Mitigation of Reverse Intermodulation Products at Colocated Base Stations

In a co-located setting, large jamming signals from one transmitter can radiate into the antenna system of a second transmitter. The signals enter the second transmitter in the reverse direction and mix in the output stage of its power amplifier to produce intermodulation products. These ‘reverse’ intermodulation products get radiated from the antenna system and may fall on the victim receiver´s desired channel. The paper proposes an architecture that regenerates an estimate of the reverse intermodulation products using the fundamental jammer components and mitigates them in a baseband postdistortion cancellation circuit. A novel multiple-front-end receiver architecture is developed to overcome the high sample rate requirements if the jammers are well out of band. However, this leads to a frequency offset problem in the regenerated distortion estimate. Signal correlation is used to align the frequency, phase and amplitude of the distortion estimate with the interfering reverse intermodulation product. Simulations and theoretical analysis show the output signal-to-interference ratio (SIR) of the system is independent of the input SIR but dependent on the equivalent number of uncorrelated samples in the averaging block. A hardware prototype demonstrated a 16 dB reduction of the interfering reverse intermodulation product.