Outage probability of CDMA-TDD micro cells in a CDMA-FDD environment

We investigated the feasibility of the coexistence of CDMA-TDD micro cells under a CDMA-FDD macro cell network with both sharing the same frequency band. A dynamic channel allocation (DCA) algorithm decides on the basis of mutual interference which FDD band is to use for the micro cell TDD channel. On the assumption of giving the macro cell higher priority we investigated the lower bound probability of total micro cell loss (outage) dependent on the distance between the micro- and macro cell BS. We also considered wall loss at the boundary of the micro cell and studied the effect of different propagation conditions on the assumption of equally distributed macro cell subscribers. A mathematical model for outage is developed and the results are compared with simulations. The results reveal a good match between the theoretical approximation and the simulations. This, in turn, indicates that micro cell outage is mainly caused by a single macro cell mobile within a certain area around the micro cell BS. It can be seen that the outage goes up to about 15% when the micro cell is located at the macro cell boundary. This value exponentially decreases when the micro cell is moved towards the macro cell BS. Walls around micro cells do have a significant influence on outage. It has been shown, that outage converges towards a value determined by the micro cell size.