In a multi user multiple input multiple output MU-MIMO wireless communication network comprising an access point and subscriber modules, a method of channel sounding comprises 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 determined based on previously determined channel estimates. Each MU-MIMO beam is used to transmit a respective sounding symbol, each respective sounding symbol being different from each other sounding symbol of the first sounding group. A first sounding symbol is received at a first subscriber module of the first sounding group, and it is determined whether to send a response message to the access point on the basis of a determination of a received level of interference to the first sounding symbol.