针对低信噪比环境下编码辅助载波同步算法存在复杂度高和实时性差的问题, 本工作引入估计积分器不断补偿信号向量、减小待估偏移值. 在此基础上,提出新的同步迭代停止准则;当LDPC码的各变量节点的对数似然比绝对值的均值趋于稳定时,提前停止迭代. 仿真结果表明,该算法在缩小频率搜索范围的同时,大大减少同步迭代次数,且与理想同步译码相比,性能损失不超过0.12 dB.
We consider high complexity and long delay of the code-aided carrier synchronization algorithm at low SNR(signal to noise ratio). We use the estimation integrator for compensating signal vector and decreasing offset to be estimated. A new iterative synchronization stopping rule is proposed. The system will stop iteration if the mean of the absolute values of LLR (log-likelihood ratio) message at the output of LDPC decoder keeps steady. The simulation results indicate that this algorithm narrows the frequency search range and the average number of iterations is reduced greatly. It also shows that the performance loss is not larger then 0.12 dB.
[1] Mackay D J C, Neal R M. Near shannon-limit performance of low-density parity check codes[J]. Electron Lett, 1996, 32(18): 1 645-1 646.
[2] Herzet C, Noels N, Lottici V. Code-aided turbo synchronization[J].Proceedings of the IEEE, 2007, 95(6): 1 255-1 271.
[3] Shi Z P, Tang F J, Yan H, et al. Optimization of code aided carrier synthronization algorithm[J]. Journal of University of Electronic Science and Technology of China, 2012, 41(3): 342-347(in Chinese). 史治平,唐发建,晏辉,等.编码辅助载波同步算法的优化设计[J].电子科技大学学报,2012,41(3):342-347.
[4] Ebert J, Schlemmer H, Gappmair W. The code-aided FEPE algorithm for joint frequency and phase estimation at low SNR[C].IEEE ASMS and SPSC, 2012, 2012: 350-354.
[5] Wymeersch H, Moeneclaey M. Iterative code-aided ML phase estimation and phase ambiguity resolution[J]. EURASIP Journal on Applied Signal Processing, 2006: 981-988.
[6] Rahamim Y, Freedman A, Reichman A. ML iterative soft-decision-directed (ML-ISDD): A carrier synchronization system for short packet turbo coded communication[J]. IEEE Transactions on Communications, 2008, 56(7): 1 169-1 177.
[7] Noels N, Lottici V, Dejonghe A, et al. A theoretical framework for soft-information-based synchronization in iterative receivers[J]. EURASIP Journal on Wireless Communications and Networking, 2005: 117-129.
[8] Herzet C, Ramon V, Vandendorpe L. A theoretical framework for iterative synchronization based on the sum-product and the expectation-maximization algorithms[J].IEEE Transactions on Signal Processing, 2007, 55(5): 1 644-1 658.
[9] Valles E L, Wesel R D, Villasenor J D, et al. Carrier and timing synchronization of BPSK via LDPC code feedback[C]//Proc 40th Asilomar Conference on Signal, System and Computers. Pacific Grove, CA, 2006: 2 177-2 181.
[10] Valles E L, Wesel R D, Villasenor J D, et al. Pilotless carrier phase-synchronization via LDPC code feedback[C]. Military Communications Conference, 2010: 2 068-2 073.
[11] Yan H, Tang F J, Zhang Z P. A low complexity code-aided carrier synchronization algorithm[J]. Journal of Electronics & Information Technology, 2010, 32(12):2 959-2 963(in Chinese). 晏辉,唐发建,张忠培.一种低复杂度编码辅助载波同步算法[J].电子与信息学报,2010, 32(12): 2 959-2 963.
[12] Lechner G, Sayir J. On the convergence of log-likelihood values in iterative decoding[J]. Mini-Workshop on Topics in Information Theory, Sept 2002: 457-463.
[13] Li J, You X H, Li J. Early stopping for LDPC decoding: convergence of mean magnitude (CMM)[J]. IEEE Communications Letters, 2006, 10(9): 667-669.
