符号化MIMO-OFDM準最適検出におけるビット系列対数尤度比計算アルゴリズム

摘要

符号化MIMO-OFDM移動通信において，符号化されたビット系列の復号に不可欠な対数尤度比（LLR）を効率よく求めるアルゴリズムを提案する．近似的なLLRの計算では，符号化されたビットが1か0の条件の下に，対数尤度関数を最大にする送信信号候補の対を求める必要がある．従来の計算法とは異なり，これらの送信信号候補を一括で探索する方法を提案する．具休的には，MMSE受倍を基点としてMMSEの雑音強調の方向に探索を行う．幕乗法により，受信信号の自己相関行列の逆行列について，その固有ベクトルを雑音強調の方向ベクトルとして複数求める．次に，各固有ベクトルの方向に1次元探索と硬判定を行い，上記の送信信号候補の対を対数尤度関数に基づき確定する．計算機シミュレーションにより，4×4MIMO-OFDM，QPSK変調，符号化率1／2の畳み込み符号において，提案アルゴリズムは，最尤検出（MLD）による厳密なLLR計算法と比べて演算圭を約1／8に低減し，また平均パケット誤り率10~(-3)を達成するE_b／N_0の劣化を0．6dBに抑えられることを示す．%This report proposes a low-complexity algorithm to calculate log likelihood ratios (LLRs) of coded bits, which is necessary for channel decoding in coded MIMO-OFDM mobile communications. An approximate calculation of LLR needs to find a pair of transmitted signal candidates that can maximize the log likelihood function with a coded bit equal to either one or zero. The proposed algorithm can find such a pair simultaneously, while conventional ones find them individually. Specifically, the proposed method searches such candidates in the directions of the noise enhancement using the MMSE detection as a starting point. First, the inverse matrix of the autocorrelation matrix of received signal is obtained and then the power method derives eigenvectors of the inverse matrix as the directions of the noise enhancement. With each eigenvector, one-dimensional search and hard decision are performed. From the resultant signals, the transmitted signal candidates to be required are selected on the basis of the log likelihood function. Computer simulations with 4×4 MIMO-OFDM, QPSK, and the convolutional code of coding rate 1/2 demonstrate that the proposed algorithm requires only 0.6 dB more E_b/N_0 than that of the maximum likelihood detection (MLD) in order to achieve PER of 10~(-3), while reducing the complexity to about 12.5% of that of MLD.