unit1chapter1-171125094511.pdf cellular mobile radio system

CELLULAR AND
MOBILE
COMMUNICATIONS
M V S S BABU M.TECH (COMMUNICATION SYSTEMS)| ASST.
PROFESSOR DEPARTMENT OF ELECTRONICS & COMMUNICATION
UNIT – I
CHAPTER :1 CELLULAR MOBILE RADIO SYSTEMS

List of topics
Introduction to Cellular Mobile System,
Performance criteria,
uniqueness of mobile radio environment,
operation of cellular systems,
Hexagonal shaped cells,
Analog Cellular systems.
Digital Cellular systems
2

Introduction to Cellular Mobile System
Limitations of conventional mobile telephone systems:
One of many reasons for developing a cellular mobile system and deploying it in many cities is
the operational limitations of conventional mobile telephone systems:
 limited service capability,
 poor service performance, and inefficient frequency spectrum utilization.
Limited service capability:
A conventional mobile telephone system is usually designed by
 selecting one or more channels from a specified frequency allocation for use is in
autonomous geographic zones.
 The communication coverage area of each zone is normally planned to be large as possible,
which means the transmitted power should be high.
 The user who starts a call in one zone has to reinitiate the call when moving into a new
because the call will be dropped. There is no guarantee that a call can be completed without
a handoff capability.
 The handoff is a process of automatically changing frequencies as the mobile unit moves
from one frequency zone to other frequency zone so that the conversation can be continued
in a new frequency zone without reinitiating the call (redialing).
 Another disadvantage of the conventional system is that the number of active users is limited
to the number of channels assigned to a particular frequency zone.
3

4

Limitations of conventional mobile telephone systems:
Poor service performance:
 In the past, a total of 33 channels were allocated to three mobile telephone systems:
1. Mobile Telephone Service (MTS)
2. Improved Mobile Telephone Service (IMTS) MJ Systems
3. Improved Mobile Telephone Service (IMTS) MK Systems
 EX: In 1976, New York City had 6 channels of MJ servicing 320 customers, with another
2400 customers on in waiting list.
 New York City also had 6 channels of MK servicing 225 customers, with another 1300
customers on in waiting list.
 The large number of subscribers created a high blocking probability during busy hours.
Although service performance was poor, the demand was still great. A high capacity
system for mobile telephones was needed.
5

6

BASIC CELLULAR SYSTEMS: (Q. Describe the basic cellular system?)
 The basic requirement of cellular system is high capacity. High capacity is achieved by
limiting the coverage of each base station to a small geographic region called cell.
 This is different from traditional radio transmitters which would cover the entire city at the
whole city have one cell. In order to achieve higher capacity and to support larger
number of users the city must be divided into small cells and each cell must have a
transceiver called base station. In this case a single high power transmitter is replaced
by a large number of low power transmitters as shown below.
7

8
 The main goal of basic cellular system is to provide wireless communication between two moving devices,
called mobile stations or between one mobile unit and a stationary unit, commonly referred to as land line unit.
 As mobile users travel from one cell to another cell, their conversations are handed off between cells.
 Frequencies used in one cell can be reused in another cell some distance away
 A basic cellular system consists of following subsystems:
1. Mobile stations (MS): Mobile handsets which are used by user to communicate with another user. A mobile
unit contains a control unit, a transceiver, and an antenna system.
2. Cell: Each cellular service area is divided into small regions called cell.
3. Base station: The base station provides interface between the mobile telephone switching office
(MTSO)/mobile switching center (MSC) and the mobile units. It has a control unit, radio cabinets, antennas, a
power plant, and data terminals.
4. MTSO/MSC: Each base station is controlled by a switching office called mobile switching center. The mobile
switching center is also connected to public switching telephone center (PSTN) so that the mobile stations can
communicate to the base station(BS), MSC and PSTN to a stationary phone(Land line phone). The switching
office, the central coordinating element for all base stations, contains the cellular processor and cellular switch.
It interfaces with telephone company zone offices; controls call processing, provide operation and
maintenance, and handles billing activities.
5. Connections: The radio and high-speed data links connect the MS, BS and MSC. Each mobile unit can only
use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the
entire band assigned by the serving area, with each site having multichannel capabilities that can connect
simultaneously to many mobile units.

9
STATIONARY
PHONE
MOBILE SWITCHING
CENTER (MSC)
PUBLIC SWITCHIND
TELEPHONE NETWORK
(PSTN)
BS
MS
MS
MS MS
MS
MS
BS
MS
MS
MS
MS
MS
MS
BS
MS
MS
MS
MS
MS
MS

OPERATION OF CELLULAR SYSTEMS (Q. Write the different operations of basic cellular system?)
The operation can be divided into four parts and a handoff procedure.
1. Mobile unit initialization
2. Mobile originated call
3. Network originated call
4. Call termination
 Mobile unit initialization: When a user activates the receiver of the mobile unit, the
receiver scans the set-up channels. It then selects the strongest and locks on for a
certain time. Because each site is assigned a different set-up channel, locking onto the
strongest set-up channel usually means selecting the nearest cell site. This self-location
scheme is used in the idle stage and is user-independent. It has a great advantage
because it eliminates the load on the transmission at the cell site for locating the mobile
unit. The disadvantage of the self-location scheme is that no location information of idle
mobile units appears at each cell site. Therefore, when the call initiates from the land line
to a mobile unit, the paging process is longer. For a large percentage of calls originates at
the mobile unit, the use of self-location schemes is justified. After a given period, the self-
location procedure is repeated.
10

 Mobile originated call: The user places the called number into an originating register in
the mobile unit, and pushes the “send” button. A request for service is sent on a selected
set-up channel obtained from a self-location scheme. The cell site(BS) receives it, and in
directional cell sites (or sectors), selects the best directive antenna for the voice channel
to use. At the same time, the cell site sends a request to the mobile telephone switching
office (MTSO/MSC) via a high-speed data link. The MTSO selects an appropriate voice
channel for the call, and the cell site acts on it through the best directive antenna to link
the mobile unit. The MTSO also connects the wire-line party through the telephone
company zone office.
 Network originated call: A land-line party dials a mobile unit number. The telephone
company zone office recognizes that the number is mobile and forwards the call to the
MTSO. The MTSO sends a paging message to certain cell sites based on the mobile unit
number and the search algorithm. Each cell site transmits the page on its own set-up
channel. If the mobile unit is registered, the registered site pages the mobile. The mobile
unit recognizes its own identification on a strong set-up channel, locks onto it, and
responds to the cell site(BS). The mobile unit also follows the instruction to tune to an
assigned voice channel and initiate user alert.
11

 Call termination: When the mobile user turns off the transmitter, a particular signal
(signaling tone) transmits to the cell site(BS), and both sides free the voice channel. The
mobile unit resumes monitoring pages through the strongest set-up channel.
 Handoff procedure: During the call, two parties are on a traffic channel. When the
mobile unit moves out of the coverage area of a particular cell site(BS), the reception
becomes weak. The current base station requests all the neighboring base stations to
report the strength they are receiving. The current base station then transfers ownership
to the cell (new BS) getting the strongest signal and the MSC switches the call to a new
frequency channel in a new cell site without either interrupting the call or alerting the
user. The process is called handoff. The call continues as long as the user is talking. The
user does not notice the handoff occurrences.
12

Operational Channels:
In each cell, there are four types of channels that take active part during a mobile call. These
are:
 Forward Voice Channel (FVC): This channel is used for the voice transmission from the
BS to the MS.
 Reverse Voice Channel (RVC): This is used for the voice transmission from the MS to the
BS.
 Forward Control Channel (FCC): Control channels are generally used for controlling the
activity of the call, i.e., they are used for setting up calls and to divert the call to unused
voice channels. Hence these are also called setup channels. These channels transmit
and receive call initiation and service request messages. The FCC is used for control
signaling purpose from the BS to MS.
 Reverse Control Channel (RCC): This is used for the call control purpose from the MS to
the BS. Control channels are usually monitored by mobiles.
13

There are three categories for specifying performance criteria.
1. Voice Quality: Voice quality is very hard to judge without subjective tests for users’ opinions. In this technical
area, engineers cannot decide how to build a system without knowing the voice quality that will satisfy the
users. In military communications, the situation differs: armed forces personnel must use the assigned
equipment. CM: For any given commercial communications system, the voice quality will be based on the
following criterion: a set value x at which y percent of customers rate the system voice quality (from
transmitter to receiver) as good or excellent; the top two circuit merits (CM) of the five listed below.
 MOS: As the percentage of customers choosing CM4 and CM5 increases, the cost of building the system rises.
The average of the CM scores obtained from all the listeners is called mean opinion score (MOS). Usually, the
toll-quality voice is around MOS ≥4.
14
PERFORMANCE CRITERIA

2. Service Quality
Three items are required for service quality.
I. Coverage: The system should serve an area as large as possible. With radio coverage,
however, because of irregular terrain configurations, it is usually not practical to cover 100
percent of the area for two reasons:
 The transmitted power would have to be very high to illuminate weak spots with sufficient
reception, a significant added cost factor.
 The higher the transmitted power, the harder it becomes to control interference. Therefore,
systems usually try to cover 90 percent of an area in flat terrain and 75 percent of an area in
hilly terrain.
II. Required grade of service: For a normal start-up system, the grade of service is specified
for a blocking probability of .02 for initiating calls at the busy hour. This is an average value.
However, the blocking probability at each cell site will be different. At the busy hour, near
freeways, automobile traffic is usually heavy, so the blocking probability at certain cell sites
may be higher than 2 percent, especially when car accidents occur. To decrease the
blocking probability requires a good system plan and a sufficient number of radio channels.
III. Number of dropped calls: During Q calls in an hour, if a call is dropped and Q−1 calls are
completed, then the call drop rate is 1/Q. This drop rate must be kept low. A high drop rate
could be caused by either coverage problems or handoff problems related to inadequate
channel availability or weak reception.
15

3. Special Features: A system would like to provide as many special features as possible,
such as call forwarding, call waiting, voice stored (VSR) box, automatic roaming, short
message service (SMS), multimedia service (MMS), push-to-talk (PTT), or navigation
services. However, sometimes the customers have to pay extra charges for these
special services.
16

UNIQUENESS OF MOBILE RADIO ENVIRONMENT (Q. Determine the path losses in real mobile radio
environment?)
The Propagation Attenuation: In general, the propagation path loss increases not only with frequency but also
with distance.
 If the antenna height at the cell site is 30 to 100 m and at the mobile unit about 3 m above the ground, and the
distance between the cell site and the mobile unit is usually 2 km or more, then the incident angles of both the
direct wave and the reflected wave are very small, as shown in below Fig.
 The incident angle of the direct wave is Ɵ1, and the incident angle of the reflected wave is Ɵ2.
 Ɵ1 is also called the elevation angle. The propagation path loss would be 40dB/dec. This means that a 40-dB
loss at a signal receiver will be observed by the mobile unit as it moves from 1 to 10 km.
 Therefore C is inversely proportional to R4
17
UNIQUENESS OF MOBILE RADIO
ENVIRONMENT
Ɵ2
Ɵ1
30
-100m
2km or further

 The difference in power reception at two different distances R1 and R2 will result in
 and the decibel expression of Eq. is
 This 40 dB/dec is the general rule for the mobile radio environment and is easy to
remember. It is also easy to compare to the free-space propagation rule of 20 dB/dec.
The linear and decibel scale expressions are
 In a real mobile radio environment, the propagation path-loss slope varies as
 γ usually lies between 2 and 5 depending on the actual conditions, γ cannot be lower
than 2, which is the free-space condition.
18
ENVIRONMENT

EX.1: Let the maximum calls per hour Qi in one cell be 3000 and an average calling
time T be 1.76 min. The blocking probability B is 2 percent find the offered load A.
Solution:
Given Data Qi=3000 calls/hour, T = 1.76 min.
Offered Load
= 88
19
ENVIRONMENT

Basic call procedure
 Before a mobile phone can establish a call, it first needs to establish a connection to the
available service providers network. Whenever a mobile phone is switched on, it first
scans the group of forward control channels to determine the strongest one. Then it
keeps on monitoring the same channel until the signal level drops below a minimum level
after which it again starts to scan. Every mobile phone repeats this process as long as it
is kept switched on. The signal level of the current channel which the phone is locked to
will be displayed on the handset.
 Steps to make a call:
1. First of all, a call initiation request is sent on the reverse control channel. Along with this
request, the mobile phone also sends its Mobile Identification Number (MIN), Electronic
Serial Number (ESN) and the dialed telephone number.
2. The base station receives this data and sends it to the Mobile Switching Center (MSC).
The MSC validates the request by checking the MIN with the records on its database. If
it is valid, a connection to the called party is made through Public switched Telephone
network (PSTN). Then the MSC requests the base station to move the mobile phone to
an unused voice channel so that the conversation can begin.
20
Operation of cellular systems

3. Once a call is in progress, the MSC adjusts the power transmitted by the mobile phone
as it moves in and out of the coverage area of each base station. When a mobile phone
with a call in progress moves from one base station to another, the process is called
handoff.
4. Once a call is in progress, the MSC adjusts the power transmitted by the mobile phone
as it moves in and out of the coverage area of each base station. When a mobile phone
with a call in progress moves from one base station to another, the process is called
handoff.
5. Then the MIN which is the subscribers number is transmitted as a message through all
the base stations in the system. The mobile identifies the message and responds to it.
6. The base station then relays the acknowledgement to the MSC. Then the MSC
requests the base station to move the phone to an unused voice channel. During this
process, a control signal is also send which triggers the mobile phone to start ringing.
The MSC connects the call to the PSTN and the conversation can begin as soon as the
mobile user accepts the call.
21

7. When a call is made and there is no free voice channels available then you will probably
hear a message that the network is busy. This happens when a large numbers of calls
are made from within the coverage area of a single base station. All these steps
described above happens within a few seconds that the user is not able to identify it.
They almost start hearing the ringing tone as soon as the call is placed.
22

Now according to the diagram above, it can be understood simply by:
1. Mobile device is connected to BTS (Antenna).
2. BTS is connected to the Switching system called BSC.
3. BSC is connected to the main switching system called MSC.
4. MSC contains its own VLR (VLR: is a temporary database which stores the information
of the visitors under its coverage area. VLR stands for Visitor Location register. When
you roam in a different place VLR stores your user information.).
5. MSC's are connected to GMSC which is connected to HLR. (HLR stands for Home
location register, It is the main database where the documents or information of user is
stored. all the documents that you give during purchase of a SIM card is stored in this
HLR. VLR Takes your information from HLR when you Roam in other state or region.).
6. HLR also provides authentication by AuC. AuC is connected with HLR. If you initiate a
call HLR and AuC will see if you are a genuine Mobile user with valid IMEI number and
Plan.
7. and then the call is set up from source to the destination device.
8. Now this is only the basic information of a call setup.
23

Hexagonal shaped cells
 Each cellular service area is divided into small regions called cell.
 Engineers draw hexagonal shaped cells on the layout to simplify the planning and
design of a cellular system because it approaches a circular shape that is the ideal
power coverage area.
 The circular shapes have overlaps or gaps which make the drawing unclear. The
hexagonal shaped cell fit the plane nicely, as shown in fig with no gap and no overlap
between the cells.
24

Cellular system in the united states:
 there are 150 major market areas in the united states where licenses for cellular
systems can be granted by the FCC.
 They have been classified by their populations into five groups.
 Each group has 30 cities.
 top 30 markets ------ very large cities
 Top 31 to 60 markets ---------------- large sized cities.
 Top 61 to 90 markets ----------------- medium sized cities
 Top 91 to 120 markets ----------------- below medium sized cities
 Top 121 to 150 markets ---------------- small sized cities.
 each market area is planned to have two systems.
 The status of each system in each area of groups 1 to 3 as of December 1985.
 There are 305 MSAs ( Metropolitan statistical areas) and 482 RSAs (rural statistical
areas).6
25
Analog Cellular Systems

Cellular system outside the united states:
 Japan:
 “Nippon telegraph and Telephone Corporation (NTT)” developed an 800 MHz land mobile
telephone system and put it into service in the Tokyo area in 1979.
 The general system operation is similar to the AMPS system.
 It accesses approximately 40,000 subscribers in 500 cities.
 It covers 75 percent of all Japanese cities, 25 percent of inhabitable areas, and 60
percent of the population.
 In Japan , 9 automobile switching centers (ASCs), 51 mobile control stations (MCSs),
465 mobile base stations (MBSs), and 39,000 mobile subscriber stations (MSSs) where
in operation as Feb. 1985.
 The Japanese mobile telephone service network configuration is shown in following
figure.
 In metropolitan Tokyo area, about 30,000 subscribers are being served.
26

27
Cellular system outside the united states:

 The 1985 system operated over a spectrum of 30 MHz.
 The total number of channels was 600 , and the channel bandwidth was 25kHz.
 This system comprised an automobile switching center (ASC), a mobile control station
(MSC), a mobile base station (MBS) , and a mobile setup buy the government.
 However, the Japanese ministry of post and telecommunication (MFT) is considering
providing a dual competative situation similar to that in the united state.
 United Kingdom:
 In 1982 June the government of UK announced two competing national cellular radio
networks.
 The UK system is called TACS (Total Access Communications Systems).
 The total number of channels was 1000, with a channel bandwidth of 25kHz per channel.
 Among them , 600 channels are assigned and 400 are reserved.
 Two competing cellular network operators, Cellnet and Vodafone, are operating in the UK.
 The cellnet system started operating in Jan 1985.
 Cellnet has over 200 cell sites, convering 82 percent of the UK.
 Vodafone, through , which started operations late, has served the same areas as Cellnet.
28

 Canadian system:
 In 1978, a system called AURORA was designed for the Alberta government
telephone (AGT).
 The system provides province wide mobile telephone service at 400 MHz. ongoing
developmental work on the AURORA is underway at 800MHz.
 AURORA 400 system:
 It is aimed at 40,000 subscriber living in an area approximately 1920 km X 960 km.
 The AURORA 400 system initially has 40 channels and is expected to add an
additional 20 channels with frequency reuse and a seven – cell cluster plan.
 A fully implemented system has 120 cells. The 400-Mhz system does not have a
handoff capability.
 AURORA 800 system:
 The AURORA 800 system is truly frequency transparent.
 By repackaging the RF sections on the cell site, the mobile unit can be operated on
any mobile RF band up to 800 Mhz.
 The handoff capability will be implemented in this system.
29

 Nordic system:
 Built mostly by Scandinavian countries ( Denmark, Norway, Sweden, and Finland) in
cooperation with Saudi Arabia and Spain and is called the NMT network.
 It is currently a 450-MHz system , but an 800-MHz system will be implemented soon
since the frequency – transparent concept as the AURORA-800 system is used to
convert the 450-MHz system to the 800 MHz system.
 The total bandwidth is 10MHz, which has 200 channels with a bandwidth of 25kHz per
channel.
 This system does have handoff and roaming capabilities.
 It also uses repeaters to increase the coverage in a low traffic area.
 The total no.of subscribers is around 1,00,000.
 European cellular system:
All the present generation of European cellular networks is totally lacking in cross-
border compatibility . Besides the united kingdom and NMT networks, the others
include the following.
30

 Benelux-country network:
 The Netherlands served on their ATF2 network at the beginning of 1985
 It has national wide coverage using 50 cell sites with two different cell sizes , 20 and 5
k.m. radii.
 The capacity of the present system is 15,000 to 20,000 subscribers.
 Dutch PT&T is using a single ericssion AXE10 switch.
 Luxembourg came on air in august 1985.
 In 1986, Belgium joined the network and It operates at 450 MHz.
 The network is compatible among the three countries.
 France:
 A direct dial car telephone operating at 160MHz can access the system in 10 regional
areas.
 The network serves 10,000 subscribers. By the end of 1984, 450 MHz was in operation.
 In the meantime redicom 2000(digital signaling) was introduced, operating at 200 MHz
but with no handoff feature.
31

 Spain:
 It uses in NMT 450 MHz cellular network introduced in 1982
 It was first cellular system in europe.
 The number of cells in service is 13.
 There are three separate networks operating 104 channels. Each of bandwidth is 25kHz.
32

 In 1992 the digital first cellular system , GSM (Special mobile group) , was developed in
Germany.
 GSM is a European standard system.
 In united states , an NA-TDMA system (IS-54) and a CDMA system (IS-95) has been
developed.
 NA-TDMA was developed in 1993 and CDMA is planned for deployment in 1995.
 A Japanese system , PDC (Personal Digital Cellular ) , was developed in Osaka in
June 1994.
33
Digital Cellular Systems

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