Introduction to the Mobile
Cellular concept
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Distinctions in Mobile radio systems
There are three major technologies of
terrestrial mobile communications in use:
• Paging Radios and PMR systems:
Brief numeric, alphanumeric or voice
messages are sent to the subscriber
typically using simultaneous broadcasting.
• Cordless Communication:
Users are provided limited mobility from a
dedicated base station
• Cellular Radio Systems:
Users are provided wide area mobility from
multiple base stations with handover
permitted
Major weaknesses of Mobile radio systems of the 1970s
• Limited Network capacity due to limitation in frequency reuse
• The callers had to know the called party’s zone before calling, the system did
not have a database to keep records of where the mobile user was located
(visitor’s location)
• Mobile stations were bulky and hence required to be installed in vehicles
(the invention of transistor in 1948 and integrated circuit, microprocessors
provide possible solution to the gaps)
• Though IMTS provided full duplex operation, automatic dialing, and
automatic channel searching, the only 11 (or 12) channels were available
for all users of the system within a given geographic area
• The Mobile Service Area(MSA) was large hence each base‐
station antenna had to be located on a tall structure and had to transmit at
high power in order to provide coverage throughout the entire service area.
• Large storage batteries were required to drive high power instruments that
were used within the vehicle.
Standard Bodies that oversee Telecom Standards development
• ITU: International Telecommunication Union
• ITU‐R,
• ITU‐T,
• ISO/IEC ‐ International Organization for Standardization/ International Electrotechnical Commission
• North America:
•
•
•
•
FCC – Federal Communication Commission, USA
EIA/TIA ‐ Electronic Industries Alliance/Telecommunications Industry Association
ANSI ‐American National Standards Institute
IEEE – Institute of Electrical and Electronic Engineers
• Europe:
•
•
•
•
CEPT ‐European Conference of Posts and Telecommunications Administrations
ETSI ‐ European Telecommunication Standard Institute),
RACE ‐ Research in Advanced Communications in Europe
ERMES – European Radio MEssaging System
• Japan:
• TTC ‐
• RCR ‐
• Company Level Standards: WINForum, CDPD, Bluetooth SIG, WiFi, WiMedia, WiMAX are
company associations not formal standards.
Broad objectives of 1G Mobile Cellular systems
• Save or conserve the radio spectrum
• Reuse frequency as much as possible without loss of quality
• Improve system capacity to meet the ever increasing demand for
mobile radio phones
• Provide basic voice service to mobile users over large area
Early Cellular Mobile Generations
MOBILE
PHONE
SYSTEM
GENERATION CHANNEL
SPACING
1G SYSTEMS
ACCESS
METHOD
AMPS
1G
30 kHz
FDMA
NAMPS
1G
10 kHz
FDMA
TACS
1G
25 kHz
FDMA
COMMENTS
Advanced Mobile Phone System, this
analogue system was first developed and used
in the USA.
Narrow band version of AMPS chiefly used in
the USA and Israel based on a 10 kHz channel
spacing. The aim was to conserve spectrum
and thereby allow a larger number of users.
Total Access Communication System
(TACS): Analogue system developed by
Motorola was originally used in the UK. Based
around 900 MHz, this system spread world
wide. After the system was first introduced,
further channels were allocated to reduce
congestion, in a standard known as Extended
TACS or ETACS
MOBILE
PHONE
SYSTEM
GENERATION
CHANNEL
SPACING
1G SYSTEMS
ACCESS
METHOD
NMT
1G
12.5 kHz
FDMA
NTT
1G
25 kHz
FDMA
C450
1G
20 kHz
FDMA
COMMENTS
Nordic Mobile Telephone. This analogue system
was the first cellular technology to be widely used
commercially being launched in 1979. It was used
initially on 450 MHz and later at 900 MHz. It was
used chiefly in Scandinavia but it was adopted by up
to 30 other countries including Oman.
Nippon Telegraph and Telephone. This cellular
technology was used in Japan in the 900 MHz
frequency band with a 55 MHz transmit receive
spacing. (A high capacity version known as HICAP
was also developed).
The system adopted in West Germany (East
Germany was separate at this time). It used a band
in the region of 450 MHz along with a 10 MHz
receive / transmit spacing.
Mobility standard between transmitter and receiver
THE AMPS 1G SYSTEM
Concepts of 1G AMPS system
• Frequency modulation technique for voice – FM
• Frequency Channel Access – FDMA
• Duplexing (two way frequency allocation –Tx and Rx ) – FDD and TDD
• The Cellular structure
• Frequency Reuse
• Mobility ‐ Handoff/Handover
• Cellular Radio Network‐
• Network architecture
Frequency Modulation
Original speech
signal spectrum
Shifted speech
signal spectrum
• Frequency Modulation is Shifting of the Human Audible Range to the
Cellular Range for transmission
• Benefits of modulation
• shifts the spectral content of a message signal to a high frequency that is
transmittable over the radio link
• Provides a mechanism for putting information into a form that may be less
vulnerable to noise or interference
FDMA (Frequency Division Multiple Access)
• Several frequencies are allocated to the base station for use within its
cell (Mobile service area) by the system designer
• The base station would then allocates these frequencies to its client
mobile station mobile stations as they come by instructing them to auto
tune to particular frequencies for receiver and transmit
FDMA ‐ Frequency Division Multiple Access
• In FDMA systems, the base station allocates the
band (or frequency) to different terminals, based on
their needs.
• This band is divided into smaller bands, and this pool
of bands is available for all the terminals to share.
• Depending on the need, the base station allocates a
frequency (the center frequency of a band) for the
terminal to transmit.
• This is known as frequency division multiple access
(FDMA) because all MS can access and use any of
the frequencies allocated in the pool
• The system consists of a number of mobile stations.
• a base station has a pool of frequencies (here, f1 to
f4).
• All four frequencies are available for sharing by the
mobile terminals located around the base station.
• If mobile terminal A is allocated frequency f1, and
mobile terminal B will be allocated frequency f2.
Frequency division duplex, FDD
• Duplexing is a way in which radio communications are
maintained in both directions
• Frequency division duplex, FDD, uses the idea that the
transmission and reception of signals are achieved
simultaneously using two different frequencies.
• Using FDD it is possible to transmit and receive signals
simultaneously as the receiver is not tuned to the same
frequency as the transmitter as shown.
• For the FDD scheme to operate satisfactorily, it is necessary
that the frequency, i.e. channel separation between the
transmission and reception frequencies be sufficient to
enable the receiver not to be unduly affected by the
transmitter signal.
• This is known as the guard band
Shared resources through FDD and TDD
• FDD uses two frequencies for
duplexing
• These frequencies are allocated
through FDMA on a need basis
• The common signalling channel
is shared on a time basis
• The base station gives mobile
stations time to transmit their
signals one after the other
(TDD) using the same common
signalling frequency also
referred to as a channel or
signalling channel specifically
Use of FDD
• Receiver blocking is an important issue with FDD schemes, and often highly
selective filters may be required.
• For cellular systems using FDD, filters are required within the base station
and also the handset to ensure sufficient isolation of the transmitter signal
without desensitising the receiver.
• While cost is not such a significant driver for the base stations, placing a filter
into the handsets is more of an issue.
• The use of an FDD system does enable true simultaneous transmission and
reception of signals.
• However two channels are required and this may not always use the
available spectrum efficiently
Use of filters in FDD
• Filter
Characteristics of 1G systems
• Use Cellular Concept to provide service to a geographic area by
providing a number of small adjacent cells to provide better coverage
and mobility through:
• Frequency Reuse
• Handoff/Handover
• FDMA/FDD systems – Common Air Interface (CAI) standards only
• Analog Voice communications using FM
• Digital Control channels for signaling
• Adjustable Mobile Power levels
• Macro Cells : 1‐40 km radius
Cellular Radio Network
• Mobile Radio Network was
designed for wide areas
coverage through on
cellular structure
• Frequency reuse was
employed to resolve
limited frequency
availability
• Mobility between cell
areas made possible
through intelligent
handover mechanisms
The Cellular structure
• The cells are theoretically taken to
be hexagonal in shape
• the cell reuse is calculated to
avoid interference
• Reuse factor is
The Frequency reuse
• The cells are theoretically taken to
be hexagonal in shape
• distance is given by 𝐷 𝑅 3𝑁
• where R is cell radius
• N is the reuse pattern (the
cluster size or the number of
cells per cluster)
• In This case there are 7 cells per
cluster
• Reuse factor is
3𝑁
The AMPS Frequency allocation
• The FCC allocated a 40‐MHz
bandwidth in the 800‐ to 900‐MHz
frequency range for high‐capacity
mobile radiotelephone service in
AMPS
• 40‐MHz bandwidth was separated
into 2 x 20‐MHz bands
Mobile Station Transmit
• 825 to 845 = 20‐MHz band was for mobile‐radio transmit in the 825‐ to 845‐
MHz range(up link)
Base Station Transmit
• 870 to 890 = 20MHz was used for the base‐station transmit band in the 870‐
to 890‐MHz range (downlink )
Service provider allocations
• The channels were further subdivided into 666 two way channels, each
channel consisting of two frequencies having channel bandwidths of 30 kHz
each.
• To enable frequency separation between channels within a given area, the
666 channels are arranged for two operators in the form of a distribution
matrix, as illustrated below.
• The Block A and Block B operators have 333 channels each
• Among 333 channels, 21 channels are the setup channels
Mobile transmit
• Among 333 channels, 21 channels are the setup channels
• The matrix can be considered to be 21 sets of channels (columns)
• To simplify distribution, the 21 sets are arranged into 3 groups of 7
and assigned suffix letters A, B, and C, respectively
Basic cell block
• R = radius of each cell;
• D = distance between two
adjacent frequency‐reuse
cells;
• K = number of cells in a
basic cell block.
• K = 7 in this illustration,
and D/R = 4.6.
Channel assignment
• The distribution of channels and channel frequencies obtained by this
arrangement ensures that assignments within one geographic cell area will not
interfere with channels assigned in adjacent cell locations.
• Cells that are separated by a minimum distance determined by propagation
variables can simultaneously use the same channels with no risk of interference.
• The mobile operator serving a particular population center could provide mobile‐
radio coverage to large numbers of users based on cellular reuse of assigned
channel frequencies.
• Each block of seven cells is repeated in such a manner that corresponding
numbered cells in adjacent seven‐cell blocks are located at a predetermined
distance from the nearest cell having the same number.
• Correspondingly, the 20‐MHz‐bandwidth radio spectrum is divided into seven
disjoint sets, with a different set allocated to each one of the seven cells in the
basic block.
• With a total of 333 channels in 21 sets available, it is possible to assign as many as
three sets to each of the seven cells constituting the basic block pattern.
Cluster size
• The cluster size or the number of
cells per cluster is given by
• N = i2 + ij + j2 where i and j are
integers.
• N = 1, 3, 4, 7, 9, 12, 13, 16, 19,
21, 28, …, etc.
The popular value of N being 4
and 7
Cell Splitting
Cell Sectoring by Antenna Design
Base station directional Antennas
Cell Sectoring by Antenna Design
Cells in AMPS
• In actual practice, the cell boundaries are defined by the minimum required
signal strength at distances determined by the reception threshold limits.
• In the AMPS, base stations are referred to as cell sites because they perform
supervision and control in addition to the transmitting and receiving
functions normally associated with the conventional base station.
• Mobile‐telephone subscribers within a given cell are assigned to a particular
cell site serving that cell simply by the assignment of an idle channel
frequency under the control of the mobile telephone switching office
(MTSO).
• When a mobile unit crosses a cell boundary, as determined by the signal
reception threshold limits, a new idle channel frequency is assigned by the
new serving cell site.
• This automatic switching control function is referred to as a “handoff.”
Cellular Handover (EU) / Handoff (USA)
• The basic requirement of the Mobile system is that
• as the mobile handset moves out of one cell to the next,
• it must be possible to hand the call over from the base station of the first cell,
to that of the next with no discernable disruption to the call
• Failure of proper reliable handover can result in dropped calls, and
this is one of the key factors that can lead to customer dissatisfaction
and exit from the network provider
• Thus handover or handoff is one of the key performance indicators
monitored as an indication of a robust cellular handover / handoff
regime
Hand over
• The cellular network needs to decide when handover or handoff is necessary, and to
which cell.
• Also when the handover occurs it is necessary to re‐route the call to the relevant base
station along with changing the communication between the mobile and the base
station to a new channel.
Handoff/ Handover Region
Parameters that need to be known to determine handover are:
• The signal strength of the base station with which communication is being made,
• The signal strengths of the surrounding stations.
• The availability of channels in the next base station to takeover the call
Hand over process
• The mobile continually monitors the signal strengths of the base stations it can
hear, including the one it is currently using, and it feeds this information back to
the base station
• When the strength of the signal from the base station that the mobile is using
starts to fall to a level where action needs to be taken the cellular network looks at
the reported strength of the signals from other cells reported by the mobile.
• It then checks for channel availability, and if one is available it informs this new cell
to reserve a channel for the incoming mobile.
• When ready, the current base station passes the information for the new channel
to the mobile, which then makes the change.
• Once done the mobile sends a message on the new channel to inform the network
it has moved over to the new channel.
• If this message is successfully sent and received then the network shuts down
communication with the mobile on the old channel, freeing it up for other users
• The concerned mobile now communicates through the new channel to the new
base station
Hard Handover
• Hard Handoff (break
before make)
• Releasing current
resources from the
prior BS before
acquiring resources
from the next BS.
• Used in FDMA and
TDMA
1G systems used Hard handover / handoff
• The existing connection must be broken before the new one is established
• Also called break before make
• One example of hard handover is when frequencies are changed.
• As the mobile will normally only be able to transmit on one frequency at a
time, the connection must be broken before it can move to the new channel
where the connection is re‐established.
• This is often termed and inter‐frequency hard handover.
• Although there is generally a short break in transmission, this is normally
short enough not to be noticed by the user.
• It is also possible to have intra‐frequency hard handovers where the
frequency channel remains the same in 3G systems that will be looked into
later
Types of handover / handoff
Hard handover
• The existing connection must be broken before the new one is established
• Also called break before make
• One example of hard handover is when frequencies are changed.
• As the mobile will normally only be able to transmit on one frequency at a
time, the connection must be broken before it can move to the new channel
where the connection is re‐established.
• This is often termed and inter‐frequency hard handover.
• It is also possible to have intra‐frequency hard handovers where the
frequency channel remains the same.
• Although there is generally a short break in transmission, this is normally
short enough not to be noticed by the user.
Cellular Radio Network Architecture
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MS – Mobile Station
BTS – Base Transceiver Station
BSC – Base Station Controller
MSC – Mobile Switching Centre
VLR‐ Visitor Location Register
HLR – Home Location Register
Authentication Centre
Equipment Identity Register
PSTN –Public Switched Telephone Network
ISDN – Integrated Systems Digital Network
Network elements
VLR/HLR
• VLR contains information about all visiting MSs in that particular area of MSC.
• VLR has pointers to the HLR’s of visiting MS
• VLR helps in billing and access permission to the visiting MS.
Moving from HLR to VLR areas
Registration
• Wireless system needs to know whether MS is currently located
in its home area or some other area (routing of incoming calls).
• This is done by periodically exchanging signals between BS and
MS known as Beacons.
• BS periodically broadcasts beacon signal (1 signal per second)
to determine and test the MSs around.
• Each MS listens to the beacon, if it has not heard it previously
then it adds it to the active beacon kernel table.
• This information is used by the MS to locate the nearest BS.
• Information carried by beacon signal:
• Cellular network identifier
• Timestamp
• Gateway address ID of the paging area, etc.
Beaconing
• Beacons are signals sent between the MS and the BS through the
signalling channel
Registration process
• After receiving the BS beaconing
• The MS responds on the reserved paging or control channel to have access to the network
• the mobile registers with the cellular network for its existence to be known
and calls directed
• After completion of registration the BS sends the "attach" message.
• Thus at registration the HLR/VLR registers are updated
• Even when the mobile is in what is termed its idle mode it will periodically
communicate with the network to update its position and status.
• When the mobile is switched off it sends a “detach” message.
• This informs the network that it is switching off, and enables the network to
update the last known position for the mobile.
From Home MSC to Visiting MSC
• The HLR is updated in the Home MSC
• On hand over the next MSC takes over
• Thus the VLR is updated in the MSC
Base transceiver station, BTS
• The BTS provides the direct communication with the mobile phones
• The base transceiver station or system, BTS consists of a number of different
elements.
• The first is the part is the antenna and the feeder to connect the antenna to
the BTS Unit.
• The second the part is BTS Unit consisting of the electronics section normally
located in a container at the base of the antenna tower.
• This contains the electronics for communicating with the mobile handsets and
includes radio frequency amplifiers, radio transceivers, radio frequency
combiners, control and power supplies with back up
• Furthermore the BTS unit contains the interface between the BTS and its BSC,
as well as the communication links to the BSC.
• The interface consists of control logic and software as well as the cable link to
the controller.
Base station controller BSC
• This unit acts as a small centre to route calls to the required BTS, and it
also makes some decisions about which of the base station is best suited
to a particular call.
• The links between the BTS and the BSC may use either land lines or even
microwave links.
• Often the BTS antenna towers also support a small microwave dish
antenna used for the link to the BSC.
• The BSC is often co‐located with a BTS.
• The BSC interfaces with the mobile switching centre.
• The MSC makes more widespread choices about the routing of calls and
interfaces to the land line based PSTN as well as the HLR and VLR
Mobile Switching Center (MSC)
• The MSC is the control centre for the cellular system, coordinating the
actions of the BSCs, providing overall control, and acting as the switch and
connection into the public telephone network.
• As such it has a variety of communication links into it which will include
fibre optic links as well as some microwave links and some copper wire
cables.
• These enable it to communicate with the BSCs, routing calls to them and
controlling them as required.
• It also contains the Home and Visitor Location Registers, the databases
detailing the last known locations of the mobiles.
• It also contains the facilities for the Authentication Centre, allowing
mobiles onto the network.
• In addition to this it will also contain the facilities to generate the billing
information for the individual accounts
Call setup initialisation from MS to BS
• Communication 1 to 3 occurs on the reserved signalling channel
Call setup initialisation from BS to MS
• Communication 1 to 4 occurs on the reserved signalling channel
• Process 5 occurs on the allocated channel
Roaming
• To move from a cell controlled by one MSC area to a cell connected to another
MSC.
• Beacon signals and the use of HLR‐VLR allow the MS to roam anywhere provided
the roaming service is allowed (service provider and corresponding frequency
band)
• The two registers are required, one for mobiles for which the network is the home
to i.e. the one with whom the contract exists, and the other for visitors.
• If there was only one register then every time the mobile sent any message to the
foreign network, this would need to be relayed back to the home network and this
would require international signalling (slow process).
• The approach which is adopted is to send a message back to the HLR when the
mobile first enters the new country saying that the mobile is in a different network
and that any calls for that mobile should be forwarded to the foreign visited
network.
• By undergoing a registration procedure when the mobile is turned on, the cellular
network is able to communicate correctly with it, provide access for outgoing calls,
and also route any incoming calls to it in the most efficient manner.
• Registration also only allows those mobiles that have permission to access the
network to communicate with it.