Lei Wen

Bio: Lei Wen is an academic researcher from National University of Defense Technology. The author has contributed to research in topics: Low-density parity-check code & Codebook. The author has an hindex of 7, co-authored 18 publications receiving 146 citations. Previous affiliations of Lei Wen include University of Surrey & University of Defence.

Design of SCMA Codebooks Based on Golden Angle Modulation

Abstract: In this paper, we propose to use golden angle modulation (GAM) points to construct codebooks for uplink and downlink sparse code multiple access (SCMA) systems. We provide two categories of codebooks with one and two optimization parameters, respectively. The advantages of the proposed design method are twofold: $1)$ the number of optimization variables is independent of codebook and system parameters; and $2)$ it is simple to implement. In the downlink, we use GAM points to build a multidimensional mother constellation for SCMA codebooks, while in the uplink GAM points are directly mapped to user codebooks. The proposed codebooks exhibit good performance with low peak to average power ratio compared to the codebooks proposed in the literature based on constellation rotation and interleaving.

Uplink Spatial Modulation SCMA System

Abstract: Multiple-input multiple-output (MIMO) and sparse code multiple access (SCMA) are two effective techniques supporting high spectral efficiency and massive connectivity, respectively. Although the combination of these two techniques, namely MIMO-SCMA, can further improve the spectral efficiency, it deteriorates the error performance and greatly increases the computational costs. In this letter, in order to enhance the error performance and reduce the receiver complexity of the uplink MIMO-SCMA system, we propose a new scheme named spatial modulation sparse code multiple access (SM-SCMA). At the receiver, a low complexity joint message passing algorithm based on the principle of the maximum a posterior probability and the simplified factor graph is proposed. Simulation results show that the SM-SCMA can achieve a better bit-error-rate performance while significantly reducing the receiver complexity compared with the conventional MIMO-SCMA.

Design of Joint Sparse Graph for OFDM System

Abstract: Low density signature orthogonal frequency division multiplexing (LDS-OFDM) and low density parity-check (LDPC) codes are multiple access and forward error correction (FEC) techniques, respectively. Both of them can be expressed by a bipartite graph. In this paper, we construct a joint sparse graph combining the single graphs of LDS-OFDM and LDPC codes, namely joint sparse graph for OFDM (JSG-OFDM). Based on the graph model, a low complexity approach for joint multiuser detection and FEC decoding (JMUDD) is presented. The iterative structure of JSG-OFDM receiver is illustrated and its extrinsic information transfer (EXIT) chart is researched. Furthermore, design guidelines for the joint sparse graph are derived through the EXIT chart analysis. By offline optimization of the joint sparse graph, numerical results show that the JSG-OFDM brings about 1.5–1.8 dB performance improvement at bit error rate (BER) of $10^{-5} $ over similar well-known systems such as group-orthogonal multi-carrier code division multiple access (GO-MC-CDMA), LDS-OFDM, and turbo structured LDS-OFDM.

Multi-Dimensional Space-Time Block Coding Aided Downlink MIMO-SCMA

Abstract: Multiple-input multiple-output (MIMO) and sparse code multiple access (SCMA) are two key techniques supporting high spectral efficiency and massive connectivity. In this paper, we consider the downlink MIMO communication scenario where SCMA is used for multiuser access. We aim to improve the performance of the traditional single-input single-output SCMA (SISO-SCMA) by exploiting the transmit diversity of multiple antennas at the transmitter. Specifically, we propose a multi-dimensional space-time block code (MSTBC) aided downlink MIMO-SCMA and derive the design criteria of MSTBC that can achieve full diversity while keep the symbol rate same as the traditional SISO-SCMA system. In particular, the MSTBC-2 for two transmit antennas and the MSTBC-4 for four transmit antennas are designed by extending the Alamouti code and quasi-orthogonal STBC into multi-dimension. Due to the orthogonality/quasi-orthogonality of the two MSTBCs, a linear decoder is available at the receiver, which keeps the complexity of the proposed MSTBC aided MIMO-SCMA almost the same as the conventional SISO-SCMA. Furthermore, the performance of the proposed schemes is evaluated based on both Monte Carlo simulations and analytical results. Simulation results show that the proposed MSTBC aided MIMO-SCMA can significantly outperform the conventional SISO-SCMA, STBC-MIMO-OFDM, and multiplexing MIMO-SCMA schemes.

Joint Iterative Detection and Decoding Receiver for Polar Coded SCMA System

Abstract: SCMA and polar coding are possible candidates for 5G systems. In this paper, we develop a joint iterative detection and decoding (JIDD) receiver for the uplink polar coded sparse code multiple access (PC-SCMA) system. In JIDD receiver, messages are exchanged between SCMA detector and polar decoder during each iteration. This is the first joint design for PC-SCMA which has no inner iterations of SCMA detector and polar decoder. But only outer iterations are needed. Simulation results demonstrate that JIDD has lower complexity and better performance than the separate scheme. It obtains 4.8dB performance gain with code length and code rate of polar code are $N=256$ and $R=1/2$ and 3.4dB performance gain with $N=1024$ and $R=1/3$, respectively. Especially, under 150$\text{\%}$ system loading, JIDD only has 0.3dB performance loss compared with the single user uplink PC-SCMA system. The JIDD receiver only needs 5 iterations to converge, which is much less than the other joint receiver for LDPC coded or Turbo coded SCMA systems (need dozens of iterations to converge). Thus, the whole complexity of JIDD for PC-SCMA is much lower than the other two joint receivers.

Non-Orthogonal Multiple Access (NOMA) for cellular future radio access

Abstract: Radio access technologies for cellular mobile communications are typically characterized by multiple access schemes, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and OFDMA. In the 4th generation (4G) mobile communication systems such as Long-Term Evolution (LTE) (Au et al., Uplink contention based SCMA for 5G radio access. Globecom Workshops (GC Wkshps), 2014. doi:10.1109/GLOCOMW.2014.7063547) and LTE-Advanced (Baracca et al., IEEE Trans. Commun., 2011. doi:10.1109/TCOMM.2011.121410.090252; Barry et al., Digital Communication, Kluwer, Dordrecht, 2004), standardized by the 3rd Generation Partnership Project (3GPP), orthogonal multiple access based on OFDMA or single carrier (SC)-FDMA is adopted. Orthogonal multiple access was a reasonable choice for achieving good system-level throughput performance with simple single-user detection. However, considering the trend in 5G, achieving significant gains in capacity and system throughput performance is a high priority requirement in view of the recent exponential increase in the volume of mobile traffic. In addition the proposed system should be able to support enhanced delay-sensitive high-volume services such as video streaming and cloud computing. Another high-level target of 5G is reduced cost, higher energy efficiency and robustness against emergencies.

Block Sparse Bayesian Learning Based Joint User Activity Detection and Channel Estimation for Grant-Free NOMA Systems

Abstract: This paper concerns uplink grant-free nonorthogonal multiple access, where the handshaking procedure is not required to reduce control signaling overhead and transmission latency. In especially the dynamic scenarios, e.g., Internet of vehicles, the active users have to be identified and their channel state information needs to be estimated before performing multiuser detection. We investigate the joint user activity detection (UAD) and channel estimation (CE), which provides necessary information for data detection. In this paper, the joint UAD and CE is formulated as a block sparse signal recovery problem. First, the block orthogonal matching pursuit (BOMP) algorithm is studied for this problem, but its complexity grows with the fourth power of active user number, which hinders its application. Then, block sparse Bayesian learning (BSBL) is investigated to solve this problem, and in particular a low complexity message passing based implementation of BSBL with belief propagation and mean field is developed. The proposed message passing based BSBL (MP-BSBL) algorithm has a complexity independent of active user number, which can be significantly lower than that of the BOMP algorithm. In addition, MP-BSBL provides an estimate of the noise power, which can be readily used for data detection. Simulation results show that the MP-BSBL algorithm delivers almost the same performance as BOMP with the exact knowledge of active user number and can reach the performance bound for channel estimation.

Multidimensional Constellations for Uplink SCMA Systems—A Comparative Study

Abstract: Sparse code multiple access (SCMA) is a class of non-orthogonal multiple access (NOMA) that is proposed to support uplink machine-type communication services. In an SCMA system, designing multidimensional constellation plays an important role in the performance of the system. Since the behavior of multidimensional constellations highly depends on the type of the channel, it is crucial to employ a constellation that is suitable for a certain application. In this paper, we first highlight and review the key performance indicators (KPIs) of multidimensional constellations that should be considered in their design process for various channel scenarios. We then provide a survey on the known multidimensional constellations in the context of SCMA systems with their design criteria. The performance of some of those constellations are evaluated for uncoded, high-rate, and low-rate LTE turbo-coded SCMA systems under different channel conditions through extensive simulations. All turbo-coded comparisons are performed for bit-interleaved coded modulation, with a concatenated detection and decoding scheme. Simulation results confirm that multidimensional constellations that satisfy KPIs of a certain channel scenario outperform others. Moreover, the bit error rate performance of uncoded systems, and the performance of the coded systems are coupled to their bit-labeling. The performance of the systems also remarkably depends on the behavior of the multiuser detector at different signal-to-noise ratio regions.

Uplink Nonorthogonal Multiple Access Technologies Toward 5G: A Survey

Abstract: Owing to the superior performance in spectral efficiency, connectivity, and flexibility, nonorthogonal multiple access (NOMA) is recognized as the promising access protocol and is now undergoing the standardization process in 5G. Specifically, dozens of NOMA schemes have been proposed and discussed as the candidate multiple access technologies for the future radio access networks. This paper aims to make a comprehensive overview about the promising NOMA schemes. First of all, we analyze the state-of-the-art NOMA schemes by comparing the operations applied at the transmitter. Typical multiuser detection algorithms corresponding to these NOMA schemes are then introduced. Next, we focus on grant-free NOMA, which incorporates the NOMA techniques with uplink uncoordinated access and is expected to address the massive connectivity requirement of 5G. We present the motivation of applying grant-free NOMA, as well as the typical grant-free NOMA schemes and the detection techniques. In addition, this paper discusses the implementation issues of NOMA for practical deployment. Finally, we envision the future research challenges deduced from the recently proposed NOMA technologies.

On the Design of Near-Optimal Sparse Code Multiple Access Codebooks

Abstract: Sparse code multiple access (SCMA) is one of the competitive non-orthogonal multiple access (NOMA) techniques for the next generation of communication systems. In this paper, a systematic construction procedure for the design of SCMA codebooks under various channel environments is proposed in order to achieve near-optimal designs, especially for cases which consider small-scale SCMA parameters. The procedure includes the rotation angle design, the construction of multi-dimensional codebooks, and the design of labelling rules. The detailed design criteria and the related properties are derived and investigated. A low-complexity construction algorithm is further proposed for the cases of fading channels. Numerical results show that the designed SCMA codebooks provide improvements in error performance for all of the considered channel conditions, when compared to the codebooks presented in the prior literature.