| We investigate the relationship between RF front-end filtering and cellular/PCS system performance in the presence of inter-system interference caused by intermodulation (IM). The IM-induced interference may be significant if the RF front-end filter is not capable of sufficiently suppressing signals coming from the competing system operating in an adjacent frequency band. Our study includes all three major multiple access methods: FDMA, TDMA, and CDMA. We first derive the input-output spectral relationship for the RF front-end receiver and illustrate the presence of the IM term, which may cause severe degradation to system performance if it dominates the desired signal term. The third-order intercept point is used to characterize the nonlinearity of an RF front-end component in this analysis. We then apply the analytic result to dual- and multiple-CDMA system environments and show that CDMA capacity can be substantially hampered by IM-induced inter-system interference and that superior RF front-end filtering can greatly minimize this negative impact. Essentially, the strong inter-system interference results from the combination of CDMA power control (for overcoming intra-system near-far problems), imperfect RF filtering, and RF amplifier/mixer nonlinearities. We also analyze the impact of IM-induced interference in dual-FDMA and dual-TDMA system environments and conclude that superior RF filtering can reduce the guardband requirement for a given level of performance. The benefits of superior RF filtering in a mixed FDMA-CDMA and a mixed TDMA-CDMA environments prove to be less significant as the existing guardbands provide adequate protection from significant IM-induced inter-system interference. Finally, we investigate the impact of a strong adjacent-band interference, such as AGC (Air-to-ground Communication) signals near an airport area, on a cellular system performance. We conclude that superior RF front-end filtering is essential in protecting the system capacity from such strong interference. |
