System identification for wireless propagation channels in tunnels

Wireless communication channels in large tunnels at microwave frequencies involve a large number of guided wave modes. It has been reported that spatial records of field strength versus axial distance in such tunnels can be divided into two zones. Close to the transmitting source, the field consists of many modes interacting in such a way as to produce rapid decay and strong local variations. For greater distances, the field appears dominated by only a few principal modes, or a single mode, with less rapid decay and smoother variations. We apply methods of system identification to estimate parameters in a model that includes these characteristics in the two regions. We validate our model with theoretical data from a circular tunnel with lossy walls.