User Scheduling Algorithm Research Articles

Simplified Capacity-Based User Scheduling Algorithm for Multiuser MIMO Systems with Block Diagonalization

In multiple-input multiple-output (MIMO) systems, the multiuser MIMO (MU-MIMO) systems have the potential to provide higher channel capacity owing to multiuser and spatial diversity. Block diagonalization (BD) is one of the techniques to realize MU-MIMO systems, where multiuser interference can be completely cancelled and therefore several users can be supported simultaneously. When the number of multiantenna users is larger than the number of simultaneously receiving users, it is necessary to select the users that maximize the system capacity. However, computation complexity becomes prohibitive, especially when the number of multiantenna users is large. Thus simplified user scheduling algorithms are necessary for reducing the complexity of computation. This paper proposes a simplified capacity-based user scheduling algorithm, based on analysis of the capacity-based user selection criterion. We find a new criterion that is simplified by using the properties of Gram-Schmidt orthogonalization (GSO). In simulation results, the proposed algorithm provides higher sum rate capacity than the conventional simplified norm-based algorithm; and when signal-to-noise power ratio (SNR) is high, it provides performance similar to that of the conventional simplified capacity-based algorithm, which still requires high complexity. Fairness of the users is also taken into account. With the proportionally fair (PF) criterion, the proposed algorithm provides better performance (sum rate capacity or fairness of the users) than the conventional algorithms. Simulation results also shows that the proposed algorithm has lower complexity of computation than the conventional algorithms.

MIMO Beamforming for QoS Enhancement via Analog, Digital and Hybrid Relaying

Quality-of-service (QoS) of multimedia broadcast/multicast service (MBMS) is the most challenging issue that needs to be addressed for next generation wireless broadcasting. Multiple-input/multiple-output (MIMO) and multihop relay technologies are viable options for improving QoS of MBMS. For stable MBMS, the average throughput of overall cell region must be guaranteed. Nevertheless, severe throughput degradation may occur in the outer region of multicells due to significant interference from neighboring cells or relay stations, thereby limiting the overall performance. This paper describes an effective technique, which combines MIMO and two-hop relay, to guarantee the QoS of edge users. We assume that the channel between the base station (BS) and the fixed relay station (FRS) is line-of-sight (LOS). First, we define a criterion needed to install the FRS. Second, we construct a weight matrix for hybrid relaying having lower complexity than digital relaying. Finally, we propose a user scheduling algorithm which can be adapted to analog relaying (amplify and forward), digital relaying (decode and forward) and hybrid relaying (filter, amplify and forward). The simulation results show that the proposed method can improve both the QoS of edge users and the sum-rate.

Efficient Transmission Schemes for Multiuser MIMO Downlink with Linear Receivers and Partial Channel State Information

Downlink of a multiuser MIMO system is considered, in which the base station (BS) and the user terminals are both equipped with multiple antennas. Efficient transmission schemes based on zero-forcing (ZF) linear receiver processing, eigenmode transmission, and partial channel state information (CSI) at the BS transmitter are proposed. The proposed schemes utilize a handshaking procedure between the BS and the users to select (schedule) a subset of users and determine the precoding matrix at the BS. The advantage of the proposed limited feedback schemes lies in enabling relatively low-complexity user scheduling algorithms and high sum-rate throughput, even for a small pool of users. For large user pools and when the number of antennas at each user terminal is at least equal to the number of antennas at the BS, we show that the proposed scheme is asymptotically optimum.

Multiuser Diversity for Successive Zero-Forcing Dirty Paper Coding: Greedy Scheduling Algorithms and Asymptotic Performance Analysis

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Dirty paper coding (DPC) scheme is the capacity achieving transmission technique in multiuser MIMO downlink channels. As a suboptimal solution to DPC, successive zero-forcing DPC (SZF-DPC) has been proposed recently. The zero-interference constraint in designing the precoding matrices limits the number of supportable users. In this correspondence, we propose three low-complexity suboptimal user scheduling algorithms to exploit the multiuser diversity gain in SZF-DPC as the number of users grows. The first algorithm greedily maximizes the true sum rate. The second algorithm is based on eigenvalues. The third algorithm relies on the diagonal elements of the effective channel matrix since eigenvalues and diagonal entries of a Hermitian matrix have a strong relationship. Simulation results show that the proposed scheduling algorithms can obtain a significant fraction of sum rate of the optimal solution. Furthermore, the performance analysis is provided to prove that the proposed user selection algorithms can achieve the same asymptotic sum rate as that of DPC. </para>

Unitary Beamforming Multi-User MIMO System with Efficient User Scheduling Algorithm

This letter proposes a new practical multiuser MIMO (MU-MIMO) scheme, which is an evolution of the well-known Per User Unitary beamforming Rate Control (PU2RC) proposed for 3GPP-LTE and IEEE802.16m standards. The proposed scheme includes an efficient user scheduling algorithm which alleviates the major weakness of the conventional PU2RC. Numerical results show that the proposed scheme provides notable performance improvement especially with small and medium user pool since it effectively exploits the benefit from large codebook size.

Distributed Scheduling Algorithm for Multiuser MIMO Downlink with Adaptive Feedback

We propose a distributed scheduling algorithm for multiuser MIMO downlink with adaptive feedback. In this algorithm each mobile station (MS) selects transmission scheme, including beamforming (BF) and spatial multiplexing (SM) according to its channel state and then feeds back channel state information (CSI) adaptively. The base station (BS) receives CSI from each MS and selects user subset based on minimum spatial correlation criteria or system capacity maximization. The proposed user scheduling algorithm is carried out by MSs and BS corporately. The system feedback load depends on each MS’s channel state. Compared with existing schemes the proposed algorithm can achieve high system capacity as well as good BER performance.

Asymptotic analysis of SDMA systems with near-orthogonal user scheduling (NEOUS) under imperfect CSIT

In this paper, we focus on the asymptotic cross layer analysis of multi-antenna systems with transmit MMSE (Tx- MMSE) beamforming, near orthogonal scheduling and outdated CSIT. To capture the effect of the potential packet outage, we introduce the average system goodput, which measures the average b/s/Hz delivered to the mobiles successfully, as the system performance objective. We derive closed-form expressions for the optimal power and rate allocations as well as a low complexity near orthogonal user scheduling (NEOUS) algorithm to solve the cross-layer optimization problem.We derive the asymptotic order of growth in system goodput for general CSIT error variance σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and found that for sufficiently large nT (number of antennas at the base station) and K (number of users) where K = g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (nT) for some strictly increasing function g(x) = o(x), the the system goodput grows in the order of nT log[(1-σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) logK] when σ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> < 1. This is the same order of growth as the optimal order of growth in broadcast channels with perfect CSIT and hence, the NEOUS is order-optimal. On the other hand, we need exponentially larger K to compensate for the penalty in multiuser diversity gain due to CSIT errors.

Channel Asymmetry in Cellular OFDMA-TDD Networks

This paper studies time division duplex- (TDD-) specific interference issues in orthogonal frequency division multiple access- (OFDMA-) TDD cellular networks arising from various uplink (UL)/downlink (DL) traffic asymmetries, considering both line-of-sight (LOS) and non-LOS (NLOS) conditions among base stations (BSs). The study explores aspects both of channel allocation and user scheduling. In particular, a comparison is drawn between the fixed slot allocation (FSA) technique and a dynamic channel allocation (DCA) technique for different UL/DL loads. For the latter, random time slot opposing (RTSO) is assumed due to its simplicity and its low signaling overhead. Both channel allocation techniques do not obviate the need for user scheduling algorithms, therefore, a greedy and a fair scheduling approach are applied to both the RTSO and the FSA. The systems are evaluated based on spectral efficiency, subcarrier utilization, and user outage. The results show that RTSO networks with DL-favored traffic asymmetries outperform FSA networks for all considered metrics and are robust to LOS between BSs. In addition, it is demonstrated that the greedy scheduling algorithm only offers a marginal increase in spectral efficiency as compared to the fair scheduling algorithm, while the latter exhibits up to 20% lower outage.

A low complexity user scheduling algorithm for uplink multiuser MIMO systems

A low complexity user scheduling algorithm based on a novel adaptive Markov chain Monte Carlo (AMCMC) method is proposed to achieve the maximal sum capacity in an uplink multiple-input multiple-output (MIMO) multiuser system. Compared with the existing scheduling algorithms, our algorithm is not only more efficient but also converges to within 99% of the optimal capacity obtained by exhaustive search. We demonstrate the convergence of the proposed scheduling algorithm and study the tradeoff between its complexity and performance.

Antenna and User Subset Selection in Downlink Multiuser Orthogonal Space-Division Multiplexing

Block diagonalization (BD) and successive optimization (SO) are two suboptimal but more practical (compared to dirty paper coding (DPC)) orthogonal linear precoding techniques for the downlink of multiuser MIMO systems. Since the numbers of users supported by BD or SO for a given number of transmit antennas are limited, BD or SO should be combined with scheduling so that a subset of users is selected at a given time slot while meeting the dimensionality requirements of these techniques. On the other hand, receive antenna selection (RAS) is a promising hardware complexity reduction technique. In this paper, we consider user scheduling in conjunction with receive antenna selection. Since exhaustive search is computationally prohibitive, we propose simplified and suboptimal user scheduling algorithms for both BD and SO. For BD, we propose capacity and Frobenius-norm based suboptimal algorithms with the objective of sum rate maximization. Starting from an empty set, each step of proposed algorithms adds the best user from the set of users not selected yet until the desired number of users have been selected. Proposed receive antenna selection works in conjunction with user scheduling to further enhance the sum rate of BD. For SO, a Frobenius-norm based low complexity algorithm is proposed, which maximizes the ratio of the squared Frobenius norm of the equivalent channel (projected to the joint null space of the previously selected users) to the sum of the squared Frobenius norms of the previously selected users' preprocessed channels. Simulation results demonstrate that the proposed algorithms achieve sum rates close to exhaustive search algorithms with much reduced complexity. We also show that in addition to reduced hardware complexity at the receiver, antenna selection enhances multiuser diversity gain that is achieved with user scheduling.

A Novel User Scheduling Algorithm in Inhomogeneous Networks

In this paper, a novel user scheduling algorithm for maximizing the sum-rate capacity of inhomogeneous network is investigated. In order to extract the multi-user diversity order and reduce the feedback quantity, selective feedback scheme is adopted. An algorithm of key parameter, the prescribed threshold, is proposed. Numerical simulations show that when adopted the proposed threshold in the inhomogeneous networks, selective feedback scheme can still preserve the majority of the sum-rate capacity of the full back scheme, while the feedback load is significantly reduced.

Power duality for multi-antenna OFDM system in broadcast channel with user scheduling

This paper deals with design and analysis of user scheduling and power allocation for multi-antenna OFDM systems with DPC, ZF-DPC, ZF-BF and TDMA transmit strategies. We consider the general multi-user downlink scheduling problem and power minimization with multi-user rate constraints. According to the channel state, it is shown that there is a power optimal policy which selects a subset of users in each scheduling interval. We present user selection algorithms for DPC, ZF-DPC, ZF-BF and TDMA for multi-antenna OFDM system in broadcast channels, and we also present the practical water-filling solution in this paper. By the selected users with the consideration of fairness, we derive the power optimization algorithm with multi-user rate constraints. We also analyze the power duality of uplink-downlink for the transmit strategies of DPC, ZF-DPC and ZF-BF. Simulation results show that the present user-scheduling algorithm and power minimization algorithm can achieve good power performance, and that the scheduling algorithm can guarantee fairness.

Performance Analysis of the Extended Low Complexity User Scheduling Algorithm over Up-Link Multi-User MIMO OFDMA Systems

The low complexity tree-structure based user scheduling algorithm is extended into up-link MLD-based multi-user multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing access (OFDMA) wireless systems. The system sum capacity is maximized by careful user selection on a defined tree structure. The calculation load is reduced by selecting the M most possible best branches and sampling in frequency dimension. The performances of the proposed scheduling algorithm are analyzed within three kinds of OFDMA systems and compared with conventional throughput-based algorithm. Both the theoretical analysis and simulation results show that the proposed algorithm obtains better performance with much low complexity.

A Low Complexity Tree-Structure Based User Scheduling Algorithm for Up-Link Multi-User MIMO Systems

The paper describes a low complexity tree-structure based user scheduling algorithm in an up-link transmission of MLD-based multi-user multiple-input multiple-output (MIMO) wireless systems. An M-branch selection algorithm, which selects M most-possible best branches at each step, is proposed to maximize the whole system sum-rate capacity. To achieve the maximum capacity in multi-user MIMO systems, antennas configuration and user selection are preformed simultaneously. Then according to the selected number of antennas for each user, different transmission schemes are also adopted. Both the theoretical analysis and simulation results show that the proposed algorithms obtain near optimal performance with far low complexity than the full search procedure.

Low Complexity Scheduling Algorithms for Downlink Multiuser MIMO System

Downlink multiuser MIMO system has attracted considerable attention recently for its potential to increase the system capacity. However, due to the limitation on the number of transmit antennas, when there are more users than can be supported simultaneously in a cell, other multiple access schemes, such as TDMA, must be applied in combination with multiuser MIMO. In this paper, we aim to design practical user scheduling algorithms to maximize the system capacity. Because the brute-force search for optimal user allocation is computationally prohibitive, we propose three low complexity suboptimal scheduling algorithms that offer both low complexity and high performance.