393 research outputs found
Design and optimization of joint iterative detection and decoding receiver for uplink polar coded SCMA system
SCMA and polar coding are possible candidates for 5G systems. In this paper, we firstly propose the joint iterative detection and decoding (JIDD) receiver for the uplink polar coded sparse code multiple access (PC-SCMA) system. Then, the EXIT chart is used to investigate the performance of the JIDD receiver. Additionally, we optimize the system design and polar code construction based on the EXIT chart analysis. The proposed receiver integrates the factor graph of SCMA detector and polar soft-output decoder into a joint factor graph, which enables the exchange of messages between SCMA detector and polar decoder iteratively. Simulation results demonstrate that the JIDD receiver has better BER performance and lower complexity than the separate scheme. Specifically, when polar code length N=256 and code rate R=1/2 , JIDD outperforms the separate scheme 4.8 and 6 dB over AWGN channel and Rayleigh fading channel, respectively. It also shows that, under 150% system loading, the JIDD receiver only has 0.3 dB performance loss compared to the single user uplink PC-SCMA over AWGN channel and 0.6 dB performance loss over Rayleigh fading channel
SCMA Codebook Design
Multicarrier CDMA is a multiple access scheme in which modulated QAM symbols
are spread over OFDMA tones by using a generally complex spreading sequence.
Effectively, a QAM symbol is repeated over multiple tones. Low density
signature (LDS) is a version of CDMA with low density spreading sequences
allowing us to take advantage of a near optimal message passing algorithm (MPA)
receiver with practically feasible complexity. Sparse code multiple access
(SCMA) is a multi-dimensional codebook-based non-orthogonal spreading
technique. In SCMA, the procedure of bit to QAM symbol mapping and spreading
are combined together and incoming bits are directly mapped to
multi-dimensional codewords of SCMA codebook sets. Each layer has its dedicated
codebook. Shaping gain of a multi-dimensional constellation is one of the main
sources of the performance improvement in comparison to the simple repetition
of QAM symbols in LDS. Meanwhile, like LDS, SCMA enjoys the low complexity
reception techniques due to the sparsity of SCMA codewords. In this paper a
systematic approach is proposed to design SCMA codebooks mainly based on the
design principles of lattice constellations. Simulation results are presented
to show the performance gain of SCMA compared to LDS and OFDMA.Comment: Accepted for IEEE VTC-fall 201
SCMA with Low Complexity Symmetric Codebook Design for Visible Light Communication
Sparse code multiple access (SCMA) is attracting significant research
interests currently, which is considered as a promising multiple access
technique for 5G systems. It serves as a good candidate for the future
communication network with massive nodes due to its capability of handling user
overloading. Introducing SCMA to visible light communication (VLC) can provide
another opportunity on design of transmission protocols for the communication
network with massive nodes due to the limited communication range of VLC, which
reduces the interference intensity. However, when applying SCMA in VLC systems,
we need to modify the SCMA codebook to accommodate the real and positive signal
requirement for VLC.We apply multidimensional constellation design methods to
SCMA codebook. To reduce the design complexity, we also propose a symmetric
codebook design. For all the proposed design approaches, the minimum Euclidean
distance aims to be maximized. Our symmetric codebook design can reduce design
and detection complexity simultaneously. Simulation results show that our
design implies fast convergence with respect to the number of iterations, and
outperforms the design that simply modifies the existing approaches to VLC
signal requirements
Simplified Multiuser Detection for SCMA with Sum-Product Algorithm
Sparse code multiple access (SCMA) is a novel non-orthogonal multiple access
technique, which fully exploits the shaping gain of multi-dimensional
codewords. However, the lack of simplified multiuser detection algorithm
prevents further implementation due to the inherently high computation
complexity. In this paper, general SCMA detector algorithms based on
Sum-product algorithm are elaborated. Then two improved algorithms are
proposed, which simplify the detection structure and curtail exponent
operations quantitatively in logarithm domain. Furthermore, to analyze these
detection algorithms fairly, we derive theoretical expression of the average
mutual information (AMI) of SCMA (SCMA-AMI), and employ a statistical method to
calculate SCMA-AMI based specific detection algorithm. Simulation results show
that the performance is almost as well as the based message passing algorithm
in terms of both BER and AMI while the complexity is significantly decreased,
compared to the traditional Max-Log approximation method
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