Research On Coded Modulation Technology For Non-orthogonal Multiple Access Systems

The development of the mobile Internet and the Internet of Things brings the challenge of higher spectral efficiency and massive connections.In tradition orthogonal multiple schemes,the number of supported users is limited by the number of available orthogonal resources,which cannot cater for the requirement of high throughput and high connectivity density.Meanwhile,novel non-orthogonal multiple access(NOMA)schemes allow the users sharing the same orthogonal time,frequency,or code domain resources so that overloading connections can be implemented practically.Power-domain(PD)-NOMA and code-domain(CD)-NOMA are two main categories of the existing NOMA solutions.Compared with CD-NOMA,it is easier to implement PD-NOMA without complex resource allocation while interference is inevitably introduced because all the access users have to muliplex on the whole resources.On the other hand,with the CD-NOMA scheme,i.e.,sparse code multiple access(SCMA),the users' signals can be partially overlapped instead of fully multiplexing on the whole resources.Both of the PD-NOMA and CD-NOMA can achieve better performance with the help of the well-designed codebook and the enhanced multiuser detection.However,most of the existing works assuming continuous Gaussian distributed transmit signals will lead to a mismatch of the achievable rate when inputs are substituted by discrete constellations.Therefore,for the above PD-NOMA and CD-NOMA schemes,the paper aims to study both the codebook design mechanism and the advanced multiuser detection and decoding algorithms.Herein,the main work and contributions are summarized as follows:1?Power-domain NOMA: design and analysis of the probabilistic shaping(PS)scheme for uplink NOMA systemsA PS-based uplink NOMA strategy is proposed.The ergodic constellation constrained(CC)capacity for each user is derived,and the optimal probability mass function(PMF)of the discrete constellations is also obtained.The error performance for the PSbased NOMA scheme is theoretically analyzed.In detail,the transmit signals from different users are non-equiprobable distributed and the successive interference cancellation-based maximum a posterior detection is implemented at the receiver.We first theoretically analyze the ergodic channel capacity with Gaussian-distributed inputs and the CC capacity with discreteconstellation inputs,respectively.And then,a multi-step optimization scheme for maximizing the ergodic CC capacity is proposed to obtain the optimal PMF of these input signals.Furthermore,based on this non-equiprobable transmission scheme,the closed-form pairwise error probability(PEP)expressions for each user are obtained for the first time.Besides,the exact PEP results based on the assumption of Gaussian distributed interference signals are also derived.It is shown that the error performance in the PS-based NOMA system depends on not only the power level of the interference but also the employed modulation schemes.Finally,a practical low-density parity-check code(LDPC)coded modulation scheme is also developed to implement the PS-based NOMA scheme.The simulations show the superiority of the proposed scheme in terms of the bit error rate(BER).2?Code-domain NOMA:codebook design for low-density signature(LDS)systems and SCMA systemsFor LDS and SCMA systems,the paper firstly derives the CC capacity from the perspective of information theory and then propose a codebook design method for maximizing the CC capacity.Both the transmission efficiency and the error performance are improved.In detail,the single user mutual information(SMI)for the LDS system with a generic detection is first derived.Then,we analyze the relationship between the SMI and multiuser detection algorithms and prove that the SMI can achieve the maximal value iff the detector is the individual optimal detector.And also,considering the quality of service requirements for each user,we propose a sparse signature design criterion that maximizes the minimum SMI.Furthermore,different from the linear spreading LDS scheme,we propose a multi-step codebook design method for non-linear SCMA systems.On each dimension,constellation rotation between different users is utilized to achieve better superposition CC capacity with the help of the modified multi-start interior-point algorithm.Furthermore,we propose a dimensional permutation switching algorithm to efficiently combine 1-dimensional constellation sets into multi-dimensional codewords,which verifies the significance of the dimensional permutation to achieve higher multi-dimensional shaping gain.Consequently,the proposed codebook immensely narrows the gap to the Shannon limit in medium and low signal to noise ratio region and outperforms the existing codebooks in terms of the CC capacity and the BER performance.3?Code-domain NOMA:multiuser detection for LDS systems and SCMA systemsAiming at the multiuser detection problem,the paper proposes a symbol-based extrinsic information transfer(EXIT)tool to analyze the convergence of the SCMA iterative detection,and also design an iterative joint detection and decoding scheme in conjunction with LDPC codes.In detail,we divide the SCMA detector into two concatenated decoders:the variable node detector and the function node detector under the iterative decoding principle,and derive the mutual information flows in each iteration to visualize the converge property.The a priori information functions are modeled by applying independent Gaussian noise in conjunction with the character of the complex multi-dimensional codewords.By leveraging this tool,the optimal iteration-stopping criteria are provided for different codebooks and different channel conditions.Furthermore,to approach the capacity,we develop an LDPC-based SCMA receiver.During each outer loop,only partial inner iterations in the detector and the decoder are implemented.With the help of the EXIT chart,we find the best decoding strategy among variable combinations of different numbers of the inner and outer iterations.Without increasing the calculation complexity,remarkable performance gain can be achieved over the existing scheme with the concatenated SCMA detection and LDPC decoding.