Characterization of Passive Intermodulation Distortion in MultiBand FDD Radio Systems

Sammanfattning: Intermodulation distortion (IMD) is a phenomenon that results in generation of spurious distortion signals when two or more signals of different frequencies pass through a nonlinear system. In general, IMD occurs in active circuits of a radio system, however, passive wireless components such as filters, transmission lines, connectors, antennas, attenuators etc., can also generate IMD particularly when transmit power is very high. The IMD in the latter case is referred to as passive intermodulation (PIM) distortion. With the continuing advancement of radio system coupled with the radio spectrum scarcity, PIM interference is recognized as a potential obstacle to achieving the full capacity of a radio network. The assiduous enhancement of radio systems for faster data speeds and higher capacity further exacerbate the PIM interference problem. Features like carrier aggregation (CA) and multiple input multiple output (MIMO) make the PIM a more problematic issue. In modern co-sites, radio systems are often coupled together, operating in multiple bands. Furthermore, in frequency division duplex (FDD) systems, transmitter (Tx) and receiver (Rx) operate simultaneously. In such scenarios, PIM is likely to occur in the signal’s path and may potentially hit multiple Rx bands causing undesired interference. The PIM sources in the BS radio systems can be divided into two groups, namely internal and external. Internal sources are the passive components within the radio such as filters, transmission lines, connectors, antennas, etc. External sources, on the other hand, are the passive elements beyond the BS antenna but within the RF signal path such as metallic and rusty objects in antenna near field. For both types of sources, the high power current flowing through such passive objects can prompt a nonlinear behavior that in turn generates IMD. This thesis addresses PIM distortion in multiband BS radio systems by devising a characterization. For this purpose, the research begins by establishing a mathematical model for IMD and reviewing the physics that prompt nonlinear behavior in these sources. Afterwards, the enhancements in radio systems that enlarge the bandwidths and exacerbate PIM are discussed. In particular, how IMD is worsen in broadband radios is highlighted, and to complement this discussion, a review of PIM mitigation techniques is also presented. In the final part of this work, extensive lab measurement results are presented where effects of PIM with external PIM sources are analyzed and discussed. Overall, this thesis helps to build a better understanding of PIM interference problem in radio systems by providing useful insights into the nonlinear mechanisms in passive components causing PIM.