Transcript More on Cellular Telephony

More on Cellular
Telephony
LUCID Summer Workshop
July 28, 2004
Overview of Last Time
We introduced the cellular concept.
 Geographic area is broken into smaller cell.
 Cell phone users in each cell communicate with
base station, which has a high antenna tower.
 As users move from one cell to the next, call is
handed off from one base station to the next.
 Frequency reuse is used to support a large
number of users over a large coverage area using
limited spectrum.
 Local base stations are connected to and
controlled by a mobile switching center (MSC).
Outline for Today

We will continue our discussion on AMPS, the
analog first generation cellular system deployed in
the U.S.

We will also look at how the second generation
digital systems differ from AMPS.

Finally, we consider what components make up a
cell phone and what a base station looks like.
Cellular Network
MSC
Public (Wired)
Telephone
Network
MSC
MSC
MSC
Overview (Cont’d)
We began looking at AMPS, U.S.’s first cellular
system.
 Analog system based on FDMA
 832 frequency channels available; 416 in each
direction.
 42 of them are control channels (used for
coordination between user terminals and base
station).
 Remaining are voice channels (that actually carry
phone conversations).
Cellular Identifiers

In AMPS, a cell phone subscription is identified
using three numbers:
 Electronic Serial Number (ESN): unique 32 bit
number programmed into the cell when it was
manufactured.
 Mobile Identification Number (MIN): 10 digit
phone number.
 System Identification Code (SID): Unique 5 bit
code assigned by the FCC to each service
provider.
What happens when you
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.
Call 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.
Location 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.
Call 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.
Call 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.
Call 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.
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.
Call Handoff
Roaming

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.
Cell 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-topeer connection).
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.
Cell to Cell Call (Cont’d)
Florida Phone
Registration Request
Susquehanna
Phone
Registration
Request
Susquehanna
MSC
Assign
Frequency
Location
Registry
Verify
Phone
Verified
Location
Registry
Wired
Network
Verify
Phone
Verified
Florida
MSC
All of this happens in the matter of a few seconds!
Cell to Cell Call (Cont’d)
Florida Phone
Susquehanna
Phone
Summary of Digital Cellular
Systems
Recall: Analog vs Digital

In analog cellular, the audio signal (conversation) is
converted into a radio wave directly. The speech is
embedded by varying the frequency of the radio
wave (FM modulation).

In digital cellular, the audio speech signal is first
converted to a sequence of 0s and 1s. The
transmitted radio waves conveys this sequence of
0s and 1s over the air from the transmitter to the
receiver.
What’s the benefit of going
digital?




Digital cell phones use same radio technology as
analog phones, but they use it in a different way.
Analog systems do not fully utilize the bandwidth
between the phone and the cellular network.
For example, analog signals cannot be compressed
and manipulated as easily as a true digital signal.
Digital signals are basically more efficient.
Quick Aside: Review Multiple
Access Methods

There are three common technologies used by cellphone networks for transmitting information:





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Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
FDMA puts each call on a separate frequency band.
TDMA assigns each call a certain portion of time on
a designated frequency band.
CDMA gives a unique code to each call and spreads
it over the available frequencies.
TDMA: A Digital Cellular
Method




The benefit of digital cellular systems over the
FDMA analog systems can be seen by looking a
simple gain offered by a digital TDMA system.
This gain is result of a property of human speech.
Human speech (generally sound) is caused by
minute changes in the air pressure.
When a caller speaks on his/her phone, the
microphone records these varying pressure changes
into varying electronic signals.
Speech Signals


Microphone outputs an analog electrical signal.
For example, here is a graph showing the analog
wave created by saying the word "hello“.
A to D Conversion

In digital systems, this analog signal is converted to
a sequence of 0’s and 1’s.

One way this can be done is to sample the analog
signal in fixed time intervals:
Sampling instances
A to D Conversion (Cont’d)

At each sampling instant, the value of the analog
signal (a number) is recorded and converted into
binary format.

This produces a digital speech signal.

The receiver than performs the reverse process to
get an (approximate) analog signal.

It can be shown that the received analog signal
sounds like the original speech if the sampling is
done at least 8000 times per second, i.e., once
every 0.125 milliseconds (ms).
Sampling Rate


So, a cell phone needs to convert its speaker’s
voice into a binary number only once every 0.125
ms.
In other words, if the cell phone transmits the digital
signal once every 0.125 ms, it will be received by
the base once every 0.125 ms and the base will be
able to reconstruct an almost perfect replica of the
speaker’s words.
Packing in More Users on a
Frequency Channel



In analog cellular systems, we give a cell phone
user exclusive access to a frequency channel.
If we first convert the analog signal to digital (as in
digital systems), then a cell phone user only needs
access to a frequency channel every 0.125 ms.
We can then use the frequency channel during the
remaining time to support other cell phone users in
the cell.
Result: TDMA




Thus a frequency channel is able to support multiple
voice users by allowing round robin access.
This is just TDMA.
We see therefore that by converting analog speech
to digital speech, we are able to use TDMA to
improve the total capacity of the cellular system.
Another benefit of digital speech is that once a
signal of 0s and 1s has been generated
compression techniques can be used to reduce the
number of binary digits the signal contains.
Digital Compression



Digital phones convert your voice into binary
information (1s and 0s) and then compress it.
This process and subsequent compression allows
between three and 10 digital cell-phone calls to
occupy the space of a single analog call.
There are several compression techniques used to
further pack in more phone conversations over the
same frequency band.
Voice Activity Detection: A
Method to Pack in More Users




In phone conversations, we actually speak only 40%
of the time.
The remaining 60% is silence.
In analog system, the frequency given to a phone
call sits idle, i.e., unused, during these silences.
In digital systems, during silences there is no
“signal” to encode into 1’s and 0’s. Thus, there is no
need to create a signal during silences.
Voice Activity Detection



Further, it is really easy to insert another phone
conversation over the same frequency band during
these silences.
Thus, for the same frequency band, the digital
system can support more phone conversations than
typical analog systems. This method is used in
CDMA cellular systems.
Techniques like voice activity detection can be easily
implemented in digital systems, using relatively
inexpensive digital electronics.
Modulation Differences

Analog systems use FM modulation, i.e., frequency
of transmitted radio wave changes as the amplitude
of the analog signal changes.

Many digital cellular systems rely on digital
modulation schemes, e.g, binary phase shift keying
(BPSK) or frequency shift keying (FSK).

FSK uses two frequencies, one for 1s and the other
for 0s, alternating rapidly between the two to send
digital information.
IS-136: 2G TDMA in the U.S.




TDMA is the access method used by Interim
Standard 136 (IS-136), 2G standard in the U.S.
Using TDMA, a frequency band that is 30 kHz wide
is split time-wise into three time slots, each slot is
6.67 ms long.
Thus, each conversation gets the radio channel for
one-third of the time.
Once again, this is possible because voice data that
has been converted to digital information is
compressed so that it takes up significantly less
transmission space.
North American TDMA System

Therefore, TDMA has three times the capacity of an
analog system using the same number of channels.

TDMA systems operate in either the 800-MHz or
1900-MHz frequency bands.

By the time second generation cellular came about,
the FCC had allocated another chunk of spectrum
(in the 1900 MHz range) for mobile telephony.
IS-136
GSM: The Other TDMA
System

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
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.
SIM Cards



In covered areas, cell-phone users can buy one
phone that will work anywhere where the standard is
supported.
To connect to service providers in different
countries, GSM users simply switch subscriber
identification module (SIM) cards.
SIM cards are small removable disks that slip in and
out of GSM cell phones. They store all connection
data and identification numbers you need to access
a particular wireless service provider.
IS-95: The 2G CDMA Standard

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
All users transmit in the same wide-band chunk of
spectrum.
Each user's signal is spread over the entire
bandwidth by a unique spreading code.
At the receiver, that same unique code is used to
recover the signal.
CDMA systems need to put an accurate time-stamp
on each piece of a signal; they reference the GPS
system for this information.
IS-95 (Cont’d)



Between eight and 10 separate calls can be carried
in the same channel space as one analog AMPS
call.
CDMA technology is the basis for Interim Standard
95 (IS-95) and operates in both the 800-MHz and
1900-MHz frequency bands.
cdma200, a 3G cellular standard, is an enhanced
version of IS-95.
Improved Security

Another benefit of digital cellular is that it makes
phone conversations much more secure.

Security features help ensure eavesdropping does
not occur. Also reduces fraudulent use.

Encryption methods make it difficult for an
undesirable user to hear someone else’s phone
conversation.

Authentication methods make sure that both the cell
phone and the cell subscription are legitimate.
Some Nomenclature Relating
to Cellular Networks
Cellular 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.
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.
Dual 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.
Dual Band versus Dual Mode
(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.
Dual Band versus Dual Mode
(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.
Tri-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.
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.
Cell Phones and Base Stations
Inside 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.
Inside of Cell Phone (Cont’d)

If you take a digital cell phone apart, you find
that it contains just a few parts:


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A circuit broad containing the brains of the phone
An antenna
A liquid crystal display (LCD screen)
A keyboard
A microphone
A battery
The Circuit Board

The circuit board is the heart of the system.

Circuit boards contain several computer chips.

There are some analog-to-digital and digital-toanalog chips that
 Translate outgoing audio signal to digital and
 Incoming digital signal to audio
Circuit Board (Cont’d)

There is also a digital signal processing chip that
does customized signal manipulation calculations at
high speeds.

There is a microprocessor chip that
 handles all housekeeping chores for the keyboard
and display,
 deals with control signaling to the base station,
and
 coordinates rest of functions on circuit board.
Other Chips on Circuit Board

ROM and Flash memory chips provider storage for
the phone’s operating system and features like the
phone directory.

Operating system is software that controls the
phone’s hardware.
Other Cell Phone Components

The radio frequency (RF) and power section
handles
 power management and recharging, and
 also deals with the hundreds of FM channels.

RF amplifiers handle signals traveling to and from
the antenna.
Other Components (Cont’d)




The cell phone display has grown considerably in
size as the number of features in cell phones have
increased.
Most current phones offer built-in phone directories,
calculators and even games.
Many phones incorporate some type of PDA or Web
browser.
New phones also contain cameras.
Other Components (Cont’d)

Cell phones have tiny speakers and microphones.


Speaker is about the size of a dime and the
microphone is no larger than the watch battery
beside it.

A watch battery is used by the cell phone's internal
clock chip. Although some phones also have GPS
receivers to coordinate timing and location.
Summary on Phone
Components
“What is amazing is that all of this functionality -which only 30 years ago would have filled an entire
floor of an office building -- now fits into a package
that sits comfortably in the palm of your hand!”
- Howstuffworks.com
Cell 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.
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.
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.
Cell Towers (Cont’d)


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.
Cell Towers (Cont’d)

One sure sign that
multiple providers share
this tower is the fiveway latch on the gate.

Any one of five people
can unlock this gate to
get in!
Cell Towers (Cont’d)

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.
Cell Towers (Cont’d)

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).
High-Power, Low-Power Cell
Towers

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.
A 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.
Example Multitier System
Macrocell: cell radii from 1 mi to 10 mi
Microcell: cell radii from 0.1mi to 1mi
Picocell: cell radii from 10’s of meters
More 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.
Picture of a Microcell Base
Station (Toronto area)
Anecdote about Microcells



According to Telephony Magazine, AT&T began
splitting their macrocell based New York City
network in 1994.
Starting in Midtown Manhattan, the $30 million-plus
project added 55 microcells to 3 square mile area by
1997, with 10 more on the way.
Lower Manhattan got a "few dozen." Microcells in
lower Manhattan sought to increase signal quality,
while Midtown improvements tried to increase
system capacity.
Anecdote (Cont’d)

An AT&T engineer said "a macrocell costs $500,000
to $1 million to build, a microcell one-third as much
and you don't have to build a room around it."

Microcell antennas were easy to camouflage and
got placed on buildings between 25 and 50 feet
above street level.
Another Anecdote on
Deployment Costs



Omnipoint's PCS network was deployed for the
greater New York city area in mid-late 90’s.
To cover the 63,000-square-mile service area,
Ericsson says Omnipoint installed over 500 cell
sites, with their attendant base stations and
antennas, three mobile switching centers, one home
location register.
The New York Times reported the entire system cost
$680 million dollars, although they didn't say if that
included Omnipoint's discounted operating license.
Some Problems with Cell
Phones



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.
More 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).
Some Statistics

Currently there are 166,429,576 cellular & PCS
subscribers in the U.S. This means roughly 56%
penetration rate.

The U.S. has the second largest population of cell
phone users.

The highest cellular penetration rates are in
Scandinavian countries (usually Finland, Iceland,
Norway) or Taiwan, Singapore. In these places the
penetration rates have been reported somewhere in
80%.
Some Statistics (Cont’d)

The largest number of cell phone users are in China.
Its penetration rate is only about 15%.

The penetration rate in China for standard wired
phone is about the same.

Because of the large costs of laying down copper
wires to each home, the Chinese government has
decided to push cellular telephony as the primary
method of telephony.
Next Time

This concludes our discussion on cellular telephony.

Next time, we will look at the Global Positioning
System.

To do this effectively, we will also review the basics
of satellite communications.