introduction to information technology
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TRANSCRIPT
IT-101Section 001
Lecture #11
Introduction to Information Technology
Overview Finish chapter 13
Generations of cellular systems Satellite telephones
Chapter 16 Radio-Frequency and Satellite Systems
-Satellite systems-GPS
Generations of Cellular Systems
Generations of cellular systems include: AMPS 1st generation GSM 2nd generation W-CDMA 3rd generation
Cellular systems operate based on various protocols, and use RF (radio frequency) waves that propagate through the air for transmission of information. These systems typically use the 800-900 MHz or 1800-1900 MHz frequency band of the radio spectrum.
But what is the radio spectrum?
The Radio Spectrum
The Radio spectrum is composed of many frequency bands
Communication systems have the liberty to transmit signals at various frequencies, and the FCC (Federal Communications Commission) regulates which frequencies to use
Source:http://howstuffworks.lycoszone.com/radio-spectrum1.htm
Radio Spectrum Bands
MF•AM Radio•Maritime
HF•Maritime•Amateur Radio
VHF•Television•FM radio•Aviation
UHF/SHF•Satellite•Television•Cell Phones•Microwave
EHF•Astronomy
Low Frequency=large period High Frequency=small period
~400-2400 MHz frequency range
FM radio stations between: 88-108 MHz
AMPS: Advanced Mobile Phone System The only system available in the United States until
about 1997 The first system used for cellular telephony- is analog Uses the 800 MHz frequency band of the spectrum Is still being used widely - The number of subscribers
began to decrease in 1999 due to migration to digital Utilizes FDMA (Frequency division multiple access) to
separate users In FDMA, users are separated in frequency. i.e.
mobile phones communicate at different frequencies than the others within each cell. The radio spectrum is shared among users
1st generation
GSM: Originally: “Group Special Mobile”, now: “Global System for Mobile Communications”
Is the most popular system worldwide Originally developed in Europe Was later introduced into the United States Is a digital system for both voice and data
transmission Uses the 900 and 1800 MHz frequency bands Utilizes TDMA (Time division multiple access) In TDMA, users are separated in time. i.e. mobile
phones communicate in a different time slot than the others within each cell. Time is shared among the users
2nd generation
W-CDMA Wideband code division multiple access: Provides much higher data rates Supports a larger number of users Enables features such as video, internet access, web
browsing and complete worldwide operability Widespread introduction expected to begin by 2005 Utilizes CDMA (Code division multiple access) In CDMA, users are separated by a unique code
assigned to them. i.e. each mobile phone can utilize the entire chunk of spectrum, and the unique code of each user is used to encode and decode the information during transmission
3rd generation
Satellite Telephones Satellite telephones are utilized in circumstances
where there is no access to a telephone or cellular network
Examples include ships at sea Previously, these remote users would
communicate via High frequency (HF) systems. A licensed radio operator would need to be present on board to operate these systems, and would need to know morse code
Using radio waves in the HF range of the spectrum, communication could be realized between any two points on the earth
This was possible due to the unique property of HF waves to travel all the way around the world
Under appropriate conditions, HF waves can travel around the world by reflection between the earths surface and the ionosphere (the ionospheric reflection is analogous to the internal reflection that occurs in optical fibers-we will learn about fibers in chapter 15 )
This method of communication however is unreliable, and currently satellites are utilized for communication in remote places
Using satellites, a ship at sea is easily reachable as any point on land
INMARSAT is one of the main providers of satellite radio communications for remote users
A Satellite based system called the Iridium system, conceived by Motorola corp. was proposed as an alternative to the conventional cellular systems
In the Iridium system, 66 low earth orbiting satellites are used for relaying information, analogous to a switching center in conventional cellular and telephone systems
The call does not pass through land lines, increasing the efficiency over other cellular systems
The cell sites in the Iridium system are overhead and are in constant motion
Let us see how iridium works: http://www2.sis.pitt.edu/~jkabara/tele-2100/iridium/iridium_final.html
The Iridium system began operations in 1999, and presented the first generally available global system
However, the relatively high cost of service (around $3 per minute) and the relatively large size of the handsets drove the system to bankruptcy
Iridium failed to attract a sufficient amount of customers in competition with the rapidly spreading cellular systems across the globe
Shortly after this failure, a group of investors bought the system for $25 million, which had actually cost Motorola more than $ 5 billion to build
The system is expected to be profitable within a couple of years
Satellite Systems Communication satellites provided the first long-
distance, wide-bandwidth communication service Testar 1 was the first commercial communications
satellite to be launched into orbit Satellites can be distinguished into 3 primary
categories, depending on the type of orbit they reside on:
LEO MEO GEO
LEO: Low Earth Orbit satellites have a small area of coverage. They are positioned in an orbit approximately 3000km from the surface of the earth
They complete one orbit every 90 minutes The large majority of satellites are in low earth orbit The Iridium system utilizes LEO satellites The satellite in LEO orbit is visible to a point on the
earth for a very short time MEO: Medium Earth Orbit satellites have orbital
altitudes between 3,000 and 30,000 km. They are commonly used used in navigation systems
such as GPS
GEO: Geosynchronous (Geostationary) Earth Orbit satellites are positioned over the equator. The orbital altitude is around 30,000-40,000 km
They complete one orbit every 24 hours. This causes the satellite to appear stationary with respect to a point on the earth, allowing one satellite to provide continual coverage to a given area on the earth's surface
One GEO satellite can cover approximately 1/3 of the world’s surface
They are commonly used in communication systems Let’s look at a GEO animation: http://www.jpl.nasa.gov/basics/bsf5-1.htm
The Global Positioning System
GPS is funded and controlled by the Department of Defense (DOD).
Although there are many thousands of civil users of GPS worldwide, the system was originally designed for the U. S. military
GPS provides special satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position and velocity
Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver
For around $100, you can get a hand held gadget that will tell you exactly what point you are on the earth
Source:http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html
The orbital period of the GPS satellites is around 12 hours The satellites move with respect to the receivers on the earth,
but at a slower speed compared to LEO’s Typical civilian accuracy is around 100 ft, with occasional
errors of up to 300 ft For applications requiring higher accuracy such as aircraft
landing etc., enhancements in the systems are made that can reduce the error to around 1 cm
Some applications of GPS include: Aircraft navigation Marine navigation Driving Surveying Farming