The present subject matter relates to an apparatus for a communication system. The communication system comprises transmitters being configured to operate in multiple transmission frequency bands. The apparatus comprises means being configured for: capturing a set of over-the-air signals at distinct frequency bands of the multiple transmission frequency bands; aligning in time the set of signals using delays between the set of signals and a received signal at a selected receiver of the communication system; combining the set of aligned signals to generate a composite signal; and estimating an interference signal that is caused by the set of signals at the selected receiver by weighting the composite signal using a set of one or more calibrated parameters for the estimation.