Installation Issues

In any wireless communication system there are always a host of antenna
installation problems ranging from space restrictions to tower compression
or shearing loading factors or even the physical ports available to be used.
However, the installations more common are associated with physically
mounting the antennas. The introduction of 2.5G and 3G systems have
guaranteed that the installation issues of the past will continue.
One of the prevalent issues associated with 2.5G and 3G is the lack of the
number of physical antennas that will be available from which to utilize.As
with the introduction of any new radio access platform, each technology has
its own special issues.
For the CDMA2000 antenna systems there are some different considerations
to take into account when migrating from an IS-95 system to a
CDMA2000 system if it is an AMPS or PCS spectrum. The desire is to utilize
transmit diversity, and this will be achieved either by a STD or OTD
method. However, the STD method is the preferred version.

Figure 11-4 shows a typical situation where there are two or three antennas
per sector available for use. Sometimes there is only one antenna if it is
a cross pole antenna.With an AMPS system as the underlying legacy system,
the use of a STD transmit diversity scheme is possible with a configuration
shown in Figure 11-4 with the exception that only one carrier is used
for CDMA. If a second carrier is added, then OTD diversity is utilized and
the configuration shown in Figure 11-4 is used. Now, if the operator has
been able to secure more antennas per sector, that is, five, then the configuration
shown in in the figure is the desired method where the AMPS and
CDMA systems are bifurcated. The use of STD or OTD is again dependant
upon the number of carriers required at the site.
Regarding the deployment of GPRS into an existing Global System for
Mobile Communications (GSM) network, the migration is rather straightforward
from an antenna aspect because the carriers and fundamental
infrastructure issues remain the same. The only difference lies in the
amount of antennas that may need to be added due to transmitter combing
losses. However, there is no unique antenna configuration issues that need
to be adhered to other than standard GSM deployment schemes.
However, when implementing a GPRS system over a IS-136 system or
migrating from GPRS to WCDMA,there are antenna issues that need to be
thought about prior to acquiring the cell site or installing antennas. What
needs to be thought about is the fundamental problem that GPRS or IS-136
relies on a different modulation scheme and therefore has different performance
parameters and design guidelines.
A lesson learned with IS-95 deployment into an Advanced Mobile Phone
System (AMPS) environment is that for performance and optimization reasons,
a set of separate antennas should be sought were possible. It is not
that the technologies cannot share the same antenna but that the optimization
techniques for same GPRS or IS-136 is different than that envisioned
forWideband CDMA (WCDMA). Therefore, if the antenna system is
not separated, performance compromises will be experienced in both the
WCDMA and the legacy systems.
Figure 11-5 shows a typical situation where there are two or three antennas
per sector available for use. Sometimes there is only one antenna but it
is a cross pole antenna. With a GSM or IS-136 system as the underlying
legacy system, the use of a STD transmit diversity scheme is possible with
a configuration shown in Figure 11-5 with the exception that only one carrier
is used for WCDMA. If a second carrier is added, then OTD diversity is
utilized and the configuration shown in Figure 11-5 is used. Now if the
operator has been able to secure more antennas per sector, that is, five, then the configuration shown in the figure is the desired method where the
GSM/IS-136 and WCDMA systems are bifurcated. The use of STD or OTD
is again dependant upon the number of carriers required at the site.
Both WCDMA and CDMA2000 systems are all envisioned to be deployed
as three sectors only. Although there is the capability to deploy more sectors,
the practicality of the situation favors three sectors.
The determination of where to place antennas or the methodology that is
used to place the antennas is often encountered when not utilizing a monopole
or tower installation. Figure 11-6 shows an example of an omni site or
single sector involving a transmit and two receive antennas. The transmit
antenna is installed in the center while the receive antennas are aligned as
best as reasonably possible to provide maximal diversity reception for the
major road shown in the figure. Obviously the example is more relevant for
the small stretch of highway and different installation schemes can be
implemented.
The diagram shown in Figure 11-7 is a slight modification from that
shown in Figure 11-6. The change addresses the issue of when the antennas
cannot be installed at the edge of the building’s roof and needs to be
installed on the penthouse of the building.
When installing on a penthouse or any building installation where the
antennas are not installed at the edge of the building for either visual or
structural reasons, a setback rule needs to be followed. The setback rule involves the relationship between the antennas installation point, its
height above the roof top, and of course the distance between the antenna
and the roof edge.
Figure 11-8 shows the relationship in a simplistic drawing of the
antenna placement to the roof edge when installing on a roof. The concept
is to avoid violating the first fresnel zone for the antenna, however, because
each antenna has a different pattern, and there are different operating frequencies.
The relationship shown next will provide the necessary clearance.
a
5  b
Examination of the equation draws the conclusion that the farther the
antenna is from the roof edge, the higher it will need to be installed to
obtain the necessary clearance.