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     

OFDMA系统中多波束机会波束成形研究

机会波束成形（Opportunistic Beamforming,OBF）技术通过加大、加快信道的波动来提高多用户分集增益,将之使用在正交频分复用多址接入（OFDMA）系统中,可以使得系统的吞吐量最大化。在OFDMA系统中,通过对所有子载波进行分簇,可以实现在牺牲系统性能很少的情况下,大大降低系统反馈量。研究了对分簇OFDMA系统使用OBF技术来提高系统吞吐量,并进一步研究了在训练阶段使用多个随机加权向量进行多波束成形,然后从中选择最好的一个来实现数据阶段的波束成形。仿真证明,采用多波束机会波束成形技术能提高OFDMA系统吞吐量。     

多小区大规模协同功率分配及波束成形算法

针对大规模多输入单输出的多点协作下行系统,本文主要研究协同波束成形和功率控制,以达到最大化最差用户信干噪比的目的。为了求解原始下行的非凸优化问题,首先将原始优化问题转化成等价的上行优化问题进行求解。尽管在有限系统里可通过迭代算法获得波束矢量和发射功率,但是该算法依赖于瞬时信道信息,功率也需要瞬时更新。为了减少功率更新计算复杂度,本文进一步利用随机矩阵理论,提出了只需要依赖统计信道信息的算法来获得发射功率。数值仿真验证了单基站功率约束下所提算法的有效性以及相对于最大比发送算法的优越性。      

机会波束成形系统中一种新的调度方案

主要研究机会波束成形系统中的调度算法,将宽带无线IP网络调度算法应用于机会波束成形系统。提出了一个保证用户QoS要求的新方案。数值分析表明,所提的方案较之传统的方案在保证系统吞吐量和用户间公平性的同时,也保证了用户的QoS。     

Performance Analysis of an OFDMA Transmission System in a Multicell Environment

The paper deals with design and performance analysis of orthogonal frequency-division multiple-access (OFDMA)-based downlink cellular wireless communications. Due to a high degree of user mobility, the base station is assumed to have only a statistical knowledge of the users' channels. Relying on the ergodic capacities connected to the user rates, a subcarrier and power allocation that minimizes the total transmitted power is proposed. The allocation strategy requires only the knowledge of the channel statistics and the rate requirements for all users. An extension and a performance analysis of this allocation algorithm in a multicell environment working with a frequency reuse factor equal to one is also conducted. A condition for the multicell network to be able to satisfy all rate requirements is derived     

Opportunistic Beamforming with Limited Feedback

This work investigates the following question: subject to strictly limited (finite-rate) feedback in a multi-user multi-antenna system, what channel state information (CSI) should we send back to the transmitter, and how should it be used? Considering the class of single-beam systems, we suggest a combination of beamforming (array gain) and multi-user diversity. It has been shown that in single antenna systems, one bit of feedback per user can capture almost all gains available due to multi-user diversity, therefore we propose and analyze a compound strategy that uses one bit for multi-user diversity and any further feedback bits for beamforming. We obtain the scaling laws of this compound strategy, showing that it scales as well as any single-beam system with full transmit-CSI.     

A Distributed Opportunistic Access Scheme and its Application to OFDMA Systems

Multiuser systems can provide multiuser diversity gains by assigning channels to users with higher channel gains. To avoid the extensive information exchange with the access point for the uplink access in centralized approaches, we propose in this paper a distributed opportunistic access scheme. Through a judicious design of a novel backoff mechanism to utilize the channel information and reduce collisions, significant multiuser diversity gains are achieved. To a user, the higher the channel gain is, the smaller the backoff time-slot and, hence, the higher the access priority of that user is. In addition, for heterogeneous systems, our proposed scheme can realize multiuser diversity gains and achieve fairness among the users at the same time. Finally, we design two distributed opportunistic access schemes for OFDMA systems. Users contend on all sub-channels in the first scheme and only on several strongest sub-channels in the second scheme. Compared with traditional centralized OFDMA systems and other distributed access schemes, our proposed schemes reduce overhead and achieve a higher throughput.     

On the Optimality of Multi-User Coordinated Zero-Forcing Beamforming Multicell Systems with Limited Feedback in Interference Channels

Wireless Personal Communications - An efficient multicell coordinated zero-forcing channel feedback scheme is proposed in this paper. The objective of the proposed feedback design is to control the...     

有限反馈机会中继通信系统的中断概率性能

该文针对放大转发机会中继通信系统,研究了有限反馈条件下系统的中断概率及分集性能。在有限反馈条件下,量化误差将影响最优中继的选择进而恶化机会中继通信系统的性能。该文首先推导了有限反馈条件下机会中继系统的中断概率近似表达式,并在此基础上分析获得两个结论：在有限反馈条件下,无论备选中继数目为何,系统的分集阶数可达且只可达到2;当备选中继个数一定时,随着反馈比特数的增加,系统中断概率将趋近于选择最佳中继节点所能获得的性能。仿真结果验证了中断概率的理论表达式及其分析结论,可为有限反馈条件下的中继传输协议设计提供理论支持。     

Performance Analysis of Quantized Beamforming MIMO Systems

Multiple-input multiple-output (MIMO) wireless systems can achieve significant diversity and array gain by using single-stream transmit beamforming and receive combining. A MIMO beamforming system with feedback using a codebook based quantization of the beamforming vector allows practical implementation of such a strategy in a single-user scenario. The performance of this system in uncorrelated Rayleigh flat fading channels is studied from the point-of-view of signal-to-noise ratio (SNR) and outage probability. In this paper, lower bounds are derived on the expected SNR loss and the outage probability of systems that have a single receive antenna or two transmit antennas. For arbitrary transmit and receive antennas, approximations for the SNR loss and outage are derived. In particular, the SNR loss in a quantized MIMO beamforming system is characterized as a function of the number of quantization bits and the number of transmit and receive antennas. The analytical expressions are proved to be tight with asymptotically large feedback rate. Simulations show that the bounds and approximations are tight even at low feedback rates, thereby providing a benchmark for feedback system design     

Non-iterative Coordinated Beamforming for Multicell MIMO Heterogeneous Networks

Wireless Personal Communications - This paper presents ultra-wideband channel characteristics in the presence of float glass slab as obstacle of varied thickness (4 mm, 8 mm,...     

Cooperative Multicell Zero-Forcing Beamforming in Cellular Downlink Channels

In this work, a multicell cooperative zero-forcing beamforming (ZFBF) scheme combined with a simple user selection procedure is considered for the Wyner cellular downlink channel. The approach is to transmit to the user with the ldquobestrdquo local channel in each cell. The performance of this suboptimal scheme is investigated in terms of the conventional sum-rate scaling law and the sum-rate offset for an increasing number of users per cell. We term this characterization of the sum-rate for large number of users as high-load regime characterization, and point out the similarity of this approach to the standard affine approximation used in the high-signal-to-noise ratio (SNR) regime. It is shown that, under an overall power constraint, the suboptimal cooperative multicell ZFBF scheme achieves the same sum-rate growth rate and slightly degraded offset law, when compared to an optimal scheme deploying joint multicell dirty-paper coding (DPC), asymptotically with the number of users per cell. Moreover, the overall power constraint is shown to ensure in probability, equal per-cell power constraints when the number of users per-cell increases.     

Improved Opportunistic Beamforming in Ricean Channels

In order to improve the performance of opportunistic beamforming in Ricean channels, we propose an improved opportunistic beamforming scheme which forms beams intelligently to the users. First, a new concept of the generalized Ricean distribution is introduced. On the basis of this theory, we propose a maximum-likelihood (ML) estimator of the directions of arrival of the users, and we derive the Cramer-Rao lower bound to evaluate the performance of the ML estimator. Furthermore, the generalized Ricean K-factors are estimated by a moment-based method. With the same pilot overhead in the downlink and with the same feedback overhead in the uplink, the improved scheme considerably outperforms conventional opportunistic beamforming     

Corrections to “Opportunistic Beamforming and Scheduling for OFDMA Systems” [May 07 941-952]

In the above titled paper (ibid., vol. 55, no. 5, pp. 941-952, May 07), a change to equation (7), that was requested by the author, was omitted in the final version. The paragraph leading up to, and including the equation, is repeated here.     

Opportunistic Beamforming over Rayleigh Channels with Partial Side Information

In recent years, diversity techniques have evolved into highly attractive technology for wireless communications in different forms. For instance, the channel fluctuations of the users in a network are exploited as multiuser diversity by scheduling the user with the best signal-to-noise ratio (SNR). When fading is slow, beamforming at a multiple antenna transmitter is used to induce artificial channel fluctuations to ensure multiuser diversity in the network. Such a beamforming scheme is called opportunistic beamforming since the transmitter uses random beamforming to artificially induce opportunism in the network [1]. Opportunism requires a large number of users in the system in order to reach the performance of the true beamforming that uses perfect channel state information (CSI). In this paper we investigate the benefit of having partial CSI at an opportunistic transmitter. In the investigation, we focus on the maximum normalized SNR scheduling where user?s feedback consists of SNR relative to its channel gain. We show that opportunism can be beneficially used to increase the average throughput of the system. Simulations support the analytical average throughput results obtained as the amount of CSI and the number of users vary.     

过时信道状态信息下的机会式中继选择系统的安全性能分析

在 Nakagami m衰落信道下,目的端和窃听者采用最大比合并策略,本文研究了在机会式自适应解码转发中继选择安全协作系统中的安全性能。由于实际信道中的反馈延迟,最优的合法中继选择基于合法信道反馈的过时信道状态信息。为了评价机会式中继选择在改善安全性能上的表现,分别推导了准确的正安全容量概率和准确的安全中断概率闭合表达式。此外,针对两种不同情况, 推导了形式简单的渐近表达式,并明确给出安全分集阶数和安全阵列增益。理论分析和数值仿真表明,增加中继个数和目的节点的天线数能够改善安全中断概率的性能表现,且在信道状态信息过时的条件下,系统的安全分集阶数与中继数无关。      

Coordinated Scheduling and Power Allocation in Downlink Multicell OFDMA Networks

We consider a multicell orthogonal frequency-division multiple-access (OFDMA) wireless network with universal frequency reuse and treat the problem of cochannel interference mitigation via base station coordination in the downlink. Assuming that coordinated access points only share channel quality measurements but not user data symbols, we propose to select the set of cochannel users and the power allocation across tones to maximize the weighted system sum rate subject to per-base-station power constraints. Since this is a nonconvex combinatorial problem, efficient suboptimal algorithms are presented and discussed, each requiring a different level of coordination among base stations and a different feedback signaling overhead. Simulation results are provided to assess the performances of the proposed strategies.      

Block-Based Performance Measures for MIMO OFDM Beamforming Systems

In this paper, we consider an adaptive modulation system with multiple-input–multiple-output (MIMO) antennas in conjunction with orthogonal frequency-division multiplexing (OFDM) operating over frequency-selective Rayleigh fading environments. In particular, we consider a type of beamforming with a maximum ratio transmission/maximum ratio combining (MRT-MRC) transceiver structure. For this system, we derive a central limit theorem for various block-based performance metrics. This motivates an accurate Gaussian approximation to the system data rate and the number of outages per OFDM block. In addition to the data rate and outage distributions, we also consider the subcarrier signal-to-noise ratio (SNR) as a process in the frequency domain and compute level crossing rates (LCRs) and average fade bandwidths (AFBs). Hence, we provide fundamental but novel results for the MIMO OFDM channel. The accuracy of these results is verified by Monte Carlo simulations, and applications to performance analysis and system design are discussed.      

TDD MISO系统中波束成形的性能分析

文中首先指出在时分双工多输入单输出系统中,发送机和接收机的信道状态信息存在时延,而该时延会影响波束成形的性能增益。随后,通过推导接收机误比特率的闭合表达式,说明时延会损害接收机的性能。最后,用数值仿真验证了所做的性能分析的正确性。     

多波束机会波束成形系统性能比较

研究了在多输入单输出（MISO）系统中通过加大、加快信道的波动来提高多用户分集增益的机会波束成形技术,并在此基础上研究了在每个时隙中采用多个加权矢量的多波束机会波束成形技术,着重对它们的性能作了仿真比较。结果表明,多波束机会波束成形技术可以提高系统吞吐量。