Design Of The Optical Field Reconstruction And Frame Synchronization Algorithms For Multi-aperture Simplified Free Space Optical Coherent Receivers

With the development of spatial information networks(SIN),traditional microwave communication technology has become unable to meet the increasing data exchange requirements of various mobile terminals.Free space optical communication(FSOC)has the merits of high rate,small terminal size and strong confidentiality,so it has good application prospects in SIN.In FSOC system,the optical signal cannot be amplified by the in-line amplifier,so a high-sensitivity optical receiver and a large-aperture optical antenna are necessary.The coherent optical receiver has the advantages of high sensitivity,but it is complex and the power consuming,so it is difficult to meet the size,weight and power(SWa P)requirements of the mobile terminals.Recently,a simplified coherent optical receiver(SCR)has been proposed,which can use a single photodetector to realize coherent detection of the optical signal field.It has the same high sensitivity of traditional coherent optical receivers,but much lower optical complexity and power consumption.At the same time,the SCR supports coherent combining of the optical signals received by different apertures.Such multi-aperture SCRs can work under an extremely low optical signal-to-noise ratio(OSNR),thereby achieving a very long transmission distance.The difference between SCR and traditional coherent optical receivers in DSP algorithms is the addition of the optical field reconstruction algorithm in the front of the DSP chain,while coherent combining requires frame synchronization under extremely low OSNR so that the same symbols from different branches of signals can be added coherently.Therefore,aiming at the requirements of FSOC systems,this paper focuses on the development of low-complexity,low-power-consuming optical field reconstruction algorithm(FRA)and frame synchronization algorithm(FSA)suitable for the multi-aperture SCR.First,it introduces the structure,working principle and DSP chain of the SCR.Secondly,the design and performance optimization methods of the FRA is investigated in details.The Hilbert transform(HT)involved in the FRA is realized by frequency domain transfer function and overlap-and-save algorithm.By this method,the edge effect caused by the block-wise processing can be mitigated.Thirdly,this paper compares a variety of FSAs in terms of the pilot sequence and timing metric designs,and proposes a FSA suitable for very low OSNR.Compared with existing FSAs,it has less pilot sequence overhead,lower computation complexity,strong noise resistance,and can realize frequency offset estimation at the same time,thus simplify the DSP chain.Finally,real-time FRA and FSA are developed using Xilinx Vivado development tools and implemented in a FPGA to evaluate their performance on the real-time platform.Simulations and experiments show that the FRA and FSA exhibit satisfactory performance.