Impact of spatial correlation on orthogonal precoding for MIMO transmissions in tunnels

Multiple-input Multiple-output (MIMO) techniques have proven their potentiality for transmissions in the transport domain even in specific contexts, such as tunnels. When perfect channel state information (CSI) is available at the transmitter, performance improvement is allowed through a precoding operation. However, the performance of such a closed-loop MIMO system depends on the spatial correlation in the channel, the quantity and the quality of the CSI. A perfect knowledge of the CSI is rarely available in practice because the CSI usually undergoes various impairments, such as channel estimation errors and quantization errors. To overcome these drawbacks, we propose the investigation of two limited feedback precoders that allow us to reduce simultaneously processing complexity and the amount of feedback information. The first, OSM is based on the orthogonalization of the channel and the second is the basic quantized single beamforming based on the Grassmannian line packing technique. The comparison is performed both for an ideal Rayleigh channel and for two typical transmission scenarios in tunnels. Simulation results show that the OSM precoder achieves better performances than the unprecoded scheme, whatever the level of correlation in tunnels. Quantized single beamforming is interesting only for the high-correlated scenario. In this case it outperforms the OSM scheme and the unprecoded scheme.