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KEYHOLES AND MIMO CHANNEL MODELLING

It has been shown that the concept of "keyholes" in MIMO channels could be responsible for a reduced transmit/receive diversity performance, and therefore a reduction of capacity with respect the maximum available radio link capacity. In the present work, we further analyse this effect in terms of coupling between an input wave impinging on the keyhole, and several output waves due to diffraction. As a consequence in terms of MIMO channels modelling within a small antenna size approximation, the channel transfer function matrix can be expressed in terms of steering matrices for the emitting and receiving arrays, and of a connecting matrix relating the complex wave amplitudes at the two antennas. The rectangular or non diagonal character of this matrix is due to junctions in the wave paths, which leads to a keyhole when the matrix is full. This approach also allows to take into account explicitly the double directional dependence of the radio scenario[1] , and lends itself easily to geometric based stochastic modelling of MIMO channels. The generalization to nonideal multiport antennas, incorporating coupling and enhanced correlations between sensors, is immediate.