Characterisation of the time-dependent urban MIMO channel in FDD communication systems

The influence of the time dependent urban transmission channel on MIMO systems is investigated in this paper. Exploiting the full potential of MIMO systems requires the knowledge of the transmission channel matrix at the transmitter and the receiver. Channel estimation is therefore necessary. It has to be performed continuously as the transmission channel changes over time. In real communications systems it is only possible to update the channel state information in certain time intervals. During these intervals the estimated channel matrix is assumed to be constant. The difference between the real channel and the estimated one leads to non-orthogonal channels in the MIMO system. The result is inter-sub-channel interference (ISCI). The ISCI is considered as a Gaussian noise random process, which is added to the common thermal noise. Due to the increased total noise power of the resulting channel, the capacity decreases. Furthermore, a sub-channel transmission gain degradation (STGD) occurs caused by the non-orthogonal sub-channels. This results in an additional capacity loss. The dependency of ISCI and STGD on the update time interval is investigated. The considerations are based on simulations of the transmission channel in an urban environment. In order to attain realistic results, a 3D ray-tracing tool is used. An FDD communication system is assumed.