Passive Intermodulation in High-Power Radio Transceivers
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Passiivinen intermodulaatio suuritehoisissa radiolähetin-vastaanottimissa
Passive intermodulation (PIM) is a phenomenon which occurs when at least two signals are fed into a nonlinear passive device or circuit. Sources for PIM can be divided into two groups, nonlinearities in metal junctions and nonlinear materials. The most common source for PIM is a loose or a bad metal connection. The problem is more in a base station side because PIM requires high powers and a base station can have high trans-mission (TX) power and receive (RX) power may be low. In addition, there are con-nectors in use at base stations and antennas with metallic junctions. Furthermore, a base station duplexer may have a high isolation between TX and RX port leading to a situa-tion where intermodulation (IM) products due to TX power amplifier are attenuated well and PIM which is generated after the TX band-pass filter becomes significant. PIM is significant in the carrier aggregation technology, which uses more than one component carrier. In carrier aggregation, component carriers can be allocated non-contiguously on one or more frequency bands. If the duplex spacing is narrow, high-power 3rd order PIM products may fall on RX frequency band and desensitize the transceiver’s own receiver. Digital IM cancellation is based on estimating how TX signals are modified at the path to the receiver by taking and processing samples of the received signal. Then the main idea is to regenerate replicas of IM products and subtract them from the received signal. The aim in this thesis is to demonstrate that PIM products, which are generated after the TX band-pass filter, can be reduced with digital cancellation. The duplexer that is used in measurements is a frequency band 1 base station duplexer which has 190 MHz duplex spacing. Because of that, lower than 7th order IM products are not in the RX frequency band. For a reproducible test setup, a nonlinear connection at the antenna port of the duplexer is emulated with a diode. The diode circuit generated high-power IM products already with +20 dBm TX power at the antenna port and with this power the TX filter completely attenuated the IM products due to the power amplifier. With the digital cancellation, the 7th order IM product was successfully attenuated by 6 dB to 14 dB depending on the TX power. These measurement results demonstrate that it is possible to reduce PIM interference with digital cancellation. However, in this thesis the duplex spacing was considerably wide and therefore the most high-power 3rd order IM products could not be measured. For future research it would be important to measure how digital cancellation works when the duplex spacing is narrow and PIM product power is higher but the order is lower.