Heterogeneous wireless systems such as a combination of mobile broadband wireless access (MBWA) and wireless local area network (WLAN) are candidates to attain higher throughput and wide coverage in the next generation mobile communication systems. A multimode receiver to demodulate signals with multiple air interfaces is required in the heterogeneous systems. In this paper, we discuss an implementation of frequency domain equalizer (FDE) in the multimode receiver on an application specific integrated circuit (ASIC). The proposed multimode FDE could realize adaptively demodulate two systems (single carrier and multi carrier) depending on propagation environment. A multimode FDE ASIC is designed and implemented on 180-nm complementary metal-oxide semiconductor (CMOS). Core size of multimode FDE chip is 17.6 mm2. The experimental results show that the ASIC yields a throughput of 48.1 Mbit/s at a sampling rate of 100 MHz. The results also show that the power penalty of the measured bit error rate (BER) performance is only 1 dB at a BER of 10-3 in a 4-paths Rayleigh fading channel. We consider that the implemented ASIC is useful to demodulate signals with different air interfaces while optimally selecting an appropriate air interface in the heterogeneous wireless system.