Detection and characterisation of passive intermodulation at a MIMO antenna array

A Multiple Input Multiple Output (MIMO) passive intermodulation (PIM) analysis test set for detecting and characterising passive intermodulation interference at a MIMO antenna array comprising a first group of antennas for operation in a first band and a second group of antennas for operation in a second band, the apparatus comprising:

• one or more receivers for receiving:
• first digital signals representing respective first MIMO signal components transmitted at a first carrier frequency from each antenna of the first group of antennas;
• second digital signals representing respective second MIMO signal components transmitted at a second carrier frequency from each antenna of the second group of antennas, and
• third digital signals representing signals received at an antenna under analysis comprising interference generated by intermodulation between the first MIMO signal components and the second MIMO signal components;
• and
• a processor configured to:
• process the first digital signals and the second digital signals to determine a plurality of partial simulated PIM signals, each partial simulated PIM signal representing intermodulation between a respective one of the MIMO signal components transmitted from one group of the first and second groups of antennas and all of the MIMO signal components transmitted from the other group of the first and second groups of antennas;
• determine a magnitude of a cross-correlation between the third digital signals representing signals comprising interference received at the given antenna and each of the plurality of partial simulated PIM signals;
• and
• generate an output indicative of the determined cross-correlations.