null data packet (802.11ac) (NDP)
Various beamforming methods are specified in 802.11n. Since the implementation of several methods is extremely complex and often faulty, many products do not implement beamforming.
To prevent this situation from occurring again with 802.11ac, only one beamforming method has been specified for this standard: Null Data Packet ( NDP) sounding with unique weighting by a compressed beamforming matrix. Since only one procedure is specified, implementation is simplified for wireless access points( WAP), the beamformers, and WLAN clients, the beamformees.
In NDP beamforming, the beamformer communicates with beamformees (WLAN clients) that it identifies in the radio range. For test purposes, a Null Data Packet (NDP) is sent out to analyze the channel behavior. The beamformee develops a matrix, which the beamformer uses as a control matrix. This matrix is used to concentrate the transmit power in the preferred direction.