This paper presents a novel technique to compensate intermodulation distortion of a self-heterodyne direct conversion OFDM system in multipath propagation environments. A self-heterodyne direct conversion system has an advantage that simple receivers can be built that are completely immune to any phase noise or frequency offset. This system, however, has a disadvantage that the nonlinear square-law detector at the receiver of the self-heterodyne direct conversion system gives rise to second order intermodulation distortion. In this study, channel estimation is performed using a training sequence and then the predistortion coefficients with regard to estimated channel parameters are derived to compensate the receiver nonlinearity. Transmit power distribution is employed to overcome multipath fading channels as well. Computer simulation demonstrates that the proposed approach improves the BER performance of the self-heterodyne direct conversion OFDM system in a multipath fading channel. This scheme gives advantage to multi-carrier systems that are much more sensitive to frequency and phase error than single-carrier systems.

Repeaters equipped with on-board digital baseband processing in a time division duplex (TDD) demand short processing time in order to alleviate inter-symbol interference resulting from having a time delay that is greater than the guard time. To accomplish this, the total system delay of the repeater should be minimized as much as possible without distorting signal quality. Conventionally, the finite impulse response (FIR) type of filter is deployed as a channelization filter, but due to the necessity of large numbers of coefficients to fulfill a prerequisite filter response with a sharp transition band characteristic, an unwanted excessive time delay intrinsically occurs. To make the processing delay as low as possible, this paper proposes a method employing a minimum-phase characterized infinite impulse response (IIR) filter whose magnitude response is almost identical to that of the original FIR filter. Furthermore, in order to linearize the phase response of the designed IIR filter, this paper also introduces an all-pass filter cascaded with the IIR filter for digital down-conversion as well as up-conversion. To achieve further simplicity, this paper introduces polyphase-style IIR filters transformed from conventional single IIR filters that have their own all-pass filters in order to linearize the phase response. The computer simulation results verify that the proposed integrated IIR filter exhibits a relatively short processing delay with a minor deterioration in signal quality-like error vector magnitude (EVM) performance.