Adaptive MIMO Transmission Scheme: Exploiting the Spatial Selectivity of Wireless Channels

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Abstract
We present a novel adaptive transmission technique for MIMO systems with the aim to enhancing the spectral efficiency for a target error rate performance and transmit power. This adaptive method employs the condition number of the spatial correlation matrix as an indicator of the spatial selectivity of the MIMO channel. The distribution of the condition number is then used to identify the prevailing channel environment. Depending on the identified channel state, our adaptive algo- rithm chooses the MIMO transmission method, among spatial multiplexing, D-STTD, and beamforming, that maximizes the spectral efficiency. Performance results show significant gains in throughput and reduced error rate compared to conventional fixed transmission schemes. In this contribution we propose a novel adaptive trans- mission technique, aiming to enhance the spectral efficiency for a fixed predefined target error rate performance. The key insight of our method is that it is possible to characterize the spatial selectivity of the channel based on a metric derived from the spatial correlation matrices. The proposed algorithm estimates the channel quality and, based on this informa- tion, adaptively switches across different MIMO transmission schemes to maximize the throughput for a fixed error rate and transmit power. This adaptive algorithm only tracks the long- term channel statistics. The practical implementation of the algorithm is based on identifying a set of link-quality regions, which represent "typical" (quantized) channel scenarios. These link-quality regions are mapped into a set of transmission modes (defined by a combination of modulation/coding and MIMO transmission schemes) through a look-up table (LUT). Since the adaptation occurs over a long time scale, minimal coordination between the transmitter and receiver is required

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