OFDMにおける低レートターボ符号を用いたときの判定帰還チャネル推定のブロック誤り率特性

摘要

低レートターボ符号は，高い符号化利得に起因して目標のブロック誤り率（BLER：Block Error Rate）を満たすための所要平均受信SNR（Signal-to-Noise power Ratio）を低減できる.しかしながら，所要受信SNRの動作領域が低くなるため，同期検波のためのチャネル推定誤差が増大し，低レートターボ符号の実質的な符号化利得が減少してしまう.そこで，本稿では，直交周波数分割多重（OFDM：Orthogonal Frequency Division Multiplexing）における低レートターボ符号に判定帰還チャネル推定（DDCE：Decision-Directed Channel Estimation）を用いたときのBLER特性を評価する.提案のDDCEでは，参照信号（RS：Reference Signal）に加えて，Max-Log-MAP（maximum a posteriori probability）復号出力の外部確率の対数尤度比（LLR：Log-Likelihood Ratio）から軟判定シンボルレプリカを生成し，受信情報シンボルを逆変調して推定したチャネル応答を用いる.DDCEは，低チャネル符号化率の符号化利得の増大に相乗的に軟判定シンボル推定値の精度を向上することができる.計算機シミュレーション結果より，チャネル符号化率がR＝l/15および1/9のとき，DDCEを用いた場合の平均BLERが10~(-2)を満たすための所要平均受信SNRは，RSのみを用いるチャネル推定に比較して約1.2dB低減できることを示す.その結果，DDCEを用いたとき，月＝1／15および1／9のときの平均BLERが10~(-2)を満たす所要平均受信SNRは，月＝1／3の場合に比較して，それぞれ約6.0，4.4dB低減できることを示す.%A low-rate turbo code can reduce the required average received signal-to-noise power ratio (SNR) that satisfies the target block error rate (BLER) due to its high coding gain. When a low-rate turbo code is used however, channel estimation (CE) error becomes a bottleneck in bringing out the potential coding gain. Therefore, this paper presents the average BLER performance using decision-directed channel estimation (DDCE) for a low-rate turbo code in orthogonal frequency division multiplexing (OFDM). In the proposed DDCE, we employ soft-decision symbol estimation based on the log-likelihood ratio (LLR) of an extrinsic probability at the Max-Log-MAP (maximum a posteriori probability) decoder output in addition to reference signals (RSs). The DDCE works synergistically with the low-rate turbo code, since the extrinsic LLR at the Max-Log-MAP decoder becomes more reliable due to the increasing coding gain. Computer simulation results show that when the turbo code with R = 1/15 (1/9) and the constraint length of K = 4 bits is used, the required average received SNR at the average BLER of 10"2 using the DDCE is decreased by approximately 1.2 (1.2) dB compared to that for the RS based CE. As a result, the required average received SNR using the DDCE for R = 1/15 and 1/9 is decreased by approximately 6.0 and 4.4 dB, respectively, compared to that for R = 1/3.