Air-Induced Passive Intermodulation in FDD MIMO Systems: Algorithms and Measurements

Abstract

In this article, we model and develop effective cancellation schemes for passive intermodulation (PIM) distortion induced by external objects in the vicinity of the transmitterreferred to as air-induced PIM-in frequency-division duplex (FDD) multiple-input-multiple-output (MIMO) systems. PIM is interference generated from the transmitted signals undergoing nonlinear transformation, which, in FDD systems, may cause desensitization of the receiver chain. First, we present a general model of the received air-induced PIM signal, with an arbitrary number of dual-carrier TX chains active and an arbitrary number of PIM sources causing interference, on all the intermodulation frequencies. Then, the derived model is used to develop a cancellation scheme based on a complete set of basis functions (BFs) in a rank-2 dual-carrier MIMO system with four active carriers. To alleviate the high complexity of the aforementioned scheme, we then propose a novel alternative cancellation scheme with much reduced complexity, leveraging on the physical modeling of the system, which is capable of handling any number of PIM sources in the system. RF measurementbased experimentations carried out with real-life equipment evidence excellent cancellation capabilities of the complete BF model, which can be retained with much reduced complexity with the proposed alternative technique.

Passive Intermodulation Effects in Modern Radio Communication Systems

“…Many active and passive components can act as sources of IM in radio systems. The type of IM caused by nonlinear passive elements is referred to as PIM, which arises specifically when a high-power transmit signal traverses a passive device in its transmission path [7,30,37,[42][43][44][45][46][47][48]. Typical PIM sources include cables, connectors, duplex filters, and antennas.…”

“…Moreover, achieving large isolation between the TX and RX ports of the duplexer is less desirable because of the associated size, cost, and design challenges [82]. In brief, FDD radios are more susceptible to interference in the UL because the transmitted signal power can be up to 130 dB larger than the RX signal [7,[83][84][85][86].…”

“…External PIM, also referred to as air-induced PIM [7,30,44,45], occurs when DL signals from the same or different antenna branches cause interference with the UL frequency of any branch. This type of PIM is primarily caused by elements located outside the radio unit, as illustrated in Fig.…”

“…OFDM-based transmissions enable wider transmission bandwidth [3]. CA allows increased bandwidth by aggregating up to five component carriers (CCs) of 20 MHz from the same operating frequency band (contiguous) or different operating frequency bands (non-contiguous) of the spectrum, 100 MHz in total, and thus increases data rates [4][5][6][7][8][9][10][11][12]. Moreover, in advanced multi-antenna technology, MIMO uses multiple antennas to transmit higher data rates to the same user equipment, both in the uplink (UL) and downlink (DL) transmission directions [11,13].…”

“…Allocating UL and DL resources strategically can mitigate PIM, but this approach limits spectrum utilization and peak data rates. Lastly, improving the quality and isolation of RF equipment incurs significant costs [8,9,[30][31][32][33][34][35][36][37][38][39].…”