For short-packet, low-latency communications over random access channels, piloting overhead significantly reduces spectral efficiency. Therefore, pilotless systems recently gained attraction. While blind phase estimation algorithms such as Viterbi-Viterbi Phase Estimation (VVPE) can correct a phase offset using only payload symbols, a phase ambiguity remains. We first show that the remaining phase rotations in a polar coded quadrature amplitude modulation (QAM) transmission with gray labeling are combinations of bit-flips and automorphisms. Therefore, the decoder is equivariant to such phase rotations and, by smartly selecting the frozen bits, one can jointly decode and resolve the phase ambiguity, without the need for pilot symbols or an outer code. Our proposed system outperforms pilot-assisted transmissions by up to 0.8 dB and 2 dB for quaternary phase shift keying (QPSK) and 16-QAM, respectively.

翻译：针对随机接入信道中的短包、低延迟通信，导频开销会显著降低频谱效率。因此，无导频系统近年来备受关注。尽管维特比-维特比相位估计（VVPE）等盲相位估计算法仅利用有效载荷符号即可校正相位偏移，但相位模糊问题依然存在。本文首先证明，在采用格雷标记的极化编码正交幅度调制（QAM）传输中，剩余的相位旋转是比特翻转和自同构的组合。由此，解码器对此类相位旋转具有等变性，通过智能选择冻结比特，可在无需导频符号或外部编码的情况下联合实现解码与相位模糊消除。与基于导频辅助的传输方案相比，所提系统在四相相移键控（QPSK）和16-QAM中分别实现了最高0.8 dB和2 dB的性能增益。