Multiuser Diversity Systems Research Articles

Dynamic Resource Scheduling Optimization With Network Coding for Multi-User Services in the Internet of Vehicles

For Internet of Vehicles (IoV) systems with multiple users, network coding can be introduced to provide efficient error control and throughput improvement services. However, if the heterogeneity characteristics and requirements of the end users (vehicles) are neglected, it will be difficult for an IoV system to provide each end user with fair system services, without which the advantages of network coding cannot be fully achieved and the performance of the multi-user diversity system will be degraded. In this paper, we propose a Dynamic Resource Scheduling Optimization (DRSO) algorithm, a dynamic fair scheduling algorithm combined with network coding for system resource allocation in a multi-user IoV system. We construct a general solution framework for service scheduling: first, we estimate the fairness index for each end user (vehicle) with the key information on Quality of Service (QoS). Second, we construct a service scheduling control model based on the service capability of control entities (multi-access edge computing servers), and propose a new utility evaluation function. Third, based on the fairness index, we select end users into multiple network coding sets. Network coding sets are the basic units of service scheduling. The optimization objective of the scheduling service is to maximize the total utility of all the network coding sets (the utility of the control entity). Finally, we establish a coding cache queue in the control entity based on the scheduling decision. To obtain the global optimal solution for active queue control, we combine a Quantum Particle Swarm Optimization (QPSO) algorithm with a Proportional Integral (PI) model. Then, the optimal scheduling decision can be made. Extensive simulation results show that DRSO outperforms related scheduling algorithms in varying traffic loads, demonstrating that DRSO can effectively guide service resource allocation.

Power Allocation for Max-Sum Rate and Max-Min Rate Proportional Fairness in NOMA

In this letter, we consider proportional fairness scheduling (PFS) for downlink non-orthogonal multiple access (NOMA) with two users. Unlike conventional multiuser diversity systems, the power allocation plays a key role in PFS for NOMA as it can support multiple users simultaneously with positive transmission rates. With different criteria, we find optimal power allocation. Among those, it is shown that the PFS scheme that maximizes the minimum normalized rate can provide not only proportional fairness, but also small variation of transmission rates.

Unified symbol error probability analysis of multiuser diversity in TAS/OSTBC systems

In this correspondence, we develop a framework to analyze the symbol error probability (SEP) of downlink multiuser diversity (MUD) transmit antenna selection with orthogonal space-time block codes (TAS/OSTBC) over Rayleigh fading channels. Two excellent transmit antennas are chosen that maximize the received signal-to-noise level. Utilizing the moment generating function, a tight closed-form analytical expression of MUD order is derived in terms of SEP. The results shows that the MUD system can achieve a gain of diversity equal to the product of the number of users, number of transmit antennas and number of receive antennas.

Statistical quality‐of‐service analysis in multiuser diversity system enhanced with queue information

In recent years, the scheduling scheme of exploiting multiuser diversity gain has attracted much attention for the purpose of efficient spectrum utilisation. The authors consider the problem of analysing the quality of service in the scheduler extracting multiuser diversity gain. The authors use not only the channel state information but also the queue information in determining the optimal user to be scheduled. Through queuing analysis, the guaranteed rate, the average delay and the delay-bound violation probability can be calculated.

Capacity Analysis and Feedback Threshold Optimization in Fair Multiuser Diversity System

The scheduling scheme of exploiting multiuser diversity gain increases network capacity drastically; however, feedback information overhead linearly increases as the number of users increases. Thus, the allocation of an optimum amount of system resources in feedback signaling is quite an important problem. We analyze the exact capacity of the contention-based multiuser diversity scheduler, which considers the quantitative feedback overhead. The optimum feedback threshold is derived based on the analyzed capacity model. Moreover, the method of searching the optimum resource to be allocated in feedback signaling is proposed.

Multiuser Switched Diversity Scheduling Schemes

Multiuser switched-diversity scheduling schemes were recently proposed in order to overcome the heavy feedback requirements of conventional opportunistic scheduling schemes by applying a threshold-based, distributed, and ordered scheduling mechanism. The main idea behind these schemes is that slight reduction in the prospected multiuser diversity gains is an acceptable trade-off for great savings in terms of required channel-state-information feedback messages. In this work, we characterize the achievable rate region of multiuser switched diversity systems and compare it with the rate region of full feedback multiuser diversity systems. We propose also a novel proportional fair multiuser switched-based scheduling scheme and we demonstrate that it can be optimized using a practical and distributed method to obtain the feedback thresholds. We finally demonstrate by numerical examples that switched-diversity scheduling schemes operate within 0.3 bits/sec/Hz from the ultimate network capacity of full feedback systems in Rayleigh fading conditions.

Effective capacity optimization for multiuser diversity systems with adaptive transmission

The effective capacity (EC) for a wireless system expresses the maximum arrival source rate that the system can reliably transmit over the wireless channel while fulfilling a probabilistic delay constraint. This paper presents an EC optimization for a multiuser diversity system exploiting proportional opportunistic scheduling at the medium access control layer in conjunction with adaptive modulation and coding (AMC) at the physical layer. We consider the downlink of a time-slotted multiuser system with two different types of channel state information (CSI) at the transmitter, that is, (i) full CSI, whereby each user feeds back its normalized signal-to-noise ratio to the transmitter and (ii) quantized CSI, where each user feeds back the AMC mode for transmission. For each case, we first derive the EC function of the queue service process for the individual users at the medium access control layer. We then design the AMC scheme with power control aiming at maximizing the said function subject to a target packet-error rate constraint. Both independent and identically distributed and time-correlated fading channels are considered. The results illustrate the superior performance of the proposed schemes, when compared with the previous systems such as opportunistic scheduling with single-rate transmission and round-robin scheduling with AMC. Copyright © 2012 John Wiley & Sons, Ltd.

Time-Energy Limited Feedback Algorithms for Multiuser Diversity Scenarios on Nakagami Fading Channels

Preserving a reduced feedback load in Multiuser Diversity systems is crucial. By establishing either single or multiple thresholds only in the energy domain, the Selective Multiuser Diversity (SMUD) schemes achieve a reduced feedback load while maintaining a high capacity performance. This paper introduces an enhanced joint time-energy domain scheduling approach for SMUD systems that maximizes the total spectral efficiency and preserves a minimal feedback load with just a few thresholds in the joint domain. This joint domain capitalizes on the energy and the non-fade duration statistics in order to establish a feedback policy that is focused on those users who have good channel condition and preserve it for a predetermined amount of time, providing thus an automatically more rational and fair use of the resources without a sophisticated scheduling with large polling/decision times. The analysis of this enhanced approach for the general case of Nakagami fading channels is developed and expressions for the average system capacity and the average feedback load are presented. The tradeoff between the feedback load and the spectral efficiency gain is analyzed. Two main scenarios are studied; the first when there are two thresholds in the energy domain side and one in the time domain side; the second when there are two thresholds in both domains.

Performance Analysis of Adaptive Modulation in Multiuser Selection Diversity Systems With OSTBC Over Time-Varying Channels

This letter analyzes the impact of feedback delay on the performance of multiuser selection diversity systems employing orthogonal space-time block coding (OSTBC) and rate adaptive modulation over time-varying MIMO Rayleigh fading channels. Rate-adaptive M -QAM modulation along with a user selection diversity is considered in this letter. Analytical expressions are derived for the average spectral efficiency, average bit error rate, and bit error outage probability of the system. Using the results obtained from the analytical expressions, as well as validation from Monte-Carlo simulations, the impact of feedback delay on the system performance is evaluated and discussed.

MIMO relay wireless networks with scheduling and transmit beamforming

AbstractIn this paper, we investigate the performance of a multiuser wireless relay network with each of its nodes equipped with N antennas and transmit beamforming at its source and relay, where multiuser diversity (MD) is exploited to further improve the system performance. In the first part of this paper, by deriving the approximate error performance expression for a single‐relay MD system without the direct link, we obtain the maximum diversity order achievable by the CSI‐assisted amplify‐and‐forward (CAF) protocol. Then, by taking the direct link into consideration, we propose a sub‐optimal transmit beamforming scheme and show the performance loss is very small. Copyright © 2010 John Wiley & Sons, Ltd.

Reduced-Complexity Radio Architectures for Enhanced Receive Selection Combining in Multiuser Diversity Systems

Although antenna selection is a simple and efficient technique for enhancing the downlink performance of multiuser diversity systems, the large antenna interelement spacing required for achieving spatial diversity is prohibitive for user terminals due to size restrictions. In order to allay this problem, we propose miniaturized switched beam receiver designs assisted by low-cost passive reflectors. Unlike conventional spatial receive diversity systems, the proposed angular diversity architectures occupy a small volume whereas the antenna system properties are optimized by controlling the strong reactive fields present at small dimensions. The systems are designed for maximum antenna efficiency and low interbeam correlation, thus yieldingNpractically uncorrelated receive diversity branches. The simulation results show that the proposed enhanced diversity combining systems improve the average throughput of a multiuser network outperforming classical antenna selection especially for small user populations and compact user terminal size.

System-Level Impact of Multi-User Diversity in SISO and MIMO-based Cellular Systems

We quantify cell-wide mean throughputs of single-input-single-output (SISO) and multiple-input-multiple-output (MIMO)-based cellular systems which employ multi-user diversity (MuD). Our study considers several practical and useful system-level design dimensions, including: number of transmit/receive antennas; antenna-pattern (omni-directional or sectorized); degree of error-protection (Shannon coding, no coding or intermediate coding strategies); allowable constellation size; Rician $\kappa$-factor; number of users and schedluling algorithm (Greedy (i.e. MAX C/I), Proportional Fair, or Equal Grade of Service) in single-cell (noise-limited) and multi-cell (co-channel-interference-limited) environments. We also provide a comparison between single-user systems having excess receive antennas and multi-user diversity systems with no excess receive antennas. Both strategies improve signal quality. Since economic costs of RF chains, mobile size and form factor limit the number of antennas a mobile receiver can have, multi-user diversity can be a more practical option. We observe that MuD with only a few scheduled users leads to comparable throughputs as receivers with excess receive antennas. By quantifying the average throughput gains that accrue from using multi-user SISO and MIMO-based cellular systems, this study serves the needs of operators to assess these promising technologies.

Capacity and Fairness Trade-off in an Outage Situation over Multiuser Diversity Systems

Without an appropriate scheduling scheme for subscriber stations (SSs) in an outage situation, the opportunistic feedback protocol degrades the system performance and it does not achieve the desired capacity and fairness trade-off. In this paper, we propose a new downlink multiuser scheduling scheme to balance a trade-off between capacity and fairness with one-bit information. In particular, we propose a high successful-feedback SS selection (HSSS) scheme using one-bit information to provide the flexible trade-off in an outage situation. As a result, the proposed scheme not only reduces a significant feedback overload but also considers scheduling in the outage situation. Simulation results show that the proposed scheme achieves the flexible trade-off between capacity and fairness in the outage situation.

Queuing Analysis in a Multiuser Diversity System With Adaptive Modulation and Coding Scheme

In wireless packet networks, the performance of multiuser diversity schedulers has been analyzed with the saturation assumption; however, the finite queue length effect should be taken into account. We propose a new scheduling scheme exploiting multiuser diversity that considers not only channel information but queue information as well. Considering the finite queue length effect and adaptive modulation and coding scheme (AMCS), we analyze the performance of the proposed scheduler.

Cooperative feedback to improve capacity and error rate in multiuser diversity systems – an OFDM case study

AbstractTo achieve multiuser diversity gains, resource allocation relies on accurate and timely feedback of the users' channel states. Obtaining such reliable feedback under tight overhead and delay constraints is non‐trivial in many wireless systems. To this end, we let users cooperate when transmitting feedback. Studying our cooperative feedback scheme exemplarily for the multiuser OFDM downlink shows that it efficiently avoids resource allocation errors. This significantly improves the error rate, ergodic sum capacity, or the system's energy efficiency even if the resources and energy required for cooperation are taken into account.

A Feedback Quantization Scheme Leveraging Fairness and Throughput for Heterogeneous Multi-User Diversity Systems

We propose a feedback quantization scheme for downlink multiuser diversity systems. The scheme is designed for heterogeneous users, i.e., users operating under different levels of mean signal-to-noise ratios (SNRs). The proposed scheme equally distributes the channel access time to user clusters and, at the same time, maximizes the sum-rate capacity under equal-channel-access-time constraint. The method is compared to a normalized SNR scheduler, full-feedback scheduler, and finite-rate feedback scheduler without fairness consideration to illustrate the performance gaps between different modes of operation.

Feedback reduction scheme for downlink multiuser diversity

A group random access-based feedback scheme is proposed for multiuser diversity systems, where users are divided into several feedback groups according to their measured channel states. Different random access probabilities and number of feedback minislots are assigned to each feedback group. Each user competes to send a feedback message through random access if its scheduling metric exceeds a threshold. The base station jointly controls the random access probabilities, the thresholds, and the number of feedback minislots to adjust the feedback load. The proposed feedback scheme shows a good system capacity with limited feedback resources regardless of the number of users.

Capacity and Fairness in Multiuser Diversity Systems with Opportunistic Feedback

An opportunistic feedback protocol is proposed for multiuser diversity systems with proportional fair scheduling and maximum-throughput scheduling. The uplink feedback period is divided into two subperiods and users randomly access the feedback minislots according to their scheduling metric. An integrated analytical model is developed for evaluating the proposed feedback protocol.

Analysis of multiuser diversity in time-varying channels

Research in recent years has been focusing on the role of multiuser diversity in enhancing the throughput of wireless networks. Channel fading is now seen as a source of randomization that can be exploited by a smart scheduler with partial channel information, for example, in terms of data rates requested by the users. In this paper, we analyze the performance of a multiuser diversity system, taking into account the feedback errors due to channel variability. Channel estimation errors are modeled as Gaussian random variables with variance depending on the Doppler spread. Based on a block fading channel model, we present an expression for the throughput, both as a function of the key system parameters, such as packet length and data rate thresholds, and channel characteristics, such as Doppler spread. From the analysis of the performance, we present a tradeoff between multiuser diversity and mobility, and also between rate adaptivity and channel estimation errors. It is shown that multiuser diversity gain is reduced for increasing receiver mobility, and that the throughput loss depends also on the distribution of the data rate thresholds. Finally, we use the expressions derived in the paper in order to adaptively reduce the loss due to feedback errors. Given a set of possible data rates, the users can choose the most suitable one based not only on the set of thresholds and signal strength, but also on mobility conditions and system parameters, such as packet length.

Capacity of multiuser diversity systems with adaptive transmission and different MIMO schemes

AbstractIn this paper, we present a comprehensive capacity analysis of the downlink of multiuser diversity (MD) systems with adaptive transmission over Rayleigh fading channels. First, the exact capacity of the single‐input single‐output (SISO) systems with MD and adaptive transmission technique is derived. The optimal power allocation scheme for such a system is shown to be a water‐filling algorithm. Next, we derive the exact closed‐form capacity expressions for different multiple‐input multiple‐output (MIMO) schemes, including the selective combining (SC), maximum ratio combining (MRC) and space‐time block codes (STBC). In order to avoid the cumbersome numerical root finding techniques in solving the optimal cutoff SNR level below which the channel is not used, we also provide the approximate expressions for the cutoff level. For the MD MIMO systems, it is observed that the optimal power allocation strategy is to focus transmit power on a single transmit antenna (e.g. Tx‐MRC/Rx‐MRC scheme) or selecting the best transmit antennas (e.g. Tx‐SC/Rx‐MRC scheme). Copyright © 2007 John Wiley & Sons, Ltd.