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Within 3GPP, the work of defining the long-term evolution (LTE) of 3G is ongoing. The fundamental targets of this evolution are to further improve the coverage and capacity, as well as increasing the data rates and reducing the latency. Another important aspect of 3G LTE is the possibility to use 3G LTE in existing GSM and WCDMA frequency bands, hence the mobile terminal selectivity filter has to be designed taking system coexistence into account. For 3G LTE, OFDM has been chosen for the down-link and a cyclic prefix is used to avoid inter symbol interference (ISI). This paper presents an adaptive selectivity filter design suitable for 3G LTE mobile terminals that utilizes the part of the cyclic prefix which is not needed to guarantee ISI free reception of the OFDM symbols to optimize the adjacent channel selectivity (ACS) for GSM interferers. Simulation results show that the above mentioned technique can improve the ACS by up to 20 dB in some scenarios compared to if a fixed selectivity filter is used.