Taesang Yoo

On the optimality of multiantenna broadcast scheduling using zero-forcing beamforming

Although the capacity of multiple-input/multiple-output (MIMO) broadcast channels (BCs) can be achieved by dirty paper coding (DPC), it is difficult to implement in practical systems. This paper investigates if, for a large number of users, simpler schemes can achieve the same performance. Specifically, we show that a zero-forcing beamforming (ZFBF) strategy, while generally suboptimal, can achieve the same asymptotic sum capacity as that of DPC, as the number of users goes to infinity. In proving this asymptotic result, we provide an algorithm for determining which users should be active under ZFBF. These users are semiorthogonal to one another and can be grouped for simultaneous transmission to enhance the throughput of scheduling algorithms. Based on the user grouping, we propose and compare two fair scheduling schemes in round-robin ZFBF and proportional-fair ZFBF. We provide numerical results to confirm the optimality of ZFBF and to compare the performance of ZFBF and proposed fair scheduling schemes with that of various MIMO BC strategies.

Capacity and power allocation for fading MIMO channels with channel estimation error

In this correspondence, we investigate the effect of channel estimation error on the capacity of multiple-input-multiple-output (MIMO) fading channels. We study lower and upper bounds of mutual information under channel estimation error, and show that the two bounds are tight for Gaussian inputs. Assuming Gaussian inputs we also derive tight lower bounds of ergodic and outage capacities and optimal transmitter power allocation strategies that achieve the bounds under perfect feedback. For the ergodic capacity, the optimal strategy is a modified waterfilling over the spatial (antenna) and temporal (fading) domains. This strategy is close to optimum under small feedback delays, but when the delay is large, equal powers should be allocated across spatial dimensions. For the outage capacity, the optimal scheme is a spatial waterfilling and temporal truncated channel inversion. Numerical results show that some capacity gain is obtained by spatial power allocation. Temporal power adaptation, on the other hand, gives negligible gain in terms of ergodic capacity, but greatly enhances outage performance.

Cross-layer design of ad hoc networks for real-time video streaming

Cross-layer design breaks away from traditional network design where each layer of the protocol stack operates independently. We explore the potential synergies of exchanging information between different layers to support real-time video streaming. In this new approach information is exchanged between different layers of the protocol stack, and end-to-end performance is optimized by adapting to this information at each protocol layer. We discuss key parameters used in the cross-layer information exchange along with the associated cross-layer adaptation. Substantial performance gains through this cross-layer design are demonstrated for video streaming.

Optimality of zero-forcing beamforming with multiuser diversity

In MIMO downlink channels, the capacity is achieved by dirty paper coding (DPQ). However, DPC is difficult to implement in practical systems. This work investigates if, for a large number of users, simpler schemes can achieve the same performance. Specifically, we show that a zero-forcing beamforming (ZFBF) strategy, while generally suboptimal, can achieve the same asymptotic sum-rate capacity as that of DPC, as the number of users goes to infinity. In proving this asymptotic result, we propose an algorithm for determining which users should be active in ZFBF transmission. These users are semi-orthogonal to one another, and when fairness among users is required, can be grouped for simultaneous transmissions to enhance the throughput of fair schedulers. We provide numerical results to confirm the optimality of ZFBF and to compare its performance with that of various MIMO downlink strategies.

Capacity of fading MIMO channels with channel estimation error

In this paper, we investigate the effect of channel estimation error on the capacity of multiple input multiple output (MIMO) systems in i.i.d. Rayleigh flat-fading channels. We study lower and upper bounds of mutual information under channel estimation error, and show that the two bounds are tight for Gaussian inputs. It is seen that the mutual information increases with the. number of antennas, but is limited by channel estimation error at high SNR. We also derive tight lower bounds of ergodic and outage capacities and optimal transmitter power allocation strategies that achieve the bounds. For the ergodic capacity, the optimal strategy is a modified waterfilling over the spatial (subchannel) and temporal (fading) domain. For the outage capacity, it is a spatial waterfilling and temporal truncated channel inversion. Numerical results show that some capacity gain is obtained by spatial power allocation. Temporal power adaptation, on the other hand, gives negligible gain in terms of ergodic capacity, but greatly enhances outage performance.

MIMO capacity with channel uncertainty: does feedback help?

We investigate ergodic capacities and optimal transmitter strategies in Rayleigh fading multiple input multiple output (MIMO) channels with spatial correlation, when there exist channel uncertainties arising from the combined effect of channel estimation error and limited feedback. We consider both covariance feedback and instantaneous feedback, and formulate optimization problems that determine the capacities and optimal transmitter designs for both cases. In the high SNR regime, the optimal solutions have simple closed form formulas that involve inverting the channel covariance and waterfilling over instantaneous channel gains. Numerical results show that instantaneous feedback gives large capacity gain at low SNR and is also helpful at high SNR. Covariance feedback, on the other hand, seems to give little gain at mid SNR, but is almost as good as instantaneous feedback at high SNR under a reasonable channel estimation quality.

Finite-Rate Feedback MIMO Broadcast Channels with a Large Number of Users

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

Sum-rate optimal multi-antenna downlink beamforming strategy based on clique search

We consider a multi-user MIMO downlink system employing zero-forcing beamforming (ZFBF) as a spatial multiplexing strategy, and propose low-complexity user subset selection methods based on a clique (fully connected subgraph) search. The proposed algorithms, maximum weighted clique (MWC)-ZFBF and greedy weighted clique (GWC)-ZFBF, are shown to achieve the asymptotic sum-capacity of MIMO downlink channels as the number of users goes to infinity. Thus, clique search based ZFBF is an appealing strategy in MIMO downlink systems with a large number of users.

Cross-layer design for video streaming over wireless ad hoc networks

We propose a cross-layer design framework for supporting delay-critical traffic over ad hoc wireless networks and analyze its benefits for video streaming. In this framework, link capacities and traffic flows are jointly allocated to minimize the congestion experienced by video packets. The optimal solution, calculated via time sharing among different transmission schemes, concentrates resources only on active links. Experimental results on a simulated network illustrate the advantages of cross-layer design over another method based on oblivious layers. With one path, the cross-layer approach yields a 10-fold gain in supported data rate or equivalently 8.5 dB improvement in PSNR of achievable received video quality. Using 3 paths, the gain is 3-fold in rate or 5 dB in video quality. While multipath routing is essential to high data rate in oblivious-layered design, cross-layer design achieves efficient resource utilization regardless of the number of routes.

CTH13-5: Coverage Spectral Efficiency of Cellular Systems with Cooperative Base Stations

Coverage spectral efficiency (CSE) characterizes the tradeoff between efficient channel reuse and the achievable rates per cell, under the assumption of detection by a single base station and intra-cell FDMA. It is well known that intra-cell FDMA is not in general optimal. In this paper we study an alternative intra- cell wide-band scheme as well as the base station cooperation in detection, which has demonstrated potential capacity gain. The effect on CSE of different schemes are then compared and the optimal reuse distance is determined for each scheme.