A system comprising: at least one receiver (120, 220, 320, 420, 520) input port and signal channel for obtaining at least one receiver signal resulting from at least one transmitter signal that has propagated through a propagation channel, each receiver signal comprising a signal taken from one or more antenna ports of a receiver, and each transmitter signal comprising a signal provided to one or more antenna ports of a transmitter (110, 210, 310, 410, 510); and a processor, the system being characterized in that the processor is configured to form at least a first signal pair which comprises a first receiver signal and a first transmitter signal other than one to which the first receiver signal is attributable, or first and second receiver signals which are obtained from spatially-separated receiver antennas, or first and second receiver signals which are attributable to different transmitter signals; determine amplitude and phase information of the same plurality of frequency components across a bandwidth of each signal in the first signal pair; determine a set of comparison values for the first signal pair by comparing respective frequency component phases or respective frequency component amplitudes of the signals in the first signal pair; and analyze a characteristic of the set of comparison values. at least one receiver (120, 220, 320, 420, 520) input port and signal channel for obtaining at least one receiver signal resulting from at least one transmitter signal that has propagated through a propagation channel, each receiver signal comprising a signal taken from one or more antenna ports of a receiver, and each transmitter signal comprising a signal provided to one or more antenna ports of a transmitter (110, 210, 310, 410, 510); and a processor, the system being characterized in that the processor is configured to form at least a first signal pair which comprises a first receiver signal and a first transmitter signal other than one to which the first receiver signal is attributable, or first and second receiver signals which are obtained from spatially-separated receiver antennas, or first and second receiver signals which are attributable to different transmitter signals; determine amplitude and phase information of the same plurality of frequency components across a bandwidth of each signal in the first signal pair; determine a set of comparison values for the first signal pair by comparing respective frequency component phases or respective frequency component amplitudes of the signals in the first signal pair; and analyze a characteristic of the set of comparison values. form at least a first signal pair which comprises a first receiver signal and a first transmitter signal other than one to which the first receiver signal is attributable, or first and second receiver signals which are obtained from spatially-separated receiver antennas, or first and second receiver signals which are attributable to different transmitter signals; determine amplitude and phase information of the same plurality of frequency components across a bandwidth of each signal in the first signal pair; determine a set of comparison values for the first signal pair by comparing respective frequency component phases or respective frequency component amplitudes of the signals in the first signal pair; and analyze a characteristic of the set of comparison values.