Channel Estimation and Optimal Power Allocation for a Multiple-Antenna OFDM System
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Channel Estimation and Optimal Power Allocation for a Multiple-Antenna OFDM System Tai-Lai Tung GlobespanVirata, Inc., 5171 California Avenue, Suite 200, Irvine, CA 92612, USA Email: [email protected]
Kung Yao Electrical Engineering Department, 68-113 Engineering IV Building, University of California, Los Angeles, CA 90095-1594, USA Email: [email protected] Received 6 June 2001 and in revised form 24 January 2002 We propose combining channel estimation and optimal power allocation approaches for a multiple-antenna orthogonal frequency division multiplexing (OFDM) system in high-speed transmission applications. We develop a least-square channel estimation approach, derive the performance bound of the estimator, and investigate the optimal training sequences for initial channel acquisition. Based on the channel estimates, the optimal power allocation solution which maximizes the bandwidth efficiency is derived under power and quality of service (QoS) (symbol error rate) constraints. It is shown that combining channel tracking and adaptive power allocation can dramatically enhance the outage capacity of an OFDM multiple-antenna system when severe fading occurs. Keywords and phrases: OFDM, multiple-antenna, channel estimation, optimal power allocation, capacity, optimal training sequences, MSE bounds.
1.
INTRODUCTION
Orthogonal frequency division multiplexing (OFDM) technique has recently gained increasing interest as an effective alternative to conventional single tone modulation methods for high-rate communications systems [1, 2, 3]. The main advantage of OFDM is allowing transmission over a frequency-selective fading channel using a low cost receiver. On the other hand, current research has shown that intelligent use of antenna arrays at both receiver and transmitter side can significantly increase the bandwidth efficiency as well as improving the transmission reliability [4]. More recent work then show that combining OFDM technique with multiple-antenna architecture can provide high-rate high performance transmission [5, 6, 7, 8]. In this paper, we first propose least-square (LS) channel estimation approach for OFDM multiple-antennas systems. Based on the channel estimates, the bandwidth efficiency is then maximized by performing optimal power allocation over multiple antennas and subbands where some practical constraints such as specified quality of service (QoS) requirements in terms of symbol error probability and limited transmit power constraints are considered. It is shown
that combining the optimal power allocation method with a closed-loop channel state information updating mechanism can dramatically increase the system outage capacity in timevarying environment. The organization of the paper is as follows. In Section 2, we first propose least-squares channel estimator. To analyze the performance of the channel estimator, the mean square error (MSE) bound is derived in Section 3. The optimal training sequences which can achieve minimum MSE channel acquisition are designed in Section 4. The tracking and BE
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