Page 441 - From GMS to LTE

Wireless Local Area Network (WLAN) 427

practical environments. Additional MIMO paths could, however, become beneficial
when used in combination with beamforming to address several devices simultaneously
as described in the next paragraph.
Beamforming is another option in the 802.11ac standard for concentrating signal
energy in the direction of a client device. This requires that the AP becomes aware of
the direction in which to concentrate the signal energy. For this purpose, the AP trans-
mits channel sounding announcements, the so‐called Null Data Packet (NDP)
announcements. The name is derived from the channel sounding method, which is
based on transmitting an empty frame whose OFDM symbols are analyzed by the
mobile device for changes they have undergone during their transmission over the air.
For this purpose, the AP sends an NDP announcement message to request beamform-
ing‐capable devices to respond. In a second step, NDP packets are sent by the AP and
received by the client devices. These devices then analyze the OFDM symbols of the
packets, calculate a response that describes how the OFDM symbols have been altered
during transmission and return the result in a response packet. Based on this feedback,
the AP can then calculate a steering matrix for each individual client device, which is
then applied to data transmissions. The steering matrix describes how to distribute the
signal energy across the available transmission chains and antennas and how to apply
phase shifts to each chain. This way, the interference of the different wavefronts
increases the signal in one direction while decreasing the overall signal in another direc-
tion. To account for changing signal conditions, this sounding procedure has to be
repeated in the order of once per 100 milliseconds [12]. As the signal level increases in
a desired direction, regulation requires that the overall power output of the AP is
reduced by 3 dB during such a transmission to ensure that the overall transmission
power limit, which is defined for an omnidirectional antenna, is not exceeded by the
amplified directional signal. It remains to be seen by how much this transmit power
restriction reduces the effect of beamforming.
Beamforming can be combined with MIMO transmission to direct several data
streams to a single device and is referred to as Single‐User (SU) beamforming. An even
more sophisticated application of beamforming is to transmit one or more streams to
several client devices simultaneously. This is referred to as Multi‐User (MU) beam-
forming. Up to four devices can be serviced simultaneously, which could be especially
useful when APs that support more MIMO streams than individual client devices sup-
port are used. This could be the case, for example, where the AP supports four MIMO
streams whereas the battery‐powered devices (in particular) support only one stream to
reduce the computational overhead of MIMO reception, in order to conserve battery
power. By transmitting independent data streams to several devices, better use can be
made of the transmission channel and thus a higher overall throughput can potentially
be reached. The preamble of such a multi‐user frame contains information regarding
which client devices it is addressed to. Afterward, the multi‐user frame stacks the indi-
vidual data streams on top of each other and uses beamforming to direct the signal
energy for each individual data stream in the desired direction. This way, the data for
one device is not seen as noise from the point of view of another device, given that the
two devices are well enough separated in space to make the streams independent of
each other. As the amount of data as well as the modulation and coding for each device
might be different, some parts of the multi‐user frame are unused and padded. One
question that arises is how the recipients of a multi‐user frame can acknowledge