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Abstract

We present an automatic repeat request (ARQ) free space optical (FSO) system, which consists of a pulse position modulation (PPM) hard decision and an ARQ. The new ARQ's data error detection is based on a PPM hard decision's results and can eliminate the traditional ARQ information redundancy. The results of the numerical simulation have a good agreement with theoretical analysis and show that the ARQ-FSO system can effectively improve the bit error rate (BER) performance of the direct hard decision PPM system. Additionally, the proposed system significantly improves the average throughput efficiency compared to traditional ARQ systems. These characteristics make the ARQ-FSO system suitable for application in low BER and complexity FSO scenarios.OCIS In recent years, free space optical (FSO) communication has drawn noticeable attention, owing to its high transmission rate, high bandwidth, license-free, quick deployment, and inherent security [1][2][3] . However, the performance of FSO communications systems is mainly restricted by scattering and absorption losses and atmospheric-induced scintillation on the atmospheric transmission. In particular, the atmospheric turbulence effect is a major impairment of FSO systems [4] . To alleviate atmospheric turbulence effects, pulse position modulation (PPM) is extensively employed in FSO communication systems [5,6] . PPM is a type of signal modulation where a shift is made in the position of the pulse signal. In practical applications, two decoding algorithms can be used for the PPM signals. A decoding algorithm is hard decision decoding that uses a threshold detector, while the other one is soft decision decoding, which uses a maximum a posteriori or maximum likelihood detector [7] . Owing to the bit error rate (BER) performance of soft decision decoding being superior to hard decision decoding, soft decision decoding is usually used in FSO communications [4][5][6] . However, the soft decision decoding receivers are generally based on a field-programmable gate array (FPGA) [8][9][10] or erbium-doped waveguide amplifier (EDWA) planar lightwave circuits (PLCs) [11] , which leads to its hardware being more complicated and more consumptive than the hard decision decoding receiver's.With the growing needs of communication quality, the study of FSO communication is no longer confined to a single modulation technology. An effective technology to control error is using automatic repeat request (ARQ) in the FSO links [12][13][14][15][16][17] . In these studies, specific theoretical analysis for ARQ systems are investigated, and the combination of ARQ and other technology such as forward error correction [that is hybrid ARQ (HARQ)] [12][13][14] , the Rateless Round Robin protocol [15] , Luby Transform codes [16] , and adaptive-rate transmission [17] , are presented. These systems do not take modulations into account, especially PPM technology. Therefore, the more efficient and comprehensive hybrid scheme, which combines ARQ and PPM, has been considered. The combination of ARQ and...