Layered Optical Spatial Modulation In Atmosphere Laser Communication

With the rapid growth of user terminals and various business data,people have put forward higher requirements on the capacity and transmission rate of wireless optical communication systems.Multiple input multiple output?MIMO?technology improves system capacity and reliability by using multiple lasers to send data simultaneously to the receiver.However,the popularization and application of traditional MIMO technology are limited due to its strong inter channel interference and high requirements for synchronization between sub channels.As a kind of MIMO transmission technology,spatial modulation?SM?extends two-dimensional mapping to three-dimensional mapping.It not only uses the traditional modulation symbols to carry information,but also uses the index number of laser to transmit part of information.Therefore,spatial modulation has become one of the hotspots of large-scale MIMO technology research,and gradually favored by the industry.Different from the radio frequency field,the current wireless optical communication system mostly adopts the intensity modulation/direct detection method,which makes the theory and method of spatial modulation in the original radio frequency field can not be directly used in WOC.Therefore,this paper introduces the idea of layering into optical spatial modulation,and improves the spectral efficiency by constructing layered optical spatial modulation schemes,and analyzes the performance of the proposed schemes under atmospheric channels.The details are as follows:?1?The layered spatial modulation technique can effectively improve the spectrum efficiency of the communication systems.In this paper,layered spatial modulation technique is introduced into free space optical communications.A layered optical spatial modulation?LOSM?system suitable for turbulent channel is proposed by simultaneously activating two lasers using pulse position modulation?PPM?and pulse amplitude modulation?PAM?,respectively.The principle of layer mapping and bit mapping is descripted in detail.The bit error rate expression of the proposed layered optical spatial modulation system is derived,and the Monte Carlo simulation method is utilized to verify its reliability.Simulation results show that the layered optical space modulation method can greatly improve the spectrum efficiency of the system.For example,the spectral efficiency of?5,4,2,4?-LOSM system is 9 times that of the?8,4,16?–spatial pulse position modulation?SPPM?system.When the spectral efficiency is 4 bit/s/Hz,the transmission rate of?5,4,2,4?-LOSM system is almost twice to that of the?8,4,2?–spatial amplitude modulation?SPAM?system,and with the same bit error rate performance.The parameters in brackets represent the number of lasers,the number of detectors,the modulation order of PPM and the modulation order of PAM.?2?LOSM system with different modulation modes on each layer can effectively improve the system spectrum efficiency,but only two lasers can be activated at a time.When the system is large,the utilization rate of the laser is far from our expectations.Therefore,combining PPAM modulation with OSM system,a multi-layer space pulse position amplitude modulation?MLSPPAM?system is proposed,and the expression of bit error rate under the system is derived.Simulation results show that the spectral efficiency of?9,4,8,2?-MLSPPAM scheme is 16 times that of?32,4,128?-SPPM scheme at the same transmission bits,When bit error rate is 10^-3,our proposal improves the signal-to-noise ratio by about 2.5 dB with employ less than 1/3 number of lasers that used in the?32,4,128?-spatial pulse position modulation scheme.?3?In view of the fact that the number of zero elements in the transmission signal of the classical SPPM system is far greater than that of non-zero elements,this paper introduces the idea of layering into the SPPM system and constructs a layered spatial pulse modulation?LSPPM?system with sparse characteristics.Based on this sparse feature,a layered joint detection algorithm based on compressed sensing theory is proposed in the receiver.The serial number of the laser and the position of the PPM modulation on each layer are obtained by operations,and then the original bit information is recovered.The system complexity is effectively reduce.By introducing a parameter to improve the algorithm,an effective compromise between computational complexity and error performance is achieved.