Simulations Of Orbital Angular Momentum(OAM) Optical Communications Based On Low Density Parity Check(LDPC) Technology

In order to address the limited frequency spectrum resources in optical communications systems, orbital angular momentum(OAM), a dimension of light wave related to the spatial helical phase structure, is introduced in optical communications. Similar to the well-known dimension resources of light waves such as frequency, polarization, amplitude and phase, OAM dimension features intrinsic orthogonality between different OAM values and in principle infinite number of available OAM states. As a consequence, OAM multiplexing could be employed in communications to provide a new approach to increasing the spectral efficiency. The channel impairment and crosstalk in OAM communications might degrade the communication performance. In this thesis, we study and simulate OAM multiplexing communications with improved performance by exploiting low density parity check(LDPC) codes.Firstly, the working principles of error correction in optical communications using LDPC encoding and decoding are studied. The impacts of different code lengths, different number of iterations and different construction methods of check matrix on the LDPC iterative decoding performance are analyzed. Compared to the decoding performance with Turbo codes, the obtained results show a lower error floor and better long code performance when employing LDPC codes.By exploiting the model of OAM free-space multiplexing communication systems, the evolution of OAM propagation in free space is analyzed. Using the Kolmogorov model of turbulence phase offset to describe the turbulence influence in free space, the atmospheric transmission characteristic and performance degradation in dense multiplexing of fractional OAM beams are studied. Meanwhile, LDPC technology is employed to improve the bit error performance of communication systems. The simulation results show system coding gain greater than 8 dB.By exploiting the model of OAM fiber multiplexing communication systems, the performance degradation of OAM fiber multiplexing communications due to the bending and different ellipticity of OAM fiber is analyzed. Meanwhile, LDPC technology is employed to improve the bit error performance of OAM fiber multiplexing communication systems. The simulation results show system coding gain greater than 10 dB.Finally, LDPC technology in m-ary phase-shift keying(mPSK) modulation formats is discussed. The OAM digital image transmission using LDPC technology is studied. The performance of LDPC code, Turbo code and space-time Trellis code(STTC) is compared with each other in OAM multiplexing communication systems.