On Scheduling for Multiple-Antenna Wireless Networks Using Contention-Based Feedback

Multiuser diversity gain is an effective technique for improving the performance of wireless networks. This gain can be exploited by scheduling the users with the best current channel conditions. However, this kind of scheduling requires that the base station (or access point) knows some kind of channel quality indicator (CQI) information for every user in the system. When the wireless link lacks channel reciprocity, each user must feed back this CQI information to the base station. The required feedback load makes exploiting multiuser diversity extremely difficult when the number of users becomes large. To alleviate this problem, this paper considers a contention-based CQI feedback where only users whose channel gains are larger than a threshold are allowed to transmit their CQI information through a spread-spectrum based contention channel. Considering the capture effect in this contention channel, it is shown that i) the multiuser diversity gain can be exploited regardless of the number of transmit antennas at the base station and ii) the total system throughput exponentially approaches that of the full feedback scheme as the spreading code length of the contention channel linearly increases. In addition, it is also shown that multiuser diversity can be maintained with the feedback delay of time-variant channels. We also consider the issue of differentiated rate scheduling, in which the base station gives different rates to different subsets of mobiles. In this scenario, mobiles feed back their CQI with some access probability, and we show this technique causes only a negligible throughput loss compared to the case without supporting differentiated rate.     

Exploiting Multiuser Diversity With Imperfect One-Bit Channel State Feedback

It has been well recognized that significant throughput gains can be leveraged in multiuser wireless communication systems by exploiting multiuser diversity with a smart scheduler. This scheduler collects channel state information (CSI) from all users and allocates the resources to the user(s) experiencing favorable channel conditions. However, for a frequency-division-duplex system with a large number of users, how to efficiently collect the required CSI will be a challenging task, especially when the feedback links are of limited capacity. In this paper, we propose a scheduling algorithm to exploit multiuser diversity with possibly imperfect one-bit channel state feedback. The basic idea is to define a threshold lambda and let each user report one-bit information to the scheduler about the comparison between its measured channel fading level and lambda. Correspondingly, the scheduler uses these feedback bits to classify all users into two sets and assigns the channel to one user belonging to the set experiencing favorable channel conditions. Several implementation schemes are developed by attacking the optimization of lambda under different system configurations, covering both the case when the one-bit feedback is perfect and those when the one-bit feedback is imperfect. Computer simulations show that when the user number is large, say, more than ten users, the proposed scheduling supports significantly larger data rate over the round-robin scheduling, while in comparison with the optimum scheduling with complete CSI, the performance loss is limited if the one-bit feedback is of high reliability. In addition, our studies show that we can effectively enhance the robustness against feedback imperfectness by incorporating the feedback reliability into optimization of lambda     

一种适用于协作多点联合传输的自适应PMI反馈方法

作为下一代无线通信系统的关键技术,协作多点传输能够有效降低相邻小区之间的干扰,提高小区边缘用户的频谱效率。在频分双工系统中,其性能依赖于各基站获取的基于有线反馈的信道状态信息的准确性。然而,所有协作基站都获得相同精确程度的信道状态信息会导致用户终端的反馈开销随着协作基站数线性增长。本文关注于如何在性能增益和反馈开销之间取得折中,提出一种适用于协作多点联合传输的自适应预编码矩阵索引反馈方法。用户终端根据不同协作基站下行链路的信道质量指示的相对大小,自适应的用不同的比特数表示不同强度链路的预编码矩阵索引,即用较多比特数表示具有较高信道质量指示的预编码矩阵索引,反之亦然。仿真结果表明,与传统反馈方案相比,该方法在几乎不牺牲频谱效率的前提下,大幅降低了反馈开销。      

Multiuser adaptive transmission technique for time-varying frequency-selective fading channels

This paper proposes an efficient multiuser adaptive modulation and coding (AMC) scheme that considers inevitable feedback delay by employing short-term and long-term channel state information (CSI) in time-varying frequency-selective fading channels. By taking the statistic of the true signal-to-noise ratio (SNR) at a given predicted SNR value into account, the required transmit power to meet the target packet-error-rate (PER) can be obtained and used for user selection, power allocation, and modulation and coding set (MCS) selection. In addition, a simple and useful approximation method of obtaining the required transmit power is proposed. The performance of the proposed scheme is shown to be much better than that of conventional schemes without considering the feedback delay or the prediction error. The proposed scheme can also reduce the feedback resource while maintaining the system throughput by allocating different feedback resources to different users according to their prediction error variances.     

Performance of Orthogonal Beamforming for SDMA With Limited Feedback

On the multiantenna broadcast channel, the spatial degrees of freedom support simultaneous transmission to multiple users. The optimal multiuser transmission, which is known as dirty paper coding, is not directly realizable. Moreover, close-to-optimal solutions such as Tomlinson-Harashima precoding are sensitive to channel state information (CSI) inaccuracy. This paper considers a more practical design called per user unitary and rate control (PU2RC), which has been proposed for emerging cellular standards. PU2RC supports multiuser simultaneous transmission, enables limited feedback, and is capable of exploiting multiuser diversity. Its key feature is an orthogonal beamforming (or precoding) constraint, where each user selects a beamformer (or precoder) from a codebook of multiple orthonormal bases. In this paper, the asymptotic throughput scaling laws for PU2RC with a large user pool are derived for different regimes of the signal-to-noise ratio (SNR). In the multiuser interference-limited regime, the throughput of PU2RC is shown to logarithmically scale with the number of users. In the normal SNR and noise-limited regimes, the throughput is found to scale double logarithmically with the number of users and linearly with the number of antennas at the base station. In addition, numerical results show that PU2RC achieves higher throughput and is more robust against CSI quantization errors than the popular alternative of zero-forcing beamforming if the number of users is sufficiently large.     

Differential and coherent decorrelating multiuser receivers for space-time-coded CDMA systems

Previously, we proposed a differential space-code modulation (DSCM) scheme that integrates the strength of differential space-time coding and spreading to achieve interference suppression and resistance to time-varying channel fading in single-user environments. In this paper, we consider the problem of multiuser receiver design for code-division multiple-access (CDMA) systems that utilize DSCM for transmission. In particular, we propose two differential receivers for such systems. These differential receivers do not require the channel state information (CSI) for detection and, still, are resistant to multiuser interference (MUI) and time-varying channel fading. We also propose a coherent receiver that requires only the CSI of the desired user for detection. The coherent receiver yields improved performance over the differential receivers when reliable channel estimates are available (e.g., in slowly fading channels). The proposed differential/coherent receivers are decorrelative schemes that decouple the detection of different users. Both long and short spreading codes can be employed in these schemes. Numerical examples are presented to demonstrate the effectiveness of the proposed receivers.     

A Strategy to Reduce the Signaling Requirements of CQI Feedback Schemes

Resource allocation in OFDMA systems involves the application of adaptive modulation/coding (AMC). AMC provides the flexibility to match the modulation-coding scheme (MCS) to each user’s channel conditions. In order to accurately determine the appropriate MCS level, full channel state information (CSI) is required by the base-station (BS). Full CSI feedback requires bandwidth to be reserved in the OFDM frame which is occupied by the users to transmit channel quality information (CQI) to the BS. Allocations and deallocations of channels reserved for CQI feedback require expensive signaling messages between the base stations and the mobile stations, and therefore should be minimized. In this paper, we show that the amount of bandwidth consumed for this type of signalling can be significantly reduced by selecting the users with the lowest average SNR to periodically feedback their instantaneous channel state information. These benefits become more evident at low values of the user velocity.     

Performance of heterogeneous multiuser MIMO system with cross-layer design and multiple outdated estimations in Rayleigh fading channel

By combining adaptive modulation and automatic repeat request protocol as well as user scheduling, a cross-layer design (CLD) scheme for multiuser MIMO system with the normalized signal-to-noise ratio (SNR)-based scheduling under heterogeneous case is developed, where perfect and imperfect channel state information (CSI) are both considered. Based on this, by exploiting previous channel estimation information, the multiple outdated estimates method is presented to improve the system performance under imperfect CSI. According to the performance analysis and the normalized-SNR based scheduling scheme, the average spectral efficiency (SE) and packet error rate (PER) of the system are derived, respectively. As a result, closed-form SE and PER expressions are attained. These expressions include the ones with conventional single outdated estimate as special cases, and can provide good performance evaluation for the multiuser CLD system under heterogeneous case. Simulation results show that the derived theoretical SE and PER are in consistent with the corresponding simulations. Moreover, the presented CLD scheme with multiple estimates can provide higher SE and lower PER than that with conventional single estimate in the presence of imperfect CSI. Besides, multiuser MIMO-CLD has superior performance over the conventional single user counterpart due to the multiuser diversity.     

Multiple antenna MMSE based downlink precoding with quantized feedback or channel mismatch

In this paper, we consider the downlink of a multiuser wireless communication system with multiple antennas at the base station and users each with a single receive antenna. It is known that when channel state information (CSI) is available at the transmitter a large performance gain can be achieved. In a system employing time-division duplexing (TDD), CSI can be obtained at the base station if there is reciprocity between the forward and reverse channels. CSI can also be conveyed from the users to the base station via a limited-rate feedback channel in a frequency-division duplexing (FDD) system. In any case, channel estimation errors are inevitable due to the presence of background noise in the estimated signal and due to the finite number of feedback bits used in a limited-rate feedback system model. In this paper, we first consider the general case when partial CSI is available at the transmitter. We derive an MMSE based precoding technique that considers channel estimation errors as an integral part of the system design. Using rate-distortion theory and the generalized Lloyd vector quantization algorithm, we then specialize our results for the more practical limited-rate feedback system model. Compared to previously proposed precoding techniques such as channel inversion and regularized channel inversion, it is shown that the proposed precoding technique significantly improves the average bit error rate (BER) in the system. Furthermore, the performance of the proposed technique is investigated in the high signal-tonoise ratio (SNR) regime, and similar to [1], [2], it is shown that the proposed technique suffers from a ceiling effect that asymptotically limits the system performance.     

On the capacity of MIMO broadcast channels with partial side information

In multiple-antenna broadcast channels, unlike point-to-point multiple-antenna channels, the multiuser capacity depends heavily on whether the transmitter knows the channel coefficients to each user. For instance, in a Gaussian broadcast channel with M transmit antennas and n single-antenna users, the sum rate capacity scales like Mloglogn for large n if perfect channel state information (CSI) is available at the transmitter, yet only logarithmically with M if it is not. In systems with large n, obtaining full CSI from all users may not be feasible. Since lack of CSI does not lead to multiuser gains, it is therefore of interest to investigate transmission schemes that employ only partial CSI. We propose a scheme that constructs M random beams and that transmits information to the users with the highest signal-to-noise-plus-interference ratios (SINRs), which can be made available to the transmitter with very little feedback. For fixed M and n increasing, the throughput of our scheme scales as MloglognN, where N is the number of receive antennas of each user. This is precisely the same scaling obtained with perfect CSI using dirty paper coding. We furthermore show that a linear increase in throughput with M can be obtained provided that M does not not grow faster than logn. We also study the fairness of our scheduling in a heterogeneous network and show that, when M is large enough, the system becomes interference dominated and the probability of transmitting to any user converges to 1/n, irrespective of its path loss. In fact, using M=/spl alpha/logn transmit antennas emerges as a desirable operating point, both in terms of providing linear scaling of the throughput with M as well as in guaranteeing fairness.     

Hybrid beamforming design based on unsupervised machine learning for millimeter wave systems

This article proposes a hybrid beamforming design with reduced channel state information (CSI) feedback. We use a beam sweeping procedure to provide channel measurements and feed a CSI report scheme. Thereby, the base station (BS) can perform an adequate estimation of the channel characteristics with reduced signaling overhead. Consequently, we need short pilot sequences and very few precoding matrix indicators (PMIs) to properly describe channel behavior. Moreover, we also evaluate different user selection strategies based on unsupervised machine learning framework that exploits the channel information provided by the proposed beam sweeping scheme. Our performance evaluation indicates that the user selection based on fuzzy c‐means is able to efficiently explore the reduced CSI. The proposed hybrid beamforming scheme reduces the multi‐user interference and achieves significant gains in total data rate as channel conditions and interference environment becomes more challenging.     

Unquantized and uncoded channel state information feedback in multiple-antenna multiuser systems

We propose a channel state information (CSI) feedback scheme based on unquantized and uncoded (UQ-UC) transmission. We consider a system where a mobile terminal obtains the downlink CSI and feeds it back to the base station using an uplink feedback channel. If the downlink channel is an independent Rayleigh fading channel, then the CSI may be viewed as an output of a complex independent identically distributed Gaussian source. Further, if the uplink feedback channel is an additive white Gaussian noise channel, and the downlink CSI is perfectly known at the mobile terminal, it can be shown that UQ-UC CSI transmission (that incurs zero delay) is optimal in that it achieves the same minimum mean-squared error distortion as a scheme that optimally (in the Shannon sense) quantizes and encodes the CSI, while theoretically incurring infinite delay. Since the UQ-UC transmission is suboptimal on correlated wireless channels, we propose a simple linear CSI feedback receiver that can be used to improve the performance of UQ-UC transmission while still retaining the attractive zero-delay feature. We provide bounds on the performance of such UQ-UC CSI feedback and study its impact on the achievable information rates. Furthermore, we explore its application and performance in multiple-antenna multiuser wireless systems, and also propose a corresponding pilot-assisted channel-state estimation scheme.     

Subcarrier Wise Scheduling Methods for Multi-antenna and Multi-carrier Systems

In this paper, multiuser scheduling algorithms are evaluated for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) networks. These scheduling schemes allocate M [number of transmit antennas at base station (BS)] number of independent data streams from BS to the M most favourable users experiencing the highest signal-to-interference-plus-noise-ratio (SINR). Here, SINR is used to convey the channel state information (CSI) to the BS. We have investigated the system throughput and feedback overhead attained by these scheduling schemes for different scenarios as: (a) the maximum CSI is sent to the BS by every user and (b) the maximum CSI sent to the BS corresponding to every BS antenna. The overall feedback overhead incurred by MIMO-OFDM system increases linearly with number of users, number of subcarriers and number of transmit antennas. Hence, to reduce the feedback overhead, a scheme is proposed where users with SINR values greater than or equal to a predefined threshold value are only allowed to feedback the channel state information to BS. The relation between system throughput and various thresholds is also studied. The achievable system throughput results are validated by comparing the probability density function of achieved SINR values by different scheduling schemes.

     

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.     

Enhanced Unitary Beamforming Scheme for Limited-Feedback Multiuser MIMO Systems

In this letter, we consider practical downlink multiuser multiple-input multiple-output systems where each user has multiple antennas and codebook-based limited channel feedback is available at base station. We propose a preprocessing scheme for downlink transmission where a unitary beamforming matrix is used. Firstly, we propose how to construct the unitary matrix that maximizes sum rate. Secondly, we propose a codebook-based channel feedback method for the proposed beamforming method. Numerical results show that the proposed scheme outperforms conventional zero-forcing beamforming scheme in terms of sum rate.     

Improved Multiuser MIMO Unitary Precoding Using Partial Channel State Information and Insights from the Riemannian Manifold

Multiple-input multiple-output (MIMO) systems can be leveraged to increase capacity in fading channels. Especially in multiuser downlink communication systems, it has been shown that knowledge of channel state information at the transmitter (CSIT) is critical to leverage the capacity gain available from multiple antennas. When duplexing is performed using time division, CSIT can often be successfully obtained when channel reciprocity is available. CSIT acquisition, however, is much more difficult in frequency division duplexing. Sending feedback on the uplink has been shown to be a powerful technique to improve downlink performance in single user MIMO systems. The basic idea is to restrict the CSIT to a B bit codebook so that the mobiles can easily transmit these bits on the uplink. In this paper, we consider the multiuser downlink model with unitary precoding when there is a codebook consisting of 2^B unitary matrices that the precoder is restricted to lie in. This codebook is designed offline and known to both the basestation and all users. Each user sends back signal-to-interference plus noise ratio (SINR) information along with binary feedback about the unitary precoder. Based on the CSIT received on the uplink, the basestation selects one of the unitary matrices in the codebook to maximize the sum-rate. For this set-up, we first analyze the sum-rate performance of the unitary precoding scheme. We then show that the codebook of unitary precoders represents a collection of points in a special kind of manifold and show how the achievable sum-rate performance relates to the minimum distance of the codebook points in this space. Finally, we present a framework for constructing the codebook to maximize this minimum distance. Monte Carlo simulation results are presented to show the sum-rate performance of the proposed codebook design.     

Rayleigh信道多天线系统的酉空时信号识别技术

针对衰落信道中酉空时调制的识别问题,提出两种酉空时信号与传统空时码的识别方案。最大似然识别法利用信道转移概率密度构造平均似然比和广义似然比分类函数,依据不同码字似然比的差异完成分类,在对数域处理从而降低计算复杂度。高阶统计特性识别法利用随机矩阵的矩生成函数产生高阶联合矩和高阶联合累积量,依据酉空时信号特殊的高阶统计特性实现识别。最大似然识别法可在无信道状态信息的条件下完成识别,当已知信道状态信息时识别性能可大幅提高;高阶统计特性识别法需要信道状态信息,同样条件下与最大似然法相比其性能较差,且其准确性会受信道估计的影响,但实现的复杂度低。通过增加接收天线数量在各种方案中均可改善识别性能,4根接收天线相对2根接收天线的增益,无CSI的最大似然法为7-10dB,有CSI的高阶统计特性法可达45dB。仿真结果验证了所提方案的有效性。      

A Simple and Efficient User-Scheduling Strategy for RUB-Based Multiuser MIMO Systems and Its Sum-Rate Analysis

We present a user-scheduling scheme for multiuser multiple-input–multiple-output (MIMO) systems with random unitary beamforming (RUB) in this paper. The new scheme, which is termed as adaptive beam activation based on the conditional best beam index feedback (ABA-CBBI), requires low average feedback load by imposing a feedback threshold on the users' signal-to-interference-plus-noise ratio (SINR) and suffers less multiuser interference by only activating those beams requested by at least one user. We derive the exact analytical expression for the sum-rate capacity of the resulting multiuser MIMO systems, based on which we examine the optimal selection of the feedback threshold in terms of sum-rate maximization. We demonstrate through selected numerical examples that the proposed ABA-CBBI scheme with optimal thresholds can achieve better sum-rate performance than existing schemes over high-signal-to-noise-ratio (SNR) regions.      

Throughput analysis of multi-user OFDMA-systems using imperfect CQI feedback and diversity techniques

In this paper, the throughput of an adaptive multiuser SISO-OFDMA/FDD system with channel quality information (CQI) signalled digitized over a feedback channel to the transmitter is investigated, where the instantaneous signal-to-noise ratio (SNR) of the different subcarriers is used as CQI to exploit multi-user diversity using adaptive subcarrier allocation. The CQI available at the transmitter is assumed to be imperfect due to estimation errors and quantization at the receiver side, time delay and feedback errors. In this paper, a closed form expression of the average throughput of an adaptive multi-user OFDMA system using imperfect CQI and uncoded M-QAM modulation is derived. Furthermore, a closed form expression of the average throughput of an OFDMA system exploiting frequency diversity, which does not require CQI at the transmitter, is presented. Both throughput performances are compared in order to identify the optimal transmission strategy depending on the grade of CQI imperfectness.     

Quality of service aware antenna selection scheme for multi-hop relay networks

Multi input multi output (MIMO) based multi-hop relay (MHR) networks has become one of the promising technologies in improving the quality of wireless links. The optimal antenna selection is one of the suitable solutions to overcome the limitations of MIMO scheme. To attain the full benefits of transmitter antenna selection schemes, perfect channel quality information (CQI) is required at the transmitter. The time varying channel and the feedback delay make CQI at the transmitter outdated and also affect the antenna selection process. In this work, we have derived the symbol error probability (SEP) of M-ary quadrature amplitude modulation (QAM) for three different antenna selection schemes by considering the effect of delayed CQI. All these derived expressions are the function of correlation between CQI at the receiver and delayed CQI at the transmitter. The simulation results show that the antenna selection gain decreases with the decrease in correlation. It is also observed that scheme 1 based antenna selection is optimal for different constellations and more suited for MHR network.