External Passive Intermodulation Suppression by Near-Field Downlink Beamforming

Frequency-division duplex (FDD) systems face the interference of passive intermodulation (PIM) caused by nonlinear distortion of transmitted signals. This type of self-interference problem is more serious in FDD multiple-input multiple-output (MIMO) systems. Because large-scale antenna arrays can further enhance the current density in the vicinity of the base stations (BSs), passive components in the radiation field of antennas may excite more powerful PIM. In this article, near-field downlink beamforming is innovatively used to suppress radiated energy at the location of passive devices outside the BS, thus avoiding the generation of external PIM (ePIM). Compared with the classic PIM cancellation algorithms, the proposed method avoids real-time computation thereby computational resources are greatly saved. To accurately obtain the location of ePIM sources, the wireless channel coefficients from the transmitter chain to the ePIM sources are estimated based on a nonlinear signal model. This signal model simulates the nonlinear process of ePIM generation for the scenarios with multiple dual-carrier TX channels and multiple ePIM sources, taking into account the convolutional channel that the signal passes through during transmission and the nonlinear process of the ePIM sources. Experiments conducted with real-life BS evidence excellent ePIM suppression capabilities. Because the user equipment (UE) signal gain is considered in the calculation of the optimized beamforming vectors, the signal gain loss in the UE direction is minimal.