Training-aided joint frame and frequency synchronization for free space optical communication signals with low OSNR

Abstract

In the photon-starving free space optical communication (FSOC) systems the optical signals received often exhibit a very low OSNR due to the large transmission loss. Thus the digital coherent combining (DCC) based digital coherent receivers (DCC-DCR) able to deal with FSOC signals with OSNR much lower than 0dB have attracted much attention in the FSOC field. As frame and frequency synchronization (FFS) algorithms are indispensable for DCC-DCRs, the FFS algorithms able to deal with FSOC signals with extremely low OSNR are of particular interest for the FSOC systems. In this paper we evaluate the performance of the existing FFS algorithms in the very low OSNR regime and propose a joint FFS algorithm with a larger noise tolerance, lower training sequence overhead and a wider frequency offset acquisition range. Numerical simulations are presented to illustrate the merits of the proposed FFS algorithm with respect to the existing algorithms.