© 2008 The McGraw-Hill Companies 1 The Cellular Concept Modified by Sunantha Sodsee

© 2008 The McGraw-Hill Companies

1

The Cellular ConceptThe Cellular Concept

Modified by Sunantha Sodsee


2

Intro: Radio ChannelIntro: Radio Channel

Radio waves can take many different paths to get from transmitter to receiver.

Transmitter

Receiver


3

Intro: Radio ChannelIntro: Radio Channel

Tthe radio waves interact with the physical environment along each of these paths.

There are typically (unless you are in free-space) many paths from the transmitter to the receiver.

Each path is called a multipath.


4

Intro: MultipathsIntro: Multipaths

The lengths of multipaths are different.

Sine waves along one path reach the receiver at different times than the same signal along a different path.

Multipath 2

Multipath 1TransmitterReceiver


5

Worst Case Example of 2 Worst Case Example of 2 MultipathsMultipaths

Received radio wave along multipath 1

Received radio wave along multipath 2

The antenna combines (sums) these two multipaths.In the example above, the output of the antenna will be:

+ =

No Signal !!


6

Another aspect of multipathsAnother aspect of multipaths

Whenever a radio wave bounces off or passes through a physical obstruction, the amplitude of the sine wave changes.

Also amplitude of sine wave shrinks the further the radio wave travels, regardless of whether there are obstructions or not.

Originally transmittedradio wave

Reflection

A

-A

Received radio wave, < 1

A

A


7

FadingFading

Fading, which is both signal attenuation and distortion, is a major challenge in wireless communications.

We have all experienced it, e.g., fading of radio station in a car radio.

Fading varies in frequency: assuming physical conditions are fixed, if a signal

transmitted at one frequency fades, it may not if transmitted at a different frequency.


8

Fading (Cont’d)Fading (Cont’d)

This type of distortion occurs anytime there is long

time spread in the multipaths.

1 0 0

TransmittedSignal

Signal along Multipath 1

Signal along Multipath 2

1 1 0

Overall ReceivedSignal


9

Fading ChallengesFading Challenges

Time spread of multipaths = delay due to echoes.

These “echoes” are particularly problematic in urban areas, due to reflections from buildings.

Also problematic in hilly, mountainous areas.

Designers of radio systems have spent a lot of time and effort trying to overcome this fading challenge.


10

Problem with DistortionProblem with Distortion

Distortion not only impacts the strength of the received signal but also changes the “shape” of the received signal.

In this digital communications, this can be especially detrimental because bits can be inverted at the receiver due to multipath.


11

One Solution: Multiple One Solution: Multiple AntennasAntennas

One way to overcome fading problem is to design receivers with multiple, say 2, antennas.

Both antennas can receive the desired radio wave.

TransmitterReceiver


12

Multiple Antennas (Cont’d)Multiple Antennas (Cont’d)

If receive antennas are adequately separated, then paths followed by radio waves to the first antenna are different from paths followed by radio waves to the second antenna.

Result: fading of received signal on first antenna is different from fading of received signal on second antenna. Chances are if one antenna experiences a deep fade, the other does not.

Receiver can adaptively choose the “stronger” antenna to determine the received radio wave at any given time.


13

Multiple Antennas on Cell Multiple Antennas on Cell TowersTowers

Cellular base station tower (antenna tower that cell phones “talk” to) use multiple antenna to improve the quality of voice signal received from cell phone users.


14

Another Solution: Adaptive Another Solution: Adaptive Antenna ArrayAntenna Array

Of all the multipath, if there is a line-of-sight (LOS) path between the transmitter and receiver, it is the strongest.

If we can design antennas receive patter, we would like to make it in the direction of the strongest multipath.

This is what adaptive antenna array do.


15

Adaptive Antenna Array Adaptive Antenna Array (Cont’d)(Cont’d)

Adaptive antenna array is a group of receive antennas that work together to form a desirable radiation pattern.

For example, if the LOS path is in a particular direction, the antennas work together to pick out as many radio waves in that direction as possible.

In doing so, the antennas ignore radio waves in non-significant directions. These waves do not contribute to the overall fading.


16

Cellular Tower ExampleCellular Tower Example

Assume that a cellular base station tower is receiving signal from four cell phone users. The tower can use its antennas to form the following radiation power (bird’s eye view).

CellularBase Station

User 1

User 2

User 3

User 4

No signalsreceived from this direction




17

Other Fading Other Fading CountermeasuresCountermeasures

Both previous schemes are antenna techniques to counter fading.

Designers have developed other methods, e.g., coding.

One simple coding method: repetition. Repetition: transmit the same signal a few times.

Chances are one of the copies will be received without deep fades.


18

How Cell Phones WorkHow Cell Phones Work


19

An Important TechnologyAn Important Technology

Cellular telephony is one of the fastest growing technologies on the planet.

Presently, we are starting to see the third generation of the cellular phones coming to the market.

New phones allow users to do much more than hold phone conversations.


20

Beyond VoiceBeyond Voice

Store contact information Make task/to-do lists Keep track of appointments Calculator Send/receive email Send/receive pictures Send/receive video clips Get information from the internet Play games Integrate with other devices (PDA’s, MP3 Players, etc.)


21

Basic ConceptBasic Concept

Cellular system developed to provide mobile telephony: telephone access “anytime, anywhere.”

First mobile telephone system was developed and inaugurated in the U.S. in 1945 in St. Louis, MO.

This was a simplified version of the system used today.


22

System ArchitectureSystem Architecture

A base station provides coverage (communication capabilities) to users on mobile phones within its coverage area.

Users outside the coverage area receive/transmit signals with too low amplitude for reliable communications.

Users within the coverage area transmit and receive signals from the base station.

The base station itself is connected to the wired telephone network.


23

First Mobile Telephone SystemFirst Mobile Telephone System

One and only onehigh power base station with which allusers communicate.

Entire Coverage Area

NormalTelephone

System

Wired connection


24

Problem with Original DesignProblem with Original Design

Original mobile telephone system could only support a handful of users at a time…over an entire city!

With only one high power base station, users phones also needed to be able to transmit at high powers (to reliably transmit signals to the distant base station).

Car phones were therefore much more feasible than handheld phones, e.g., police car phones.


25

The Core Idea: Cellular The Core Idea: Cellular ConceptConcept

The core idea that led to today’s system was the cellular concept.

The cellular concept: multiple lower-power base stations that service mobile users within their coverage area and handoff users to neighboring base stations as users move. Together base stations tessellate the system coverage area.


26

Cellular ConceptCellular Concept

Thus, instead of one base station covering an entire city, the city was broken up into cells, or smaller coverage areas.

Each of these smaller coverage areas had its own lower-power base station.

User phones in one cell communicate with the base station in that cell.


27

3 Core Principles3 Core Principles

Small cells tessellate overall coverage area.

Users handoff as they move from one cell to another.

Frequency reuse.


28

TessellationTessellation

Some group of small regions tessellate a large region if they over the large region without any gaps or overlaps.

There are only three regular polygons that tessellate any given region.


29

Tessellation (Cont’d)Tessellation (Cont’d)

Three regular polygons that always tessellate: Equilateral triangle Square Regular Hexagon

TrianglesSquares

Hexagons


30

Circular Coverage AreasCircular Coverage Areas

Original cellular system was developed assuming base station antennas are omnidirectional, i.e., they transmit in all directions equally.

Users located outsidesome distance to thebase station receive weak signals.

Result: base station hascircular coveragearea.

Weak signal

Strong si

gnal


31

Circles Don’t TessellateCircles Don’t Tessellate

Thus, ideally base stations have identical, circular coverage areas.

Problem: Circles do not tessellate.

The most circular of the regular polygons that tessellate is the hexagon.

Thus, early researchers started using hexagons to represent the coverage area of a base station, i.e., a cell.


32

Thus the Name CellularThus the Name Cellular

With hexagonal coverage area, a cellular network is drawn as:

Since the network resembles cells from a honeycomb, the name cellular was used to describe the resulting mobile telephone network.

BaseStation


33

HandoffsHandoffs

A crucial component of the cellular concept is the notion of handoffs. Mobile phone users are by definition mobile, i.e., they

move around while using the phone.

Thus, the network should be able to give them continuous access as they move.

This is not a problem when users move within the same cell.

When they move from one cell to another, a handoff is needed.


34

A HandoffA Handoff

A user is transmitting and receiving signals from a given base station, say B1.

Assume the user moves from the coverage area of one base station into the coverage area of a second base station, B2.

B1 notices that the signal from this user is degrading.

B2 notices that the signal from this user is improving.


35

A Handoff (Cont’d)A Handoff (Cont’d)

At some point, the user’s signal is weak enough at B1 and strong enough at B2 for a handoff to occur.

Specifically, messages are exchanged between the user, B1, and B2 so that communication to/from the user is transferred from B1 to B2.

B1B2


36

Frequency ReuseFrequency Reuse

Extensive frequency reuse allows for many users to be supported at the same time.

Total spectrum allocated to the service provider is broken up into smaller bands.

A cell is assigned one of these bands. This means all communications (transmissions to and

from users) in this cell occur over these frequencies only.


37

Frequency Reuse (Cont’d)Frequency Reuse (Cont’d)

Neighboring cells are assigned a different frequency band.

This ensures that nearby transmissions do not interfere with each other.

The same frequency band is reused in another cell that is far away. This large distance limits the interference caused by this

co-frequency cell.


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Example of Frequency ReuseExample of Frequency Reuse

Cells using the same frequencies


39

Multiple Access in Cellular Multiple Access in Cellular NetworksNetworks


40

Multiple Transmitters, One Multiple Transmitters, One ReceiverReceiver

In many wireless systems, multiple transmitters attempt to communicate with the same receiver.

For example, in cellular systems. Cell phones users in a local area typically communicate

with the same cell tower.

How is the limited spectrum shared between these local transmitters?


41

Multiple Access MethodMultiple Access Method

In such cases, system adopts a multiple access policy.

Three widely-used policies:

Frequency Division Multiple Access (FDMA)

Time Division Multiple Access (TDMA)

Code Division Multiple Access (CDMA)


42

FDMAFDMA

In FDMA, we assume that a base station can receive radio signals in a given band of spectrum, i.e., a range of continuous frequency values.

The band of frequency is broken up into smaller bands, i.e., subbands.

Each transmitter (user) transmits to the base station using radio waves in its own subband.

FrequencySubbands

Cell Phone User 1Cell Phone User 2::

Cell Phone User N

Time


43

FDMA (Cont’d)FDMA (Cont’d)

A subband is also a range of continuous frequencies, e.g., 824 MHz to 824.1 MHz. The width of this subband is 0.1 MHz = 100 KHz.

When a users is assigned a subband, it transmits to the base station using a sine wave with the center frequency in that band, e.g., 824.05 MHz.


44

FDMA (Cont’d)FDMA (Cont’d)

When the base station is tuned to the frequency of a desired user, it receives no portion of the signal transmitted by another in-cell user (using a different frequency).

This way, the multiple local transmitters within a cell do not interfere with each other.


45

TDMATDMA

In pure TDMA, base station does not split up its allotted frequency band into smaller frequency subbands.

Rather it communicates with the users one-at-a-time, i.e., “round robin” access.

…FrequencyBands

Time

Use

r 1

Use

r 2

Use

r 3

Use

r N


46

TDMA (Cont’d)TDMA (Cont’d)

Time is broken up into time slots, i.e., small, equal-length intervals.

Assume there are some n users in the cell.

Base station groups n consecutive slots into a frame.

Each user is assigned one slot per frame. This slot assignment stays fixed as long as the user

communicates with the base station (e.g., length of the phone conversation).


47

TDMA (Cont’d)TDMA (Cont’d)

Example of TDMA time slots for n = 10.

In each time slot, the assigned user transmits a radio wave using a sine wave at the center frequency of the frequency band assigned to the base station.

TimeSlot

User1

User2

User10

User1

User10

User1

… …Frame

…


48

Hybrid FDMA/TDMAHybrid FDMA/TDMA

The TDMA used by real cellular systems (like AT&T’s) is actually a combination of FDMA/TDMA.

Base station breaks up its total frequency band into smaller subbands.

Base station also divides time into slots and frames.

Each user is now assigned a frequency and a time slot in the frame.


49

Hybrid FDMA/TDMA (Cont’d)Hybrid FDMA/TDMA (Cont’d)

Time

…

…

Use

r 1

Use

r 2

Use

r 10

Use

r 11

Use

r 12

Use

r 20

…

…

Use

r 31

Use

r 32

Use

r 40

Use

r 21

Use

r 22

Use

r 30

Assume a base station divides its frequency band into 4 subbands and time into 10 slots per frame.

…

…U

ser

1

Use

r 2

Use

r 10

Use

r 11

Use

r 12

Use

r 20

…

…

Use

r 31

Use

r 32

Use

r 40

Use

r 21

Use

r 22

Use

r 30

…

…

…

…

Frame

Frequency Subband 1

Frequency Subband 2

Frequency Subband 3

Frequency Subband 4


50

CDMACDMA

CDMA is a more complicated scheme. Here all users communicate to the receiver at the same

time and using the same set of frequencies. This means they may interfere with each other.

The system is designed to control this interference.

A desired user’s signal is deciphered using a unique code assigned to the user.

There are two types of CDMA methods.


51

CDMA Method 1: Frequency CDMA Method 1: Frequency HoppingHopping

First CDMA technique is called frequency hopping.

In this method each user is assigned a frequency hopping pattern, i.e., a fixed sequence of frequency values.

Time is divided into slots.

In the first time slot, a given user transmit to the base station using the first frequency in its frequency hopping sequence.


52

Frequency Hopping (Cont’d)Frequency Hopping (Cont’d)

In the next time interval, it transmits using the second frequency value in its frequency hop sequence, and so on.

This way, the transmit frequency keeps changing in time.


53

Second Type of CDMA: Direct Second Type of CDMA: Direct SequenceSequence

This is a more complicated version of CDMA. Basically, each in-cell user transmits its message to the

base station using the same frequency, at the same time. Here signals from different users interfere with each

other. But the user distinguishes its message by using a

special, unique code. This code serves as a special language that only the

transmitter and receiver understand. Others cannot decipher this language.


54

Final Points on Final Points on FDMA/TDMA/CDMAFDMA/TDMA/CDMA

When users are in the middle of a phone call, the system uses FDMA/TDMA/CDMA to give them access.

But there are only so many frequencies, time-slots, or codes available to share between users in a cell.

If we divide the frequency into too many bands, or use too many time slots, or too many codes, the quality of speech heard by the end user will be

unsatisfactory.


55

ChannelsChannels

Channel is a general term which refers to a frequency in an FDMA system, a timeslot/frequency combination in TDMA, or a code in CDMA.

This way, a base station has a fixed number of channels and can support only that many simultaneous users.


56

Random Access: Another Random Access: Another Important Multiple Access Important Multiple Access

MethodMethod


57

Motivating Random Access Motivating Random Access ChannelsChannels

As mentioned earlier, FDMA/TDMA/CDMA are used when users are engaged in a phone call.

Before being assigned a frequency, timeslot, or code (i.e., a channel), a user has to ask the base station if it has a channel leftover to assign this user.

In other words, the user has to have some other way of communicating with the base station.


58

Motivating Random AccessMotivating Random Access

Of all the frequencies available at a base station, a prescribed portion of them are set aside for this purpose.

These frequencies are called control channels, as opposed to the rest of the frequencies in cell, which are called voice channels.

A user will transmit a signal to the base station on a control channel basically saying, “I’m here and I’d like to talk to you.”


59

Random Access: FailureRandom Access: Failure

There maybe other users who do this at the same time using the same frequency.

If they do, the signals will interfere with each other and the base station will not receive anything.

This indicates a failure (aka collision), when this happens, each user will backoff for some random amount of time and try again. Since they backoff for a random amount of time, chances

are they won’t retry at the same time.


60

Random Access: SuccessRandom Access: Success

If only one user transmits, then the base station will receive the user’s signals and respond to it by saying, “Okay you can talk to me, tune into this other channel and tell me what you want.”

The user will then tune this channel and be able to exclusively transmit and receive signals to the base station.


61

Random Access: Success Random Access: Success (Cont’d)(Cont’d)

This new channel assigned to the user is also a control channel.

Using this channel the user can then send a signal that says for example “I want to make a phone to this phone number.”

To which the base station will respond by assigning the user a voice channel, if there are some available.


62

Random Access SummaryRandom Access Summary

This type of competing access method is called random access.

There are different rules followed by users participating in random access.


63

Standards: Rules for a Standards: Rules for a Cellular NetworkCellular Network


64

Second Generation of CellularSecond Generation of Cellular

The second generation (2G) of cellular networks were deployed in the early 90’s.

2G cellular phones used digital technology and provided enhanced services (e.g., messaging, caller-id, etc.).

In the U.S., there were two 2G standards that service providers could choose between.


65

Second Generation (Cont’d)Second Generation (Cont’d)

The two standards used in U.S. are different from the 2G system used in Europe (called GSM) and the system used in Japan.

First U.S. standard is called Interim Standard 136 (IS-136) and is based on TDMA (time-division multiple access).

Second is called IS-95 and is based on CDMA (code-division multiple access).

Most present systems are what is called the 2.5 generation (2.5G) of cellular.


66


2.5G Cell Phone Systems The designation 2.5G refers to a generation of cell

phones between the original second-generation (2G) digital phones and newer third-generation (3G) phones.

2.5G phones bring data transmission capability to 2G phones in addition to normal voice service.

A 2.5G phone permits subscribers to exchange emails and access the Internet by cell phone. The three technologies used in 2.5G systems are

GPRS, EDGE, and CDMA2000.


67


2.5G Cell Phone Systems One popular 2.5G technology is the general packet

radio service (GPRS). This system is designed to work with GSM phones. It uses one or more of the eight time slots in a GSM

phone system to transmit data rather than digitized voice.

A faster 2.5G technology is enhanced data rate for GSM evolution (EDGE).

It uses 8-PSK modulation instead of GMSK to achieve data rates up to 384 kbps.


68

Present Cellular SystemsPresent Cellular Systems

They offer enhanced services over second generation systems (emailing, web-browsing, etc.).

Some 2.5G systems (such as AT&T’s) are compatible with the European system, Global System Mobile (GSM).

Presently, service providers are setting up third generation (3G) cellular systems.


69

Present Systems (Cont’d)Present Systems (Cont’d)

3G offers higher data rates than 2.5G. This allows users to send/receive pictures, video clips, etc.

This service is starting to become more and more available in the U.S.

There are two standards for 3G, Wideband CDMA (WCDMA) and cdma2000. These two standards have been adopted world-wide.

Both are based on CDMA principles.


70


3G Cell Phone Systems Third-generation (3G) cell phones are true packet data

phones. 3G phones feature enhanced digital voice and high-

speed data transmission capability. 3G applications include fast e-mail and Internet access. 3G phones are being packaged with personal digital

assistants (PDAs). High speed also permits the transmission of video.


71


Advanced Cell Phones Handset manufacturers

have built in a wide range of features that make the cell phone the most desired consumer electronic product ever developed.

Consider what electronic circuits and systems are needed to make these features work:

Color LCD Screens. Digital Cameras. E-mail. Games. GPS. Internet Access. MP3 Players. Push-to-Talk. FM Radio. Wireless Headsets. Video. Location-Based Technology.


72


Advanced Cell Phones: Location-Based Technology Several different systems have been adopted by the

various carriers. Most CDMA phones contain a GPS receiver that

transmits its coordinates digitally to the carrier, from which they can be forwarded to emergency services.

GSM, GPRS, and EDGE phones use a system called Uplink—Time Difference of Arrival (U-TDOA), a method of triangulation based on cell phone signals being received at three different cell sites.


73

AMPS: A Model for AMPS: A Model for Learning about Cellular Learning about Cellular

NetworksNetworks


74

Complete Cellular NetworkComplete Cellular Network

A group of local base stations are connected (by wires) to a mobile switching center (MSC). MSC is connected to the rest of the world (normal telephone system).

MSCMSC

MSC

MSC

Public (Wired)TelephoneNetwork


75

Mobile Switching CentersMobile Switching Centers

Mobile switching centers control and coordinate the cellular network.

They serve as intermediary between base stations that may be handing off users between each other.

Base stations communicate with each via the MSC. MSC keep track of cell phone user subscription. MSC connects to the wired phone network (rest of the

world).


76

The AMPS SystemThe AMPS System

AMPS uses FDMA: a service provider is given license to 832 frequencies to use across a geographic region, say a city.

Service provider chops up the city into cells.

Each cell is roughly 10 square miles.

Each cell has a base station that consists of a tower and a small building containing radio equipment.


77

The AMPS System (Cont’d)The AMPS System (Cont’d)

AMPS uses frequency duplexing, i.e., each cell phone uses one frequency to transmit on and another frequency to receive on.

Total 832 channels are divided into half.

One half is used on the uplink, i.e., used by cell phones to transmit to the base station.

The other half is used on the downlink, i.e., used by the base to transmit to cell phone users.


78

Voice and Control ChannelsVoice and Control Channels

Of the 832/2 = 416 channels, 21 of them used as control channels.

This means that there are 416-12=395 voice channels.

Now, these voice channels are divided up among the cells based on the frequency reuse.


79

AMPS: Voice ChannelsAMPS: Voice Channels

VoiceChannels

ControlChannels

ControlChannels


80

Frequency Reuse in AMPSFrequency Reuse in AMPS

In frequency reuse, a group of local cells use different frequencies to transmit/receive signals in their cell.

This group of local cells is referred to as a cluster.


81

Clustersize of 7Clustersize of 7

Assume a clustersize of 7. This means that the total 395 voice channels are divided

into groups of seven.

Thus, each cell has about 56 voice channels. This is the most number of users that can be supported in

a cell, i.e., roughly 10 square miles in normal environments.

This may/may not be sufficient based on the distribution of users.


82

Clustersize of 7 (Cont’d)Clustersize of 7 (Cont’d)

To see what a system with clustersize of 7 looks like, color a cell with color 1.

This cell (if drawn as a hexagon) has 6 neighbors. Color each of the seven neighbors using a different color (also different from each other).

Now repeat this rule to get the overall “reuse pattern.”


83

Clustersize of 7, Reuse PatternClustersize of 7, Reuse Pattern


84

Consider a cellular system which has a total of S duplex channels.Each cell is allocated a group of k channels, .The S channels are divided among N cells.The total number of available radio channels

The N cells which use the complete set of channels is called cluster.The cluster can be repeated M times within the system. The total number

of channels, C, is used as a measure of capacity

The capacity is directly proportional to the number of replication M.The cluster size, N, is typically equal to 4, 7, or 12.Small N is desirable to maximize capacity.The frequency reuse factor is given by

Sk

kNS

MSMkNC

N/1

How to calculate related How to calculate related factorsfactors


85

• Hexagonal geometry has – exactly six equidistance neighbors– the lines joining the centers of any cell

and each of its neighbors are separated by multiples of 60 degrees.

• Only certain cluster sizes and cell layout are possible.

• The number of cells per cluster, N, can only have values which satisfy

• Co-channel neighbors of a particular cell, ex, i=3 and j=2.

22 jijiN

How to calculate related How to calculate related factorsfactors


86

What if we had a smaller What if we had a smaller cluster?cluster?

Now consider a system with a cluster of 4.

Then the number of voice channels per cell is 395/4, which is roughly 98.

Thus, in theory, we can hold more users per cell if this were true.

But there is a problem with a clustersize.


87

Problem with Smaller Problem with Smaller ClustersizeClustersize

Interfering cells are closer by when clustersize is smaller.


88

Problem with Smaller Problem with Smaller Clustersize (Cont’d)Clustersize (Cont’d)

If interfering cells are closer, then the total interference power will be larger.

With higher interference power, the quality of the speech signal will deteriorate.

To reduce the interference power, we can make the cells larger.

With larger cell, the number of users covered per unit area reduces. So, the gain (total number of users supported) of a smaller

clustersize is not as high as we think.


89

Directional AntennaDirectional Antenna

One way to get more capacity (number of users) while maintaining cell size is to use directional antenna.

Assume antenna which radiates not in alldirections (360 degrees) but rather in 120 degrees only.


90

Directional Antenna at Base Directional Antenna at Base StationStation

With 120 degree antenna, we draw the cells as:


91

Directional Antenna (Cont’d)Directional Antenna (Cont’d)

Because these directional antenna only receive signals in particular direction, the amount of interference power they receive assuming a clustersize of 7 is reduced by 1/3.

With less interference power, the speech quality is much better than it needs to be.

So we can reduce the clustersize (increase interference power) and still have good speech quality.


92

Directional AntennaDirectional Antenna

Trials show that in systems with 120 degree antenna, the clustersize can be as small as 3.

This allows more users to be supported, while keeping cell size fixed.

Because of the benefits offered by 120 degree antenna, these are most readily used by base station towers.


93

120 Degree Antenna Towers120 Degree Antenna Towers


94

What happens when you What happens when you receive a call?receive a call?

When you first power the phone, it listens for an SID on the control channel.

Recall: control channel is special frequency that the phone and base station use to talk to each other about things like call setup and channel changing, etc.

If phone cannot find any control channels, then it is out of range and it lights up the “No Service” light.


95

Call ReceptionCall Reception

When it receives in SID, the phone compares it to the SID programmed in the phone.

If it matches, then phone is in the home system. If it does not match, the phone is roaming.

Phone transmits a registration request to the base, which forward this request to the MSC.


96

Location RegistryLocation Registry

MSC uses this registration request to update a large database (called the location registry) which keeps track of the latest location of the cell phone.

This helps network find a phone when a call comes in for it.

Also it informs the MSC if the cell phone user is valid (legitimate paying customer).

MSC also learns of phone subscription features, like caller-id, etc., from the MSC.


97

Call ReceptionCall Reception

Assume a call comes in for the phone. The MSC tries to find the phone by looking up the

database. MSC uses a frequency in the cell in which the phone

was last in, and transmits an “incoming call” message over the control channel with the phone’s ESN and MIN numbers.

This message also tells the phone which frequency to switch to communicate with the base and complete the conversation.


98

Call Reception; Call HandoffCall Reception; Call Handoff

The phone and base station tower switch to these frequencies and the call is connected.

Now assume the phone user is moving around and moves to the edge of its serving cell.

Base station notes that the strength of the radio waves from this phone is diminishing.

Meanwhile, a nearby base station notes that the signal strength to this phone is increasing.


99

Call HandoffCall Handoff

All base stations constant monitor the signal strength on all voice channels (all 416) in order to pinpoint users who may be moving into their coverage area.

When the signal gets weak enough at the first base station and strong enough at the second base station, the base stations send a signal to the MSC.

The MSC determines the new frequency in the new cell that user should switch to.


100

Call Handoff (Cont’d)Call Handoff (Cont’d)

The new frequency is conveyed to the phone.

The phone switches to the new frequency (seamlessly) and the new base station tunes into this frequency and starts receiving signals from the phone.

This way the phone gets handed-over to the new base station.


101

RoamingRoaming

When SID of the phone does not match the SID of the nearest base station, the phone knows its roaming.

The MSC of the system that the phone is roaming in contacts the MSC of the phone’s home system.

The home MSC verifies the phone (valid, paying user, etc.) to the local MSC. The local MSC then keeps track of the phone as it moves thru the local system. Each time updating the database at the home system.


102

Cell to Cell CallCell to Cell Call

Let’s say there is a phone in a cell that wishes to talk to another phone in that cell.

Assume that both the phones are in a cell of their home system (thereby, they both have the same home system).

These two phones must talk to each other via the base station.

Future cell phones systems (perhaps 4G) may allow phones to connect directly with each other (peer-to-peer connection).


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Cell to Cell Call (Cont’d)Cell to Cell Call (Cont’d)

Now assume the two phones are in the same cell, but current cell is part of home system for only one of the phones.

Assume current cell is in Susquehanna County.

The other cell phone user is visiting from Florida, where its home system is.

Assume that the phone from Florida makes a phone call to the Susquehanna phone.


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Cell to Cell Call (Cont’d)Cell to Cell Call (Cont’d)

SusquehannaPhone

Florida Phone

SusquehannaMSC

Wired Network

FloridaMSC

RegistrationRequest

Verify Phone

VerifyPhone

VerifiedVerified

AssignFrequency

Registration Request

All of this happens in the matter of a few seconds!

LocationRegistry Location

Registry


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GSM: The Other TDMA GSM: The Other TDMA SystemSystem

TDMA is also used by Global System for Mobile communications (GSM). However, GSM implements TDMA in a somewhat different

and incompatible way from IS-136. GSM operates in the 900-MHz and 1800-MHz bands in

Europe and Asia, and in the 1900-MHz band in the United States.

GSM is the international standard in Europe, Australia and much of Asia and Africa. It was developed and deployed well before 2G (digital)

systems were in the U.S.


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Some Nomenclature Some Nomenclature Relating to Cellular Relating to Cellular

NetworksNetworks


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Cellular versus PCSCellular versus PCS

Personal Communications Services (PCS) is a wireless phone service very similar to cellular phone service, but with an emphasis on personal service and extended mobility.

The term "PCS" is often used in place of "digital cellular," but true PCS means that other services like paging, caller ID and e-mail are bundled into mobile telephony service.

While cellular was originally created for use in cars, PCS was designed from the ground up for greater user mobility.


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Cellular versus PCS (Cont’d)Cellular versus PCS (Cont’d)

PCS has smaller cells and therefore requires a larger number of antennas to cover a geographic area.

PCS phones use frequencies between 1.85 and 1.99 GHz (1850 MHz to 1990 MHz).

Technically, cellular systems in the United States operate in the 824-MHz to 894-MHz frequency bands; PCS operates in the 1850-MHz to 1990-MHz bands.


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Dual Band versus Dual ModeDual Band versus Dual Mode

If you travel a lot, you will probably want to look for phones that offer dual band, dual mode or both.

Dual band - A phone that has dual-band capability can switch frequencies. This means that it can operate in both the 800-MHz and 1900-MHz bands. For example, a dual-band TDMA phone could use TDMA

services in either an 800-MHz or a 1900-MHz system.


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Dual Band versus Dual Mode Dual Band versus Dual Mode (Cont’d)(Cont’d)

Dual mode - In cell phones, "mode" refers to the type of transmission technology used. So, a phone that supported AMPS and TDMA could

switch back and forth as needed. It's important that one of the modes is AMPS -- this gives

you analog service if you are in an area that doesn't have digital support.

Dual band/Dual mode - The best of both worlds allows you to switch between frequency bands and transmission modes as needed.


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Dual Band versus Dual Mode Dual Band versus Dual Mode (Cont’d)(Cont’d)

Changing bands or modes is done automatically by phones that support these options.

Usually the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at that frequency with that technology first.

If it supports dual bands, it will switch to 800 MHz if it cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the digital mode(s) first, then switch to analog.


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Tri-ModeTri-Mode

Sometimes you can even find tri-mode phones. This term can be deceptive.

It may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well as analog.

It can also mean that it supports one digital technology in two bands and also offers analog support.


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Tri-Mode (Cont’d)Tri-Mode (Cont’d)

A popular version of the tri-mode type of phone for people who do a lot of international traveling has GSM service.

Specifically, GSM is supported in the 900-MHz band for Europe and Asia and the 1900-MHz band for the United States, in addition to the analog service.


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Cell Phones and Base Cell Phones and Base StationsStations


115

Inside a Cell PhoneInside a Cell Phone

Cell phones are complex devices. Modern digital cell phones perform millions of

calculations per second in order to compress and decompress the voice stream.


116

Inside of Cell Phone (Cont’d)Inside of Cell Phone (Cont’d)

If you take a digital cell phone apart, you find that it contains just a few parts:

A circuit broad containing the brains of the phone An antenna A liquid crystal display (LCD screen) A keyboard A microphone A battery


117

Cell Phone TowersCell Phone Towers

A cell-phone tower is typically a steel pole or lattice structure that rises hundreds of feet into the air.

• This tower is used by three different cell-phone providers.


118

Cell Phone Tower (Cont’d)Cell Phone Tower (Cont’d)

The base of the tower has equipment for all service providers.

Little equipment is needed in modern systems. Older often have small

buildings at their base.


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Cell Towers (Cont’d)Cell Towers (Cont’d)

Here’s equipment for one service provider.

The box houses radio transmitters and receivers that let tower communicate with phones.

Radios connect with antenna on tower through a set of thick cables.


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If you look closely, you will see that the tower and all of the cables and equipment at the base of the tower are heavily grounded.

For example, the plate in this shot with the green wires bolting onto it is a solid copper grounding plate.


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One sure sign that multiple providers share this tower is the five-way latch on the gate.

Any one of five people can unlock this gate to get in!


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Many people have expressed concern over having cell towers near them, “not in my backyard,” also called the NIMBY problem.

This is due in part to health concerns and concerns about how they look.

There continue to be studies examining health concerns; no consensus seem to have been reached.


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Service providers have attempted to “beautify” cell towers.

Better examples are lower-power base stations, which can be embedded on sides of buildings (with brick facing).


124

High-Power, Low-Power Cell High-Power, Low-Power Cell TowersTowers

When the power of a cell tower is reduced, its coverage area is smaller.

In real cellular systems, cell sizes range from sixth tenths of a mile to thirty miles in radius.

This variation in cell sizes implies that a cellular system can be developed with a hierarchy, i.e., with multiple tiers.


125

A Multitier Cellular SystemA Multitier Cellular System

A tier is comprised of cells that have coverage areas of the same order of magnitude.

Same order of magnitude = within the same power of 10, i.e., 1, 10, 100, 1000, etc.

With cell radii ranges from 0.6 miles to 30 miles, clearly the coverage areas vary in their order of magnitude. Result: a multitier cellular network.

Different tiers are given different names: macrocell, microcell, picocell, etc.


126

Example Multitier SystemExample Multitier System

Macrocell: cell radii from 1 mi to 10 miMicrocell: cell radii from 0.1mi to 1miPicocell: cell radii from 10’s of meters


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More on Multitier SystemsMore on Multitier Systems

Picocells provide coverage to building interiors.

Microcells cover selected outdoor areas. Those that are coverage deadspots for macrocells or those regions with a high density of cell phone users (shopping mall, sporting complex, commuting bottleneck).

Macrocells provide more extensive coverage to wider areas.

A macrocell is often built first to provide coverage and smaller cells built later to improve capacity.


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Picture of a Microcell Base Picture of a Microcell Base Station (Toronto area)Station (Toronto area)


129

Some Problems with Cell Some Problems with Cell PhonesPhones

A cell phone, like any other consumer electronic device, has its problems.

Generally, non-repairable internal corrosion of parts results if you get the phone wet or use wet hands to push the buttons. Consider a protective case. If the phone does get wet, be sure it is totally dry before you switch it on so you can try to avoid damaging internal parts.

Extreme heat in a car can damage the battery or the cell-phone electronics.


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More ProblemsMore Problems

Extreme cold may cause a momentary loss of the screen display.

Analog cell phones suffer from a problem known as "cloning." A phone is "cloned" when someone steals its ID numbers and is able to make fraudulent calls on the owner's account.

Cloning is not as much of a problem with digital phones (this is because of the improved security features).


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ReferenceReference

Shalinee Kishore, LUCID Summer Workshop on Wireless Communications, Lehigh University, July 26-August 3, 2004

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Documents

© 2008 The McGraw-Hill Companies 1 The Cellular Concept Modified by Sunantha Sodsee