Zhongwei Si

Performance evaluation and parameter optimization of SoftCast wireless video broadcast

Wireless video broadcast plays an important role in multimedia communication with the emergence of mobile video applications. However, conventional video broadcast designs suffer from a cliff effect due to separated source and channel encoding. The newly proposed SoftCast scheme employs a cross-layer design, whose reconstructed video quality is proportional to the channel condition. In this paper, we provide the performance evaluation and the parameter optimization of the SoftCast system. Optimization principles on parameter selection are suggested to obtain a better video quality, occupy less bandwidth and/or utilize lower complexity. In addition, we compare SoftCast with H.264 in the LTE EPA scenario. The simulation results show that SoftCast provides a better performance in the scalability to channel conditions and the robustness to packet losses.

Recursive encoding of spatially coupled LDPC codes with arbitrary rates

Spatially coupled LDPC codes have attracted much attention due to the promising performance. Recursive encoding with low delay and low complexity has been proposed in the literature for selected node degrees. To realize the recursive encoding of spatially coupled LDPC codes with arbitrary rates, we propose in this paper a modified structure of the parity-check matrix and implement the encoding using a shift-register regardless of the node degrees. By rearranging the edge connections, the parity bits at each coupling position can be jointly determined by the information bits at the current position and the encoded bits at former positions. Performance analysis in terms of design rate and density evolution has been provided. It can be observed that the modified code structure leads to a better belief-propagation threshold. Finite-length simulation results are provided, which verify the theoretical analysis.

Fast convergence of joint demodulation and decoding based on joint sparse graph for spatially coupling data transmission

Spatially coupling data transmission (SCDT) is a multiple access technique. Since both SCDT and low density parity-check (LDPC) codes can be represented by a factor graph, a joint sparse graph (JSG) including the single graphs of SCDT and LDPC codes is constructed. Based on the JSG, the joint demodulation and decoding (JDD) by applying belief propagation (BP) algorithm in the parallel schedule is performed, but its convergence rate is slow. In order to accelerate convergence rate, a new serial schedule is proposed. The extrinsic information transfer (EXIT) charts of iterative JDD are investigated for the two schedules and employed to evaluate their converge behaviour. EXIT analysis and simulation results demonstrate that about half iteration number can be saved at the cost of marginal system performance loss. Furthermore, compared with separate demodulation and decoding, turbo-structured JDD and the uncoupling structured JDD, the JDD based on JSG for SCDT by utilizing serial schedule achieves the best performance.

NOMA Based on User Grouping and Multiple Layer Modulation

In Non-orthogonal multiple access (NOMA) system, different users utilize the same time and frequency resource to achieve a higher spectral efficiency. However, for conventional NOMA multiplexing in the power domain, the number of users carried in the system is very limited due to the requirement of perfect interference cancelation in the successive decoding. In this paper, we propose a novel NOMA scheme based on user grouping and multiple layer modulation with constellation scaling and rotation. Users are first grouped according to their channel conditions, and transmit power is allocated among the groups. Then multiple layer modulation is employed within each group where each user is identified by a unique rotation parameter. Simulation results in terms of bit error rate (BER) are provided. Compared to the conventional NOMA scheme, the proposed scheme can afford a larger number of users under the same power constraints, and it has clear advantages in both system throughput and BER performance.

Iterative Detection and Decoding for Spatially Coupled Multiuser Data Transmission

Non-orthogonal multiple access (NOMA) is a candidate multiple access scheme for 5G wireless communication. In this paper we consider applying the principle of spatially coupling in a NOMA system, where coupled data streams from different users transmit via the additive white Gaussian noise (AWGN) multiple access channel. A data stream is constructed by replicating each encoded bit for several times and then permuting the replicated bits. We propose an message-passing algorithm (MPA) based iterative multiuser detection combining with channel decoding, which is illustrated and analysed by the factor graph representation and the extrinsic information transfer (EXIT) chart, respectively. Numerical results show that lower node degree of the spatially coupled structure, higher number of iterations of the detector, and an appropriate encoding/decoding scheme jointly contribute to a better performance of the multiuser transmission system.

Rate-Splitting Non-orthogonal Multiple Access: Practical Design and Performance Optimization

In this paper we propose a novel rate-splitting non-orthogonal multiple access (RS-NOMA) scheme implemented in power-domain for practical scenarios in 5G mobile communication systems. The proposed scheme is exploited to solve the mismatch between Quality-of-Service (QoS) and channel conditions of users via rate-splitting instead of power reallocation as the conventional NOMA does. The RS-NOMA scheme contributes to both system spectrum efficiency and user flexibility on resource allocation. We investigate the signal-to-noise ratio (SNR) region division for the proposed scheme and then a splitting factor optimizing algorithm to enable the system achieving the maximum throughput. Simulation results show that the RS-NOMA scheme significantly improves the user flexibility with almost no loss to the system spectrum efficiency.

An Efficient Method to Construct Parity-Check Matrices for Recursively Encoding Spatially Coupled LDPC Codes †

Spatially coupled low-density parity-check (LDPC) codes have attracted considerable attention due to their promising performance. Recursive encoding of the codes with low delay and low complexity has been proposed in the literature but with constraints or restrictions. In this manuscript we propose an efficient method to construct parity-check matrices for recursively encoding spatially coupled LDPC codes with arbitrarily chosen node degrees. A general principle is proposed, which provides feasible and practical guidance for the construction of parity-check matrices. According to the specific structure of the matrix, each parity bit at a coupling position is jointly determined by the information bits at the current position and the encoded bits at former positions. Performance analysis in terms of design rate and density evolution has been presented. It can be observed that, in addition to the feature of recursive encoding, selected code structures constructed by the newly proposed method may lead to better belief-propagation thresholds than the conventional structures. Finite-length simulation results are provided as well, which verify the theoretical analysis.

An efficient structure for terminating spatially coupled LDPC codes

A recursive encoding of spatially coupled LDPC codes with low complexity has been proposed in recent works. But it has to solve a system of linear equations to terminate the codes. In this paper we modify the protograph of spatially coupled LDPC codes by adding extra variable nodes, called repeat-accumulate(RA) tail, which can simplify the termination without time delays. Performance analysis with density evolution and simulation results over binary erasure channel(BEC) channels are also provided. Simulation results show that, for spatially coupled LDPC codes with RA tail, the complexity of termination can be reduced with a small trade-off in performance.