Abstract
The performance of an orthogonal frequency-division multiplexing (OFDM) system can be significantly increased by using adaptive modulation and transmit diversity. An accurate estimate of the channel, however, is required at the transmitter to realize this benefit. Due to the time-varying nature of the channel, this estimate may be outdated by the time it is used for detection. This results in a mismatch between the actual channel and its estimate as seen by the transmitter. In this paper, we investigate adaptive OFDM with transmit and receive diversities, and evaluate the detrimental effects of this channel mismatch. We also describe a robust scheme based on using past estimates of the channel. We show that the effects of the mismatch can be significantly reduced with a combination of diversity and multiple channel estimates. In addition, to reduce the amount of feedback, the subband approach is introduced where a common channel estimate for a number of subcarriers is fedback to the transmitter, and the effect of this method on the achievable rate is analyzed.
Highlights
The performance of wireless communication systems can be significantly improved by adaptively matching the transmission parameters such as rate, power level, or coding type to the channel frequency response [1,2,3]
The system capacity can be maximized if the data rate and power level of each subcarrier are adjusted according to the channel gain of that subcarrier
Due to the time-varying nature of the wireless channel, if the Doppler is large enough, this information may be outdated by the time it is used for detection resulting in an imperfect channel state information (CSI) at the transmitter
Summary
The performance of wireless communication systems can be significantly improved by adaptively matching the transmission parameters such as rate, power level, or coding type to the channel frequency response [1,2,3]. One of the options is to use an antenna array at the transmitter to form a beam in a specific direction to maximize the signal power at the receiver This type of transmit diversity is called beamforming and it requires an accurate estimate of the channel response. It is common for most high-speed wireless systems to suffer from imperfections in CSI due to Doppler, constraints on the size of the CSI data that can be fedback to the transmitter, or channel estimation errors This has led many researchers to investigate the robust optimization of transmission strategies with imperfect CSI for single-carrier and OFDM systems, possibly with multiple antennas, and several solutions have been presented [11,12,13,14,15,16,17,18].
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