UNITED STATES DEPARTMENT OF AGRICULTURE

**Rural Electrification Administration**

**BULLETIN 1751H-705**

**SUBJECT: [Fundamentals of Cellular Radio Service]{.underline}**

**TO:** Telephone Borrowers

REA Telephone Staff

**EFFECTIVE DATE:** Date of Approval

**EXPIRATION DATE:** Three years from the effective date.

**OFFICE OF PRIMARY INTEREST:** Transmission Branch, Telecommunications
Standards Division

**PREVIOUS INSTRUCTIONS:** None - this bulletin is new information.

**FILING INSTRUCTIONS:** Place this document in Section 1751H,
Transmission Design Bulletins.

**PURPOSE:** Provide a tutorial on Cellular Radio Systems and include
definitions, fundamentals, system architecture and equipment, and future
developments.

Michael M. F. Liu 12-27-91

‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑ ‑‑‑‑‑‑‑‑‑‑‑‑‑‑

**Acting** **Administrator Date**

**FUNDAMENTALS OF CELLULAR RADIO SYSTEMS**

**TABLE OF CONTENTS**

.Begin Table C.

1\. Introduction 7

> 1.1 General 7
>
> 1.2 Overview 7
>
> 1.3 History 9
>
> 1.4 IMTS 10

2\. Fundamentals of the Cellular Mobile Radio System 11

> 2.1 General Description 11
>
> 2.2 Frequency Allocation 12
>
> 2.3 Cellular Geometry 14
>
> 2.4 Frequency Reuse 16
>
> 2.5 Channel Assignments 18
>
> 2.6 Cell Splitting 21
>
> 2.7 Cellular System Parameters 21
>
> 2.8 Handoff 24
>
> 2.9 Roaming 24

3\. System Control Techniques 26

> 3.1 Network Structure 26
>
> 3.2 Call Setup 27
>
> 3.3 Fixed Station Originated Call 28
>
> 3.4 Mobile Originated Call 29
>
> 3.5 Call Disconnect 30

4\. Mobile Switching Center 30

> 4.1 General Description 30
>
> 4.2 The Switching System 32
>
> 4.3 Program Control 33

5\. The Cell Site 33

> 5.1 General Description 33
>
> 5.2 Cell Site Controller 34
>
> 5.3 Radio Channel Units 35
>
> 5.4 Radio Frequency System 36

6\. The Mobile Unit 36

> 6.1 General Description 36
>
> 6.2 Transceiver Unit 37
>
> 6.3 Control Unit 37

7\. Future Trends 38

> 7.1 Fixed Cellular 38
>
> 7.2 Basic Exchange Telecommunications Radio Service 40
>
> 7.3 Digital Cellular 40
>
> 7.4 Narrow Advanced Mobile Phone Service 41
>
> 7.5 Personal Communication Services 42

REFERENCE INDEX 43

.End Table C.

INDEX: Cellular

Cellular, Fundamentals

Mobile, Cellular

Radio, Cellular

**DEFINITIONS**

**Access** The process of trying to gain entrance to the cellular
network for the purpose of transferring information or data.

**BETRS** Basic Exchange Telecommunications Radio Service. A rural radio
telephone service.

**Bypass** The use of any facilities or services as an alternative to
the local telephone company.

**Call Traffic** Customer ac­tivity on the cellular network.

**Cell** Depicted as hexagonal in shape, it represents the coverage area
of a base station within a cellular system.

**CGSA** Cellular Geographic Service Area. The service area of a
cellular radio system operator.

**Central Office** (CO) Loca­tion where the switching system and other
related equipment are housed and user lines are terminated in a PSTN.

**Channel** A pair of frequencies used to carry a conversation, that is,
to transmit and receive simultaneously.

**Cluster** A group of cells that forms a pattern that can be duplicated
many times in the same CGSA.

**Co-channel Interference** Interference caused by two or more
transmissions sharing the same channel.

**CDMA** Code Division Multi­ple Access. Modulation technique where
several calls are able to be transmitted simultaneously over a wide
bandwidth with each call assigned its own unique code.

**Common Carrier** A company that furnishes telecommu­nications
facilities or service to the public and cannot control message content
over the network.

**CPU** Central Processing Unit. The part of the computer that contains
the logic, computation, and control circuits.

**CT-2** Second Generation Cordless Telephone.

**CTIA** Cellular Telecommunications Industry Association.

**Directional Antenna** An antenna that radiates power in a specific
area.

**Duplex Transmission** Simultaneous but independent two-way
transmission in both directions.

**FCC** Federal Communications Commission.

**FDMA** Frequency Division Multiple Access. Modu-lation technique where
several signals are sent simultaneously by dividing the frequency.

**Frequency Reuse** The prin­ciple of using the same frequencies in
different cells within the same CGSA.

**Handoff** The process of automatically changing the voice channel of a
mobile unit to minimize call degradation as the unit travels throughout
the service area.

**IMTS** Improved Mobile Telephone Service.

**Interference** The reception of unwanted signals mixed with the
desired signal.

**MSA** Metropolitan Statistical Area. As defined in cellular telephony,
a region defined as an urban area that is designated by the FCC to have
two operators licensed to provide cellular service.

**Off-hook** Condition indi­cating a seizure, request for service, or a
busy signal on a telephone line or trunk.

**On-hook** Condition indicating a disconnect, unanswered call, or an
idle signal on a telephone line or trunk.

**Omnidirectional Antenna** An antenna that ideally radi­ates power in a
spherical pattern.

**Paging** The process of de­termining the availability of a mobile unit
to receive a call.

**PCM** Pulse Code Modulation. Technique by which the modulating signal
is sampled, then quantized and coded.

**PCN** Personal Communications Network.

**PCS** Personal Communications Service.

**PSTN** Public Switched Telephone Network. The network of the wireline
common carrier telephone company.

**Radio Channel** A band of frequencies capable of carrying radio
communica­tions.

**Roaming** Operating the mobile unit outside of the designated \"home\"
service area.

**RSA** Rural Service Area. A region defined as a rural area that is
designated by the FCC to have two opera­tors licensed to provide cellular
service.

**RSSI** Received Signal Strength Indicator.

**SAT** Supervisory Audio Tone. One of three tones in the 6 kHz band
with one assigned to each sector.

**Sectorization** The process of creating subdivisions within a cell for
the pur­pose of reusing the allo­cated frequencies for that cell.

**ST** Signaling Tone A 10 kHz tone transmitted by the mobile unit on a
voice channel. Indicates call confirmation or call completion.

**TDMA** Time Division Multi­ple Access. Modulation technique where users
are assigned individual time slots within the same frequency band.

**TIA** Telecommunications Industry Association.

**Traffic** The volume of calls in progress.

**Transceiver** A transmitter and receiver combined into one unit such
as a mobile telephone.

**1. Introduction**

**1.1 General**

**1.1.1** Cellular mobile radio technology is complex and diverse both
in theory and application. This bulletin provides a practical and
comprehensive overview of the cellular mobile radio environment.
Cellular radio definitions, terminology, and concepts are described to
give the reader a broad understanding of the topic. In addition,
detailed descriptions of network architecture and equipment, as well as
system functions and parameters are discussed.

**1.1.2** Discussion on cellular radio system design recommendations and
application guidelines involves a further in-depth study of the cellular
network, and therefore are not discussed in this bulletin but will be
included in future REA bulletins.

**1.2 Overview**

**1.2.1** Although a variety of new services have been introduced, the
telephone is considered a means of communicating from a stationary
location. The ability to make or receive calls depended on your
proximity to a telephone set. Technology continued to mature and
inspired the concept of providing communications facilities in a
vehicle. Consequently, the mobile telephone was introduced.

**1.2.2** Evolving technology led to more compact mobile units that
resulted in the creation of portable telephones. Telephone service now
offers users a variety of communications tools to match their lifestyles
and needs.

**1.2.3** Cellular radio describes a technology primarily designed to
provide mobile radio service. Classified as a common carrier telephone
service, cellular mobile radio service provides an extension to the
wireline telephone network. Cellular radio technology was also developed
to counter the limitations associated with conventional mobile radio
service.

**1.2.4** The popularity of mobile communications proved to be a service
both desirable and practical. Although demand for mobile service was
high, available frequencies were limited. Each conversation is conducted
using a pair of frequencies, one transmitting from the base station and
the other from the mobile station. These frequencies are chosen from a
finite group of radio waves capable of carrying voice communications.
Each service area that offers mobile service is allocated a pool of
frequencies from the radio spectrum (see Figure 1). Many mobile service
customers encountered delayed and blocked calls caused by the few
available channels. In most cities, prospective mobile users had to wait
many years for service. The allocated frequency spectrum could only
handle a few conversations, consequently restricting the number of
customers served.

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**1.2.5** Since each service area was allocated a set of frequencies for
mobile service, customers had to redial when they drove outside their
original service area. Conventional mobile service was poor and at times
inconvenient. However, the demand and popularity of mobile telephones
increased in metropolitan areas. Consequently, the telecommunications
industry reassessed the potential impact of mobile communications and
directed further attention on promoting more efficient mobile services.

**1.2.6** One means of developing improved mobile service was to address
the problem of limited radio spectrum. More frequencies could not be
created so a scheme to maximize usage of what was available needed to be
explored. Consequently, the development and implementation of cellular
radio increased spectral efficiency over conventional mobile radio
technology.

**1.2.7** Usually, conventional mobile service had one base station with
a high-power transmit antenna (see Figure 1). Antennas were located on
high structures to serve a wide area. The high level of power emitted
often caused interference problems in neighboring service areas. Under
these conditions, frequency reuse in adjacent areas was nearly
impossible.

**1.2.8** On the other hand, the cellular concept uses many base
stations with low power transmit antennas. Each base station serves a
much smaller area and these regions are called cells (see Figure 2).
Frequencies can be reused increasing the traffic carrying capacity
within the service area. Delayed and blocked calls are reduced and the
quality of service is improved. Also, service can be made available to
more potential customers. Cellular radio operates in the 800 MHz band
while conventional mobile telephone service operates in the 150 MHz and
450 MHz bands.

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**1.2.9** When cellular telephones first emerged on the scene, they were
used by a limited group of executives and status-seekers capable of
affording the high cost of service. Lower prices and increased features
attracted other types of users. Many business professionals used
cellular telephones to increase productivity and capitalize on saving
time and making money. The public is also using cellular telephones to
call towing garages or emergency personnel: promoting a sense of
additional safety and peace of mind. The capacity to answer the growing
popularity and usage of cellular has opened a window for new revenue and
marketing possibilities for those providing cellular service.

**1.3 History**

**1.3.1** Mobile radio service has been available in the United States
since 1921. Police departments were the first to use mobile radio and
they operated at a frequency of 2 MHz. Over the years, in response to
the demand, the Federal Communications Commission (FCC) made spectrum
available for mobile use between 30-50 MHz, 150-174 MHz, and 450-512
MHz.

**1.3.2** In 1946, mobile systems were allocated six radio channels in
the 150 MHz band with a channel spacing of 60 kHz. In the 1950\'s the
creation of new channels between existing channels was accomplished by
reducing the channel spacing to 30 kHz. By 1956, additional radio
channels were allocated for mobile service, including 11 channels in the
150 MHz band and 12 channels in the 450 MHz band.

**1.3.3** Initially, conventional mobile communications were manually
operated and required an operator to connect calls to and from mobile
units. Mobile units had no dialing capability or dial-tone. For rural
applications, mobile communication technology had to devise a technique
to eliminate operator assisted calls because rural exchange offices were
usually unattended.

**1.3.4** The Rural Electrification Administration (REA) issued a
specification outlining performance requirements for an automatic dial
operated system. The specification was circulated among manufacturers
with an invitation to bid on the development of this type of equipment.
Two manufacturers developed prototypes and a system was installed in
1958 and operated with favorable results.

**1.3.5** The technical and economic feasibility of this type of system
caused other companies to design and market similar systems.
Consequently, REA telephone borrowers economically implemented two-way
dial mobile telephone service through an unattended telephone exchange
using a system that was the forerunner to Improved Mobile Telephone
Service (IMTS).

**1.4 IMTS**

**1.4.1** In the mid 1960s, IMTS was introduced. IMTS was a service that
enabled users random access to any available channel. Mobile users were
permitted to automatically search all available channels and select a
specific channel for use. This technique increased the number of
subscribers served by a given number of channels.

**1.4.2** IMTS also offered automatic-dialing, eliminating the need for
operator assistance. Formerly, mobile service consisted of a
push-to-talk operation. IMTS replaced the system with channels capable
of full-duplex service. Consequently, IMTS enables users to talk and
listen simultaneously.

**1.4.3** IMTS also offered improved audio quality. The radio equipment
was designed for a wider voice frequency bandwidth which approaches that
of landline telephone systems. Although IMTS improved mobile service,
spectrum remained limited, restricting the number of available voice
channels and customers on the network. Consequently, most prospective
mobile users still experienced a considerable delay in receiving
service.

**2. Fundamentals of the Cellular Mobile Radio System**

**2.1 General Description**

**2.1.1** The cellular mobile radio system also provides telephone
service to the customer similar to the local wireline telephone network.
The local wireline telephone network is also commonly called the public
switched telephone network (PSTN). The cellular mobile radio network is
not considered a bypass but an extension to the PSTN. Services offered
by the PSTN are also enjoyed by the cellular mobile user.

**2.1.2** The basic architecture of a cellular system includes the
Mobile Telephone Switching Office (MTSO) and several base stations, all
collectively serving many mobile, portable, and fixed units. Each base
station, also commonly known as a cell site, serves the mobile units
contained within its boundaries. Each system is comprised of several
cell sites connected and operated by a single MTSO. The MTSO also links
the cellular network to the central office (CO) of the PSTN. Larger
networks contain several MTSOs but the basic configuration of the system
is the same. Figure 3 illustrates a basic cellular mobile radio system.

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**2.1.3** The cellular network allows mobile users to place and receive
calls from their vehicles or portable telephones within the system. As
mentioned before, the MTSO is the central coordinator in the cellular
system. The MTSO is the switching office that serves as a link between
the PSTN and each of the cell sites. The cell site is an interface
between the MTSO and the mobile units providing cellular coverage to its
specific area. The cell site coordinates all activities in the cell and
operates with the MTSO. The mobile and portable units provide the user
with an interface to the cellular radio network and a communication link
to the PSTN.

**2.1.4** Radio and high-speed transmission paths connect the three
subsystems as illustrated in Figure 4. A radio link carries voice and
data signaling between the mobile unit and the cell site. Microwave
links, T1 carriers, or fiber optics carry both voice and data between
the cell site and the MTSO. Voice trunks connect the MTSO to other
cellular systems or to the PSTN.

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**2.2 Frequency Allocation**

**2.2.1** The frequency spectrum is a limited resource, nearly saturated
by the demands of television and radio broadcasting, mobile and fixed
radio telephone service, and satellite communications to name a few. New
technology and innovative applications place further demands on an
already saturated spectrum. Such demands and competition require
government regulation and spectrum allocation.

**2.2.2** The regulatory body responsible for frequency allocation in
the United States is the Federal Communications Commission (FCC). The
FCC establishes regulations for public mobile service and regulates the
licensing and operation of cellular systems in the 824-849 MHz and
869-894 MHz bands.

**2.2.3** The FCC chose the 800 MHz band for cellular radio because the
spectrum between 30 and 400 MHz was already crowded with other services.
Television broadcasting services also contributed to the crowding and
their usage extended to the 900 MHz band. Furthermore, problems with
severe rain activity would make portions of the spectrum in the
gigahertz range poor candidates for providing mobile service.
Specialized programing once shown on channels 73 to 83 in the broadcast
TV spectrum are now provided on cable television. Cellular now uses
these channels. Therefore an opening for cellular services was created
in the 800 MHz band.

**2.2.4** Two cellular systems are authorized to operate in each of the
306 Metropolitan Statistical Areas (MSAs) and 428 Rural Service Areas
(RSAs). The assignment of frequencies in the 824-849 MHz and 869-894 MHz
bands are divided into two 25 MHz groups. Cellular Systems A and B are
each allocated a block of frequencies. System B is licensed to a
wireline carrier usually affiliated with the local public landline
telephone company. System A is usually licensed to a non-wireline
company to offer competitive cellular service in the same area. In some
instances, System A is licensed to a wireline carrier that does not
provide public landline telephone service in that same area.

**2.2.5** The cellular frequency bands are divided between System A and
B as shown in Table 1. Each block of frequencies contains 416 frequency
pairs with a 30 kHz channel spacing. There are 21 control channel pairs
as-signed to each system. Originally, there were only 312 voice channel
pairs allocated per system. The FCC added 25 percent to the cellular
radio spectrum to compensate for the sudden demand created in already
congested metropolitan areas. 83 voice channels were added to each
system totaling 395 voice channel pairs. The additional voice channels
are currently used only in the most congested MSAs such as New York
City, Los Angeles, and Chicago.

**2.2.6** Each system uses a different band of frequencies for cell site
transmit and mobile unit transmit. There is a spacing of 45 MHz between
transmit and receive channels. The non-wireline company, System A
designee, uses the band of frequencies 825-835 MHz for the mobile
station transmitters and cell site receivers; and 870-880 MHz for the
cell site transmitters and mobile station receivers. Cellular System B,
used by wireline services, uses the 835-845 MHz band for mobile station
transmitters and cell site receivers, and 880-890 MHz for cell site
transmitters and mobile station receivers. Each mobile unit is capable
of using any of the 333 channels on either System A or B as well as any
of the additional 83 channels allocated.

.D. **Band Mobile Base Channel Number**
‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑ A 824-835
869-880 1-333, 667-716 845-846.5 890-891.5 991-1023 B 835-845 880-890
334-666 846.5-849 891.5-894 717-799
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--Control
Channel A 834.390-834.990 313-333 879.390-879.990 Control Channel B
835.020-835.620 334-354 880.020-880.620
\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--System
A = Non-wireline OperatorSystem B = Wireline OperatorDuplex Spacing = 45
MHzChannel Spacing = 30 kHzSystem A IDs end with an odd number.System B
IDs end with an even number

**Table 1 - Transmission Frequency Allocation**

**2.3 Cellular Geometry**

**2.3.1** Within a cellular system the service area is divided into
smaller subdivisions called cells. Theoretically, cells can be any shape
and they do not need to be uniform. However, a geometric pattern would
maintain design consistency with growth of the system. Circular shapes
would represent the ideal power coverage area but would create gaps and
overlaps. On the other hand, hexagonal-shaped cells would cover an area
with no gaps or overlaps. Hexagonal-shaped cells are used on a layout to
simplify the planning and design of a cellular system. The hexagon is a
fictitious and not a real or even idealized representation of signal
coverage (see Figure 5).

**2.3.2** Each hexagonal-shaped cell depicts an area of signal coverage.
The area is covered by signals emanating from the cell site usually
located at the center of each cell. There may be one or more antennas
located at each cell site.

**2.3.3** A cellular system in the early stages will be able to use the
available frequency spectrum without any difficulty. Thus,
omnidirectional antennas are usually used at the cell site to serve a
cell.

**2.3.4** When call-traffic increases, the available frequency spectrum
needs to be efficiently utilized. Directional antennas usually
accomplish this task. Each cell can be subdivided into three or six
sectors by using three or six directional antennas at the cell site.
Each sector is assigned a set of frequencies from the original group of
frequencies allocated to the cell. Three directional antennas create
three 120 degree angle sectors. Six directional antennas create six 60
degree angle sectors (see Figure 6).

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**2.4 Frequency Reuse**

**2.4.1** As previously discussed, conventional mobile radio systems
used a powerful antenna situated on an elevated structure to cover a
large service area. Each call is assigned to an available radio channel.
A radio channel consists of a pair of frequencies to provide full-duplex
operation. A fixed frequency spectrum restricts the number of
subscribers that can be served in a given service area. A powerful
antenna serves a large area but causes interference in adjacent service
areas inhibiting the reuse of channels. Thus, fewer potential customers
can obtain service.

**2.4.2** In contrast, the cellular concept divides a service area into
smaller regions known as cells. Each cell contains its own low-powered
antenna reducing the coverage area radius. Each cell is responsible for
providing mobile telephone service within its region. As a mobile unit
moves within the CGSA, movement between cells may occur. Such cell to
cell movement causes the cellular network to automatically switch the
call to an available channel within the CGSA. Each service area has a
fixed number of channels but the number of cells may vary. More cells
carry more calls. The cellular concept allows channels to be reused
which was virtually impossible in conventional mobile radio systems.

**2.4.3** Frequency reuse can be applied in different situations. The
same frequency can be assigned in different geographic locations, such
as radio and television stations using the same frequency in different
cities. Frequency reuse also can be applied repeatedly in the same
general area. The service area is divided into smaller regions called
cells. Cells are grouped together to form clusters or frequency reuse
patterns (see Figure 7). Within one cluster no channel is used twice,
but the pattern of channel assignments may be repeated in another
cluster. The number of different channels in each cell is calculated by
dividing the total number of allocated channels by the number of cells.
For example, 416 channels divided by 12 cells gives approximately 34
channels per cell.

**2.4.4** Frequency reuse is a fundamental principle of the cellular
concept. The same set of frequencies can be used in different cells.
Initially, omnidirectional antenna systems in low density areas use a
12-cell frequency reuse pattern. As call traffic increases, directional
antennas are used to create a 7-cell frequency reuse pattern for 120
degree sectorization or a 4-cell frequency reuse pattern for 60 degree
sectorization. (Refer to paragraph 2.3.4 for the number of transmit
antennas required.) Other frequency reuse patterns are also possible.
However, frequency reuse is prohibited in adjacent cells to prevent
co-channel interference. Co-channel interference is minimized by a large
geographic separation. Figure 8 illustrates frequency reuse in a CGSA.

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**Figure 7 - Frequency Cluster Patterns**

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**2.5 Channel Assignments**

**2.5.1** A radio channel is composed of a pair of frequencies that
provides full-duplex operation. The pair of frequencies corresponds to
the different bands of transmit and receive frequencies. For example,
the two frequencies assigned to channel 1 are 825.030 MHz (mobile
transmit) and 870.030 MHz (cell site transmit). The pair of frequencies
is always separated by a spacing of 45 MHz.

**2.5.2** There are 333 channels assigned to each system with an
additional 83 channels used in crowded metropolitan areas (see Figure
9). As previously discussed, channels can be reused in different cells
within the same CGSA. Consequently, a systematic technique for channel
assignments is necessary to avoid the development of co-channel
interference. The occurrence of the common use of the same channel can
be minimized by maintaining adequate geographic separation. Therefore,
frequency reuse cannot be applied to adjacent cells and adjacent
channels must never be used in the same cell.

**2.5.3** Channels are systematically assigned to each cell to maximize
channel separation. For example, a 7-cell cluster system uses 21 channel
sets. Each cell contains 3 channel sets. The total number of voice
channels in a cell is about 45. Cell #1 would use channel sets 1, 8, and
15. Cell #2 would use channel sets 2, 9, and 16. Cell #3 would use
channel sets 3, 10, and 17. The channel assignment pattern would
continue for all seven cells in the cluster. In this instance, the
minimum separation between adjacent channel sets within a cell is 7
channels (see Figure 10).

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**2.5.4** Channel sets can be assigned to each cell or to antennas
located at the cell site. Adjacent channels assigned to adjacent cells
could create interference problems as shown in Figure 11. This situation
can be avoided by using directional antennas to create sectors in each
cell. For example, a cluster of 7 cells can be divided into 21 sector
cells (see Figure 12). By using directional antennas and sectorization,
the distance between co-channel cells decreases and more channels are
allowed per cell. Consequently, more call traffic can be carried per
cell.

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**2.6 Cell Splitting**

**2.6.1** Cellular geographic areas are composed of cells combined into
a series of clusters. Each cluster has access to all frequencies
allocated for cellular radio use. These frequencies are divided among
the cells within the cluster. A cell that lies outside the cluster or is
part of another cluster may reuse the same frequencies.

**2.6.2** When the capacity to handle calls on the allocated channels
within a cell is exceeded, cell splitting is necessary. A single cell
within a cluster can be subdivided into smaller cells as shown in Figure
13. The number of channels allocated to the original cell is equally
divided among the new subcells created. Cell splitting decreases the
area of each cell. Consequently, the number of simultaneous calls can be
increased without additional spectrum allocation. Thus, frequencies are
reused more often in a given service area.

**2.6.3** Cell splitting accommodates an increase in mobile telephone
traffic. However, if a new cell site is too costly, cell splitting may
prove to be economically prohibitive.

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**2.7 Cellular System Parameters**

**2.7.1** The FCC has mandated that mobile units used for cellular
mobile service be limited to a maximum effective radiated power (ERP) of
7 watts. (ERP is not real power but radiated power relative to uniform
spherical propagation in free space.) The maximum ERP for a cell site
transmitter in a MSA is 100 watts. However, cell sites in RSA\'s that
are located more than 24 miles (38.62 kilometers) from an adjacent MSA
may operate at a maximum ERP of 500 watts. Radiated power at the cell
site must be further reduced if the transmitting antenna is more than
500 feet (152.4 meters) above the average terrain. Usually the average
antenna height is between 100 and 200 feet (30.48-60.96 meters) above
the ground.

**2.7.2** The cell radius is a parameter that affects the call traffic
capacity of a service area. A system just starting up usually has a
large cell radius with very little, if any, frequency reuse. The cell
radius depends upon the ERP and antenna height at the cell site. A
maximum cell radius of 30 miles (48.28 kilometers) is possible in areas
with very little foliage and a flat terrain. Ultimately, a maximum cell
radius is desired that will allow ample coverage without compromising
transmission quality. More cells are created through cell splitting or
the formation of clusters to accommodate increased calling traffic. A
minimum cell radius of 1 mile (1.61 kilometers) can usually increase
traffic carrying capacity without succumbing to co-channel interference
problems.

**2.7.3** The effectiveness of a frequency reuse cell pattern, or
cluster, is measured by the D/R ratio. To accommodate areas with high
calling traffic, the minimum distance that allows frequency reuse
between cells is desired. The minimum distance is determined by the type
of terrain, antenna height, and transmitted power at each cell site.
Minimum distance is desired without creating objectionable co-channel
interference. As noted in Figure 14, a set of frequencies used in a cell
with a coverage radius R can be used in another cell with the same
coverage radius at a distance D. The D/R ratio can be calculated by the
expression:

D/R = (3N)1/2 (1)

where N is the number of cell sites in the frequency reuse pattern or
cluster. Table 2 lists cluster patterns and their corresponding D/R
ratio. When the D/R ratio increases, co-channel interference decreases.

**2.7.4** Assume the area of a cluster remains the same. As the number
of cells per cluster increases, the frequency reuse distance D
decreases. Decreasing the distance increases the chance of co-channel
interference. Since the number of allocated channels is fixed, a large
cluster means few channels are assigned to each cell in the cluster.
Although, theoretically a large cluster would accommodate heavy traffic
demands, co-channel interference might become objectionable. A small
number of cells per cluster is preferred.

**2.7.5** Another parameter effecting transmission quality is the
carrier to interference (C/I) ratio. According to FCC regulations,
co-channel sites must maintain the appropriate D/R ratio to achieve at
least a C/I ratio of 17dB over 90% of the coverage area for reliable
transmission quality. This condition is usually met with a 12-cell
cluster using omnidirectional antennas or a 7-cell cluster using
directional antennas.

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**2.8 Handoff**

**2.8.1** The cellular system uses a feature called handoff to provide
continuous service to the mobile unit as it travels within a service
area. While a call is in progress, a mobile unit may cross the
boundaries of sectors within a cell or from one cell into another as
illustrated in Figure 15a. Handoff is the process of transferring the
mobile unit from one channel to another at the same or different cell
site. Calls are handed-off to a new cell to maintain the clarity of the
call.

**2.8.2** The cell site monitors the received signal strength of all
calls in progress and stores the signal as location information. This
location measurement determines whether an antenna should continue to
serve a mobile unit or handoff to another antenna at the same or
different cell site. This information is continuously sent to the MTSO.

**2.8.3** The MTSO monitors the number of voice channels available in
each adjacent cell and the signal strength of each mobile unit. If the
signal strength falls below a predetermined threshold, a report is sent
to the cell site. The cell site places the call on a handoff candidate
list and advises the MTSO. If the MTSO decides a handoff is necessary,
then the call is transferred to an available voice channel in the same
cell or to an adjacent cell.

**2.8.4** The MTSO sends the new cell a set-up message and the old cell
a handoff message. The cell site controller in the original cell sends
an initiate handoff message to the mobile unit using a blank and burst
technique. This technique blanks the voice signal for about 50ms, while
a burst of data informing the mobile unit of the new channel is sent
over the voice path. Once acknowledged, the cell site signals the MTSO
to switch from the old to the new channel. The mobile unit tunes to the
new channel. The new cell site sends confirmation to the MTSO that the
mobile unit is tuned to the new channel. The handoff process is
completed. The entire process occurs within a fraction of a second
without noticeable interruption in the call.

**2.8.5** Handoff is possible between cells within a single system.
However, the Telecommunications Industry Association (TIA) has developed
a standard, IS-41, that would allow handoff between neighboring cellular
systems using equipment from different manufacturers as shown in Figure
15b. Intersystem handoff trials have been completed by several major
cellular manufacturers and the equipment is now in commercial service.

**2.9 Roaming**

**2.9.1** Each mobile unit operates in its assigned CGSA considered
\"home.\" Use of the mobile unit outside of the customer\'s subscribed
service area is considered roaming. Nationwide roaming capability would
allow mobile units to place or receive calls while traveling in any
cellular system across the country.

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**2.9.2** Recently, all cellular licenses have been issued to the 428
RSAs, many of which are beginning to offer service. The 306 MSAs serve
an area that covers approximately 76 percent of the U.S. population.
Over 350 wireline owned cellular systems and over 250 non-wireline owned
cellular systems are in operation. It is estimated that 90 percent of
the U.S. population and interstate highway system will be covered with
cellular systems by the mid 1990\'s. Consequently, cellular subscribers
will be able to use cellular telephones almost everywhere they travel.
Cellular subscriber growth and cellular system expansion will increase
the demand for roaming capability and draw attention to the potential
revenue generated by roaming.

**2.9.3** The FCC requires that all cellular system operators serve
roamers. Roaming agreements between carriers have facilitated the
placement of outgoing calls made by roaming mobile telephone users. On
the other hand, roamers can receive calls in two ways.

**2.9.4** A roamer can receive calls through a call forwarding system,
when such a system exists in the area traveled. A caller from the mobile
subscriber\'s home area simply dials the mobile unit\'s telephone number
to make a connection. The subscriber\'s home cellular system forwards
the call to the cellular system where the mobile unit is roaming. The
host roaming cellular system pages the mobile subscriber until it finds
the cell in which it is traveling. The mobile subscriber normally pays
any associated long distance charges.

**2.9.5** The second way involves leaving an itinerary with potential
callers. The itinerary lists cities of travel and roamer access numbers
for each city. Local callers that wish to call the roaming mobile unit
must first dial the roamer access number. After hearing a second
dial-tone, the ten digit mobile number is dialed. This allows local
callers to call the roaming mobile unit free of long distance charges.

**2.9.6** Currently, there are several cellular roaming handbooks that
list cellular systems in operation, roaming charges, phone numbers,
dialing patterns, and other related information. They serve as useful
references both to the mobile user and those wishing to contact a mobile
subscriber.

**2.9.7** Roaming agreements are temporary arrangements used by cellular
operators until TIA standard IS-41 is implemented by all vendors. The
TR45.2 subcommittee of the Telecommunications Industry Association (TIA)
created the IS-41 standard for intersystem communication. This standard
will facilitate communication between switches from different
manufacturers. Integrated switching would create regional roaming
networks benefiting both the subscriber and cellular operator.

**3. System Control Techniques**

**3.1 Network Structure**

**3.1.1** The cellular network links the mobile subscriber to the PSTN
through the MTSO and cell site. Dedicated trunks for voice and data link
the cell sites to the MTSO. These trunks are T-carrier or microwave
links. Trunks linking the MTSO to the PSTN are usually T-carrier
systems.

**3.1.2** There are two types of radio channels available for
transmission, voice, and control channels. Each is a full-duplex channel
using one pair of carrier frequencies. A duplex voice channel is used to
carry conversation and messages while a call is in progress. The duplex
voice channel is divided into a forward voice channel for transmission
from the cell site to the mobile unit and a reverse voice channel from
the mobile unit to the cell site. Similarly, the control channel is also
subdivided. Data is delivered using the forward control channel from the
cell site to the mobile unit and the reverse control channel from the
mobile unit to the cell site. Figure 16 illustrates the major system
connections.

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**3.2 Call Setup**

**3.2.1** There are two systems assigned to each Cellular Geographic
Service Area (CGSA). Each system is allocated 416 voice channels and 21
control channels. At least one control channel is assigned to each cell
site. This channel is used for the exchange of control information
between the mobile unit and cell site. The control channel is also
needed to setup or establish calls.

**3.2.2** As previously discussed, the control channel is divided into a
forward control channel and a reverse control channel. The forward
control channel is used to transmit from the cell site and is used to
complete a call. The reverse control channel is used to transmit from
the mobile unit and used to originate a call.

**3.2.3** Call setting-up, either by the mobile user or the one calling
the mobile user, is provided by paging or access. Paging is the process
of determining a mobile unit\'s availability to receive an incoming
call. In contrast, access is the process by which the mobile unit
initiates the call. Access is initiated when a mobile unit wishes to
make a call or respond to a page. For most cellular systems, paging and
access messages share the same set of control channels. In heavy traffic
areas access and paging messages can occupy separate channels but this
is seldom implemented today.

**3.2.4** The mobile unit continually scans the control channel for a
paging message indicating someone wishes to call the mobile unit. A
potential phone call causes the cell site to transmit a paging command
over the forward control channel. Paging signals contain a binary
representation of the mobile unit\'s 10 digit telephone number. Also as
part of the paging information, the control channel transmits a message
called the overhead word. This message includes the identification of
the mobile service area, the number of control channels to be scanned,
and the first available channel for access.

**3.2.5** As previously stated, the reverse control channel is used to
transmit from the mobile unit, either to originate a call or respond to
a page. All mobile units in a cell compete for the same reverse control
channel during access. Collisions occur when two or more messages arrive
simultaneously causing system disruption and delays in calls. There is a
process performed by the system called collision avoidance to minimize
these occurrences.

**3.2.6** First, the mobile unit must be available to initiate or
respond to a call. Then a message is transmitted over the forward
control channel. The cell site acknowledges the readiness status of the
mobile unit. At this time, the mobile unit attempts to seize the reverse
control channel. The mobile unit opens a window in time where it expects
to see a message in the forward control channel go from idle to busy. If
this does not happen within the time window, the seizure attempt is
cancelled. An arbitrary number of automatic attempts will continue at
random intervals as permitted by the overhead word. If the attempt is
still not successful, a busy signal will be heard by the mobile unit.

**3.3 Fixed Station Originated Call**

**3.3.1** Contact with a mobile unit from a fixed station begins by
dialing the mobile unit\'s 7 or 10 digit number. The call is routed
through the PSTN to the home MTSO of the mobile unit. The MTSO converts
the number to the mobile unit\'s identification number. Cell sites
containing available paging channels are instructed to page the mobile
unit over the forward control channel.

**3.3.2** This paging signal is sent throughout the service area.
Recognizing its page, the mobile unit will attempt to seize the reverse
control channel. Successful seizure of the reverse control channel
causes the mobile unit to send its identification number to the cell
site. The cell site then forwards the paging reply to the MTSO over a
high-speed data link.

**3.3.3** Communicating through the cell site over the forward control
channel, the MTSO selects an idle voice channel for the mobile unit. The
mobile unit is instructed to tune to the designated voice channel.
During voice transmission, two out-of-band tones are used for
supervision, the supervisory audio tone (SAT) and signaling tone (ST).
Both are used only on voice channels when data is not being transmitted.

**3.3.4** The SAT consists of a continuous frequency modulation that is
one of three frequencies, 5970 Hz, 6000 Hz, and 6030 Hz. Only one is
used at each cell site with all cell sites within a given cell cluster
using the same SAT frequency.

**3.3.5** The SAT originates at the cell site and is returned when
received, filtered, and modulated by the mobile unit. Failure to receive
the same SAT sent by the cell site indicates the mobile unit is in an
area of interference or the mobile transmitter is off.

**3.3.6** The signaling tone (ST) consists of a burst of 10 kHz and
originates at the mobile unit. When both the ST and SAT are received at
the cell site, the mobile is available to receive or initiate a call.

**3.3.7** The mobile unit\'s readiness status to receive a call is
forwarded to the MTSO through the cell site. The MTSO then directs the
cell site to send a data message over the forward control channel to
ring the mobile unit signaling an incoming call.

**3.3.8** When the subscriber answers, the ST is removed from the mobile
unit. The cell site sets the landline trunk to an off-hook state. An
off-hook signal indicates a seizure, request for service, or a busy
condition. An on-hook signal indicates a disconnect, unanswered call, or
an idle condition. The MTSO acknowledges an off-hook signal by removing
the ringing message and establishing a connection for conversation to
begin.

**3.4 Mobile Originated Call**

**3.4.1** A mobile subscriber that wishes to make a call enters the
appropriate telephone number into the mobile unit\'s register. This
process, preorigination dialing, allows the dialing and storing of the
number before going off-hook. Since no dial tone is used in the cellular
system, preorigination dialing does not tie up voice channels and
enables errors to be corrected before going off-hook. Also a number can
be resent without redialing if the line is busy.

**3.4.2** The call is initiated by the mobile subscriber by depressing
the SEND button on the unit. A message is sent over the reverse control
channel to the cell site. The message includes the mobile unit\'s
identification number, the called number, and an access request. The
cell site communicates this information to the MTSO over its data link.
The MTSO indicates through the cell site the assigned voice channel for
the mobile subscriber\'s use.

**3.4.3** The mobile unit then tunes to the assigned voice channel. The
MTSO determines routing and charging information by analyzing the dialed
number. The call is routed through the PSTN by the MTSO. A successful
connection enables communication between the mobile subscriber and
called party.

**3.5 Call Disconnect**

**3.5.1** When the mobile subscriber places the telephone on-hook, or
hangs-up, the mobile unit transmits ST and turns off its transmitter.
After receiving ST, the cell site places an on-hook signal on the
landline trunk. The MTSO responds to the on-hook signal by idling all
switching office components associated with the call and transmitting
the necessary disconnect signal to the wireline network. The MTSO
concludes the disconnect by commanding the cell site to turn off the
radio transmitter associated with the call.

**3.5.2** When a caller from a fixed station goes on-hook, a disconnect
signal from the wireline network is received by the MTSO. All switching
components associated with the call are idled by the MTSO and a
disconnect order is sent over the data link to the serving cell. The
cell site transmits the disconnect message over the voice channel to the
mobile unit. The mobile unit responds by initiating the mobile
disconnect (see 3.5.1) sequence. The MTSO recognizes the mobile unit\'s
successful disconnection. A command is sent through the landline data
link to turn off the cell site transmitter used for the call.

**4. Mobile Switching center**

**4.1 General Description**

**4.1.1** The mobile telephone switching office (MTSO) is the master
control coordinator for the cellular system. The MTSO controls the
switching between the PSTN and the cell sites, processing all
wireline-to-mobile, mobile-to-wireline, and mobile-to-mobile calls. The
MTSO is linked to each cell site and the PSTN. The switch communicates
to the cell site through redundant data links. The links operate from
2400 to 9600 bits per second depending on the facilities available and
anticipated traffic loads.

**4.1.2** The MTSO processes data received from the cell site while
noting the availability of the mobile unit to receive or initiate a
call. Based on data processed from the cell site, the MTSO sends
instructions through the cell site to the mobile unit to change channels
or power levels. The MTSO also commands each cell site to perform
automatic self-diagnostic tests periodically or upon command when
necessary. Other functions include processing diagnostic information and
compiling billing and statistical data.

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**4.1.3** The MTSO is composed of several subsystems integrated to
function as the heart of the cellular system (see Figure 17). The MTSO
and the wireline central office use the same standard digital switch
hardware. However, the software used in the MTSO is designed
specifically for cellular system operation. Each manufacturer\'s MTSO
and operating system is differentiated by his software and switching
capabilities. Currently, the MTSO and cell site equipment must be
manufactured by the same company. However, standards have been developed
and manufacturers are designing equipment that will be compatible in
different systems. Although available features and switching
architectures may vary between manufacturers, the basic components and
cellular system operation are the same.

**4.2 The Switching System**

**4.2.1** The switching system maintains administrative and system
control of the network. The switching sub-system contains processors,
memory, a switching network, trunk circuits, and miscellaneous service
circuits. Circuits are contained on modules easily inserted and replaced
within the switch. The switching system also contains software that
provides the logic to control the processing of telephone calls. The
central processing unit and memory are usually duplicated to maintain
the constant reliability of the system.

**4.2.2** The central processing unit (CPU) is responsible for all
control and routing of calls within the switch. It controls all other
parts of the system through instructions received from the programs
stored in the memory modules. The CPU can have a standby backup and
either one can continue full system operation if the other fails. Only
the active CPU can communicate with the processor memories. Systems that
incorporate redundant CPUs have data ports compare information between
the CPUs. If there is an inconsistency, the faulty CPU is identified,
the system reconfigured, and all processing ability is recovered. Such a
system is designed to maintain system integrity in the event of a
component or power failure without disrupting service.

**4.2.3** The MTSO memory accumulates and stores call processing
information and customer data. Call processing information includes
originating trunk, called number, terminating trunk, channel numbers,
and notes the origination and termination of each call. Customer data
includes the class of service, phone numbers, trunk identification, and
the terminal to which the CPU is connected.

**4.2.4** The switching network also interfaces trunks. Trunks between
switching offices terminate on the switching network through channel
banks or are direct digital interfaces. Four-wire telephone circuits are
usually used to connect the MTSO and the cell sites. T1-carrier, which
handles the equivalent of 24 channels, allows the transmission of
digital data without conversion back to analog voice signals. The
switching network transfers pulse code modulated (PCM) voice signals,
processes the PCM bit stream, and interfaces with the CPU over data
links to allow the passage of a telephone conversation. The switching
network also controls digitized audio signaling.

**4.2.5** Peripheral modules interconnect the switching network with
analog trunks, service circuits, and digital carrier facilities.
Peripheral modules contain digital trunk controllers, trunk modules, and
maintenance trunk modules. Digital trunk controllers connect digital
trunk lines directly to remote units. Trunk modules and maintenance
trunk modules house service electronics such as receivers, audio tone
detectors, announcing trunks, and testing circuits.

**4.2.6** The input/output controller provides an interface between the
central control and device controllers such as magnetic disks for
billing, teleprinters, and visual display terminals.

**4.3 Program Control**

**4.3.1** The switching system operates under the control of stored
software programs. These programs are stored in modules for easy access
to update, modify, or correct. The program store modules are program
instructions for call processing, administration, maintenance, and
system operation.

**4.3.2** Call processing functions include call setup and termination
by selecting channels and initiating ringing. Other functions include
mobile unit monitoring and locating, handoff, and roaming.

**4.3.3** Administration functions include telling the CPU how to obtain
data stored in memory for updates and changes. Other related functions
are storing customer information, collecting billing data, and
monitoring system traffic.

**4.3.4** Maintenance functions include recognizing failures and
substituting standby units to maintain a working system. Maintenance
modules are also responsible for running diagnostics on the radio and
switching subsystems.

**4.3.5** Maintenance and administration modules also perform routine
testing, fault detection, analysis, diagnostic reporting, and system
status reporting.

**5. The Cell site**

**5.1 General Description**

**5.1.1** The cell site, also called the base station, functions under
the direction of the MTSO. Each cell site serves a designated area
providing the mobile user with cellular communication with the PSTN.
Many simultaneous telephone conversations are handled by the cell site
through duplex radio voice channels. Unlike the MTSO where switching
occurs, the cell site provides a way-station for voice and data
communication links.

**5.1.2** Each cell site provides services for radio frequency
transmission, reception, and call distribution. It provides a data
communication link between the MTSO and mobile units. The cell site
performs handoff and locating functions, also handles equipment control
and system administration within the cell. Furthermore, the cell site
performs voice processing functions, call setup, call supervision, and
call termination functions. Other functions include equipment testing
and maintenance within the cell.

**5.1.3** Each cell site consists of a cell site controller, an antenna
system, radio channel units, RF components, and other peripheral
components (see Figure 18).

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**5.2 Cell Site Controller**

**5.2.1** Each cell has a cell site controller (CSC), or processor, that
receives, interprets, and executes instructions from the MTSO. The CSC
unit is usually duplicated to assure continuation of service in the
presence of equipment failure. The CSC contains subsystems acting as
data links, controllers, and radio channel units.

**5.2.2** The CSC manages all voice and control channels within the cell
site. It supervises calls by monitoring and reporting signal strength
readings from within the cell and reporting handoff availability to the
MTSO. The CSC controls transmitters and receivers and sends data on the
control channels to mobile users and the MTSO. It also performs
diagnostic tests on cell site equipment reporting failures to the MTSO.

**5.2.3** The subsystems operate cohesively to allow the CSC to maintain
system reliability and organization in the cell. The data links are
usually T-carrier, fiber optic links, or microwave radio systems
connecting the cell site with the MTSO. Even though the cell site
requires analog input, channel banks are usually used to connect the
link from the MTSO. The controllers are similar to the CPU of the MTSO
in operation, administration, and construction. The radio channel units
transmit voice, data, and location information.

**5.3 Radio Channel Units**

**5.3.1** There are two types of radio channels, voice and control
channels. Each channel has two receivers and one transmitter. Two
receivers are used to minimize the effects of fading and to achieve
maximum signal strength. Each radio channel is an interface between the
cell site controller and the MTSO and is physically connected to the
cable trunks or microwave carrier.

**5.3.2** The control channel provides a continuous stream of data to
the mobile units and receives any messages transmitted by the mobile
units. This data informs the mobile unit or the cell site controller
that a call is being processed and which channels are being assigned to
the call.

**5.3.3** The voice channel permits conversation between the mobile user
and others. The main functions are controlling audio paths and providing
call processing signaling. The voice channel also must monitor SAT, ST,
and the received signal strength indicator (RSSI). SAT uses 5970, 6000,
or 6030 Hz frequencies outside the cellular frequency band to monitor
the availability of a mobile unit on a channel. ST is 10 kHz and used
for acknowledgment from the mobile unit. Other control functions include
noting whether a mobile phone is on-hook or off-hook. The RSSI
determines when a mobile unit needs to be handed-off to another cell or
a signal power level change. Consequently the CSC is kept informed on
the mobile unit\'s status.

**5.3.4** To maintain good voice quality and data transmission, the
mobile unit must be connected to the appropriate cell site to receive a
strong signal. The locating receiver measures the SAT and the RSSI on a
channel specified by the MTSO via the CSC every few seconds on each
call. When the signal becomes weak, the call becomes eligible to be
handed-off to an adjacent cell or antenna capable of offering a stronger
signal. The CSC makes locating measurements when commanded by the MTSO.
The MTSO in turn determines the best cell site antenna to handle the
continuation of the call.

**5.4 Radio Frequency System**

**5.4.1** Each cell contains at least one radio transmitter and two
radio receivers. Both receivers are tuned to the same frequency but the
one receiving the stronger radio signal will be the one selected. Both
are components of the space diversity antenna system. The cell\'s radio
frequency system also consists of transmitter combiner and receiver
multicoupler to allow the connection of multiple radios to one antenna.

**5.4.2** Cellular radio antennas usually reside on free standing towers
that are 100 to 500 feet (30.48 - 152.4 meters) high or on tall
buildings in downtown areas. Initially, an omnidirectional antenna is
customarily used in each cell and is employed in areas that handle light
to moderate traffic loads. As the system expands, directional antennas
are often used to radiate more power in a concentrated area. This is a
technique used to increase traffic capacity within the cell. Directional
antennas are more prevalent in metropolitan and other densely populated
areas to enhance area coverage and minimize interference with
neighboring cells. Portable units that only emit 600 mW often experience
interference from buildings. Thus, directional antennas also increase
signal transmission for portable units.

**6. The mobile unit**

**6.1 General Description**

**6.1.1** The cellular mobile unit allows the mobile subscriber to
communicate with the landline telephone system and other mobile
subscribers. The mobile unit consists of a telephone handset combined
with the control unit, transceiver with logic unit, antenna port, and
power supply. There are three types of mobile units: the car phone,
transportable phone, and hand-held portables. The car phone delivers the
most powerful signal and is hardwired to the individual vehicle. The
transportable phone matches the car phone in components and signal
output. Portable units, on the other hand, have a lower power output
capacity and a less powerful antenna system. Many units are being
designed to function both as a mobile station within the vehicle and as
a transportable unit.

**6.1.2** Once the mobile unit is installed, a variety of services
becomes available to the user. The transceiver is usually installed in
the trunk or under the seat. Placement in the trunk rather than under
the seat involves a shorter feedline to the antenna. Since transmission
occurs at such high frequencies, a shorter feedline will provide a
clearer and stronger signal. The telephone handset is mounted on the
dashboard or console, while the antenna is mounted on the roof, trunk,
or rear window. The antenna is connected via a coaxial cable to the
transceiver.

**6.1.3** Each mobile unit contains a unique permanent serial number and
a 10 digit directory telephone number stored in the random access memory
of the unit. The identity number is set by the manufacturer, giving the
mobile unit a unique identification within the CGSA. The MTSO uses the
number to check the mobile unit\'s identification. This minimizes the
possibility of mobile subscribers changing assigned telephone numbers
and other fraudulent usage.

**6.2 Transceiver Unit**

**6.2.1** The transceiver unit is responsible for all control and
communication between the mobile unit and the cell site controller. This
includes determining the frequencies for transmitting and receiving
signals. The transceiver operates on one duplex channel either to
transmit or receive. It is capable of operating on any of the channels
allocated by the FCC for cellular radio system operation. The frequency
bands of operation are from 824 MHz to 845 MHz to transmit and from 869
MHz to 894 MHz to receive. The transceiver uses a frequency synthesizer
to tune to the assigned channel.

**6.2.2** The transceiver consists of two sections: a radio section and
a logic section. The radio section is responsible for generating the
carrier frequency to be transmitted and contains the antenna port. The
logic unit interprets customer actions and system commands, and manages
the transceiver and control units. The logic unit also reads the
contents of the random access memory, such as called telephone number,
and transmits information to the MTSO. The logic unit also monitors and
controls the power response output of the transmitter. Increasing or
reducing the power helps keep the transmitter at a usable minimum to
prevent possible interference in adjacent cells (see Figure 19).

**6.3 Control Unit**

**6.3.1** The control unit is the user interface to the mobile station.
The unit contains the handset, various user controls, and indicators
allowing the subscriber to communicate with the mobile unit and receive
a response to instructions. User instruction buttons located on the
control unit include SEND, LOCK, STORE, and END. Indicator lights
include IN USE, NO SERVICE, and ROAM.

**6.3.2** Enabling the subscriber to have control over the system, the
control unit is connected to the transceiver by a multiwire cable. The
control unit may consist of one or several components. The telephone
headset, control unit or even a speakerphone can be combined into one
unit or separated.

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**7. Future Trends**

**7.1 Fixed Cellular**

**7.1.1** Cellular radio technology has provided reliable and high
quality mobile service. However, technology, equipment, and the
increasingly ubiquitous nature of telecommunications has generated
interest towards using cellular for fixed applications. Fixed cellular
opens new avenues for growth within the cellular industry and creates a
viable potential market.

**7.1.2** Currently all 306 MSAs and probably 90% of all RSAs will have
operating cellular systems by the end of 1991. There are still locations
in rural areas where telephone service is not available or only
multiparty service is offered. Fixed cellular could provide single party
service in situations where landline service is unavailable or
economically prohibitive. Fixed cellular also can be used for backup
service to the landline network, emergency call boxes, monitoring
facilities, temporary service, and payphones on public transportation.

**7.1.3** Of course, cellular mobile equipment can be used for fixed
cellular stations. However, to create a transparent environment for the
end user, interface equipment is used to connect the cellular network
with standard customer premise equipment. Currently, there are companies
that manufacture these special interface devices. The cost of single
line units is between \$1200 and \$2000.

**7.1.4** The basic components of a fixed cellular installation include
a power supply, transceiver, adapter card, interface circuit, ring
generator, antenna, RJ11 jack, and a standard telephone (see Figure 20).
The power supply typically includes a battery and charging circuit that
provides 10 amp-hours of battery backup. Similar to standard telephone
service, this equipment provides the end user with telephone ringing and
dial-tone. Consequently, cellular service is available to virtually any
customer premise equipment by using a RJ11 connecting jack.

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**7.1.5** The interface circuit, together with the adapter card,
differentiate the fixed cellular station from the mobile or portable
unit. These circuits generate 3W of 20 Hz ringing signal and decode
dialing signals, either tone or pulse at 5pps to 25pps. In effect, the
circuitry functions much the same as a landline central office.

**7.1.6** The interface circuit, also known as the master board,
contains a logic module. This module reads as many as 16 digits being
dialed, recognizes number validity, and determines when dialing is
completed. The module also forwards a SEND command to the transceiver
when dialing is completed.

**7.1.7** The adapter card makes the master board compatible with the
transceiver. It has a logic interface with the transceiver, converts
receiver audio signals to recognizable messages and speech, and controls
the ringing voltage generator.

**7.2 Basic Exchange Telecommunications Radio Service (BETRS)**

**7.2.1** Basic Exchange Telecommunications Radio Service (BETRS) is a
rural radio service that provides fixed telecommunications service to
subscribers. BETRS equipment is licensed by the FCC to operate in the
150, 450, and 800 MHz bands under Part 22 for Public Land Mobile
Service. BETRS equipment in the 800 MHz band is also licensed under Part
90 for Private Land Mobile Service.

**7.2.2** BETRS equipment licensed under Part 90 must operate at least
100 miles (160.93 kilometers) from the border of the 54 largest MSAs.
BETRS can only be offered by local exchange carriers or others with
state authorization. However, cellular operators and resellers can offer
fixed BETRS in RSAs and rural parts of MSAs as regulated under Part 22.
For further information on BETRS systems and equipment, refer to REA
Bulletin 1751H-703, BETRS Radio Systems.

**7.3 Digital Cellular**

**7.3.1** To many, the cellular telephone is a useful business tool.
With advanced technology and lower prices, cellular has begun to broaden
its consumer market base to those beyond the work environment. The
largest MSAs are experiencing subscriber and traffic overload on their
cellular systems currently using analog technology. An influx of data
communications and increased mobile subscriber usage has driven carriers
to consider more efficient spectrum utilization, thus the development of
digital cellular. Digital cellular is being developed to address system
overcrowding and provide improved features required by cellular
subscribers.

**7.3.2** The Telecommunications Industry Association (TIA), an
organization of cellular manufacturers, is responsible for developing
standards for cellular radio equipment, cellular system operation, and
the implementation of digital cellular. The Cellular Telecommunications
Industry Association (CTIA) is an organization consisting of
representatives from cellular industry carriers. TIA works closely with
CTIA to answer the problems facing the booming cellular market. They
address new technological advances needed to keep pace with demand and
quality of service. TIA Cellular Standards Subcommittee TR45.3 is
working towards a standardized transmission format for digital cellular.
By the end of the 1990\'s, CTIA projects digital cellular equipment will
be installed and operational in many metropolitan service areas.

**7.3.3** Time division multiple access (TDMA) was chosen to be the
standard mode of transmission for digital cellular by the TIA. The
architecture of TDMA segments the 30 kHz radio frequency channel into 6
time slots occurring every 24 milliseconds. The 6-slot frame would
produce a total bit rate of 48 kb/s. TDMA allows 3 calls to be carried
on each channel. By the mid 1990\'s, industry experts project TDMA will
allow six 8 kb/s calls per voice channel with improved speech coding and
compression.

**7.3.4** Speech coding and compression of information are major
features of digital cellular that respond to the need for increased
capacity and improved quality of service. Speech coding is a digital
representation of the sound of speech that provides efficient storage,
transmission, recovery, and reconstruction of the original voice signal.
For example, the analog representation of the voice spoken into the
cellular phone is sampled and converted to a digital representation
inside the cellular transceiver. Compressing the information can triple
the capacity of the voice channel and allow transmission at a bit rate
of 8 kb/s. The compressed information is transmitted over communication
links and reconverted back to analog. Voice compression can also cause
noise levels to increase, data speed to decrease, and overall network
delay to increase.

**7.3.5** Digital cellular can increase spectrum and channel efficiency.
Analog cellular uses a 30 kHz channel bandwidth but digital would only
require a 10 kHz channel thus tripling the capacity. Consequently,
digital transmission allows three times as many phone conversations on a
single channel.

**7.3.6** Other characteristics of digital cellular include transmission
that is less sensitive to noise, crosstalk, and distortion: interference
factors that increase with system growth. Digital cellular is also less
susceptible to conversation tapping common with wireless portable
phones. Digital technology allows the equipment to be smaller and
lighter, an added convenience to the subscriber.

**7.4 Narrow Advanced Mobile Phone Service (NAMPS)**

**7.4.1** The evolution of cellular service will eventually turn towards
digital cellular to accommodate increased usage and service. While TIA
has already chosen TDMA as the mode of transmission for digital
cellular, Frequency Division Multiple Access (FDMA) and Code Division
Multiple Access (CDMA) have not been removed from consideration. The
industry is moving towards digital to accommodate overcrowded systems.
However, discussions continue on which modulation technique to use and
when digital can be practically implemented.

**7.4.2** Motorola, a cellular manufacturer, has developed NAMPS to
operate on current systems and provide an immediate solution to system
overcrowding. Based on Narrow Channel Frequency Division Multiple Access
(NC-FDMA), NAMPS is able to reduce the amount of radio frequency used by
using narrow channels. In other words, the present 30 kHz channel is
subdivided into three 10 kHz bands within the original channel. Using
this method, the currently available 832 channels can be expanded to
2412 channels. (There are 42 signaling channels that are not divided
therefore {{832-42}\*3}+42=2412.) The added advantage is the telephones
using NAMPS are also compatible with current analog cellular systems.

**7.5 Personal Communication Services (PCS)**

**7.5.1** The widespread popularity of mobile communications is an
indication of the public\'s desire for portability. Both business and an
increasing number of residential customers are taking advantage of the
new advances in wireless voice technologies. There are many innovative
developments such as personal communication networks (PCNs), second
generation cordless telephones (CT-2), wireless private branch exchanges
(PBXs) and local area networks (LANs). These are a few examples of
personalizing communications and possibly creating a competitive medium
for local exchange carriers.

**7.5.2** The forerunner among the cluster of technological innovations
in the arena of PCSs is a group of technologies called personal
communications networks (PCNs). Similar to existing mobile cellular
radio networks, PCNs also divide the service area into cells. PCNs
contain many low-powered microcells usually spaced 200 meters apart.
This architecture enables PCNs to deploy a high degree of frequency
reuse.

**7.5.3** PCNs are intended to complement and not compete with cellular
telephone service. PCNs will be able to serve portable hand-held units
as well as mobile units. However, handoff is not possible in vehicles
traveling faster than 20 miles per hour. PCNs intend to make the
telephone number an attribute of the person and not an outlet. Some
companies are willing to commercially implement PCNs using the FCC\'s
Part 15 ruling. This regulation allows cellular companies to use their
allocated 25 MHz for any communication service as long as the quality of
cellular service is not adversely effected.

**7.5.4** Second generation cordless telephone technology (CT-2) is the
technical standard used for telepoint service. Telepoint compares with
cellular and paging services, allowing the subscriber to use a
pocket-sized cordless telephone to make calls from any location.
However, subscribers cannot receive calls and handoff is not possible
with telepoint service. Outgoing calls can be made within 200 yards
(182.88 meters) of the base station. Charges are billed to the caller\'s
home or office. Base stations connected to the public network would
widen the subscriber\'s calling service area.

**7.5.5** Future developments in the area of wireless communications
will expand service usage. Technological innovations such as spread
spectrum and other modulation techniques will augment available services
to the mobile user. Cellular systems will likewise improve and only
benefit from these developments.

REFERENCE INDEX

.Begin Index.

Access\....28

BETRS\....40

Cell site\....33

Cell splitting\....21

Cells\....14

Cellular

> Digital\....40
>
> Fixed\....38

Channel

> Assignments\....18
>
> Control\....27
>
> Radio\....35
>
> Voice\....27

Frequency allocation\....12

Handoff\....24

Improved Mobile Telephone Service\....10

Metropolitan Statistical Areas\....13

Mobile Telephone Switching Office\....11, 30

Mobile unit\....36

NAMPS\....41

Paging\....28

PCNs\....42

PCS\....42

Roaming\....24

Rural Service Areas\....13

Sectorization\....14

Signaling tone\....29

Supervisory audio tone\....29

.End Index.