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Cellular
Communications |
6. Digital
Systems
As demand for mobile telephone service has increased, service
providers found that basic engineering assumptions borrowed from
wireline (landline) networks did not hold true in mobile systems.
While the average landline phone call lasts at least 10 minutes,
mobile calls usually run 90 seconds. Engineers who expected to
assign 50 or more mobile phones to the same radio channel found that
by doing so they increased the probability that a user would not get
dial tone—this is known as call-blocking probability. As a
consequence, the early systems quickly became saturated, and the
quality of service decreased rapidly. The critical problem was
capacity. The general characteristics of time division multiple
access (TDMA), Global System for Mobile Communications (GSM),
personal communications service (PCS) 1900, and code division
multiple access (CDMA) promise to significantly increase the
efficiency of cellular telephone systems to allow a greater number
of simultaneous conversations. Figure 8 shows the
components of a typical digital cellular system.
The advantages of digital cellular technologies over analog
cellular networks include increased capacity and security.
Technology options such as TDMA and CDMA offer more channels in the
same analog cellular bandwidth and encrypted voice and data. Because
of the enormous amount of money that service providers have invested
in AMPS hardware and software, providers look for a migration from
AMPS to digital analog mobile phone service (DAMPS) by overlaying
their existing networks with TDMA architectures.
Time Division Multiple Access (TDMA)
North American digital cellular (NADC) is called DAMPS and TDMA.
Because AMPS preceded digital cellular systems, DAMPS uses the same
setup protocols as analog AMPS. TDMA has the following
characteristics:
- IS–54 standard specifies traffic on digital voice channels
- initial implementation triples the calling capacity of AMPS
systems
- capacity improvements of 6 to 15 times that of AMPS are
possible
- many blocks of spectrum in 800 MHz and 1900 MHz are used
- all transmissions are digital
- TDMA/FDMA application 7. 3 callers per radio carrier (6
callers on half rate later), providing 3 times the AMPS capacity
TDMA is one of several technologies used in wireless
communications. TDMA provides each call with time slots so that
several calls can occupy one bandwidth. Each caller is assigned a
specific time slot. In some cellular systems, digital packets of
information are sent during each time slot and reassembled by the
receiving equipment into the original voice components. TDMA uses
the same frequency band and channel allocations as AMPS. Like NAMPS,
TDMA provides three to six time channels in the same bandwidth as a
single AMPS channel. Unlike NAMPS, digital systems have the means to
compress the spectrum used to transmit voice information by
compressing idle time and redundancy of normal speech. TDMA is the
digital standard and has 30-kHz bandwidth. Using digital voice
encoders, TDMA is able to use up to six channels in the same
bandwidth where AMPS uses one channel.
Extended Time Division Multiple Access (E–TDMA)
The E–TDMA standard claims a capacity of fifteen times that of
analog cellular systems. This capacity is achieved by compressing
quiet time during conversations. E–TDMA divides the finite number of
cellular frequencies into more time slots than TDMA. This allows the
system to support more simultaneous cellular calls.
Fixed Wireless Access (FWA)
FWA is a radio-based local exchange service in which telephone
service is provided by common carriers (see Figure 9).
It is primarily a rural application—that is, it reduces the cost of
conventional wireline. FWA extends telephone service to rural areas
by replacing a wireline local loop with radio communications. Other
labels for wireless access include fixed loop, fixed radio access,
wireless telephony, radio loop, fixed wireless, radio access, and
Ionica. FWA systems employ TDMA or CDMA access technologies.
Personal Communications Service (PCS)
The future of telecommunications includes PCS. PCS at 1900 MHz
(PCS 1900) is the North American implementation of digital cellular
system (DCS) 1800 (GSM). Trial networks were operational in the
United States by 1993, and in 1994 the Federal Communications
Commission (FCC) began spectrum auctions. As of 1995, the FCC
auctioned commercial licenses. In the PCS frequency spectrum, the
operator's authorized frequency block contains a definite number of
channels. The frequency plan assigns specific channels to specific
cells, following a reuse pattern that restarts with each
nth cell. The uplink and downlink bands are paired mirror
images. As with AMPS, a channel number implies one uplink and one
downlink frequency (e.g., Channel 512 = 1850.2-MHz uplink paired
with 1930.2-MHz downlink).
Code Division Multiple Access (CDMA)
CDMA is a digital air interface standard, claiming 8 to 15 times
the capacity of analog. It employs a commercial adaptation of
military, spread-spectrum, single-sideband technology. Based on
spread spectrum theory, it is essentially the same as wireline
service—the primary difference is that access to the local exchange
carrier (LEC) is provided via wireless phone. Because users are
isolated by code, they can share the same carrier frequency,
eliminating the frequency reuse problem encountered in AMPS and
DAMPS. Every CDMA cell site can use the same 1.25-MHz band, so with
respect to clusters, n = 1. This greatly simplifies
frequency planning in a fully CDMA environment.
CDMA is an interference-limited system. Unlike AMPS/TDMA, CDMA
has a soft capacity limit; however, each user is a noise source on
the shared channel and the noise contributed by users accumulates.
This creates a practical limit to how many users a system will
sustain. Mobiles that transmit excessive power increase interference
to other mobiles. For CDMA, precise power control of mobiles is
critical in maximizing the system's capacity and increasing battery
life of the mobiles. The goal is to keep each mobile at the absolute
minimum power level that is necessary to ensure acceptable service
quality. Ideally, the power received at the base station from each
mobile should be the same (minimum signal to interference).
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