Existing Orthogonal Frequency-Division Multiplexing (OFDM) variants based on cyclic prefix (CP) allow for efficient time synchronization, but suffer from lower power efficiency compared to zero-padded (ZP)-OFDM. Because of its power efficiency, ZP-OFDM is considered as an appealing solution for the emerging low-power wireless systems. However, in the absence of CP, time synchronization in ZP-OFDM is a very challenging task. In this paper, the non-data-aided (NDA) maximum-likelihood (ML) time synchronization for ZP-OFDM is analytically derived. We show that the optimal NDA-ML synchronization algorithm offers a high lock-in probability and can be efficiently implemented using Monte Carlo sampling (MCS) technique in combination with golden-section search. To obtain the optimal NDA-ML time synchronization algorithm, we first derive a closed-form expression for the joint probability density function (PDF) of the received ZP-OFDM samples in frequency-selective fading channels. The derived expression is valid for doubly-selective fading channels with mobile users as well. The performance of the proposed synchronization algorithm is evaluated under various practical settings through simulation experiments. It is shown that the proposed optimal NDA-ML synchronization algorithm and its MCS implementation substantially outperforms existing algorithms in terms of lock-in probability.

翻译：现有的基于循环前缀（CP）的正交频分复用（OFDM）变体能够实现高效的时间同步，但相比零填充（ZP）-OFDM，其功率效率较低。由于其功率效率优势，ZP-OFDM被视为新兴低功耗无线系统的有前景解决方案。然而，在缺少CP的情况下，ZP-OFDM的时间同步是一项极具挑战性的任务。本文从解析角度推导了ZP-OFDM的非数据辅助（NDA）最大似然（ML）时间同步方法。研究表明，最优的NDA-ML同步算法具有高锁定概率，且可通过结合蒙特卡洛采样（MCS）技术与黄金分割搜索进行高效实现。为获得该最优算法，我们首先推导了频率选择性衰落信道中接收ZP-OFDM样本联合概率密度函数（PDF）的闭式表达式。该表达式同样适用于移动用户的双选择性衰落信道。通过仿真实验，在多种实际场景下评估了所提同步算法的性能。结果表明，所提出的最优NDA-ML同步算法及其MCS实现方式在锁定概率方面显著优于现有算法。