From another point of view, beamforming techniques can
be divided into open-loop and closed-loop methods. In openloop
methods, receivers do not participate in finding the proper
orientation of transmitted signals and the transmitter has to
calculate the direction according to received data signals, while
in closed-loop methods, there is a feedback channel between
the two ends to help estimate the direction of the signal.
The IEEE 802.11n standard defines these two methods as
Implicit Feedback and Explicit Feedback beamforming [2].
In either case, by detecting the direction of clients, signals
can be focused at a particular angle to avoid wasting the
signal energy in other directions. Moreover, by knowing the
appropriate angle for each client, signals from other directions
can be regarded as interference and the reception performance
as well as the range of transmission are increased considerably