A method comprising: generating statistical channel correlation matrices R TX, R RX, R Del, from tensor unfoldings H, H, H of a multi-dimensional array or tensor describing a channel impulse or frequency response at a transmit-side, receive-side and across a delay-spread of a multiple-input, multi-output wireless transmission system (1, 20) having a plurality of N TX transmit antennas and a plurality of N RX receive antennas for a defined period, wherein the channel correlation matrices comprises a transmit-side matrix, a receive-side matrix and a delay-spread matrix, wherein the tensor unfoldings describe channel behaviour in single-input multiple output, multiple-input single-output, and delay-spread respectively, as follows: H 1 = h 111 ⋯ h 11 D h 121 ⋯ h 12 D ⋯ h 1 N Tx 1 ⋯ h 1 N Tx D ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ h N Rx 11 ⋯ h N Rx 1 D h N Rx 2 D ⋯ h N Rx 2 D ⋯ h N Rx N Tx 1 ⋯ h N Rx N Tx D H 2 = h 111 ⋯ h N Rx 11 h 112 ⋯ h N Rx 12 ⋯ h 11 D ⋯ h N Rx 1 D ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ h 1 N Tx 1 ⋯ h N Rx N Tx 1 h 1 N Tx 2 ⋯ h N Rx N Tx 2 ⋯ h 1 N Tx D ⋯ h N Rx N Tx D H 3 = h 111 ⋯ h 1 N Tx 1 h 211 ⋯ h 2 N Tx 1 ⋯ h N Rx 11 ⋯ h N Rx N Tx 1 ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ h 11 D ⋯ h 1 N Tx D h 21 D ⋯ h 2 N Tx D ⋯ h N Rx 1 D ⋯ h N Rx N Tx D eigen-decomposing each of said correlation matrices to generate an eigen-decomposed transmit-side matrix, an eigen-decomposed receive-side matrix and an eigen-decomposed delay-spread matrix; extracting transmit-side, receive-side and delay-spread eigenvectors from the eigen-decomposed transmit-side, receive-side and delay-spread correlation matrices; generating differential unitarity measures using transmit-side, receive-side and delay-spread eigenvectors extracted at different time points, wherein generating differential unitarity measures comprises generating an indication of changes in unitary of respective eigenbases as determined from the generated differential unitarity measures; and generating an output indicative of presence or activity within a defined space based on said differential unitarity measures. generating statistical channel correlation matrices R TX, R RX, R Del, from tensor unfoldings H, H, H of a multi-dimensional array or tensor describing a channel impulse or frequency response at a transmit-side, receive-side and across a delay-spread of a multiple-input, multi-output wireless transmission system (1, 20) having a plurality of N TX transmit antennas and a plurality of N RX receive antennas for a defined period, wherein the channel correlation matrices comprises a transmit-side matrix, a receive-side matrix and a delay-spread matrix, wherein the tensor unfoldings describe channel behaviour in single-input multiple output, multiple-input single-output, and delay-spread respectively, as follows: H 1 = h 111 ⋯ h 11 D h 121 ⋯ h 12 D ⋯ h 1 N Tx 1 ⋯ h 1 N Tx D ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ h N Rx 11 ⋯ h N Rx 1 D h N Rx 2 D ⋯ h N Rx 2 D ⋯ h N Rx N Tx 1 ⋯ h N Rx N Tx D H 2 = h 111 ⋯ h N Rx 11 h 112 ⋯ h N Rx 12 ⋯ h 11 D ⋯ h N Rx 1 D ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ h 1 N Tx 1 ⋯ h N Rx N Tx 1 h 1 N Tx 2 ⋯ h N Rx N Tx 2 ⋯ h 1 N Tx D ⋯ h N Rx N Tx D H 3 = h 111 ⋯ h 1 N Tx 1 h 211 ⋯ h 2 N Tx 1 ⋯ h N Rx 11 ⋯ h N Rx N Tx 1 ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ ⋮ ⋱ ⋮ h 11 D ⋯ h 1 N Tx D h 21 D ⋯ h 2 N Tx D ⋯ h N Rx 1 D ⋯ h N Rx N Tx D eigen-decomposing each of said correlation matrices to generate an eigen-decomposed transmit-side matrix, an eigen-decomposed receive-side matrix and an eigen-decomposed delay-spread matrix; extracting transmit-side, receive-side and delay-spread eigenvectors from the eigen-decomposed transmit-side, receive-side and delay-spread correlation matrices; generating differential unitarity measures using transmit-side, receive-side and delay-spread eigenvectors extracted at different time points, wherein generating differential unitarity measures comprises generating an indication of changes in unitary of respective eigenbases as determined from the generated differential unitarity measures; and generating an output indicative of presence or activity within a defined space based on said differential unitarity measures.