Multiuser Diversity in Downlink Channels: When does the Feedback CostOutweigh the Spectral Efficiency Gain? (original) (raw)

Exploiting multiuser diversity in MIMO broadcast channels with limited feedback

We consider a multiple antenna broadcast channel in which a base station equipped with M transmit antennas communicates with K ≥ M single-antenna receivers. Each receiver has perfect channel state information (CSI), whereas the transmitter has partial channel knowledge obtained via a limited rate feedback channel. Built upon multiuser interference bounds, we propose scalar feedback metrics that incorporate information on the channel gain, the channel direction, and the quantization error, with the goal to provide an estimate of the received signal-to-noise plus interference ratio (SINR) at the transmitter. These metrics, combined with efficient user selection algorithms and zero-forcing beamforming on the quantized channel are shown to achieve a significant fraction of the capacity of the full CSIT case by exploiting multiuser diversity. A multi-mode scheme that allows us to switch from multiuser to single-user transmission is also proposed as a means to compensate for the capacity ceiling effect of quantization error and achieve linear sum-rate growth in the interference-limited region.

Multiuser Diversity - Multiplexing Tradeoff in MIMO Broadcast Channels with Limited Feedback

2006 Fortieth Asilomar Conference on Signals, Systems and Computers, 2006

We consider joint scheduling and beamforming in a broadcast channel with multiple antennas at the transmitter and a single antenna at the mobile receiver. Perfect channel knowledge is assumed to be available at the receiver while the transmitter is provided with partial channel state information (CSIT) through a limited rate feedback channel. Each user feeds back quantized information regarding the channel vector direction (from a codebook) and a quantized (scalar) channel quality indicator. We identify the tradeoff between multiuser diversity and spatial multiplexing gain given a limited amount of feedback bits. Scaling laws of the above parameters are derived in order to achieve a target rate performance. Our results reveal useful design guidelines for the split of feedback bits for channel quantization and channel quality.

Multi-antenna broadcast channels with limited feedback and user selection

2007

We analyze the sum-rate performance of a multi-antenna downlink system carrying more users than transmit antennas, with partial channel knowledge at the transmitter due to finite rate feedback. In order to exploit multiuser diversity, we show that the transmitter must have, in addition to directional information, information regarding the quality of each channel. Such information should reflect both the channel magnitude and the quantization error. Expressions for the SINR distribution and the sum-rate are derived, and tradeoffs between the number of feedback bits, the number of users, and the SNR are observed. In particular, for a target performance, having more users reduces feedback load.

Multi-antenna broadcast channels with limited feedback and user selection,” to appear

2007

Diversity-multiplexing tradeoff in multi-user scenario with selective feedback

2006

In this paper, we explore the existing tradeoff in using either OSTBC or opportunistic beamforming techniques with multiple beams (RB-MUX) in multiuser systems with selective feedback. We derive a closed-form expression of the ergodic system capacity for OSTBC and an approximation for the high-SNR regime for RB-MUX. By doing so, we analytically assess the impact of the number of terminals and bandwidth restrictions in the feedback channel on both the OSTBC and RB-MUX approaches. In particular, we show that RB-MUX schemes are more effective for exploiting multiuser diversity gain. However, OSTBC approaches give more benefits when either the feedback channel is restricted or the SNR of the system is increased.

Opportunistic feedback for downlink multiuser diversity

IEEE Communications Letters, 2005

This paper proposes a random access based feedback protocol for achieving multiuser diversity with a fixed number of feedback slots. Active users send a feedback message with some probability if their channel power is above a threshold. The threshold and random access probability are jointly designed to maximize the average sum rate of all users.

How Much Feedback Is Required for TDD Multi-Antenna Broadcast Channels with User Selection?

EURASIP Journal on Advances in Signal Processing, 2010

The enormous gains in a multi-antenna transmitter broadcast channel require the Channel State Information at the Transmitter (CSIT). Although the fundamental question "How much feedback is required for a broadcast channel?" has been treated in the literature to some extent, a more comprehensive treatment is certainly desirable. We study the time-division duplex broadcast channel with initial assumption of channel state information (CSI) neither at the base station (BS) nor at the users' side. We provide two transmission strategies through which the BS and the users get necessary CSI. We derive novel lower and upper bounds for the sum rate reflecting the rate loss compared to a perfect CSIT system. Corresponding approximate sum rate expressions are also developed for both schemes. These expressions fully capture the benefits of the CSIT feedback, allowing multi-user diversity gain and better inter-user interference cancellation, and the cost of exchange of information required. These expressions can be optimized for any set of system parameters to unveil the trade-off between the cost and the gains associated to feedback. Thus they allow to characterize the optimal amount of feedback which maximizes the sum rate of the broadcast channel, a well-accepted metric of system performance at the physical layer.

Opportunistic Feedback for Multiuser MIMO Systems With Linear Receivers

IEEE Transactions on Communications, 2000

A novel multiuser scheduling and feedback strategy for the multiple-input multiple-output (MIMO) downlink is proposed in this paper. It achieves multiuser diversity gain without substantial feedback requirements. The proposed strategy uses per-antenna scheduling at the base station, which maps each transmit antenna at the base station (equivalently, a spatial channel) to a user. Each user has a number of receive antennas that is greater than or equal to the number of transmit antennas at the base station. Zero-forcing receivers are deployed by each user to decode the transmitted data streams. In this system, the base station requires users' channel quality on each spatial channel for scheduling. An opportunistic feedback protocol is proposed to reduce the feedback requirements. The proposed protocol uses a contention channel that consists of a fixed number of feedback minislots to convey channel state information. Feedback control parameters including the channel quality threshold and the random access feedback probability are jointly adjusted to maximize the average throughput performance of this system. Multiple receive antennas at the base station are used on the feedback channel to allow decoding multiple feedback messages sent simultaneously by different users. This further reduces the bandwidth of the feedback channel. Iterative search algorithms are proposed to solve the optimization for selection of these parameters under both scenarios that the cumulative distribution functions of users are known or unknown to the base station. Index Terms-Diversity methods, feedback, multiple-input multiple-output (MIMO) systems, multiuser channels, scheduling. I. INTRODUCTION M ULTIPLE-INPUT multiple-output (MIMO) is an attractive physical layer (PHY) technology that supports high data rate communications through the use of multiple transmit and receive antennas. The MIMO downlink in cellular systems is one instance of the MIMO broadcast channel, where a point with multiple transmit antennas sends data to multiple destinations, each with one or more multiple receive antennas. Research on the MIMO broadcast channel capacity in [1]-[5] Paper approved by Y. (G.

A multi-threshold scheme for feedback load reduction in multiuser MIMO broadcast channel

21st Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, 2010

In a multiuser system, independent time-varying channels among different users can be exploited to provide multiuser diversity (MUD) gain and increase the system throughput. For the downlink channel, this requires the users to feedback their channel state information (CSI) to the base station (BS), which incurs a large aggregate feedback load, especially when the number of users is large. In this paper, we propose a novel scheme to reduce the feedback load in a downlink orthogonal space division multiple access (SDMA) system by allowing the users to dynamically determine the number of feedback bits to use according to multiple thresholds. These thresholds are derived from the statistics of the signal-to-interference-plus-noise ratio (SINR). Through simulation, we demonstrate that the proposed scheme is more efficient than the existing feedback methods, and it achieves the same sum rate growth as the full CSI feedback scheme. Furthermore, a fast bit allocation method is also proposed for the multi-threshold feedback scheme. Simulation results show that this method achieves almost the same sum rate as that achieved by the the optimal bit allocation algorithm.

Achieving Multi-user Diversity Gain using User-Identity Feedback

IEEE Transactions on Wireless Communications, 2000

ABSTRACT A user-identity feedback method is proposed to achieve multi-user diversity in broadcast channels. This method associates the feedback slots with pre-determined thresholds that diminish with time, and allows feedback from those users whose channel states exceed the threshold assigned to the current slot. Since the threshold can approximate the channel state, this method requires only user-identity feedback. In Rayleigh fading channels, this method is mathematically shown to achieve the sum-rate capacity asymptotically with a large number of users, while requiring only a few feedback slots and little transmission power.

Multiuser Diversity in Downlink Channels: When does the Feedback CostOutweigh the Spectral Efficiency Gain? (original) (raw)

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