Dageng Chen

On Receiver Design for Uplink Low Density Signature OFDM (LDS-OFDM)

Low density signature orthogonal frequency division multiplexing (LDS-OFDM) is an uplink multi-carrier multiple access scheme that uses low density signatures (LDS) for spreading the symbols in the frequency domain. In this paper, we introduce an effective receiver for the LDS-OFDM scheme. We propose a framework to analyze and design this iterative receiver using extrinsic information transfer (EXIT) charts. Furthermore, a turbo multi-user detector/decoder (MUDD) is proposed for the LDS-OFDM receiver. We show how the turbo MUDD is tuned using EXIT charts analysis. By tuning the turbo-style processing, the turbo MUDD can approach the performance of optimum MUDD with a smaller number of inner iterations. Using the suggested design guidelines in this paper, we show that the proposed structure brings about 2.3 dB performance improvement at a bit error rate (BER) equal to 10-5 over conventional LDS-OFDM while keeping the complexity affordable. Simulations for different scenarios also show that the LDS-OFDM outperforms similar well-known multiple access techniques such as multi-carrier code division multiple access (MC-CDMA) and group-orthogonal MC-CDMA.

Multi-Dimensional SCMA Codebook Design Based on Constellation Rotation and Interleaving

Sparse code multiple access (SCMA) is a new non- orthogonal multiple access scheme, which effectively exploits the shaping gain of multi-dimensional codebook. In this paper, a multi-dimensional SCMA (MD-SCMA) codebook design based on constellation rotation and interleaving method is proposed for downlink SCMA systems. In particular, the first dimension of mother constellation is constructed by subset of lattice Z2. Then the other dimensions are obtained by rotating the first dimension. Further, the interleaving is used for even dimensions to improve the performance in fading channels. In this way, we can design different codebooks for the aim of spectral efficiency or power efficiency. And the simulation results show that the bit error rate (BER) performance of MD-SCMA codebooks outperforms that of the existing SCMA codebooks and low density signature (LDS) in downlink Rayleigh fading channels.

Performance evaluation of Low Density Spreading Multiple Access

In this paper, we evaluate the performance of Multicarrier-Low Density Spreading Multiple Access (MC-LDSMA) as a multiple access technique for mobile communication systems. The MC-LDSMA technique is compared with current multiple access techniques, OFDMA and SC-FDMA. The performance is evaluated in terms of cubic metric, block error rate, spectral efficiency and fairness. The aim is to investigate the expected gains of using MC-LDSMA in the uplink for next generation cellular systems. The simulation results of the link and system-level performance evaluation show that MC-LDSMA has significant performance improvements over SC-FDMA and OFDMA. It is shown that using MC-LDSMA can considerably reduce the required transmission power and increase the spectral efficiency and fairness among the users.

Spherical Codes for SCMA Codebook

In this paper, we investigate the application of spherical codes in a sparse code multiple access (SCMA) system. We use spherical codes to build multidimensional mother constellations for SCMA codebooks, to lower peak to average power ratio (PAPR) as well as to improve the overall spectrum efficiency. Furthermore, we introduce four approaches to construct good spherical codes. These codes are easy and have large coding gains. We make extensions to some of these spherical codes such that they are suitable for usage in SCMA systems. The performance of the constructed codebooks are compared to known codebooks through simulations, and the numerical results show that spherical codes based codebooks can effectively improve the system performance.

Subcarrier and Power Allocation for LDS-OFDM System

Low Density Signature-Orthogonal Frequency Division Multiplexing (LDS-OFDM) has been introduced recently as an efficient multiple access technique. In this paper, we focus on the subcarrier and power allocation scheme for uplink LDS-OFDM system. Since the resource allocation problem is not convex due to the discrete nature of subcarrier allocation, the complexity of finding the optimal solutions is extremely high. We propose a heuristic subcarrier and power allocation algorithm to maximize the weighted sum-rate. The simulation results show that the proposed algorithm can significantly increase the spectral efficiency of the system. Furthermore, it is shown that LDS-OFDM system can achieve an outage probability much less than that for OFDMA system.

A New Constellation Shaping Method and Its Performance Evaluation in BICM-ID

Using equiprobable M-PAM/QAM constellation instead of Gaussian-distributed signal suffers performance loss, and the ultimate performance loss in high SNR regime is 1.53dB. In this paper, we propose a new shaping method to optimize equiprobable constellations. The obtained PSK-PAM constellations can achieve performance gain in sense of mutual information by up to 0.3dB and 0.57dB over 16QAM and 64QAM, respectively. The new constellations are then applied to bit-interleaved coded modulation with iterative decoding (BICM-ID) and the performance gain over M-QAM is verified.

Joint Iterative Interference Alignment and SCMA Technique for MIMO-OFDM Systems

Interference alignment (IA) is a well-known transmission strategy for interference channels, with which the sum rate can linearly scale with the number of users at high signal-to-noise ratio (SNR). However, most existing works on IA considered only the maximization of the sum rate without taking into account the bit error rates (BER) performance. In this work, we firstly propose a novel IA algorithm to improve the BER performance for MIMO-OFDM interference channels on a per-subcarrier basis. The proposed IA algorithm is then combined with the sparse code multiple access (SCMA) technique to further enhance the BER performance by exploiting the inherent frequency diversity in the MIMO-OFDM systems. At the receiver, a linear filter is first used to mitigate the inter-user aligned interference and separate the spatial streams, and then an iterative frequency-domain message passing algorithm (MPA) is applied to efficiently separate the overlapped multiple codewords. Simulation results demonstrate that the proposed joint IA and SCMA strategy can provide significant BER gain compared to the existing IA algorithms.

A Mapping Scheme of Users to SCMA Layers for D2D Communications

Sparse code multiple access (SCMA) is a non- orthogonal multiple access scheme, which has been considered as one of the key 5G technologies to improve spectral efficiency. Moreover, device-to- device (D2D) communication is introduced as a vital technology component for 5G cellular communication system to improve spectral reuse. D2D communication combined with SCMA will be a significant part of 5G technology to support higher spectral efficiency. However, these benefits depend on efficiently allocating SCMA layers for cellular users and D2D pairs to share corresponding spectrum resources. In this paper, a mapping scheme of users to SCMA layers for D2D communication is proposed to maximize the system sum rate, in which after assigning SCMA layers to cellular users, a coalition game based scheme is proposed to obtain the solution for mapping of D2D pairs to SCMA layers. Simulation results validate the effectiveness of the proposed scheme and show that the proposed scheme outperforms other schemes, and the D2D communication combined with SCMA can obtain significant gain over the one in LTE networks in terms of system sum rate.

An Optimized Design of Irregular SCMA Codebook Based on Rotated Angles and EXIT Chart

In this paper, an optimized codebook design based on rotated angles and extrinsic information transfer (EXIT) chart for a non-orthogonal multiple access scheme, called irregular sparse code multiple access (IrSCMA), is presented. Unlike regular SCMA, in IrSCMA, the defined degree of layers resource nodes is different, which is beneficial to system performance.It is demonstrated that the new codebook can greatly improve the BER performance especially when the signal-to-noise ratio(SNR) is high, without sacrificing the low detection complexity, compared with the existing codebooks and LDS.

Simultaneous Blind Estimation of Antenna Mutual Coupling and Direction of Arrival

In this paper, we propose a blind estimation method for the antenna mutual coupling matrix and the direction of arrival. First of all, the higher order cumulants based blind estimation method is used to calculate the steering vector of the array. The estimated steering vector can differ from the true steering vector by a factor of -1 or by exchanging of the real and imaginary parts. These differences as well as the direction of arrival and the mutual coupling matrix are estimated afterwards using the least square error criteria. Simulations show the proposed algorithm can converge to the exact solution very quickly and is suitable for the closely aligned antenna arrays for the massive multiple input and multiple output (MIMO) systems.