CDMA

The operators who chose to deploy CDMA systems had basically two methods
to use in deploying CDMA, IS-95 systems. The first method is to deploy
CDMA in every cell site for the defined service areas on a 1:1 basis. The
other method available is to deploy CDMA on a N:1 basis. Both the 1:1 and
the N:1 deployment strategies had their advantages and disadvantages.Of
course, a third method involved a hybrid approach to both the 1:1 and N:1
methods.

The introduction of CDMA into an existing AMPS system also required
the establishment of a guard band and guard zone. The guard band and
guard zone are required for CDMA to ensure that the interference received
from the AMPS system does not negatively impact the ability for CDMA to
perform well.

The specific location that the CDMA channel or channels occupy in a
cellular system is dependent upon a multitude of issues. The first issue is
how much spectrum will be dedicated to the use of CDMA for the network.
The spectrum issue ties into the fact that one CDMA channel occupies 1.77
MHz of spectrum, 1.23 MHz per CDMA channel, and 0.27 MHz of guardband
on each side of the CDMA channel. With a total of 1.77 MHz per
CDMA, the physical location in the operator’s band that CDMA will operate
in needs to be defined. For the B-band carrier, wireline operators, two
predominant locations were utilized. The first location in the spectrum is
the band next to the control channels, and the other section is in the lower
portion of the extended AMPS band. The upper end of the AMPS band is
not as viable due to the potential of AGT interference because AGT transmit
frequencies have no guard band between AMPS receiving and AGT
transmitting. The lower portion of the AMPS band has the disadvantage of
receiving A band mobile to base interference, which will limit the size of
the CDMA cell site.
The other issue with the guard band ties into the actual amount of spectrum
that will be unavailable for use by AMPS subscribers in the cellular
market.With the expansive growth of cellular, assigning 1.77 MHz of spectrum
to CDMA reduces the spectrum available for AMPS usage by 15 percent
or to 59 radio channels from the channel assignment chart. The
reduction in the available amount of channels for regular AMPS requires
the addition of more cell sites to compensate for the amount of radio channels
no longer available in the AMPS system. Utilizing a linear evaluation,
the reduction in usable spectrum by 15 percent involves a reduction on the
traffic-handling capacity by the AMPS system by a maximum of 21 percent
at an Erlang B 2-percent Grade of Service (GOS) with a maximum of 16
channels per sector verse 19. The reduction of 21 percent in the initial
AMPS traffic-handling capacity requires the need to build more analog cell
sites to compensate for this reduction in traffic-handling capabilities. The
only way to offset the reduction in the traffic-handling capacity experienced
by partitioning the spectrum is to preload the CDMA subscriber utilizing
dual mode phones or to build more analog cell sites.

The guard zone is the physical area outside the CDMA coverage area
that can no longer utilize the AMPS channels now occupied by the CDMA system. Figure 3-5 shows an example of a guard zone versus a CDMA
system coverage area. The establishment and size of the guardzone is
dependent upon the traffic load expected by the CDMA system. The guard
zone is usually defined in terms of a signal strength level from which analog
cell sites operating with the CDMA channel sets cannot contribute to
the overall interference level of the system. The interesting point about the
guard zone is when the operator on one system wants to utilize CDMA and
must require the adjacent system operator to reduce his or her channel utilization
in the network to accommodate his or her neighbor’s introduction
of this new technology platform.

However, regardless of the method chosen for the implementation of the
CDMA into an existing 1G analog system, part of the radio frequency spectrum
needed to be cleared of existing analog radio usage. The impact to this
situation, as discussed,was the need to build more cell sites with lower traffic-
carrying capacity due to the spectrum reduction, or to increase the blocking
percentage that the system would be allowed to operate at. Obviously,
the mix of both increased blocking as well as additional cell sites was the
method that was followed by the wireless operators.
In addition to the reduction in spectrum that is used for existing 1G subscribers
for base stations which had CDMA installed there are also numerous sites which did not have CDMA installed but still had to surrender the
use of spectrum to accommodate the introduction of CDMA in the system.
Naturally, if the system was a complete 1:1 system, then there would be no
need for the implementation of a guard zone and only a guard band. But
when the CDMA system butted up to another radio access system like
TDMA or even analog where another operator decided not to implement 2G
systems, the need for a guard zone and guard band was required. 60