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Principles of Electronic
Communication Systems
Third Edition
Louis E. Frenzel, Jr.
© 2008 The McGraw-Hill Companies
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Chapter 20
Cell Phone Technologies
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Topics Covered in Chapter 20
 20-1: Cellular Telephone Systems
 20-2: The Advanced Mobile Phone System (AMPS)
 20-3: Digital Cell Phone Systems
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20-1: Cellular Telephone Systems
 Wireless refers primarily to the cellular telephone
industry.
 The cell phone is the largest-volume consumer
electronics device.
 It has changed the way that we communicate.
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20-1: Cellular Telephone Systems
 In 2005, cell phone subscribers numbered more than
wired telephone subscribers.
 As the data speed of the newer digital cell phone
transmissions increases, more cell phone applications
are possible, including cameras, Internet access, emails, audio, gaming, and video.
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20-1: Cellular Telephone Systems
 A cellular radio system provides standard telephone
service by two-way radio at remote locations.
 Cellular radios or telephones were originally installed
in cars or trucks, but today most of them are available
in handheld models.
 Cellular telephones permit users to link up with the
standard telephone system, which permits calls to any
part of the world.
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20-1: Cellular Telephone Systems
 Cellular radio telephone service is available
worldwide.
 The original U.S. cell phone system, known as the
advanced mobile phone system, or AMPS, was
based on analog technologies.
 AMPS has been phased out and replaced by secondgeneration (2G) and third-generation (3G) digital cell
phone systems.
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20-1: Cellular Telephone Systems
Cellular Concepts
 The basic concept behind the cellular radio system is
that rather than serving a given geographical area with
a single transmitter and receiver, the system divides the
service area into many small areas known as cells.
 The typical cell covers only several square miles and
contains its own receiver and low-power transmitter.
 The coverage of a cell depends upon the density
(number) of users in a given area.
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20-1: Cellular Telephone Systems
Cellular Concepts
 Each cell is connected by telephone lines or a
microwave radio relay link to a master control center
known as the mobile telephone switching office
(MTSO).
 The MTSO controls all the cells and provides the
interface between each cell and the main telephone
office.
 As the person with the cell phone passes through a cell,
it is served by the cell transceiver.
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20-1: Cellular Telephone Systems
Cellular Concepts
 The telephone call is routed through the MTSO and to
the standard telephone system.
 As the person moves, the system automatically
switches from one cell to the next.
 The receiver in each cell station continuously monitors
the signal strength of the mobile unit.
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20-1: Cellular Telephone Systems
Cellular Concepts
 When the signal strength drops below a desired level, it
automatically seeks a cell where the signal from the
mobile unit is stronger.
 The computer at the MTSO causes the transmission
from the person to be switched from the weaker cell to
the stronger cell. This is called a handoff.
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20-1: Cellular Telephone Systems
Figure 20-1: The area served by a cellular telephone system is divided into small
areas called cells. Note: Cells are shown as ideal hexagons, but in reality they have
circular to other geometric shapes. These areas may overlap, and the cells may be of
different sizes.
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20-1: Cellular Telephone Systems
Frequency Allocation
 Cellular radio systems operate in the UHF and
microwave bands as assigned by the Federal
Communications Commission (FCC).
 The original frequency assignments were in the 800- to
900-MHz range previously occupied by the mostly
unused UHF TV channels 68 through 83.
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20-1: Cellular Telephone Systems
Frequency Allocation
 The frequencies between 824 and 849 MHz are
reserved for the uplink transmissions from the cell
phone to the base station. These are also called the
reverse channels.
 The frequencies between 869 and 894 MHz are the
downlink bands from base station to cell phone.
 Two blocks of 60 MHz between 1850 and 1990 MHz are
referred to as the personal communications systems
(PCS) channels.
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20-1: Cellular Telephone Systems
Figure 20-4: Additional U.S. cell phone spectrum. (a) 890 to 960 MHz and (b) 1850 to
1990 MHz are called the personal communication system PCS band.
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20-1: Cellular Telephone Systems
Multiple Access
 Multiple access refers to how the subscribers are
allocated to the assigned frequency spectrum.
 Access methods are the ways in which many users
share a limited amount of spectrum.
 The techniques include:
 Frequency reuse
 Frequency-division multiple access (FDMA)
 Time-division multiple access (TDMA)
 Code-division multiple access (CDMA)
 Spatial-division multiple access (SDMA).
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20-1: Cellular Telephone Systems
Multiple Access: Frequency Reuse
 In frequency reuse, individual frequency bands are
shared by multiple base stations and users.
 This is possible by ensuring that one subscriber or base
station does not interfere with any others.
 This separation achieved by controlling such factors as
transmission power, base station spacing, and antenna
height and radiation patterns.
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20-1: Cellular Telephone Systems
Multiple Access: Frequency-Division Multiple Access
 FDMA systems are like frequency-division multiplexing.
 They allow many users to share a block of spectrum by
dividing it up into many smaller channels.
 Each channel of a band is given an assigned number or
is designated by the center frequency of the channel.
 One subscriber is assigned to each channel.
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20-1: Cellular Telephone Systems
Multiple Access: Time-Division Multiple Access
 TDMA relies on digital signals and operates on a single
channel.
 Multiple users use different time slots.
 Because the audio signal is sampled at a rapid rate, the
data words can be interleaved into different time slots.
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20-1: Cellular Telephone Systems
Multiple Access: Code-Division Multiple Access
 CDMA is just another name for spread spectrum.
 A high percentage of cell phone systems use direct
sequence spread spectrum (DSSS).
 Here the digital audio signals are encoded in a circuit
called a vocoder to produce a 13-kbps serial digital
compressed voice signal.
 It is then combined with a higher-frequency chipping
signal.
 A unique pseudo-random chipping code is used to
identify multiple subscribers who use the same
spectrum.
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20-1: Cellular Telephone Systems
Multiple Access: Spatial-Division Multiple Access
 This form of access is actually an extension of
frequency reuse.
 It uses highly directional antennas to pinpoint users and
reject others on the same frequency.
 Very narrow antenna beams at the cell site base station
are able to lock in on one subscriber but block another
while both subscribers are using the same frequency.
 Modern antenna technology using adaptive phased
arrays makes this possible.
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20-1: Cellular Telephone Systems
Duplexing
 Duplexing refers to the ways in which two-way radio or
telephone conversations are handled.
 Telephone communications have always been full
duplex, where both parties can simultaneously send
and receive. All cell phone systems are full duplex.
 To achieve full duplex operation, frequency-division
duplexing (FDD) or time-division duplexing (TDD)
must be implemented.
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20-1: Cellular Telephone Systems
Duplexing
 In FDD, separate frequency channels are assigned for
the transmit and receive functions.
 The transmit and receive channels are spaced so that
they do not interfere with one another inside the cell
phone or base station circuits.
 TDD is less common. The system assigns the transmit
and receive data to different time slots, both on the
same frequency.
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20-2: The Advanced Mobile
Phone System (AMPS)
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Typical AMPS Handset
 Although AMPS cell phones are due to be phased out
beginning in 2007, millions are still in use.
 An AMPS unit consists of five major sections:
transmitter, receiver, synthesizer, logic unit, and control
unit.
 Mobile radios derive their operating power from built-in
rechargeable batteries.
 The transmitter and receiver share a single antenna.
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20-2: The Advanced Mobile
Phone System (AMPS)
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Figure 20-10: General block
diagram of a typical AMPS
unit (cellular radio).
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20-2: The Advanced Mobile
Phone System (AMPS)
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Typical AMPS Handset: Transmitter
 The transmitter is a low-power FM unit operating in the
frequency range of 825 to 845 MHz.
 The transmitter’s output power is controllable by the cell
site and MTSO.
 Special control signals picked up by the receiver are
sent to an automatic power control (APC) circuit that
sets the transmitter to one of eight power output levels.
 The APC feature permits optimum cell site reception
with minimal power and helps minimize interference
from other stations in the same or adjacent cells.
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20-2: The Advanced Mobile
Phone System (AMPS)
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Typical AMPS Handset
 The receiver is typically a dual-conversion
superheterodyne.
 The frequency synthesizer section develops all the
signals used by the transmitter and receiver.
 The logic unit contains the master control circuitry.
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20-2: The Advanced Mobile
Phone System (AMPS)
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Typical AMPS Handset
 All cellular radios contain a programmable read-only
memory (PROM) chip called the number assignment
module (NAM).
 The NAM contains the mobile identification number
(MIN), which is the telephone number assigned to the
unit.
 The control unit contains the handset with speaker and
microphone.
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20-3: Digital Cell Phone Systems
 All new cell phones and systems use digital rather
than analog methods.
 All-digital systems were developed to expand the
capacity of existing cell phone systems.
 Digital techniques provide several ways to multiplex
many users into the same spectrum space.
 Digital systems are more reliable in a noisy
environment.
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20-3: Digital Cell Phone Systems
 Digital circuits can be made smaller and more power-
efficient, so handsets can be compact and can
operate for longer times on a single battery charge.
 Digital cell phones greatly facilitate the transmission of
data, so services such as e-mail and Internet access
are possible with a cell phone.
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20-3: Digital Cell Phone Systems
2G Cell Phone Systems
 Three basic second-generation (2G) digital cell
phone systems are in wide use today.
 Two of them use time-division multiplexing (TDM), and
the third uses spread spectrum (SS).
 The TDM systems are the Global System for Mobile
Communications (GSM) and the IS-136 standard for
time division multiple access (TDMA).
 The SS system is code-division multiple access
(CDMA).
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20-3: Digital Cell Phone Systems
2G Cell Phone Systems: Vocoder
 To use digital data transmission techniques first requires
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that the voice be digitized.
The circuit that does this is a vocoder, a special type of
analog-to-digital (A/D) converter and digital-to-analog
(D/A) converter.
The converted serial data signal, representing the voice,
modulates the carrier and the composite signal
transmitted over the assigned channel.
The main function of a vocoder is data compression.
All 2G and 3G phones contain a vocoder.
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20-3: Digital Cell Phone Systems
2G Cell Phone Systems: IS-136 TDMA
 IS-136 is the Telecommunications Industry Association
(TIA) standard that describes the time-division multipleaccess (TDMA) cell phone system.
 IS-136 operates concurrently on the same 800- to 900MHz band channels used by AMPS and is also used in
the PCS-1900 bands.
 The IS-136 system provides for six time slots in the
TDMA frame. Two time slots are assigned to each of
three users.
 Spectral efficiency is achieved with π/4-DQPSK
modulation, one of the most efficient modulation
methods currently available.
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20-3: Digital Cell Phone Systems
2G Cell Phone Systems: GSM
 The most widely used 2G digital system is GSM, or
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Global System for Mobile Communications.
In the US, GSM is widely implemented in both the 800and 1900-MHz personal communication system band.
It has mostly replaced the IS-136 systems in the US.
Like IS-136, GSM uses TDMA.
The modulation method, known as Gaussian minimum
shift keying (GMSK), is similar to FSK but allows
higher speeds to be transmitted in a narrower channel.
GSM is the dominant cell phone technology in the
world.
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20-3: Digital Cell Phone Systems
2G Cell Phone Systems: IS-95 CDMA
 The IS-95 CDMA TIA cell phone standard is called
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code-division multiple access (CDMA) and is also
known as CDMA One.
CDMA uses direct sequence spread spectrum (DSSS)
with a 1.2288-MHz chipping rate that spreads the signal
over a 1.25-MHz channel.
Up to 64 users can use this band simultaneously with
little or no interference or degradation of service.
The CDMA system uses FDD for duplexing.
A key part of a CDMA system is APC.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits
 Digital cell phones are quite different from analog
phones.
 Because they use digital techniques and pulse
modulation methods, and since massive growth in
cellular usage has caused spectrum crowding and
interference problems, new architectures and circuits
have been developed.
 A variety of different circuits have been created to
accommodate numerous standards.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits
 Three major trends dominate the cell phone evolution:
 Increased digital processing.
 Increased integration of circuitry on a few chips.
 Multimode/multiband phones.
 Some new digital phones also contain AMPS circuitry.
 If a subscriber roams into an area lacking a carrier that
uses digital technology, the phone reverts to analog,
which is still supported in most areas.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits
 In a 2G cell phone, the RF section contains the
transmitter and receiver circuits including mixers, local
oscillators or frequency synthesizers for channel
selection, the receiver LNA, and the transmitter power
amplifier.
 The baseband section contains the vocoder with its A/D
and D/A converters plus a DSP chip that handles many
processing functions.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits
 An embedded controller handles all the digital control
and signaling, handoffs, and connection and
identification operations.
 The controller also runs the display and keyboard and
all other user functions such as number storage, auto
dialing, and caller ID.
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20-3: Digital Cell Phone Systems
Figure 20-19: Block diagram for a 2G digital cell phone.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits: Direct Conversion
 The direct-conversion or zero IF design sets the LO
frequency to the incoming signal frequency so that the
translation is made directly to the baseband signal.
 Since direct conversion works only with doublesideband suppressed (DSB) AM signals, changes have
been made to accommodate FSK, BPSK, QPSK, and
other forms of digital modulation.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits: Direct Conversion
 Direct conversion eliminates the need for an expensive
and physically large selective IF filter.
 Direct conversion eliminates the imaging problem so
common in superheterodyne designs, especially in the
crowded multiband cellular spectrum.
 With direct conversion, baseband filtering can be
accomplished using simple low-pass RC filters and/or
DSP filters.
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20-3: Digital Cell Phone Systems
Figure 20-20: A direct-conversion receiver.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits: Low IF
 When an IF is used near the baseband frequencies,
filtering is simple and very effective.
 Most 2G and later phones are multiband phones that
can operate in three or four bands, thereby permitting
widespread roaming.
 The signal passes through one of three SAW filters and
feeds into a mixer.
 An image reject mixer uses a technique similar to the
phasing method of generating a single sideband (SSB)
signal.
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20-3: Digital Cell Phone Systems
Digital Cell Phone Circuits: Low IF
 Simple integrated RC low-pass filters are used to
eliminate the sum signals resulting from the conversion.
 Programmable gain amplifiers (PGAs) equalize the
signal levels, then the low-IF signals are applied to two
delta-sigma A/D converters.
 The digitized signals go to the baseband circuit and are
demodulated by the DSP.
 The recovered digital data is then sent to the vocoder,
where the voice signal is recovered.
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20-3: Digital Cell Phone Systems
2.5G Cell Phone Systems
 The designation 2.5G refers to a generation of cell
phones between the original second-generation (2G)
digital phones and newer third-generation (3G) phones.
 2.5G phones bring data transmission capability to 2G
phones in addition to normal voice service.
 A 2.5G phone permits subscribers to exchange emails
and access the Internet by cell phone.
 The three technologies used in 2.5G systems are
GPRS, EDGE, and CDMA2000.
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20-3: Digital Cell Phone Systems
2.5G Cell Phone Systems
 One popular 2.5G technology is the general packet
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radio service (GPRS).
This system is designed to work with GSM phones.
It uses one or more of the eight time slots in a GSM
phone system to transmit data rather than digitized
voice.
A faster 2.5G technology is enhanced data rate for
GSM evolution (EDGE).
It uses 8-PSK modulation instead of GMSK to achieve
data rates up to 384 kbps.
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20-3: Digital Cell Phone Systems
2.5G Cell Phone Systems: CDMA2000
 A third, different form of 2.5G digital cell phone is
designated CDMA2000. This standard is an extension
of the widely used IS-95 CDMA standard (cdmaOne).
 The basic CDMA2000 data transmission method uses
1.25-MHz-wide channels but changes the modulation
and coding formats to double the voice capacity.
 The data capability is packet-based and permits a data
rate of up to 144 kbps which is comparable to EDGE.
 The more recent version is called 1×EV-DO or
Evolution-Data Optimized. It has a data rate of about
3.1 Mbps downlink and an uplink rate up to 1.8 Mbps.
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20-3: Digital Cell Phone Systems
3G Cell Phone Systems
 Third-generation (3G) cell phones are true packet data
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phones.
3G phones feature enhanced digital voice and highspeed data transmission capability.
3G applications include fast e-mail and Internet access.
3G phones are being packaged with personal digital
assistants (PDAs).
High speed also permits the transmission of video.
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20-3: Digital Cell Phone Systems
UMTS 3G
 The ITU recommended one worldwide version known
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as wideband CDMA (WCDMA) in implementing 3G.
This system is also known as the Universal Mobile
Telecommunications Service (UMTS).
WCDMA is a direct sequence spread spectrum
technology.
In the most popular configuration, it is designed to use a
3.84-MHz chipping rate in 5-MHz-wide bands.
Duplexing is FDD, requiring the matching of 5-MHz
channels. The modulation is QPSK.
It can achieve a packet data rate up to 2 Mbps.
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20-3: Digital Cell Phone Systems
UMTS 3G
 A key problem in implementing 3G is the need for huge
portions of spectrum.
 The exact 3G spectrum varies widely depending on
which part of the world you are in, making it extremely
difficult to design a cell phone that is fully operable
worldwide.
 The UMTS 3G standard also defines a TDD version
known as TD-SCDMA for time-division synchronous
code-division multiple access.
 The primary benefit of TD-SCDMA is that less spectrum
is needed.
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20-3: Digital Cell Phone Systems
UMTS 3G
 Because of the need for faster systems, a new system
compatible with WCDMA has been developed.
 Known as high-speed downlink packet access
(HSDPA), this so-called 3.5G technology is an add-on
to WCDMA systems.
 HSDPA uses an adaptive coding and modulation
scheme with QPSK and 16-QAM.
 When a fast uplink is needed, a companion standard
known as high-speed uplink packet access (HSUPA)
is used.
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20-3: Digital Cell Phone Systems
Advanced Cell Phones
 Color LCD Screens.
 Handset manufacturers
 Digital Cameras.
have built in a wide range
of features that make the
cell phone the most
desired consumer
electronic product ever
developed.
 Consider what electronic
circuits and systems are
needed to make these
features work:
 E-mail.
 Games.
 GPS.
 Internet Access.
 MP3 Players.
 Push-to-Talk.
 FM Radio.
 Wireless Headsets.
 Video.
 Location-Based Technology.
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20-3: Digital Cell Phone Systems
Advanced Cell Phones: Location-Based Technology
 Another feature included in modern phones is
enhanced 911 (E911) capability.
 This system is mandated by the U.S. government.
 All cell phone carriers must have a system that makes it
possible to locate any cell phone position automatically.
 This permits emergency medical services or automobile
towing crews to find the cell phone used to make the
911 emergency call.
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20-3: Digital Cell Phone Systems
Advanced Cell Phones: Location-Based Technology
 Several different systems have been adopted by the
various carriers.
 Most CDMA phones contain a GPS receiver that
transmits its coordinates digitally to the carrier, from
which they can be forwarded to emergency services.
 GSM, GPRS, and EDGE phones use a system called
Uplink—Time Difference of Arrival (U-TDOA), a method
of triangulation based on cell phone signals being
received at three different cell sites.
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20-3: Digital Cell Phone Systems
Base Stations
 The most complex and expensive part of any cellular
telephone system is the network of base stations that
carriers must have to make it all work.
 Base stations consist of multiple receivers and
transmitters so that many calls can be handled on many
different channels simultaneously.
 The most visible feature of a base station is its antenna
on a tower.
 Base station antennas have become directional, which
helps to increase subscriber capacity with minimal cost.
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20-3: Digital Cell Phone Systems
Figure 20-27: Horizontal radiation and reception pattern of a typical cell site antenna.
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