On the capacity and BER performance of multiuser scheduling over MIMO Nakagami-m fading channels

This paper presents a performance analysis for evaluating multiuser and spatial diversities achieved in multiuser MIMO systems employing orthogonal space-time block coding (OSTBC) over Nakagami-m fading channels. Two multiuser scheduling schemes are considered, absolute SNR-based scheduling (AS) and normalized SNR-based scheduling (NS) schemes for both heterogeneous and homogeneous wireless networks. Analytical expressions are derived for the average channel capacity and average bit error rate (BER) of these systems. The considered scheduling schemes are numerically evaluated and compared in terms of average capacity, average BER and fairness. It is shown that in the heterogeneous case, unlike the AS scheme, the NS scheme can guarantee fairness to the users. It is also shown that in the heterogeneous case, the AS scheme yields a higher average capacity and a lower average BER compared to the NS scheme and to the homogeneous case.     

A Unified Capacity Analysis for Wireless Systems With Joint Multiuser Scheduling and Antenna Diversity in Nakagami Fading Channels

A Unified Capacity Analysis for Wireless Systems With Joint Multiuser Scheduling and Antenna Diversity in Nakagami Fading Channels In this paper, we present a cross-layer analytical framework to jointly investigate antenna diversity and multiuser scheduling under the generalized Nakagami fading channels. We derive a unified capacity formula for the multiuser scheduling system with different multiple-input multiple-output antenna schemes, including: 1) selective transmission/selective combining (ST/SC); (2) maximum ratio transmission/maximum ratio combining (MRT/MRC); 3) ST/MRC; and 4) space–time block codes (STBC). Our analytical results lead to the following four observations regarding the interplay of multiuser scheduling and antenna diversity. First, the higher the Nakagami fading parameter, the lower the multiuser diversity gain for all the considered antenna schemes. Second, from the standpoint of multiuser scheduling, the multiple antennas with the ST/SC method can be viewed as virtual users to amplify multiuser diversity order. Third, the boosted array gain of the MRT/MRC scheme can compensate the detrimental impact of the reduced amount of fading gain on multiuser scheduling, thereby resulting in greater capacity than the ST/SC method. Last, employing the STBC scheme together with multiuser diversity may cause capacity loss due to the reduced amount of fading gain, but without the supplement of array gain.     

Throughput and Channel Access Statistics of Generalized Selection Multiuser Scheduling

To provide a near-optimal low-complexity solution to parallel multiuser scheduling in code-division multiple-access (CDMA), we propose generalized selection multiuser diversity (GSMuD) schemes with multi-code channel assignment and analyze their performance. The proposed GSMuD (Lc, L) schemes rank a total of L users awaiting transmissions by their signal-tonoise ratios (SNRs) and select the Lc (1 ? Lc ? L) users with the largest absolute (or normalized) SNRs for parallel channel access, which achieve near-optimal sum rate with a low scheduling complexity. The sum and individual channel throughput rates, second order statistics, fairness, and channel access statistics of the proposed GSMuD schemes are derived, taking into account different types of generalized fading channels. Compared to the round robin (RR) scheduling without SNR ranking, the GSMuD with normalized SNR ranking achieves a substantially higher sum rate while maintaining fairness. GSMuD also significantly improves the channel access performance and the degree of fairness than selective multiuser diversity (SMuD), which selects one best user only at each time slot.     

Scheduling with base station diversity and fairness analysis for the downlink of CDMA cellular networks

Efficient packet scheduling in CDMA cellular networks is a challenging problem due to the time variant and stochastic nature of the channel fading process. Selection diversity is one of the most effective techniques utilizing random and independent variations of diverse channels to improve the performance of communication over fading channels. In this paper, we propose two packet scheduling schemes exploiting base station selection diversity in the downlink of CDMA cellular networks. The proposed schemes rely on the limited instantaneous channel state information (CSI) to select the best user from the best serving base station at each time slot. This technique increases the system throughput by increasing multiuser diversity gain and reducing the effective interference among adjacent base stations. Results of Monte Carlo simulations are given to demonstrate the improvement of system throughput using the proposed scheduling schemes. In addition, we investigate fairness issue of wireless scheduling schemes. Due to different characteristics of wireless scheduling schemes, the existing fairness indexes may result in misleading comparison among different schemes. We propose a new fairness index to compare the overall satisfaction of the network users for different scheduling schemes. Copyright © 2006 John Wiley & Sons, Ltd.     

Impacts of Radio Channel Characteristics, Heterogeneous Traffic Intensity, and Near–Far Effect on Rate Adaptive Scheduling Algorithms

Applying adaptive modulation combined with scheduling in a shared data channel can substantially improve the spectral efficiency for wireless systems. This performance gain results from the multiuser diversity, which exploited independent channel variations across multiple users. In this paper, we present a cross-layer analysis to integrate physical-layer channel characteristics, media access control (MAC) layer scheduling strategies, and the network layer issue of heterogeneous traffic intensity across near–far users. Specifically, for radio channel characteristics, we take account of path loss, slowly varying log-normal shadowing and fast-varying Nakagami fading. We also evaluate the impact of selective transmit diversity on the throughput and fairness of wireless data networks. Furthermore, we consider three MAC schedulers: random scheduler, greedy scheduler (GS), and a newly proposed queue-length-based scheduler (QS). By applying the proposed cross-layer analytical framework, the following insights can be gained. First, for the three considered schedulers, channel fluctuations induced by Nakagami fading or log-normal shadowing can improve both total throughput and fairness. Second, using selective transmit diversity can improve throughput, but is unfavorable for the fairness performance. Third, the GS and the QS methods can improve throughput at the expense of unfairness to the far users. However, the throughput improvement from using the GS and the QS decreases as the traffic intensity of the far user increases. In summary, this cross-layer analysis can be used to develop new scheduling mechanisms for achieving better tradeoff between the fairness and throughput for wireless data networks.     

空分复用多输入多输出系统中的分组多用户分集

该文针对基于空分复用的多用户多输入多输出系统下行链路,研究了空间衰落相关和信道估计误差对系统容量的影响,提出了两种新的分组多用户分集方案。所提出的方案通过合理选择共道用户,能有效提升系统容量,同时兼顾公平性。分析和仿真结果表明,所提出的方案对衰落相关性以及信道估计误差具有较好的稳健性;其中,第2种方案与第1种相比较,容量性能更优,但具有较高的计算复杂度。     

Fairness based resource allocation for multiuser MISO-OFDMA systems with beamforming

Resource allocation problem in multiuser multiple input single output-orthogonal frequency division multiple access (MISO-OFDMA) systems with downlink beamforming for frequency selective fading channels is studied. The article aims at maximizing system throughput with the constraints of total power and bit error rate (BER) while supporting fairness among users. The downlink proportional fairness (PF) scheduling problem is reformulated as a maximization of the sum of logarithmic user data rate. From necessary conditions on optimality obtained analytically by Karush-Kuhn-Tucker (KKT) condition, an efficient user selection and resource allocation algorithm is proposed. The computer simulations reveal that the proposed algorithm achieves tradeoff between system throughput and fairness among users.     

Opportunistic Beamforming and Scheduling for OFDMA Systems

Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser diversity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by applying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fairness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity     

Distributed approaches for exploiting multiuser diversity in wireless networks

In wireless fading channels, multiuser diversity can be exploited by scheduling users to transmit when their channel conditions are favorable. This leads to a sum throughput that increases with the number of users and, in certain cases, achieves capacity. However, such scheduling requires global knowledge of every user's channel gain, which may be difficult to obtain in some situations. This paper addresses contention-based protocols for exploiting multiuser diversity with only local channel knowledge. A variation of the ALOHA protocol is given in which users attempt to exploit multiuser diversity gains, but suffer contention losses due to the distributed channel knowledge. The growth rate of the sum throughput for this protocol is characterized in a backlogged system under both short-term and long-term average power constraints. A simple "fixed-rate" system is shown to be asymptotically optimal and to achieve the same growth rate as in a system with an optimal centralized scheduler. Moreover, asymptotically, the fraction of throughput lost due to contention is shown to be 1/e. Also, in a system with random arrivals and an infinite user population, a variation of this ALOHA protocol is shown to be stable for any total arrival rate, given that users can estimate the backlog.     

多天线系统中的多用户分集方案的性能分析及比较

在多天线多用户无线通信系统中,可以利用用户之间彼此独立的随机衰落信道的特点,结合随机波束成型技术,或者空间分集技术,或者天线选择技术,获得多用户分集增益。然而不同多用户分集方案的系统性能不尽相同。该文分析了在多输入单输出(MISO)信道中目前几种典型的多用户分集方案的性能,给出系统吞吐率的定量表达式,并综合比较了不同用户数和不同天线数对各种多用户分集方案性能的影响,为实际多天线多用户系统设计提供了有益的参考。     

Space-Time-Block-Coded Transmissions for Downlink MC-CDMA Transmissions Over Frequency-Selective Fading Channels

Mitigation of multipath fading effects and suppression of multiuser interference (MUI) constitute major challenges in the design of the third generation wireless mobile systems. Space-time (ST) coding offers a attractive solution to cope with mutipath fading, but most existing ST coding schemes assume flat fading channels that may not be realistic for wide-band communications. Especially multiuser ST block-coded transmissions through multipath fading channels present unique challenge in suppressing not only MUI but also intersymbol/chip interference. In this paper, we design ST multiuser transceivers for MC-CDMA quasi-synchronous systems, capable to reliably transmit over frequency-selective multipath downlink channels. The proposed system is able to provide transmit diversity and to guarantee symbol recovery in multiuser environments, regardless of unknown multipath. Unlike existing approaches, the mobile does not need to know the channel of other users. In addition to decoding simplicity, computer simulations show the performance merits of the proposed transceiver.     

一种低复杂度的多用户分集调度算法

利用多用户分集增益可以显著的提高系统容量,但最大化吞吐量的调度算法会导致用户不公平的占用资源,比例公平算法在损失较大容量的条件下可以让用户公平的分享资源。本文重点考虑多用户分集系统中的系统容量和公平性的折中,结合最大化吞吐量和比例公平调度算法的思想,提出一种低复杂度的加权调度算法,并推导出门限反馈和全反馈时的系统容量和公平性的理论表达式。仿真结果表明,所提算法在损失较少容量的同时,可以保持较高的公平性。并且在用户数较少的同时,性能优于已有的折中算法,而在用户数较多时,可以通过设置反馈门限来降低用户的反馈开销,同时不会带来容量和公平性的损失。     

Spatial-multiplexing phase-sweep transmit diversity (SM-PSTD) for multiantenna base stations with mobile-assisted scheduling and incremental rate feedback

Phase-sweep transmit diversity (PSTD) has been widely studied in 3G1X and Universal Mobile Telecommunications Systems (UMTS) for its performance gain and simplicity. However, conventional PSTD systems are designed based on asingle-sweeping beam, and only one user is selected for transmission at any time. Therefore, it fails to fully exploit the available spatial degree of freedom. In this study, a novel spatial-multiplexing phase-sweep transmit diversity (SM-PSTD) scheme with partial feedback mobile-assisted scheduling is being proposed. The downlink performance (system capacity and proportional fairness) of a multiantenna base station has been its focus. The spatial-channel sweeping facilitates the multiuser selection diversity between users, while the spatial multiplexing significantly enhances the system capacity. As the number of transmit antennas n/sub T/ is increased, the SM-PSTD system performance is gradually limited by the multiuser interference because it is more and more difficult to find a set of users perfectly orthogonal within the n/sub T/ spatial channels. Yet, the multiuser interference could be reduced by increasing the number of active users K in the system. Asymptotically, at large n/sub T/ and K, it shows that the system capacity scales linearly instead of logarithmically with respect to the transmitted power.     

Antenna diversity in multiuser data networks

We consider the use of multiple antennas at the transmitter and/or the receiver to provide open-loop spatial diversity in a multiuser wireless data network. With channel quality information (CQI) available to the transmitter, and by always scheduling the transmission to the active user having the best channel conditions at the time of scheduling, another form of diversity, termed multiuser diversity, is obtained in a data system. This paper provides an analysis of the interaction between these two forms of diversity. From a network point of view, we prove that the asymptotic sum rate, in the limit of a large number of active homogeneous users and subject to the same average total transmit power, is inversely related to the number of transmit antennas for independent and identically distributed (i.i.d.) flat Rayleigh fading channels. In the case of i.i.d. flat Rician fading, the asymptotic sum rate also depends inversely on the number of transmit antennas, but directly on the number of receive antennas. Numerically, we show that the total diversity gain is also constrained by finite CQI quantization and channel fading statistics.     

On Capacity and Fairness of Quality-Based Channel-State Reporting with Different Thresholds in Non-Identical Rayleigh Fading Channels

The quality-based channel-state reporting (QBR) that prevents the users with signal strength below a threshold from sending feedback induces a fairness problem in non-identically distributed fading channels. This letter extends QBR so as to achieve a fair throughput among users to whom different values of the threshold are assigned. With numerical investigation, the proposed extension is shown to achieve both multiuser diversity and a near absolute fairness where all the users have the same individual throughput.     

A flexible downlink scheduling scheme in cellular packet data systems

Fast downlink scheduling algorithms play a central role in determining the overall performance of high-speed cellular data systems, characterized by high throughput and fair resource allocation among multiple users. We propose a flexible channel-dependent downlink scheduling scheme, named the (weighted) alpha-rule, based on the system utility maximization that arises from the Internet economy of long-term bandwidth sharing among elastic-service users. We show that the utility as a function of per-user mean throughput naturally derives the alpha-rule scheme and a whole set of channel-dependent instantaneous scheduling schemes following different fairness criteria. We evaluate the alpha-rule in a multiuser CDMA high data rate (HDR) system with space-time block coding (STBC) or Bell Labs layered space-time (BLAST) multiple-input multiple-output (MIMO) channel. Our evaluation shows that it works efficiently by enabling flexible tradeoff between aggregate throughput, per-user throughput, and per-user resource allocation through a single control parameter. In other words the Alpha-rule effectively fills the performance gap between existing scheduling schemes, such as max-C/I and proportional fairness (PF), and provides an important control knob at the media-access-control (MAC) layer to balance between multiuser diversity gain and location-specific per-user performance.     

一种多用户MIMO系统的公平调度算法

多用户多输入多输出(MIMO)系统中需要对用户进行合理调度,一般基于信道增益的调度算法忽略了用户之间的公平性。基于此,提出了一种公平的多用户调度算法,其思想是先根据信道条件数初选出用户,然后统计每个用户被选择的次数,如果次数之差的绝对值大于设定门限,则选择次数少的用户。这样既能获得多用户分集增益又兼顾了公平性。仿真结果表明,提出的调度算法在系统性能无明显损失的情况下保证了用户公平性。     

Space-time block-coded multiple access through frequency-selectivefading channels

Mitigation of multipath fading effects and suppression of multiuser interference (MUI) constitute major challenges in the design of wide-band third-generation wireless mobile systems. Space-time (ST) coding offers an effective transmit-antenna diversity technique to combat fading, but most existing ST coding schemes assume flat fading channels that may not be valid for wide-band communications. Single-user ST coded orthogonal frequency-division multiplexing transmissions over frequency-selective channels suffer from finite-impulse response channel nulls (fades). Especially multiuser ST block-coded transmissions through (perhaps unknown) multipath present unique challenges in suppressing not only MUI but also intersymbol/chip interference. In this paper, we design ST multiuser transceivers suitable for coping with frequency-selective multipath channels (downlink or uplink). Relying on symbol blocking and a single-receive antenna, ST block codes are derived and MUI is eliminated without destroying the orthogonality of ST block codes. The system is shown capable of providing transmit diversity while guaranteeing symbol recovery in multiuser environments, regardless of unknown multipath. Unlike existing approaches, the mobile does not need to know the channel of other users. In addition to decoding simplicity, analytic evaluation and corroborating simulations reveal its flexibility and performance merits     

Performance analysis of MIMO systems with multiuser diversity

In this paper, we present a comprehensive performance analysis for multiple‐input multiple‐output (MIMO) systems with multiuser diversity over Rayleigh fading channels. We derive exact closed‐form expressions of the outage probability and the average bit error rate (BER) for different MIMO schemes, including the selective combining (SC), maximum ratio combining (MRC) and space‐time block codes (STBC). We also provide the explicit upper bounds on the BER performance. Finally, the mathematical formalism is verified by numeric results that study the interaction between the antenna diversity and the multiuser diversity. It is observed that the system performance is deteriorated as the number of transmit antennas increases in multiuser scenario, which is unlike the case in single‐user systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Quantized feedback MIMO scheduling for heterogeneous broadcast networks

One-bit quantization of signal-to-interference-plus-noise ratio is discussed in literature for user scheduling in homogeneous network where users are assumed to have equal signal-to-noise ratio (SNR). It is mentioned in literature that 1-bit quantization with fixed quantization threshold does not achieve multiuser diversity. Moreover, the system sum-rate achieved by this lags significantly behind that of full feedback scheme. Two multi-bit quantized feedback scheduling schemes are proposed for broadcast network with heterogeneous users experiencing different channel statistics. It is presented that these two schemes with fixed optimum quantization thresholds profit from the diversity provided by independent and identically distributed channels. Moreover, proposed optimistic multi-bit quantized scheduling scheme achieves higher system sum-rate than the proposed multi-bit quantized scheme by addressing the limitations of the later one. The optimum quantization thresholds depend on the number of transmit antennas and system SNR. Moreover, these multi-bit quantized feedback scheduling schemes also ensure user fairness. Simulation results are presented to support the numerical analysis.