A Low Complexity Algorithm to Compute Intermodulation Distortion of Arbitrary Signals in Frequency Domain

Intermodulation distortion (IMD) arises in all analogue circuitry, due to always-present nonlinearities. Although they may be very weak, due to near-far problem in wireless communications large level discrepancies are faced. IMD spectra may shade weak ones and turn e.g. a receiver blind and imply outage of communication. In MIMO transmitters, a problem called reverse intermodulation is faced, leading into wideband emission and signal integrity degradation. We present here an algorithm of low complexity that efficiently computes the intermodulation spectra in receivers and transmitters for NLTI (non-linear time invariant) systems. Thus, the only prerequisite we demand is memoryless behavior of system, which holds well for receivers and transmitters unless they are driven deeply into saturation. The algorithm is largely based on iFFT/FFT and Isserli’s theorem. It is capable of computing IM3, IM5 and IM7 effects. Further, it provides large insight into the compression mechanisms present in receivers and transmitters and how each IM order contributes to the overall spectrum. By that, we overcome the simple treatment of intermodulation with unmodulated two-tone test or the time consuming computation of intermodulation with modulated signals in time domain.