Article

Practical Multiuser Diversity With Outdated Channel Feedback

IEEE Transactions on Vehicular Technology (Impact Factor: 1.98). 08/2005; 54(4):1334 - 1345. DOI: 10.1109/TVT.2005.851325
Source: IEEE Xplore

ABSTRACT

Inspired by the information theoretic results concerning multiuser diversity, we address practical issues in implementing multiuser diversity in a multiple access wireless setting. Considering a channel-assigning strategy that assigns the channel only to the user with the best instantaneous SNR [3], our emphasis is on the effects of channel feedback delay in downlink transmissions. A finite set of M-ary quadrature amplitude modulation (M-QAM) constellations is adopted and a constant transmit power is assumed in this practical multiuser adaptive modulation scheme. Based on the closed-form expressions for average bit error rate (BER) and average data rate, we illustrate the impact of channel feedback delay on the achievable multiuser diversity gain with the number of users. Simple and accurate asymptotic approximations are also provided in the limit of large numbers of users. Focusing on different applications, we propose two optimization criteria for the switching thresholds, based on either an average BER, or an outage probability constraint. Two novel constant power, variable rate M-QAM schemes that are less sensitive to feedback delay are proposed using the optimal switching thresholds, which are derived to maximize the average data rate subject to these two constraints, respectively. To obtain a certain degree of fairness among the users, we also consider a fair channel-assigning strategy that assigns the channel to only the user with the greatest normalized SNR.

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    • "Under the multi-cluster subband fading model, we perform analysis on the average goodput and the average outage probability with heterogeneous partial feedback. In addition to examining the impact of imperfect feedback on multiuser diversity [26], [27], we also investigate how to adapt and optimize the average goodput in the presence of these imperfections. We consider both the fixed rate and the variable rate scenarios, and utilize bounding technique and an efficient approximation to derive near-optimal strategies. "
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    ABSTRACT: Current OFDMA systems group resource blocks into subband to form the basic feedback unit. Homogeneous feedback design with a common subband size is not aware of the heterogeneous channel statistics among users. Under a general correlated channel model, we demonstrate the gain of matching the subband size to the underlying channel statistics motivating heterogeneous feedback design with different subband sizes and feedback resources across clusters of users. Employing the best-M partial feedback strategy, users with smaller subband size would convey more partial feedback to match the frequency selectivity. In order to develop an analytical framework to investigate the impact of partial feedback and potential imperfections, we leverage the multi-cluster subband fading model. The perfect feedback scenario is thoroughly analyzed, and the closed form expression for the average sum rate is derived for the heterogeneous partial feedback system. We proceed to examine the effect of imperfections due to channel estimation error and feedback delay, which leads to additional consideration of system outage. Two transmission strategies: the fix rate and the variable rate, are considered for the outage analysis. We also investigate how to adapt to the imperfections in order to maximize the average goodput under heterogeneous partial feedback.
    Preview · Article · Feb 2013 · IEEE Transactions on Signal Processing
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    • "For non-spectrum sharing environments, there have been many studies on characterizing the multiuser diversity gains [9]-[12]. Downlink multiuser diversity in a single cell is analyzed for a large number of users in [12] and [13]. The uplink performance analysis considering the Rayleigh fading environment has been studied in [14]. "

    Full-text · Article · Jan 2011
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    • "(ST-CSI). However, it may not be feasible to share ST- CSI among adjacent cells mainly due to backhaul delay and signaling overhead problem [3]. This problem can be alleviated by using long-term (LT)-CSI for the coordination. "
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    ABSTRACT: Deployment of multiple femtocells in a confined area may cause serious interference problem to each other. To mitigate the inter-femtocell interference, we consider combined use of coordinated user scheduling with transmit beamforming in femtocell networks. Femto base stations collaboratively schedule users to minimize the interference in an average sense and employ a beamforming technique to maximize the network spectral efficiency. We assume that only the transmit spatial correlation coefficient is shared by femto base stations, making it quite feasible for implementation. The effectiveness of the proposed scheme is verifed by computer simulation in the presence of channel correlation. 1
    Full-text · Conference Paper · Jan 2011
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