Robust Downlink Power Control in Wireless Cellular Systems

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EURASIP Journal on Wireless Communications and Networking 2004:2, 261–272 c 2004 Hindawi Publishing Corporation

Robust Downlink Power Control in Wireless Cellular Systems
Mehrzad Biguesh
Department of Communication Systems, University of Duisburg-Essen, Bismarckstrasse 81, 47057 Duisburg, Germany Email: biguesh@sent5.uni-duisburg.de

Shahram Shahbazpanahi
Department of Communication Systems, University of Duisburg-Essen, Bismarckstrasse 81, 47057 Duisburg, Germany Department of Electrical & Computer Engineering, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4K1 Email: shahbaz@mail.ece.mcmaster.ca

Alex B. Gershman
Department of Communication Systems, University of Duisburg-Essen, Bismarckstrasse 81, 47057 Duisburg, Germany Department of Electrical & Computer Engineering, McMaster University, 1280 Main Street West, Hamilton, ON, Canada L8S 4K1 Email: gershman@ieee.org Received 30 November 2003; Revised 13 July 2004 A serious shortcoming of current downlink power control methods is that their performance may be severely degraded when the downlink channel information is known imprecisely at the transmitter. In this paper, a computationally and implementationally simple centralized downlink power control method is proposed for cellular wireless communication systems using code division multiple access (CDMA) or space division multiple access (SDMA). Our method provides a substantially improved robustness against imperfect knowledge of the wireless channel by means of maintaining the required quality of service for the worst-case channel uncertainty. In the SDMA case, the proposed technique can be straightforwardly combined with any of the existing transmit beamforming methods. Simulation results validate substantial robustness improvements achieved by our approach. Keywords and phrases: power control, cellular system, CDMA, SDMA, downlink beamforming.

1.

INTRODUCTION

Power control is an intelligent way of adjusting the transmitted powers in cellular systems so that the total transmitted power is minimized but, at the same time, the user signalto-interference-plus-noise ratios (SINRs) satisfy the system quality of service (QoS) requirements [1]. Depending on the location where the decision on how to adjust the transmitted powers is made, the power control algorithms can be divided into two groups: centralized and noncentralized (distributed) techniques. In distributed power control, local measurements are used to evaluate the transmitted power for each user so that all users finally meet the QoS requirements [2, 3]. In centralized power control, users channel information is sent to the central unit which computes the desired transmitted powers for each user [4, 5]. Downlink beamforming and power control techniques have been a recent focus of intensive studies in application

to cellular communication systems [4, 6, 7, 8, 9, 10, 11, 12, 13, 14]. The user SINR criterion has been adopted in these papers to optimize the transmitted powers and beamformer weights to ensure that the QoS requirements are satisfied for all users. For example, in [8, 13], the problem of optimal centralized power control and downlink beamforming is considered in the case when the exact downlink channel information is available at the base stations. Several other works consider simpler suboptimal power control and/or beamforming methods [7, 9, 10, 14]. However, a serious shortcoming of all centralized power control methods is that they assume the exact knowledge of the user downlink channel at the transmitter and, as a result, can be quite sensitive to imprecise channel knowledge [11]. In practical situations, the downlink channel may be uncertain and, in the general case when base stations are equipped with antenna arrays, the downlink channel correlation (DCC) matrices may be subject to substantial errors.

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EURASIP Journal on Wireless Communications and Networking
mth CS

As the DCC matrices are...
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