Research On MIMO Detection Techniques In MDM Systems

The rapid growth of internet protocol(IP)traffic,such as high-definition video streams,cloud computing,big-data analysis,and intelligent network communication,imposes significant requirements on the bandwidth and data rates for better customer services.This brings great challenges to the current communication systems.As the core part of backbone telecommunication system,optical fiber communication systems have always been maintaining technical innovation.In order to enhance the capacity of single mode fibers(SMF),different degrees of freedom are already used in the deployed optical transmission systems,including amplitude,phase,time,wavelength as well as polarization state of the optical field.However,the systems based on SMF are approaching their theoretical non-linear capacity limits.At present,it is a hotspot of the research to achieve the expansion of the capacity of optical fiber communication by using the spatial dimension of few mode fibers(FMF).MDM technology resorts to several spatial modes existing in FMF as the carriers for transmitting independent data streams,which therefore realizes the multiplication of channel capacity.However,the orthogonality of the propagating modes may be destroyed due to the imperfection of the inline optical components such as fibers,amplifiers and multiplexers,hence resulting in non-unitary channel crosstalk.MIMO signal processing is one of the effective measures to solve the mode crosstalk in mode division multiplexing(MDM)system.This thesis mainly focuses on the mitigation of MDL in MDM system by using the space-time coding combined with MIMO detection techniques.The detailed contents are as follows:First,a long-haul MDM system with accumulated MDL impairment is established,and the effects of different coupling strengths with or without mode scramblers are verified.Three space-time coding schemes,i.e.V-BLAST,OSTBC and TAST,are employed in the MDM system with maximum likelihood(ML)detection to compare their performance in mitigating MDL.Taking into account its BER performance and transmission rate,the TAST coding scheme can be used as an effective solution to mitigate MDL.However,the exponential computational complexity of ML detection limits its practical application.In order to design a low-complexity detection algorithm,we systematically analyse several existing MIMO detection algorithms: linear detection,interference cancellation(IC),tree search,lattice reduction(LR),semidefinite programming relaxation(SDPR),and probabilistic data association(PDA),and the performance of these detection algorithms are tested in MDM system with TAST coding schemes.For existing MIMO detection algorithms,these are inherent defects in performance or complexity in MDM systems based on TAST codes.In order to solve this problems,the difference between wireless MIMO channel and MDM channel is discussed.Based on this,the feasibility of the detection based on IC and LR for improving performance in MDM system are investigated.Then according to the analyses of the orthogonality and full diversity feature of TAST decoding equivalent channel,a hybrid detection based on IRS method integrated with DFE technique is proposed.This scheme combines the advantages of DFE detection in full diversity equivalent channel and optimizes the search set of IRS.Simulation results show that the proposed detection method can achieve near-optimal solutions with low computational complexity.Moreover,it has the attractive advantage of flexible adjustment in the trade-off between the performance and complexity.