Multiuser Transmission Scheme Research Articles

Millimeter‐wave non‐orthogonal multiple access systems with sparse antenna arrays

AbstractMillimeter wave (mmWave) non‐orthogonal multiple access (NOMA) is a multiuser transmission scheme that can increase the quality and data rate of wireless mobile networks. In this article, we introduce an mmWave‐NOMA system utilizing a sparse antenna array that not only offers small and lightweight antenna devices in the base station but also improves the outage probability performance of the system. We also introduce a low‐complex computation technique to find the optimum beamforming vector to reduce the outage probability. We provide a proof of convexity for the outage probability of the mobiles' communication links in the proposed system for which a gradient‐based algorithm can be used to find the optimum solution. The simulation results show that utilizing the proposed optimum beamforming vector reduces the outage probability of the system up to 98% in high signal‐to‐noise ratios. Our simulation and analysis results demonstrate that the system equipped with a sparse antenna array applying the optimum beamforming vector outperform conventional mmWave‐NOMA system with uniform linear array antennas in terms of outage probability and sum rates.

Sparse Vector Coding Aided Ultra-Reliable and Low-Latency Communications in Multi-User Massive MIMO Systems

Ultra-reliable and low-latency communication (URLLC) has been recognized as a key service to support delay-sensitive applications for next generation wireless systems. A critical challenge is how to support multiple users with ultra reliability in the short packet transmission. In this paper, we propose a sparse vector coding (SVC)-aided URLLC multi-user transmission scheme by employing the massive multiple-input multiple-output (MIMO) technique. To reliably acquire the support information, we formulate the SVC decoding problem in multi-user massive MIMO systems as the problem to find out nonzero positions of a sparse vector. Then, the support recovery problem is transformed into a series of sub-problems by decoupling the support identification of each user, where the successive interference cancellation based matching pursuit (SIC-MP) algorithm is proposed to estimate the support sequentially. In addition, the user sorting scheme is proposed to alleviate the inter-user interference in the SVC decoding. The complexity of the proposed SIC-MP algorithm is only linear with the system parameters.

UAV Secure Downlink NOMA Transmissions: A Secure Users Oriented Perspective

This paper proposes a secure downlink multi-user transmission scheme enabled by a flexible unmanned aerial vehicle base station (UAV-BS) and non-orthogonal multiple access (NOMA). According to their heterogeneous service requirements, multiple legitimate users are categorized as security-required users (SUs) and quality of service (QoS)-required users (QUs), while these QUs can potentially act as internal eavesdroppers which are curious about the secrecy transmissions of SUs. In such a context, our goal is to maximize the achievable minimum secrecy rate among SUs through the joint optimization of user scheduling, power allocation, and trajectory design, subject to the QoS requirements of QUs and the mobility constraint of UAV-BS. Due to the non-convexity of the problem, an efficient iterative algorithm is firstly proposed, based on the alternative optimization (AO) and successive convex approximation (SCA) methods and along with a penalty-based algorithm to deal with the introduced binary integer variables, to obtain a sub-optimal solution. Then, we propose an SUs-oriented low-complexity algorithm by taking advantage of the inherent characteristics of the optimization problem, which can efficiently reduce the computational complexity and can act as a reasonable initial solution for the previous iterative algorithm to achieve better performance. Finally, the superiority of our proposed scheme compared with the conventional orthogonal multiple access (OMA) one is validated by numerical simulation results.

Secure Alamouti Multiple Access Channel Transmissions: Multiuser Transmission and Multi-Antenna Eavesdroppers

Using the Alamouti space-time block code in conjunction with artificial noise (AN) is an effective approach to secure multiple access (MAC) transmission in the presence of eavesdroppers. Existing studies on this technique focus on the two-user transmission systems and single-antenna eavesdroppers. This letter proposes a novel approach that generalizes this technique to multiuser transmission schemes and multi-antenna eavesdroppers. Security gains are analyzed, and the minimum number of MAC users that should participate in generating the AN in order to secure their transmissions is characterized analytically.

Single Radio-Over-Fiber Link and RF Chain-Based 60 GHz Multi-Beam Transmission

An efficient multi-user transmission scheme at 60 GHz using a single-feed leaky wave antenna (LWA) and hence requiring only a single radio-over-fiber link and single RF chain is presented. A subcarrier multiplexed signal carrying different users’ data is transported over 2.2 km of optical fiber and then upconverted to the 60 GHz band for transmission to multiple spatially separated users through the beam steering characteristics of the LWA. An overall sum data rate, the combined rate from all users, of 10.6 Gb/s using 16-QAM modulation serving ten users over a transmission bandwidth of 3.05 GHz or 20 users with QPSK over 6.1 GHz span, is achieved experimentally. The theoretical sum data rates for 6.1 GHz bandwidth for different numbers of users are calculated, considering the SNR degradation due to the angularly dispersed LWA beam, showing that data rates over 30 Gb/s can be obtained. Finally, a system design that improves coverage and spectrum efficiency through operating multiple LWAs with a single RF chain is demonstrated.

Using OFDMA for MU-MIMO User Selection in 802.11ax-Based Wi-Fi Networks

Two innovative multi-user (MU) communication mechanisms, multi-user multi-input multi-output (MU-MIMO) and orthogonal frequency division multiple access (OFDMA), have recently attracted much attention as key technologies to boost the capabilities of 802.11 Wi-Fi systems. The method of MU transmission in Wi-Fi systems still suffers from severe problems with channel state information (CSI) feedback overhead, however, and this precludes obtaining much higher MU-MIMO gains through user selection. Although in 802.11ax-based Wi-Fi systems the methods used for uplink OFDMA transmission and downlink MU-MIMO transmission bear a great deal of resemblance to each other, in the sense that both are initiated by the AP, this similarity in protocol is not appropriately utilized for close collaboration between the two technologies. This paper proposes MUSE, a novel multi-user transmission scheme for 802.11ax networks. By exploiting uplink OFDMA transmissions for MU-MIMO user selection, MUSE takes advantage of both OFDMA and MU-MIMO functionalities. In particular, the AP estimates and gathers multiple CSI values from the uplink OFDMA frame, and this channel information is then used to find the optimal downlink MU-MIMO receiver group to maximize the system utility. To realize MUSE, a new OFDMA resource allocation algorithm and frame structure are developed. Extensive MATLAB simulation results show that MUSE significantly improves the network throughput, even in dense network scenarios, and also works effectively in co-existence with legacy nodes.

Multiuser 3D massive MIMO transmission in full-duplex cellular system

In this paper, we provide an effective multiuser transmission scheme in three-dimensional (3D) massive multiple-input multiple-output (MIMO) cellular systems, where the full-duplex (FD) base station (BS) equips two separate large-scale uniform planar antenna array (UPA). In order to reduce the computational and implementation complexity, we investigate the characteristic of the beam-domain (BD) 3D massive MIMO channels and self-interference (SI) channel. We propose the 3D multiuser beamspace transmission (MUBT) scheme that requires the spatial angular information of the users and the SI channel. We show that, due to the reduced dimension property of the effective beamspace channel, the overhead for channel estimation is reduced. Furthermore, a user scheduling algorithm is proposed to enable the 3D MUBT scheme in the FD systems. Finally, both the theoretical analysis and simulation results show the the SI can be effectively reduced and demonstrate the effectiveness and superiority of the proposed 3D MUBT scheme on spectral efficiency compared with the conventional half-duplex (HD) and FD transmission schemes.

Investigation of Transmission Schemes for Millimeter-Wave Massive MU-MIMO Systems

A massive multiuser multiple-input–multiple-output (MU-MIMO) system working at millimeter-wave frequencies is a new concept that has appeared in recent years. In such systems, the sparsity of the beamspace channel matrix makes the channel rank reduction and limited feedback realizable. Based on these preferable features, this paper introduces two kinds of beamspace transformation and exploits their corresponding beamspace characters. Then, the spatial division multiple access (SDMA) and the interference suppression precoding are investigated separately. Considering the advantages and disadvantages of these two traditional schemes, the authors focus on the design of the limited-feedback-based multiuser transmission schemes for millimeter-wave massive MIMO systems. An enhanced SDMA scheme is first proposed in this paper, which utilizes the beamspace sparsity and overcomes the challenges of channel information acquisition and interference control from a general perspective. Based on these, we analyze the interuser interference condition and further propose a joint transmission scheme, which incorporates a double-precoding-based SDMA and an interference suppression precoding. The performances of the proposed schemes are also evaluated via simulation, which show that high system sum-rate capacity could be achieved with only low computational complexity and a small amount of feedback.

Coordinated multi-user spectrum sharing in distributed antenna-based cognitive radio systems

Spectrum sharing for efficient reuse of licensed spectrum is an important concept for cognitive radio technologies. In a spectrum-sharing system (SSS), deploying the antennas in a distributed manner can offer a new spatial dimension for the efficient reuse of licensed frequency bands. To improve the whole performance of multiple secondary users (SUs), this paper addresses the problem of coordinated multi-SU spectrum sharing in a distributed antenna-based SSS. By adopting the Hungarian method, the primal decomposition method and pricing policy, we propose a coordinated multi-user transmission scheme, so as to maximize the sum-rate of SUs. Simulation results show that the proposed method can significantly enhance the system performance, and the computational complexity is low.

A Spectrum Efficient Multi-User Transmission Scheme for 5G Systems With Low Complexity

To maximize the system throughput for downlink multi-user transmission considering fairness among the users, superposition coding (SC) with all the users scheduled simultaneously is optimal. But it would also lead to high implementation complexity at receivers. In this paper, a simplified downlink multi-user transmission scheme is proposed via combining SC and time division multiplexing (TDM). All the users are divided into multiple user groups and only the users within each user group are scheduled simultaneously via SC. Closed-form expression for the system throughput by the proposed scheme is derived and numerical results are provided to demonstrate the difference in the system throughputs by the proposed scheme and the optimal SC as well as TDM. With the proposed scheme, a good tradeoff between performance and complexity can be achieved. It is shown that the proposed scheme can reduce the implementation complexity considerably at the cost of a little system throughput loss.

Uni-MUMAC: a unified down/up-link MU-MIMO MAC protocol for IEEE 802.11ac WLANs

Due to the dominance of the downlink traffic in Wireless Local Area Networks (WLANs), a large number of previous research efforts have been put to enhance the transmission from the Access Point (AP) to stations (STAs). The downlink Multi-User Multiple-Input Multiple-Output (MU-MIMO) technique, supported by the latest IEEE amendment-802.11ac, is considered as one of the key enhancements leading WLANs to the Gigabit era. However, as cloud uploading services, Peer-to-Peer (P2P) and telepresence applications get popular, the need for a higher uplink capacity becomes inevitable. In this paper, a unified down/up-link Medium Access Control (MAC) protocol called Uni-MUMAC is proposed to enhance the performance of IEEE 802.11ac WLANs by exploring the multi-user spatial multiplexing technique. Specifically, in the downlink, we implement an IEEE 802.11ac-compliant MU-MIMO transmission scheme to allow the AP to simultaneously send frames to a group of STAs. In the uplink, we extend the traditional one round channel access contention to two rounds, which coordinate multiple STAs to transmit frames to the AP simultaneously. 2-nd round Contention Window (CW2nd), a parameter that makes the length of the 2-nd contention round elastic according to the traffic condition, is introduced. Uni-MUMAC is evaluated through simulations in saturated and non-saturated conditions when both downlink and uplink traffic are present in the system. We also propose an analytic saturation model to validate the simulation results. By properly setting CW2nd and other parameters, Uni-MUMAC is compared to a prominent multi-user transmission scheme in the literature. The results exhibit that Uni-MUMAC not only performs well in the downlink-dominant scenario, but it is also able to balance both the downlink and uplink throughput in the emerging uplink bandwidth-hungry scenario.

Superposition Coding Strategies: Design and Experimental Evaluation

We design and implement a software-radio system for Superposition Coding (SC), a multiuser transmission scheme that deliberately introduces interference among user signals at the transmitter, using a library of off-the-shelf point-to-point channel codes. We experimentally determine the set of rate-pairs achieved by this transmission scheme under a packet-error constraint. Our results suggest that SC can provide substantial gains in spectral efficiencies over those achieved by orthogonal schemes such as Time Division Multiplexing. Our findings also question the practical utility of the Gaussian approximation for the inter-user interference in Superposition-Coded systems.

Particle Swarm Optimisation Aided MIMO Multiuser Transmission Designs

Bio-inspired computational methods have found wide-ranging applications in signal processing and other walks of engineering. The main attraction of adopting bio-inspired computational intelligence algorithms is that they may facilitate global or near global optimal designs with affordable computational costs. In this contribution, particle swarm optimisation (PSO) is invoked for designing optimal multiuser transmission (MUT) schemes for multiple-input multiple-output communication. Specifically, we consider the minimum bit-error-rate (MBER) linear MUT using PSO and we design a PSO aided MBER generalised vector precoding for nonlinear MUT. These PSO aided MUT techniques compare favourably with the state-of-the-art conventional schemes, in terms of performance and complexity.

CSI Feedback for Dynamic Switching Between Single User and Multiuser MIMO

We consider channel state information (CSI) feedback in 3GPP Long Term Evolution (LTE)-Advanced context. In LTE-Advanced, switching between single user and multiuser transmission schemes is possible without higher layer signaling, which means that the feedback should support both single user and multiuser transmissions. Typically, the CSI feedback consists of a precoding matrix index (PMI) and channel quality indication(s) (CQI). For PMI feedback, we consider different PMI selection schemes and study whether there is a tradeoff between single user and multiuser specific codeword selection metrics. For multiuser CQI, we consider different CQI estimation strategies for two paired users, which is the primary case in LTE-Advanced. The schemes include single user single stream and two stream CQIs and several multiuser specific CQI estimation options. We find that estimating the multiuser CQI as an average over unitary pairs or as the minimum of the signal-to-interference and noise ratios of the unitary pairs offers a stable, well-performing options for different signal-to-noise ratio (SNR) ranges and antenna correlation values.

Opportunistic Network-Coded Cooperative Transmission with Demodulate-and-Forward Protocol in Wireless Channels

In this letter, we consider a multiuser cooperative transmission scheme with network coding over wireless channels. Network coding based on decode-and-forward protocol is a promising technique to improve network throughput. However, decoding errors at the relay cause error propagation, which degrades the performance of network-coded cooperations. To mitigate the error propagation in the network-coded signal, we first express a single equivalent gain, and then the opportunistic relay selection and proportional combining schemes are performed with the equivalent channel and the relay-destination channel. Simulation results show that all cooperative users can obtain a diversity gain of order two by using single network-coded transmission, even when there are detection errors at the relay.

High user capacity collaborative code-division multiple access

In this study, the authors propose a novel collaborative multi-user transmission and detection scheme for the uplink of code-division multiple access (CDMA) that exploits the differences between users' fading channel signatures to increase the user capacity well beyond the spreading length in multiple access interference (MAI) limited environment. The authors show that it is possible to achieve this increase at a low complexity and high bit error rate (BER) performance in flat fading channels, which is a major research challenge for overloaded CDMA systems. In this approach, instead of using one sequence per user as in conventional CDMA, the authors group a small number of users to share the same spreading sequence and enable group spreading and despreading operations. The proposed collaborative multi-user receiver consists of two stages: group multi-user detection (MUD) stage to suppress the MAI between the groups and a low complexity maximum-likelihood detection stage to recover jointly the co-spread users' data using minimum Euclidean distance measure and users' channel gain coefficients. The scheme is investigated analytically and by extensive simulations and comparisons with conventional CDMA, overloaded CDMA using group pseudo-decorrelation (G-PD) and layered space–time (LAST) MUD. It is shown that the total number of full-rate users supported by the scheme is substantially higher than the available sequences. Moreover, it achieves much lower complexity and significantly improved BER for the same user capacity compared with G-PD. It is also shown that, with antenna diversity reception, collaborative CDMA considerably outperforms LAST scheme, particularly when more transmit antennas per group than available receive antennas are employed.

Effects of Channel Estimation on Multiuser Virtual MIMO-OFDMA Relay-Based Networks

A practical multiuser cooperative transmission scheme denoted as Virtual Maximum Ratio Transmission (VMRT) for multipleinput multiple-output-orthogonal frequency division multiple access (MIMO-OFDMA) relay-based networks is proposed and evaluated in the presence of a realistic channel estimation algorithm and using low-density parity-check (LDPC) codes. It is shown that this scheme is robust against channel estimation errors. It offers diversity and array gain, keeping the complexity low with a multiuser and multiantenna channel estimation algorithm that is simple and efficient. In addition, the combination with LDPC codes provides improved gains; diversity gains larger than 6 dB can be easily obtained with a reduced number of relays. Thus, this scheme can be used to extend coverage or increase system throughput by using simple cooperative OFDMA-based relays.

Interference Cancellation at the Relay for Multi-User Wireless Cooperative Networks

We study multi-user transmission and detection schemes for a multi-access relay network (MARN) with linear constraints at all nodes. In a $(J, J_a, R_a, M)$ MARN, $J$ sources, each equipped with $J_a$ antennas, communicate to one $M$-antenna destination through one $R_a$-antenna relay. A new protocol called IC-Relay-TDMA is proposed which takes two phases. During the first phase, symbols of different sources are transmitted concurrently to the relay. At the relay, interference cancellation (IC) techniques, previously proposed for systems with direct transmission, are applied to decouple the information of different sources without decoding. During the second phase, symbols of different sources are forwarded to the destination in a time division multi-access (TDMA) fashion. At the destination, the maximum-likelihood (ML) decoding is performed source-by-source. The protocol of IC-Relay-TDMA requires the number of relay antennas no less than the number of sources, i.e., $R_a\ge J$. Through outage analysis, the achievable diversity gain of the proposed scheme is shown to be $\min\{J_a(R_a-J+1),R_aM\}$. When {\small$M\le J_a\left(1-\frac{J-1}{R_a}\right)$}, the proposed scheme achieves the maximum interference-free (int-free) diversity gain $R_aM$. Since concurrent transmission is allowed during the first phase, compared to full TDMA transmission, the proposed scheme achieves the same diversity, but with a higher symbol rate.

High rate open-loop MIMO multi-user downlink transmission scheme based on blind precoding quasi-orthogonal space time block code

The authors first propose a new class of full-rate full-diversity quasi-orthogonal space time block code (QO-STBC), namely QO-STBC with blind precoding (BP-QO-STBC). Based on this code, a novel open-loop multiple-input multiple-output (MIMO) multi-user downlink transmission scheme is derived. Presuming M blocks of BP-QO-STBC (which could be of different code lengths) are transmitted from the access point simultaneously, thus achieving a multiplexing gain of M, we show that the multiplexed signal can be separated by any user with only mR≥M receive antennas, which is much smaller than that required by conventional linear schemes. The proposed scheme is extendable by simple iterative construction to an arbitrary number of transmit antennas. By exploiting the special structure of BP-QO-STBC, we derived two decoding algorithms with different levels of complexities, namely an optimal maximum likelihood decoder that achieves the lowest decoding complexity known for all full-rate full-diversity QO-STBC and a suboptimal linear decoder with even lower complexity. These two decoders combined with the simple construction method of BP-QO-STBC provide flexible tradeoffs between complexity and performance, as well as between diversity and multiplexing gains. A comparison between the proposed scheme and random beamforming in terms of bit error rate and achievable sum throughput is shown via simulation, and the results support our proposed scheme as a better practical candidate for high-speed wireless network.

Channel-aware multi-user uplink transmission scheme for SIMO-OFDM systems

The problem of medium access control (MAC) in wireless single-input multiple-output-orthogonal frequency division multiplexing (SIMO-OFDM) systems is addressed. Traditional random access protocols have low overheads and inferior performance. Centralized methods have superior performance and high overheads. To achieve the tradeoff between overhead and performance, we propose a channel-aware uplink transmission (CaUT) scheme for SIMO-OFDM systems. In CaUT, users transmit request-to-send (RTS) at some subcarriers whose channel gains are above a predetermined threshold. Using the channel state information provided by RTS, access point performs user selection with receive beamforming to decide which users can access and then broadcasts the selection results via clear-to-send (CTS) to users. We present a distributed power control scheme by using a simple fixed modulation mode. We optimize the modulation order and channel gain thresholds to maximize the separable packets subject to the bit-error-rate (BER) and temporal fairness requirements and the individual average transmit power constraints. The performance of CaUT scheme is analyzed analytically and evaluated by simulations. Simulation results show that CaUT can achieve more significant throughput performance than traditional random access protocols.