TDMA (IS-54/IS-136)

IS-136, an enhancement of IS-54, is the digital cellular standard developed
in the United States using TDMA technology. Systems of this type operate
in the same band as the AMPS systems and are used in the PCS spectrum
also. IS-136 therefore applies to both the cellular and PCS bands, as well as
in some unique situations to down-banded IS-136, which operates in the
SMR band.
TDMA technology enables multiple users to occupy the same channel
through the use of time division. The TDMA format utilized in the United
States follows the IS-54 and IS-136 standards and is referred to as the
North American Dual Mode Cellular (NADC). IS-136 is an evolution to the
IS-54 standard and enables a feature-rich technology platform to be utilized
by the current cellular operators.
TDMA, utilizing the IS-136 standard, is currently deployed by several
cellular operators in the United States. IS-136 utilizes the same channel
bandwidth, as does analog cellular: 30 kHz per physical radio channel. However,
IS-136 enables three and possibly six users to operate on the same
physical radio channel at the same time. The IS-136 channel presents a
total of six time slots in the forward and reverse direction. IS-136 at present
utilizes two time slots per subscriber at this time with the potential to go to
half-rate vocoders that require the use of only one time slot per subscriber.
IS-136 has many advantages in its deployment in a cellular system:

■ Increased system capacity, up to three times over analog
■ Improved protection for adjacent channel interference
■ Authentication
■ Voice privacy
■ Reduced infrastructure capital to deploy
■ Short message services

Integrating IS-136 into an existing cellular system can be done more easily
than for the deployment of CDMA. The use of IS-136 in a network
requires the use of a guardband to protect the analog system from the
IS-136 signal. However, the guardband required consists of only a single channel on either side of the spectrum block allocated for IS-136 use.
Depending on the actual location of the IS-136 channels in the operator’s
spectrum, it is possible to only require one or no guardband channel.
The IS-136 has the unique advantage of affording the implementation of
digital technology into a network without elaborate engineering requirements.
The implementation advantages mentioned for IS-136 also facilitate
the rapid deployment of this technology into an existing network.
The implementation of IS-136 is further augmented by requiring only
one channel per frequency group as part of the initial system offering. The
advantage with only requiring one channel per sector in the initial deployment
is the minimization of capacity reduction for the existing analog network.
Another advantage with deploying one IS-136 channel per sector
initially eliminates the need to preload the subscriber base with dual mode,
IS-136 handsets. 56