The effects of interference between the TDD and FDD mode in UMTS at the boundary of 1920 MHz

The Universal Mobile Telecommunications System (UMTS) is composed of an FDD and TDD mode. The spectrum allocation is in such a way that both modes have an adjacent carrier at 1920 MHz. The implications thereof with respect to system capacity are investigated. In this context the separation distance of the TDD and FDD base station (BS) and frame synchronisation are varied. It is shown that the most detrimental effects for the FDD interface are for small BS separations. In contrast, the optimum with respect to TDD capacity is found for the co-location of both base stations. This yields a trade-off for the optimal BS locations. Moreover, an adjacent channel protection factor of 30 dB in an interference limited system such as CDMA is shown to be too low unless a significant capacity loss is acceptable.