Page 63 - Antennas for Base Stations in Wireless Communications

36 Chapter Two

2.2.1 Control of Antenna Parameters
We now examine how each of the performance parameters required by
the user can be provided by the antenna design engineer.

2.2.1.1 Azimuth Radiation Pattern The azimuth beamwidth of an array
is determined by the design of the radiating element(s) used for each
vertical unit (tier), together with its relationship to the reflecting surface
behind it, used to create a unidirectional beam. The width and shape of
the reflector will also control the front-to-back ratio and to some extent
the rate of roll-off off the pattern beyond the –3 dB points of the azimuth
pattern.
In order to obtain predictable network hand-off behavior, the shape
of the azimuth radiation pattern should change as little as possible
over the operating frequency band. The MS measures the level of the
Broadcast Control Channel (BCCH) GSM or Pilot Channel (CDMA)
received from different cells, both of which are functions of the beam-
width of the BS antenna at the BS transmit (TX) frequency; the system
assumes that a handoff made by comparative measurements on these
channels will be accompanied by a matching change in the BS receive
(RX) channel gain, and a significant difference between antenna gain
on the TX and RX frequencies may result in problems with uplink qual-
ity, unstable handoff performance, or dropped calls. In a CDMA system,
the overlap between cells has a critical effect on the proportion of traffic
that uses soft/softer handoff modes, a parameter that directly affects
network capacity. For these reasons, it is usual to specify tight limits on
the azimuth beamwidth as a function of frequency, typically 65° ±3° or
90° ±5° over the whole band. The beamwidth at −10 dB is sometimes also
specified, but this is relatively difficult to control by antenna design.
The typical front-to-back (F/b) ratio of a 65° antenna is 30 dB, and
this is specified to control the frequency re-use characteristics of the
base station. Whatever antenna is chosen for use, the network engineer
should use measured pattern data in system coverage modeling and
should check on the coverage implications of any change in radiation
patterns and gain over the band.
If the required F/b ratio is greater than is offered in an otherwise
suitable antenna, the effective ratio can be increased by selecting an
antenna with a larger electrical beamtilt than is required and mounting
it in an uptilted attitude. This places the main beam with the desired
beamtilt in the forward direction while tilting the unwanted rearlobe
well below the horizontal in the rearward direction.
The usual convention is that the azimuth pattern is specified and
measured at an elevation angle that contains the elevation beam maxi-
mum, so for an antenna with an electrical downtilt of 5°, the azimuth
pattern is plotted on the surface of a cone with a half angle 85° from the