve diversity gain, the system needs extra resource (e.g., antenna, frequency, time). However, the proposed transmit diversity can obtain diversity gain over block Rayleigh fading channel without some extra resource. In the proposed system, each data is encoded with individual encoder and superimposed onto the other codeword with XOR operation. Concretely speaking, each codeword is superimposed onto the next codeword. In the proposed system, the receiver has two decoding architectures. The one is for the case when the receiver successfully decoded the signal at the previous time-slot. The other is for the case when the receiver failed to decode. In the case when the receiver successfully decoded the signal at the previous time-slot, since the superimposed factors consist of the previous codewords, the receiver already knows the superimposed factors and can simply subtract them from the received signal. Therefore, compared to the conventional system, the proposed system has no disadvantage except for additional XOR operation. While, in the case when the receiver failed to decode the signal at the previous time-slot, the receiver must estimate the present and previous data. However, the receiver can utilize the decoding results calculated at previous time-slot as a-priori information in the decoding operation. From the outage analysis, the proposed system can achieve diversity gain and improve its performance without any extra resources.