Page 164 - Antennas for Base Stations in Wireless Communications

Advanced Antennas for Radio Base Stations 137

4.5 Higher Order Receive Diversity
Coverage is an important issue, especially in sparsely populated areas.
One method to increase uplink coverage is to employ higher order receive
diversity in base stations in order to improve receiver sensitivity. In sys-
tems where coverage is uplink limited, a solution based on higher order
receive diversity can provide a balance in the uplink-downlink perfor-
mance. The four-branch receive diversity uses four complete RF receiver
chains. The four antenna signals may be obtained by a combination of
polarization and space diversity combined at baseband. Figure 4.4a shows
an antenna configuration with two polarization diversity antennas, with
a separation of 10 to 20 wavelengths. However, space diversity is not
crucial for the performance in environments with rich multipath, and in
these cases, those two polarization antennas can be combined into one
10
unit under a single radome, as shown in Figure 4.4b. The advantages of
the combined antenna unit are its compact size and easier installation.
For higher order receive diversity systems, i.e., receive diversity sys-
tems with more than two antennas, the additional statistical diversity
gain decreases with increasing number of diversity branches for many
macro site propagation environments, whereas the gain related to the
additional antenna area, power gain, will still be available. The result-
ing improvement of the receiver sensitivity performance may not be
motivated as it comes at the cost of additional receiver chains.

10–20 wavelengths
(a) (b)
Figure 4.4 Four-branch receive diversity antenna configurations with
dual-polarized antenna columns: (a) separate antenna units and (b)
single antenna unit