Acs Unit 4 Part1

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ACS UNIT 4
The basic concepts of two-way mobile telephone are quite simple; however, mobile telephone systems involve intricate and rather complex communications networks comprised of
analog and digital communications methodologies, sophisticated computer-controlled
switching centers, and involved protocols and procedures. Cellular telephone evolved from
two-way mobile FM radio.
The fundamental concepts of cellular telephone service:
Cellular services include
standard cellular telephone service (CTS),
personal communications systems (PCS), and
personal communications satellite systems (PCSS).
ABBREVIATIONS:
standard cellular telephone service (CTS),
personal communications systems (PCS), and
personal communications satellite systems (PCSS).
Mobile Telephone Systems or Manual Telephone Systems(MTSs)
Push-to-talk (PTT)
public switched telephone network (PSTN).
Improved Mobile Telephone System (IMTS)
Signaling System No. 7 (SS7)
frequency reuse factor (FRF).
Mobile Telephone Switching Office(MTSO)
 Federal Communications Commission(FCC)











MOBILE TELEPHONE SERVICE
 Mobile telephone services began in the 1940s and were called MTSs (mobile telephone sys- tems or
sometimes manual telephone systems, as all calls were handled by an operator). MTS systems utilized
frequency modulation and were generally assigned a single carrier frequency in the 35-MHz to 45-MHz range
that was used by both the mobile unit and the base station.
 The mobile unit used a push-to-talk (PTT) switch to activate the transceiver. Depressing the PTT button turned
the transmitter on and the receiver off, whereas releasing the PTT turned the receiver on and the transmitter
off. Placing a call from a MTS mobile telephone was similar to making a call through a manual switchboard in
the public telephone network.
 When the PTT switch was depressed, the transmitter turned on and sent a carrier frequency to the base station,
illuminating a lamp on a switchboard. An operator answered the call by plugging a headset into a jack on the
switchboard. After the calling party verbally told the operator the telephone number they wished to call, the
operator connected the mo- bile unit with a patchcord to a trunk circuit connected to the appropriate public
telephone network destination office. Because there was only one carrier frequency, the conversation was
limited to half-duplex operation, and only one conversation could take place at a time.
 MTS mobile identification numbers had no relationship to the telephone numbering
system used by the public telephone network. Local telephone companies in each state,
which were generally Bell System Operating Companies, kept a record of the numbers
as- signed to MTS subscribers in that state. MTS numbers were generally five digits long
and could not be accessed directly through the public switched telephone network
(PSTN).
 In 1964, the Improved Mobile Telephone System (IMTS) was introduced, which used
several carrier frequencies and could, therefore, handle several simultaneous mobile
con- versations at the same time. IMTS subscribers were assigned a regular PSTN
telephone number; therefore, callers could reach an IMTS mobile phone by dialing the
PSTN directly, eliminating the need for an operator.
 IMTS and MTS base station transmitters outputted powers in the 100-W to 200-W range,
and mobile units transmitted between 5 W and 25 W. Therefore, IMTS and MTS mobile
telephone systems typically covered a wide area using only one base station
transmitter.
 In early radio terminology, the term mobile suggested any radio transmitter, receiver,
or transceiver that could be moved while in operation. The term portable described a
relatively small radio unit that was handheld, battery powered, and easily car- ried by a
person moving at walking speed.
 the modern, all-inclusive definition of mobile telephone is any wireless telephone
capable of operating while moving at any speed, bat- tery powered, and small enough
to be easily carried by a person.
CELLULAR TELEPHONE VS TWO WAY MOBILE:
 Cellular telephone is similar to two-way mobile radio in that most communications
occurs between base stations and mobile units..
 Cellular telephone is best described by pointing out the primary difference between it
and two-way mobile radio. Two-way mobile radio systems operate half- duplex and use
PTT transceivers. With PTT transceivers, depressing the PTT button turns on the
transmitter and turns off the receiver, whereas releasing the PTT button turns on the
receiver and turns off the transmitter.
 With two-way mobile telephone, all transmissions (unless scram- bled) can be heard by
any listener with a receiver tuned to that channel. Hence, two-way mobile radio is a
one-to-many radio communications system. Examples of two-way mobile radio are
citizens band (CB), which is an AM system, and public land mobile radio, which is a twoway FM system such as those used by police and fire departments.

Another limitation of two-way mobile radio is that transmissions are limited to relatively
small geographic areas unless they utilize complicated and expensive repeater
networks.
 On the other hand, cellular telephone offers full-duplex transmissions and operates
much the same way as the standard wireline telephone service provided to homes and
businesses by local telephone companies.
 Mobile telephone is a one-to-one system that permits two-way simultaneous
transmissions and, for privacy, each cellular telephone is assigned a unique telephone
number. Coded transmissions from base stations activate only the in- tended receiver.
With mobile telephone, a person can virtually call anyone with a telephonenumber,
whether it be through a cellular or a wireline service.


Fundamental Concepts of Cellular Telephone
 The fundamental concepts of cellular telephone are quite simple. The FCC originally
defined geographic cellular radio coverage areas on the basis of modified 1980 census
figures. With the cellular concept, each area is further divided into hexagonal-shaped
cells that fit together to form a honeycomb pattern as shown in Figure.
 The hexagon shape was chosen because it provides the most effective transmission by
approximating a circular pattern while eliminating gaps inherently present between
adjacent circles. A cell is defined by its physical size and, more importantly, by the size
of its population and traffic patterns.
 The number of cells per system and the size of the cells are not specifically defined by
the FCC and has been left to the providers to establish in accordance with anticipated
traffic patterns. Each geographical area is allocated a fixed number of cellular voice
channels.
 The physical size of a cell varies, depending on user density and calling patterns. For
example, large cells (called macrocells) typically have a radius between 1 mile and 15
miles with base station transmit powers between 1 W and 6 W. The smallest cells
(called microcells) typically have a radius of 1500 feet or less with base station transmit
powers between 0.1 W and 1 W. Figure shows a cellular configuration with two sizes of
cell.
 Microcells are used most often in high-density areas such as found in large cities and inside buildings. By
virtue of their low effective working radius, microcells exhibit milder propagation impairments, such as
reflections and signal delays. Macrocells may overlay clusters of microcells with slow-moving mobile units
using the microcells and faster-moving units using the macrocells. The mobile unit is able to identify itself as
either fast or slow moving, thus allowing it to do fewer cell transfers and location updates.

1

2

1

6

2

2

1
7
35
4

6

6

7
7
3

5

3

5

4

(a)

4

(b)

(a) Honeycomb cell pattern;
two sizes of cells

(b) honeycomb pattern with

FREQUENCY REUSE basic definition:
Frequency reuse is the process in which the same set of frequencies (channels) can be
allocated to more than one cell, provided the cells are separated by sufficient distance.
CELLULAR TELEPHONE:
 The key principles of cellular telephone (sometimes called cellular radio) were
uncovered in 1947 by researchers at Bell Telephone Laboratories and other
telecommunications companies throughout the world when they developed the basic
concepts and theory of cellular telephone.
 It was determined that by subdividing a relatively large geographic market area, called a coverage zone, into
small sections, called cells, the concept of frequency reuse could be employed to dramatically increase the
capacity of a mobile telephone channel.
 In essence, cellular telephone systems allow a large number of users to share the
limited number of common-usage radio channels available in a region.
NOTE:
When designing a system using hexagonal-shaped cells, base station transmitters can be
located in the center of a cell, or on three of the cells’ six vertices or corners. Omnidirectional antennas are normally used in center-excited cells, and sectored directional
antennas are used in edge- and corner-excited cells (omnidirectional antennas radiate and
receive signals equally well in all directions).

(a)

(b)

(c)

(a) Center excited cell; (b) edge excited cell; (c)corner excited cell
Cellular telephone is an intriguing mobile radio concept that calls for replacing
a single, high-powered fixed base station located high above the center of a city with multiple,
low-powered duplicates of the fixed infrastructure distributed over the coverage area on sites
placed closer to the ground. The cellular concept adds a spatial dimension to the simple cabletrunking model found in typical wireline telephone systems.
FREQUENCY REUSE
Frequency reuse is the process in which the same set of frequencies (channels) can be
allocated to more than one cell, provided the cells are separated by sufficient distance.
Reducing each cell’s coverage area invites frequency reuse. Cells using the same set of radio
channels can avoid mutual interference, provided they are properly separated. Each cell base
station is allocated a group of channel frequencies that are different from those of neighboring
cells, and base station antennas are chosen to achieve a desired coverage pattern within its
cell. However, as long as a coverage area is limited to within a cell’s boundaries, the same
group of channel frequencies may be used in different cells with-out interfering with each
other, provided the two cells are sufficient distance from one another.

Cluster 1
Coverage
area

B
G

C
A

F
BE
GCB AG

Cluster 2

D

C
FDA
EFD E

Cluster 3

FIGURE Cellular frequency reuse concept
Figure illustrates the concept of frequency reuse in a cellular telephone system. The figure
shows a geographic cellular radio coverage area containing three groups of cells called
clusters. Each cluster has seven cells in it, and all cells are assigned the same number of fullduplex cellular telephone channels. Cells with the same letter use the same set of channel
frequencies. The letters A, B, C, D, E, F, and G denote the seven sets of frequencies
The frequency reuse concept can be illustrated mathematically by considering a sys- tem with
a fixed number of full-duplex channels available in a given area. Each service area is divided
into clusters and allocated a group of channels, which is divided among N cells in a unique
and disjoint channel grouping where all cells have the same number of chan- nels but do not
necessarily cover the same size area. Thus, the total number of cellular chan- nels available in
a cluster can be expressed mathematically as
F = GN…………
(1)
where F = number of full-duplex cellular channels available in a cluster
G = number of channels in a cell
N = number of cells in a cluster
The cells that collectively use the complete set of available channel frequencies make up the
cluster. When a cluster is duplicated m times within a given service area, the channel capacity
can be expressed mathematically as
C = mGN
or= mF………..
(2)
where C = total channel capacity in a given area
m = number of clusters in a given area
G = number of channels in a cell
N = number of cells in a cluster/cluster size
From Equations 1 and 2, it can be seen that the channel capacity of a cellular telephone
system is directly proportional to the number of times a cluster is duplicated in a given service
area(i.e m). The factor N is called the cluster size and is typically equal to 3, 7, or 12. When
the cluster size is reduced and the cell size held constant, more clusters are required to cover
a given area, and the total channel capacity increases. The frequency reuse factor of a cellular
telephone system is inversely proportional to the number of cells in a cluster (i.e., 1/N).
Therefore, each cell within a cluster is assigned 1/Nth of the total available channels in the
cluster.
The number of subscribers who can use the same set of frequencies (channels) in nonadjacent cells at the same time in a small area, such as a city, is dependent on the total

number of cells in the area. The number of simultaneous users is generally four, but in
densely populated areas, that number may be significantly higher. The number of users is
called the frequency reuse factor (FRF). The frequency reuse factor is defined mathematically
as

FRF =

N

C
where FRF = frequency reuse factor (unitless)
N = total number of full-duplex channels in an area
C = total number of full-duplex channels in a cell
Cells use a hexagonal shape, which provides exactly six equidistant neighboring cells, and the
lines joining the centers of any cell with its neighboring cell are separated by multiples of 60.
Therefore, a limited number of cluster sizes and cell layouts is possible. To connect cells
without gaps in between (tessellate), the geometry of a hexagon is such that the number of
cells per cluster can have only values that satisfy the equation
N = i2 + ij + j2
where
N = number of cells per cluster ;; i and j = nonnegative
integer values
The process of finding the tier with the nearest co-channel cells (called the first
tier) is as follows and shown in Figure:
 Move i cells through the center of successive cells.
 Turn 60° in a counterclockwise direction.
 Move j cells forward through the center of successive cells.

A
j = 2 60

i=3
A

FIGURE : Locating first tier co-channel cells

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