Page 194 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 167

4.11 Steered Beam Array Antenna
A steered beam antenna is characterized by the use in downlink of a
beam shape, on a per-user basis, that is adapted to the existing traf-
fic situation in the network. This adaptation is in terms of adjusting
only pointing direction for simpler systems but also beam shape and
sidelobe levels for more advanced antenna systems. 7,44−46 As each user
is served by a unique beam, the beamforming is performed at baseband
because this is the only place in the transmission chain where user
signals are separately controllable. Baseband beamforming means that
coherency, i.e., sufficiently small errors in amplitude, phase, and time
between signal paths, is needed from baseband to the antenna aperture.
Calibration is likely necessary to fulfill the coherency requirements for
most implementations.
A steered beam system in uplink is similar to a fixed multibeam
reception system. However, the configuration depends on whether or
not a Butler matrix is used in the antenna unit. One major purpose
of using a Butler matrix in a steered beam implementation is to relax
coherency requirements.
For systems with uplink and downlink separated in frequency more
than a fraction of the correlation bandwidth of the propagation channel,
for example, frequency-division duplex (FDD) systems, reciprocity will
typically not hold; i.e., the uplink signal state over the antenna (ampli-
tude and phase distribution) cannot be directly applied to the downlink
signals. Either a frequency compensation corresponding to the duplex
distance has to be applied, if at all possible, or, which is more likely,
higher level information about the user device may be extracted from
the uplink signals, such as knowledge about the spatial location of the
user. The beam used for downlink transmission may then be based on
the direction information for the user device of interest. Other infor-
mation that is useful is the spatial distribution of traffic load, which
may contain information about other user devices within the cell, for
example, their location, data rates, and power allocation, and possibly
also information about user devices in adjacent cells served by the same
base station.
Figure 4.26 shows a functional chart of one implementation of a
steered beam antenna system. This system employs beamsteering on
downlink and has fixed beamforming on uplink that performs angular
prefiltering of the signals to facilitate direction of arrival (DOA) esti-
mation.
This antenna system requires one individual transmitter for each
antenna element or column as well as phase coherency of the branches
on both the receiving and transmitting sides. The advantage is that down-
link beamforming is not limited to a fixed set of beams or beam shapes.