On the Characterization of Mu-mimo Channels

In this work we study the divergence of different links in wide-band multi-user multiple-input multiple-output (MUMIMO) channels. The divergence is measured on several levels: (i) spatial separation of the user’s correlation matrices, (ii) co-linearity of the MIMO channel matrices, and (iii) correlation of large scale fading. The measurement data has been acquired using Eurecom’s MIMO Openair Sounder (EMOS). The EMOS can perform real-time MIMO channel measurements synchronously over multiple users. For this work we have used an outdoor measurement with two transmit antennas and two users with two antennas each. Several measurements with different distances between users were acquired. We find that the structure of the MIMO channel matrices changes significantly with the inter-user distance. This is best captured by the co-linearity measure. The transmit and the full correlation matrix also show some dependence on the inter-user distance whereas the receive correlation matrices are independent of the inter-user distance. The shadowing correlation was found to be very low in all cases. These findings are important for MU-MIMO precoding and scheduling algorithms.