Research On Coded Modulation Technology Combining Unconventional Modulation And LDPC Codes

With the rapid development of fifth generation(5G)mobile communications,it is urgent to improve the reliability of data transmission.The coded modulation technology can effectively integrate forward error correction(FEC)coding and modulation to obtain superior error performance.Low-density parity-check(LDPC)code has been adopted by many wireless communications standards due to its superior performance close to the Shannon limit.Non-power-of-two modulation has attracted attention for increasing the flexibility of adaptive modulation.However,since the number of constellation points of non-standard modulation is not a power of prime number,it is difficult to construct a matching LDPC code in finite fields.In this thesis,the LDPC coding scheme that matches with the 6-order modulation is studied and a two-level coded modulation scheme for matching the unconventional 6-order modulation is proposed.Further,the overall performance of the coded modulation scheme is optimized from the aspects of constellation modulation and decoding method.First,a two-level coded modulation scheme for unconventional 6-order modulation constellation is proposed.At the transmitter,aiming at a 6-order quadrature amplitude modulation(6-QAM)based on the origin plus a regular pentagon,the Galois field GF(2)LDPC code and GF(3)non-binary LDPC(NB-LDPC)code is adopted for layered encoding,and the two kinds of LDPC codewords are mapped to senary codewords for 6-order modulation.At the receiver,according to the decoding order of GF(2)LDPC code and GF(3)NB-LDPC code,two decoding methods based on twolevel LDPC codes are proposed.Simulation results show that,under the Additive White Gaussian Noise(AWGN)channel and the Rayleigh fading channel,the two-level LDPC codes achieve matching with the unconventional 6-order modulation,and the proposed two-level coded modulation scheme obtains great symbol error rate performance.Then,to improve the decoding performance of the coded modulation scheme,constellation diagram optimization and iterative decoding method are used.On one hand,the 6-QAM constellation diagram is optimized by balancing the squared Euclidean distance between the intrasubset and the intersubset,and the performance of the scheme combining 6-QAM and LDPC code is compared with that of the combination scheme of optimized 6-QAM and LDPC code.On the other hand,an iterative decoding method between two-level LDPC codes is proposed.The decoding information between GF(2)LDPC code and GF(3)NB-LDPC code is transferred to each other.Therefore,the accuracy of LDPC decoding is improved.Furthermore,the6-QAM and optimized 6-QAM are combined with the iterative decoding method,and the performance of the coded modulation scheme based on 6-QAM and the scheme with optimized 6-QAM is compared.Simulation results show that,with the increase of the overall number of iterations,the optimized 6-QAM combined with LDPC code scheme obtains a greater performance gain,which effectively improves the error correction performance of the senary coded modulation scheme.