An Opportunistic Channel Quality Feedback Scheme for Proportional Fair Scheduling

In this paper, we propose a channel quality information (CQI) feedback load reduction scheme for proportional fair scheduling (PFS) in wireless systems. The proposed scheme induces select users to feed back CQI only when the probability of selection is high. It is shown that the proposed scheme achieves system throughput comparable to the conventional scheme, where each and every user feeds back its CQIs, with significant feedback load reduction for each of the users.     

Proportional fair scheduling with reduced feedback

An opportunistic feedback technique is proposed for proportional fair (PF) scheduling in a downlink multi-user system. This technique requires each mobile station, the scheduling metric of which is higher than a threshold, to send one-bit feedback for PF scheduling. The scheduler determines the optimal modulation and coding scheme for a scheduled user based on channel quality indicator (CQI) update, which is sent on a longer period than the scheduling period. The simulation results indicate that PF scheduling with the proposed update scheme can achieve similar performance as PF scheduling with full CQI feedback. The feedback overhead can be reduced in comparison with typical PF scheduling provided that an optimal CQI update period is chosen.     

An enhanced channel-quality indication (CQI) reporting scheme for HSDPA systems

A major evolution of UMTS standard is the high speed downlink packet access (HSDPA). One of key techniques supporting HSDPA is the adaptive modulation and coding (AMC) in which the modulation scheme and the coding rate are adaptively changed according to the downlink channel quality reported by the user equipment (UE). Therefore, the channel quality indication (CQI) reporting scheme is directly related to the accuracy of AMC and the performance of HSDPA. This letter proposes an enhanced CQI reporting scheme that can be used when a proportional fair scheduling algorithm (PFA) is used as a packet scheduling algorithm. The proposed scheme uses a dynamic threshold to filter off redundant CQI reports. With the proposed scheme, the battery capacity of UK can be conserved, maintaining the performance of traditional CQI reporting scheme.     

Analytical evaluation of hybrid feedback scheme for OFDMA system

This paper first introduces the performance analysis of two classical channel quality indicator （CQI） feedback schemes which are best-n feedback and the threshold based feedback and derives the mathematical expressions of average capacity which is described by Theorem 1 and 2. Then, a reduced feedback scheme is designed for multiple traffics and multi-channel. The novel scheme combines the best-n feedback and the threshold based feedback together to reduce the feedback overhead. The proposed scheme can not only guarantee the quality of service （QoS） requirement of real time （RT） traffic but also reduce feedback overhead at the cost of a marginal increased downlink overhead. Simulation results demonstrated the good performance of the proposed feedback scheme.     

Design of a fair scheduler exploiting multiuser diversity with feedback information reduction

In this letter, we propose a new downlink fair scheduling scheme exploiting the multiuser diversity to enhance the transmission capacity. In the proposed scheme, only the MSs (mobile stations) whose normalized SNR (signal-to-noise ratio) values are larger than a given threshold feedback one-bit information to the BS (base station). As a result, while achieving the strict fairness, the proposed scheme can efficiently utilize the spectrum by reducing the considerable amount of the feedback information, compared to the proportional fair scheduling scheme where all the MSs feed back the normalized SNR values to the BS. Numerical studies show that the transmission capacity in the proposed scheme with a suitable value of the threshold is very close to that in the proportional fair scheduling scheme.     

Adaptive Max SNR Packet Scheduling for OFDM Wireless Systems

In this paper, we consider scheduling and resource allocation for a downlink in a wireless OFDM system. If each broadcast sub channel is allocated to a user according to max SNR selection, optimal system throughput is obtained for the cost of a significant loss in fairness among users. As a solution to resolve this issue and in an attempt at reaching a compromise between fairness and throughput, we propose to add to the max SNR scheme a weak control based on user QoS requirements. In this work, user latency between two successive channel accesses is considered as a parameter for the control. The feedback of quantized channel state information (CSI) is proposed to reduce the feedback burden. Performance analysis of the proposed scheme has been presented to illustrate the capacity-fairness-feedback trade-off of the considered scheme compared to max SNR and proportional fair algorithms used as benchmark.

On capacity of quality-based channel-state reporting in mobile systems with greedy transmission scheduling

Greedy transmission scheduling achieves great capacity by maximally exploiting independent time-varying channels across different mobile users. The improvement in capacity, however, depends on the degree of completeness of the channel quality information (CQI) fed back from the receiver to the transmitter. To be motivated by an insight that too many CQI feedbacks may rather impair the capacity gain, due to causing congestion in feedback link, this letter proposes a quality-based CQI reporting (QBR) scheme where the CQIs are fed back to the transmitter only for receivers whose signal quality is above a predefined threshold. The capacity is provided in terms of the threshold and feedback-error rate. The results show that QBR achieves outstanding performance when the feedback error is present. In addition, it quickly approaches an unimpaired ideal capacity, as the number of users increases if the error is not assumed.     

Flexible block‐wise loading algorithm for effective resource allocation and reduction of uplink feedback information in OFDMA systems

Adaptive modulation and coding (AMC) is a well‐known technique that selects a suitable modulation level and coding rate according to the user's channel quality. The utilization of AMC in downlink (DL) requires the channel information of each user on DL to properly select the modulation and coding scheme (MCS) level. However, under a practical OFDMA system environment, it is unsuitable to feedback all the channel information over the whole frequency resources due to the limited uplink bandwidth. In this paper, to optimize the performance of the AMC system and to reduce the uplink feedback requirement, we propose a flexible block‐wise loading (FBL) algorithm combined with a novel channel quality information (CQI) feedback scheme requiring less number of CQI feedback bits, which is suitable for the general OFDMA system with AMC. The proposed FBL algorithm dynamically allocates groups of sub‐carriers to each user in DL based on the channel quality information encapsulated in a newly defined feedback signal format. As a result, the sector throughput and outage performance is improved due to the flexibility on the sub‐channel allocation and increased accuracy on the CQI feedback information by the proposed FBL algorithm. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

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.     

Efficient CQI feedback via network coding for wireless relay networks

For the efficacy of radio resource management in downlink wireless relay networks, the channel quality indicator (CQI) between a mobile station (MS) and a relay station (RS) should be fed back from the RS to its base station (BS), at the cost of additional feedback overhead. In this letter, we propose an efficient feedback scheme based on network coding between a pilot sequence and the CQI of MS-RS link, through which the BS can have both CQIs of MS-RS and RS-BS links without overhead. Numerical results reveal that the proposed CQI reporting scheme has as good feedback performance as conventional one which generally requires additional feedback burden.     

一种基于NACK的HSDPA自适应信道质量反馈技术

作为一种B3G技术，HSDPA已经受到广泛的关注，该技术可以提供高速的分组下行数据传输。其关键技术包括AMC，HARQ，FCS等。在HSDPA系统中，其调制方式和编码可以随着下行链路的质量状况自适应改变。因此，信道质量反馈就直接影响AMC的准确度和系统的性能。本文介绍了TD—SCDMA模式下HSDPA中CQI的工作机制和一种基于NACK的自适应信道质量反馈技术。     

Network coding‐based channel quality indicator reporting for two‐way multi‐relay networks

This paper considers channel quality indicator (CQI) reporting for data exchange in a two‐way multi‐relay network. We first propose an efficient CQI reporting scheme based on network coding, where two terminals are allowed to simultaneously estimate the CQI of the distant terminal‐relay link without suffering from additional overhead. In addition, the transmission time for CQI feedback at the relays is reduced by half while the increase in complexity and the loss of performance are negligible. This results in a system throughput improvement of 16.7% with our proposed CQI reporting. Upper and lower bounds of the mean square error (MSE) of the estimated CQI are derived to study performance behaviour of our proposed scheme. It is found that the MSE of the estimated CQI increases proportionally with the square of the cardinality of CQI level sets although an increased number of CQI levels would eventually lead to a higher data rate transmission. On the basis of the derived bounds, a low‐complexity relay selection (RS) scheme is then proposed. Simulation results show that, in comparison with optimal methods, our suboptimal bound‐based RS scheme achieves satisfactory performance while reducing the complexity at least three times in case of large number of relays. Copyright © 2012 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.     

CSI Feedback for Dynamic Switching Between Single User and Multiuser MIMO

We consider channel state information (CSI) feedback in 3GPP Long Term Evolution (LTE)-Advanced context. In LTE-Advanced, switching between single user and multiuser transmission schemes is possible without higher layer signaling, which means that the feedback should support both single user and multiuser transmissions. Typically, the CSI feedback consists of a precoding matrix index (PMI) and channel quality indication(s) (CQI). For PMI feedback, we consider different PMI selection schemes and study whether there is a tradeoff between single user and multiuser specific codeword selection metrics. For multiuser CQI, we consider different CQI estimation strategies for two paired users, which is the primary case in LTE-Advanced. The schemes include single user single stream and two stream CQIs and several multiuser specific CQI estimation options. We find that estimating the multiuser CQI as an average over unitary pairs or as the minimum of the signal-to-interference and noise ratios of the unitary pairs offers a stable, well-performing options for different signal-to-noise ratio (SNR) ranges and antenna correlation values.     

分布式双向中继选择算法及用户功率分配

针对放大转发的瑞利双向中继信道的节点选择问题,提出了基于部分信道信息的分布式双向中继选择算法。算法通过计算双向链路的接收信噪比,推导出满足目标接收信噪比的转发阈值,各中继节点根据该阈值决定是否参与转发,从而实现分布式选择。此外,考虑用户总功率受限的情况,在分布式中继选择基础上提出了优化功率分配策略,使双向信道的接收信噪比更加接近。仿真结果表明,分布式中继选择算法与最优多中继算法的系统传输速率相似,计算复杂度大大降低,尤其是在中继数目增大的情况下更加明显。优化功率分配策略能进一步提高系统能量效率,在相同性能下可节省7%左右的功率。     

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。      

Limited feedback-based block diagonalization for the MIMO broadcast channel

Block diagonalization is a linear preceding technique for the multiple antenna broadcast (downlink) channel that involves transmission of multiple data streams to each receiver such that no multi-user interference is experienced at any of the receivers. This low-complexity scheme operates only a few dB away from capacity but requires very accurate channel knowledge at the transmitter. We consider a limited feedback system where each receiver knows its channel perfectly, but the transmitter is only provided with a finite number of channel feedback bits from each receiver. Using a random quantization argument, we quantify the throughput loss due to imperfect channel knowledge as a function of the feedback level. The quality of channel knowledge must improve proportional to the SNR in order to prevent interference-limitations, and we show that scaling the number of feedback bits linearly with the system SNR is sufficient to maintain a bounded rate loss. Finally, we compare our quantization strategy to an analog feedback scheme and show the superiority of quantized feedback.     

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.     

MIMO下行链路中的多用户调度算法

针对采用随机波束成形的MIMO下行链路,提出了一种多用户调度算法。在接收端利用每个用户在各个波束方向上的比例公平性参数来得到容量门限。仿真结果显示,这种调度算法相比其他门限调度算法,在系统性能无明显损失的情况下,有效地减小了反馈量。