Non-orthogonal multiple access (NOMA) has been considered as a study-item in 3GPP for 5G new radio (NR). However, it was decided not to continue with it as a work-item, and to leave it for possible use in beyond 5G. In this paper, we first review the discussions that ended in such decision. Particularly, we present simulation comparisons between the Welch-bound equality spread multiple access (WSMA)-based NOMA and multiuser multiple-input-multiple-output (MU-MIMO), where the possible gain of WSMA-based NOMA, compared to MU-MIMO, is negligible. Then, we summarize the 3GPP discussions on NOMA, and propose a number of methods to reduce the implementation complexity and delay of both uplink (UL) and downlink (DL) NOMA-based transmission, as different ways to improve its efficiency. Here, particular attention is paid to reducing the receiver complexity, the cost of hybrid automatic repeat request as well as the user pairing complexity. As demonstrated, different smart techniques can be applied to improve the energy efficiency and the end-to-end transmission delay of NOMA-based systems.

On the other hand, in many applications, such as vehicle-to-vehicle and vehicle-to-infrastructure communications for traffic efficiency/safety or real-time video processing for augmented reality, the codewords are required to be short (in the order of ∼ 100 channel uses) [5], [6]. Thus, it is interesting to investigate the performance of HARQ protocols in the presence of finite-length codewords [7], [8].In this letter, we study the data transmission efficiency of HARQ protocols utilizing codewords of finite length. The problem is cast as the maximization of the power-limited throughput in the presence of incremental redundancy (INR) HARQ feedback. The contributions of the paper are two-fold. 1) We use the recent results on the achievable rates of finite block-length codes [9]-[11] to analyze the throughput. With codewords of finite length, we derive closed-form expressions for the outage probabilities of the INR HARQ in different retransmission rounds and evaluate the effect of variable-length coding on the throughput. 2) We investigate the effect of feedback delay on the throughput. Particularly, we present sufficient conditions for the usefulness of HARQ protocols such that the use of HARQ increases the throughput compared to the open-loop communication setups. For a large range of HARQ feedback delays, the results show that the implementation of finite-length INR HARQ leads to throughput improvements.

Recently, substantial attention has been paid to increase the achievable rates of wireless networks using different kinds of limited channel quality information feedback. Hybrid automatic repeat request (ARQ) and quantized channel state information (CSI) feedback are two well-known approaches applied by experts to provide the limited channel quality information at the transmitter. Considering quasi-static fading channels, this paper aims to provide some comparisons between the performance of these methods from different points of view. The paper first investigates the power-and outage-limited performance of hybrid ARQ and quantized CSI schemes under short-and long-term power constraints. Then, 1) the feedback signaling load, 2) robustness and 3) the complexity of these schemes are compared under short-term transmission power constraint. Both INcremental Redundancy (INR) and Repetition Time Diversity (RTD) approaches are considered for hybrid ARQ feedback. Finally, approximate low-complexity solutions are presented for power allocation in the INR ARQ-based scheme under long-term transmission power constraint.Analytical and numerical results demonstrate the equivalency or the superiority of these approaches in different circumstances.

Recently, efficient use of energy has become an essential research topic for green communication. This paper studies the effect of optimal power controllers on the performance of delay-sensitive communication setups utilizing hybrid automatic repeat request (HARQ). The results are obtained for repetition time diversity (RTD) and incremental redundancy (INR) HARQ protocols. In all cases, the optimal power allocation, minimizing the outage-limited average transmission power, is obtained under both continuous and bursting communication models. Also, we investigate the system throughput in different conditions. The results indicate that the power efficiency is increased substantially, if adaptive power allocation is utilized. For instance, assume Rayleigh-fading channel, a maximum of two (re)transmission rounds with rates {1, 1 2 } nats-per-channeluse and an outage probability constraint 10 −3 . Then, compared to uniform power allocation, optimal power allocation in RTD reduces the average power by 9 and 11 dB in the bursting and continuous communication models, respectively. In INR, these values are obtained to be 8 and 9 dB, respectively.

This paper investigates the performance of multipleinput-multiple-output (MIMO) systems in the presence of automatic repeat request (ARQ) feedback. We show that, for a large range of performance metrics, the data transmission efficiency of the ARQ schemes is determined by a set of parameters which are scheme-dependent and not metric-dependent. Then, the results are used to study different aspects of MIMO-ARQ such as the effect of nonlinear power amplifiers, large-scale MIMO-ARQ, adaptive power allocation and different data communication models. The results, which are valid for various forward and feedback channel models, show the efficiency of the MIMO-ARQ techniques in different conditions.

CitationMakki This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Abstract-We study the performance of antenna selectionbased multiple-input-multiple-output (MIMO) networks with large but finite number of transmit antennas and receivers. Considering the continuous and bursty communication scenarios with different users' data request probabilities, we develop an efficient antenna selection scheme using genetic algorithms (GA). As demonstrated, the proposed algorithm is generic in the sense that it can be used in the cases with different objective functions, precoding methods, levels of available channel state information and channel models. Our results show that the proposed GAbased algorithm reaches (almost) the same throughput as the exhaustive search-based optimal approach, with substantially less implementation complexity.

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This paper studies the performance of delayconstrained hybrid automatic repeat request (HARQ) protocols. Particularly, we propose a fast HARQ protocol where, to increase the end-to-end throughput, some HARQ feedback signals and successive message decodings are omitted. Considering quasistatic channels and a bursty communication model, we derive closed-form expressions for the message decoding probabilities as well as the throughput, the expected delay and the error probability of the HARQ setups. The analysis is based on recent results on the achievable rates of finite-length codes and shows the effect of the codeword length on the system performance. Moreover, we evaluate the effect of various parameters such as imperfect channel estimation and hardware on the system performance. As demonstrated, the proposed fast HARQ protocol reduces the packet transmission delay considerably, compared to state-of-the-art HARQ schemes. For example, with typical message decoding delay profiles and a maximum of 2, . . . , 5 transmission rounds, the proposed fast HARQ protocol can improve the expected delay, compared to standard HARQ, by 27, 42, 52 and 60%, respectively, independently of the code rate/fading model.