A method of channel sounding for a multi user multiple input multiple output MU-MIMO wireless communication network comprising an access point and a plurality of subscriber modules (2a - 2g), comprising: forming a respective MU-MIMO beam from the access point to each subscriber module of a first sounding group of subscriber modules by applying a beamforming weightset to a weighting matrix , the beamforming weightset having been determined on the basis of previously determined channel estimates; using each MU-MIMO beam to transmit simultaneously a respective sounding symbol , each respective sounding symbol being different from each other sounding symbol of the first sounding group; receiving a first sounding symbol at a first subscriber module of the first sounding group; and determining whether to send a response message to the access point in response to the receipt of the first sounding symbol on the basis of a determination of a received level of interference to the first sounding symbol, wherein: each sounding symbol has a respective assigned set of sub-carriers, the assigned set of sub-carriers being energised and non-assigned subcarriers being suppressed, each subcarrier assigned in an assigned set for a sounding symbol is not allocated to another assigned set for another sounding symbol of the first sounding group; and said determining whether to send a response message comprises determining a level of at least one subcarrier that is not in the assigned set for the first subscriber module, wherein said determining whether or not to send the response message comprises determining whether or not subcarriers that are not in the assigned set for the first subscriber module are received above a threshold relative to a received level of subcarriers in the assigned set. forming a respective MU-MIMO beam from the access point to each subscriber module of a first sounding group of subscriber modules by applying a beamforming weightset to a weighting matrix , the beamforming weightset having been determined on the basis of previously determined channel estimates; using each MU-MIMO beam to transmit simultaneously a respective sounding symbol , each respective sounding symbol being different from each other sounding symbol of the first sounding group; receiving a first sounding symbol at a first subscriber module of the first sounding group; and determining whether to send a response message to the access point in response to the receipt of the first sounding symbol on the basis of a determination of a received level of interference to the first sounding symbol, wherein: each sounding symbol has a respective assigned set of sub-carriers, the assigned set of sub-carriers being energised and non-assigned subcarriers being suppressed, each subcarrier assigned in an assigned set for a sounding symbol is not allocated to another assigned set for another sounding symbol of the first sounding group; and said determining whether to send a response message comprises determining a level of at least one subcarrier that is not in the assigned set for the first subscriber module, each sounding symbol has a respective assigned set of sub-carriers, the assigned set of sub-carriers being energised and non-assigned subcarriers being suppressed, each subcarrier assigned in an assigned set for a sounding symbol is not allocated to another assigned set for another sounding symbol of the first sounding group; and said determining whether to send a response message comprises determining a level of at least one subcarrier that is not in the assigned set for the first subscriber module, wherein said determining whether or not to send the response message comprises determining whether or not subcarriers that are not in the assigned set for the first subscriber module are received above a threshold relative to a received level of subcarriers in the assigned set. <EOS>