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Due to limited frequency resources, the co-channel interference (CCI) from adjacent femto cells operating and coexisting with conventional macro cells in the same frequency channel would greatly degrade the bit error rate (BER) performance of mobile terminals. CCI cancellation algorithms for the 2G GSM and the 3G CDMA systems have been under intense research. However, since orthogonal frequency division multiplexing (OFDM) has been adopted in the next generation mobile communication systems, dealing with CCI is more complex in OFDM than in GSM or CDMA because it requires dynamic power control and frequency allocation with advanced coordination among base stations. In this paper, we propose a single antenna interference cancellation (SAIC) algorithm named power adaptive joint maximum a posterior (PA-JMAP) for OFDM systems. The proposed PA-JMAP algorithm jointly decodes the desired and interfering data by using a joint trellis, which defines all possible combinations of each base station's encoder trellis. Signal to interference ratio (SIR) estimated by a sequentially updated estimator is used to track power changes of the received signal. Simulation results show that the proposed PA-JMAP decoder could greatly improve the BER performance for OFDM systems even under severe CCI conditions when compared to a conventional viterbi decoder (CVD).