Iterative Signal Detection Under Timing Errors for Optical Wireless Links With High Mobility

In this paper, the problem of timing error for free-space optical links with high mobility is investigated. In particular, we consider a real scenario in which the sampling clock offset is randomly changing and, in addition, the receiver has no information about the instantaneous channel coefficient as well as instantaneous timing error. Moreover, we consider a general case with signal dependent noise. For the considered scenario, for a conventional receiver with one sampler, we propose an iterative detection method based on the expectation-maximization (EM) algorithm. In the presence of large values of sampling clock offset, shifted error is the dominant error. Hence, in order to reduce the impact of shifted error probability, we propose an iterative data detection method for a receiver which utilizes two shifted samplers. The proposed iterative detection method only requires a series of simple addition and multiplication operation and does not require to solve any integral equations. A set of simulation results are provided to study the system performance for the proposed detection methods. Simulation results demonstrate that for a receiver with two samplers, by using the proposed EM-based detector, we can achieve bit error rate performance very close to the ideal receiver that perfectly knows the instantaneous channel coefficient and sampling clock offset.