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More on Cellular Telephony
LUCID Summer WorkshopJuly 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
MSCMSC
MSC
MSC
Public (Wired)TelephoneNetwork
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-to-peer 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)
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
Cell to Cell Call (Cont’d)
SusquehannaPhone
Florida 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 cell-phone networks for transmitting information: 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
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
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:
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-to-analog 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 five-way 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 SystemMacrocell: cell radii from 1 mi to 10 miMicrocell: cell radii from 0.1mi to 1miPicocell: 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.
