Multiuser Diversity Research Articles

SER Analysis of Scheduled TAS With MRC in the Presence of Non-Identical Channel Estimation Errors

In this letter, we present a performance analysis of scheduled transmit antenna selection with maximal ratio combining (TAS-MRC) in the presence of non-identical channel estimation errors. This letter derives the probability density function (PDF) of the instantaneous effective signal-to-noise ratio for the scheduled TAS-MRC over Rayleigh fading channels with non-identical channel estimation errors. Using this PDF, we derive exact closed-form expressions for the scheduled TAS-MRC symbol error rate in the presence of non-identical channel estimation errors. The analysis results show that the order of diversity of scheduled TAS-MRC is improved in terms of transmit spatial diversity and multiuser diversity, whereas the received diversity is diminished by non-identical channel estimation errors, regardless of the modulation type.

Analysis of Energy Efficiency for Cognitive Radio Networks

CR-based technology aims to combat scarcity in the radio spectrum using dynamic spectrum access (DSA).It improves the performance of spectrum efficiency by allowing secondary users to share the spectrum owned by primary users. In this paper, different techniques have been analyzed to improve the energy efficiency of a CRNs.Gradient based iteration algorithm is used to obtain the Stackelberg equilibrium solution. Water filling factors aided search method deals with the Energy Efficient optimization problem with multiple interference power constraint. Efficient barrier method is used to provide optimal solution with reasonable complexity. Extreme-value theory characterize the impact of the multi-user diversity gain of both kinds of users.

Performance analysis of zero‐forcing‐precoded scheduling system with adaptive modulation for multiuser‐multiple input multiple output transmission

In this study, the author presents performance analysis of a zero-forcing (ZF)-precoded scheduling system with adaptive modulation for multiuser-multiple input multiple output transmission. The proposed scheduling system adapts the transmission rate and selects the best modulation to satisfy a given target bit error rate (BER). The author derive the tight closed-form expressions of the ZF-precoded system with adaptive modulation for the average spectral efficiency where user set non-scheduling and user set scheduling methods are presented. The author also presents the tight closed-form BER expressions of the non-scheduling and scheduling systems with adaptive modulation for M-ary quadrature amplitude modulation and M-ary phase shift keying modulation. From the analytical results, the author shows that the proposed scheduling system provides performance improvement of the average spectral efficiency over the non-scheduling system. Moreover, the proposed system provides the signal to noise ratio gain compared with the non-scheduling system to satisfy the target BER regardless of modulation types. This is because the diversity order of the proposed system is superior to that of the non-scheduling system by multi-user diversity.

Capacity Analysis of Opportunistic Scheduling in Nakagami-<inline-formula> <tex-math notation="LaTeX">$m$</tex-math></inline-formula> Fading Environments

The wireless medium has a time-varying feature due to fading, and individual users experience different degrees of fading. Opportunistic scheduling exploits such diversity of fading across users to improve network performance. In this paper, we explicitly analyze the capacity of opportunistic scheduling in various fading environments. To that end, we use Nakagami- ${m}$ fading as a fading channel model and analyze the multiuser diversity capacity when considering the number of users and fading parameters. Our analysis shows that the mean capacity is explicitly decomposed into three factors: the average signal-to-noise ratio (SNR), the number of users, and the capacity improvement that comes from diversity of fading across users. Our analytical results provide the precise impact of fading channels on multiuser capacity, which asymptotic analysis cannot capture. We verify the analytical results via simulations.

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.

Channel-Aware Medium Access Control in Multichannel Cognitive Radio Networks

Distributed medium access control (MAC) is important for efficient exploitation of spectrum opportunities in cognitive radio networks. This letter proposes a channel-aware distributed MAC protocol for multichannel cognitive radio networks. The transmitting nodes use channel state information to exploit the available multiuser and multichannel diversity, by contending only under relatively favorable channel conditions. Towards this end, each node utilizes a threshold that is dynamically updated according to certain rules. Simulation results corroborate the throughput gains achieved by the proposed protocol.

A three level LTE downlink scheduling framework for RT VBR traffic

In recent literature, it has been observed that obtaining low overhead LTE downlink scheduling mechanisms that ensure a descent QoS is a challenging task especially in the presence of transient overloads. This paper proposes a novel three level LTE downlink scheduling framework (called TLS). The three major objectives of the proposed framework includes low overall traffic flow selection and mapping overheads at each Transmission Time Interval (TTI), maintaining acceptable QoS levels for various real-time (RT) variable bit rate (VBR) flows even during transient overloads through graceful degradation, and maximizing spectral efficiency by effectively exploiting multi-user diversity. Both the outermost and the second level of TLS employs a novel adjustable frame-based approach to provide efficient service even in the face of ever changing wireless channel conditions, system workloads, traffic bit rates etc. A low-overhead O(1) look-up based scheme is used in the innermost layer of the framework to physically allocate bandwidth resources to the traffic flows at each TTI. Simulation results clearly reveal that TLS outperforms the existing schedulers with respect to QoS, goodput and spectral efficiency.

Joint Optimization of Cooperative Spectrum Sensing and Resource Allocation in Multi-channel Cognitive Radio Sensor Networks

Cooperative spectrum sensing (CSS) that utilizes multi-user diversity to mitigate channel instability and noise uncertainty is a promising technique in cognitive radio networks (CRNs). However, the spectrum-sensing parameters which affect the channel-access opportunities of secondary users (SUs) are conventionally regarded as static and treated independently from the resource-allocation strategies. In this paper, joint optimization of CSS, channel access and resource allocation is investigated in an overlay CRN in which each SU carries multi-channel spectrum sensing and transmits the detected energy to a fusion centre in the imperfect reporting channel. An access factor is introduced to describe the channel-access strategies in both cooperative and non-cooperative schemes. Based on the aggregate interference and the transmit power constraints, an optimization problem of multi-channel CSS is formulated to obtain the optimal transmit powers, allocation-access strategies, and sensing threshold of CR system for maximization of the opportunistic throughput. To solve the non-convex problems in both the single and multiple CR systems, the efficient iterative algorithms are developed by exploiting the hidden convexity of the optimization problems. Numerical results show that the performance of our approaches yields a significant enhancement compared with the equal channel-access and equal power-allocation strategy.

Fair Scheduling Policies Exploiting Multiuser Diversity in Cellular Systems With Device-to-Device Communications

We consider the resource allocation problem in cellular networks which support device-to-device communications (D2D). For systems that enable D2D via only orthogonal resource sharing, we propose and analyze two resource allocation policies that guarantee access fairness among all users, while taking advantage of multi-user diversity and local D2D communications, to provide marked improvements over existing cellular-only policies. The first policy, the Cellular Fairness Scheduling (CFS) Policy, provides the simplest D2D extension to existing cellular systems, while the second policy, the D2D Fairness Scheduling (DFS) Policy, harnesses maximal performance from D2D-enabled systems under the orthogonal sharing setting. For even higher spectral efficiency, cellular systems with D2D can schedule the same frequency resource for more than one D2D pairs. Under this non-orthogonal sharing environment, we propose a novel group scheduling policy, the Group Fairness Scheduling (GFS) Policy, that exploits both spatial frequency reuse and multiuser diversity to deliver dramatic improvements to system performance with perfect fairness among the users, regardless of whether they are cellular or D2D users.

Multicast Multigroup Precoding and User Scheduling for Frame-Based Satellite Communications

The present work focuses on the forward link of a broadband multibeam satellite system that aggressively reuses the user link frequency resources. Two fundamental practical challenges, namely the need to frame multiple users per transmission and the per-antenna transmit power limitations, are addressed. To this end, the so-called frame-based precoding problem is optimally solved using the principles of physical layer multicasting to multiple co-channel groups under per-antenna constraints. In this context, a novel optimization problem that aims at maximizing the system sum rate under individual power constraints is proposed. Added to that, the formulation is further extended to include availability constraints. As a result, the high gains of the sum rate optimal design are traded off to satisfy the stringent availability requirements of satellite systems. Moreover, the throughput maximization with a granular spectral efficiency versus SINR function, is formulated and solved. Finally, a multicast-aware user scheduling policy, based on the channel state information, is developed. Thus, substantial multiuser diversity gains are gleaned. Numerical results over a realistic simulation environment exhibit as much as 30% gains over conventional systems, even for 7 users per frame, without modifying the framing structure of legacy communication standards.

Power Allocation for a Hybrid Decode–Amplify–Forward Cooperative Communication System With Two Source–Destination Pairs Under Outage Probability Constraint

We develop a two-source–destination-pair cooperative communication scheme that can achieve multiuser diversity and cooperative diversity. In this scheme, we use hybrid decode–amplify–forward (HDAF) protocol to improve system performance. From practical point of view, HDAF protocol performs better than amplify-and-forward and decode-and-forward protocols by combining the merits of both. Meanwhile, we derive a closed-form expression of outage probability associated with HDAF protocol analyzed. Based on outage probability constraint, we investigate optimal power allocation that minimizes the total power. Numerical analysis is used to confirm the accuracy of the derived theory and to examine the performance of our proposed cooperative scheme, particularly as we analyze power savings in using optimal power allocation compared with equal power allocation, which is commonly assumed in previous studies. It is shown that significant power savings can be obtained by using the proposed power optimization method.

Symbol Error Rate Analysis of Joint Diversity System With Channel Estimation Error

In this paper, we investigate the impact of channel estimation error on symbol error rate (SER) for joint diversity (i.e., combining space–time block coding and user scheduling) in multiple-input–multiple-output systems with a channel estimation error. Under a Gaussian distributed channel estimation error, the exact expressions of the joint diversity for SER with both M-ary quadrature amplitude modulation (QAM) and phase-shift keying (PSK) modulation are derived. Using probability density function approximation, we quantify the diversity order and signal-to-noise ratio (SNR) gain of SER for joint diversity in the presence of a channel estimation error. From the analysis results, we show that the diversity order is enhanced as the number of user terminals increases, regardless of the modulation type. This is because user scheduling provides multiuser diversity. Moreover, the SNR gain decreases as the number of user terminals increases. This is because the peak effective SNR in joint diversity decreases as the number of user terminals increases.

Maximizing Energy Efficiency in Multiple Access Channels by Exploiting Packet Dropping and Transmitter Buffering

Quality of service (QoS) for a network is characterized in terms of various parameters specifying packet delay and loss tolerance requirements for the application. The unpredictable nature of the wireless channel demands for application of certain mechanisms to meet the QoS requirements. Traditionally, medium access control (MAC) and network layers perform these tasks. However, these mechanisms do not take (fading) channel conditions into account. In this paper, we investigate the problem using cross layer techniques where information flow and joint optimization of higher and physical layer is permitted. We propose a scheduling scheme to optimize the energy consumption of a multiuser multi-access system such that QoS constraints in terms of packet loss are fulfilled while the system is able to maximize the advantages emerging from multiuser diversity. Specifically, this work focuses on modeling and analyzing the effects of packet buffering capabilities of the transmitter on the system energy for a packet loss tolerant application. We discuss low complexity schemes which show comparable performance to the proposed scheme. The numerical evaluation reveals useful insights about the coupling effects of different QoS parameters on the system energy consumption and validates our analytical results.

Radio Resource Allocation on Complex 4G Wireless Cellular Networks

In this article we consider the heuristic algorithm which improves step by step wireless data delivery over LTE cellular networks by using the total transmit power with the constraint on users’ data rates, and the total throughput with the constraints on the total transmit power as well as users’ data rates, which are jointly integrated into a hybrid-layer design framework to perform radio resource allocation for multiple users, and to effectively decide the optimal system parameter such as modulation and coding scheme (MCS) in order to adapt to the varying channel quality. We propose new heuristic algorithm which balances the accessible data rate, the initial data rates of each user allocated by LTE scheduler, the priority indicator which signals delay- throughput- packet loss awareness of the user, and the buffer fullness by achieving maximization of radio resource allocation for multiple users. It is noted that the overall performance is improved with the increase in the number of users, due to multiuser diversity. Experimental results illustrate and validate the accuracy of the proposed methodology.

Uplink scheduling for joint wireless orthogonal frequency and time division multiple access networks

In this paper, we present a deterministic resource allocation model for a hybrid uplink wireless orthogonal frequency and time division multiple access network. Since the input data of the model may be affected by uncertainty, we further consider a stochastic formulation of the problem which we transform into an equivalent deterministic binary second-order conic program (SOCP). Subsequently, we use this binary SOCP to derive an equivalent integer linear programming formulation. The proposed models are aimed at maximizing the total bandwidth channel capacity subject to user power and sub-carrier assignment constraints while simultaneously scheduling users in time. As such, the models are best suited for non-real-time applications where sub-channel multiuser diversity can be further exploited simultaneously in frequency and time domains. Finally, in view of the large execution times required by CPLEX to solve the proposed models, we propose a variable neighborhood search metaheuristic procedure. Our numerical results show tight bounds and near optimal solutions for most of the instances when compared to the optimal solution of the problem. Moreover, we obtain better feasible solutions than CPLEX in the stochastic case. Finally, these bounds are obtained at a very low computational cost.

Interference Alignment in Device-to-Device LAN Underlaying Cellular Networks

This paper studies a device-to-device (D2D) LAN underlaying a cellular uplink, where multiple D2D users (DUs) intend to communicate with a D2D receiver. Two interference alignment (IA) based D2D communication schemes are proposed to manage the interference between two networks. The first scheme is referred to as “interference-free” IA scheme. In this scheme, the interference signals from DUs are designed to be aligned in the orthogonal space of cellular links at the evolved NodeB (eNB), so that the links of cellular users (CUs) are completely free from interference. The second scheme is referred to as “interferencelimiting” IA scheme. In this scheme, the DUs' signals are allowed to occupy some links of CUs, but the peak interference power on each of the “interfered” links is controlled under a certain threshold γ. The “interference-limiting” IA scheme is most efficient when there are a large number of DUs as the scheme can take advantage of multiuser diversity. Performance analysis shows that based on the proposed schemes, the interference generated on the cellular links is eliminated or well controlled, while the quality of service of the D2D LAN can also be guaranteed.

Caching gain in interference‐limited wireless networks

The authors consider the effect of caching in interference-limited wireless networks where fading is the dominant channel effect. First, the authors propose a one-hop transmission strategy for cache-enabled wireless networks, which is based on exploiting multi-user diversity gain. Then, they derive a closed-form result for throughput scaling of the proposed scheme in large networks, which reveals the inherent trade-off between cache memory size and network throughput. Their results show that substantial throughput improvements are achievable in networks with sources equipped with large cache size, compared with previous works where each transmitter could just store one file. Also, the authors provide extensive simulations through which they analyse network throughput and verify their analytical results. Their simulations suggest that the scaling results also hold true even by introducing correlation between channels, making the results more general.

On the multiuser diversity in SIMO interfering multiple access channels: Distributed user scheduling framework

Due to the difficulty of coordination in the cellular uplink, it is a practical challenge how to achieve the optimal throughput scaling with distributed scheduling. In this paper, we propose a distributed and opportunistic user scheduling (DOUS) that achieves the optimal throughput scaling in a single-input multiple-output interfering multiple-access channel, i.e., a multi-cell uplink network, with M antennas at each base station (BS) and N users in a cell. In a distributed fashion, each BS adopts M random receive beamforming vectors and then selects M users such that both sufficiently large desired signal power and sufficiently small generating interference are guaranteed. As a main result, it is proved that full multiuser diversity gain can be achieved in each cell when a sufficiently large number of users exist. Numerical evaluation confirms that in a practical setting of the multi-cell network, the proposed DOUS outperforms the existing distributed user scheduling algorithms in terms of sum-rate.

CDF Scheduling Methods for Finite Rate Multiuser Systems With Limited Feedback

In this work, simple and practical CDF scheduling methods are developed that preserve the virtues of CDF scheduling, namely fairness and effective use of multiuser diversity. They are the accelerated extended CDF scheduling (AeCS), modified non-parametric CDF scheduling (modified NPCS), and the CDF scheduling with optimized Quantizers (CSwQ) methods. The developed methods do not need a priori knowledge of the channel distribution and can work effectively in systems that support discrete number of transmission rates and are constrained by limited feedback resources. They exploit the limited feedback resource for their scheduling decisions and transmission rate selections and learn the distribution of channel quality information of each user, as needed, to best exploit multiuser diversity. The CSwQ method is shown to have desirable properties in terms of both system throughput and power savings. Analytical results for the system throughput and power savings of the CSwQ method are developed that are valid for a finite number of observation samples. Then, through numerical simulations, the developed methods are shown to effectively save transmit power and achieve high system throughput. In addition, they converge rapidly with the number of data samples to the ideal case wherein perfect knowledge of the channel CDF is assumed.

Fundamental Limits of CDF-Based Scheduling: Throughput, Fairness, and Feedback Overhead

In this paper, we investigate fundamental performance limits of cumulative distribution function (CDF)-based scheduling (CS) in downlink cellular networks. CS is known as an efficient scheduling method that can assign different time fractions for users or, equivalently, satisfy different channel access ratio (CAR) requirements of users while exploiting multi-user diversity. We first mathematically analyze the throughput characteristics of CS in arbitrary fading statistics and data rate functions. It is shown that the throughput gain of CS increases as the CAR of a user decreases or the number of users in a cell increases. For Nakagami-m fading channels, we obtain the average throughput in closed-form and investigate the effects of the average signal-to-noise ratio, the shape parameter m, and the CAR on the throughput performance. In addition, we propose a threshold-based opportunistic feedback technique in order to reduce feedback overhead while satisfying the CAR requirements of users. We prove that the average feedback overhead of the proposed technique is upper bounded by -ln(p), where p is the probability that no user satisfies the threshold condition in a cell. Finally, we adopt a novel fairness criterion, called qualitative fairness, which considers not only the quantity of the allocated resources to users but also the quality of the resources. It is observed that CS provides a better qualitative fairness than other scheduling algorithms designed for controlling CARs of users.