satellite communicatio n

© 2008 The McGraw-Hill Companies

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Satellite Communication

Modified by Sunantha Sodsee


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SatellitesSatellites

The basic component of a communications satellite is a receiver-transmitter combination called a transponder.

A satellite stays in orbit because the gravitational pull of the earth is balanced by the centripetal force of the revolving satellite.

Satellite orbits about the earth are either circular or elliptical.


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Satellite OrbitsSatellite Orbits

Satellite orbits. (a) Circular orbit. (b) Elliptical orbit.


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Satellite OrbitsSatellite Orbits

Angle of elevation.


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Orbit ShapesOrbit Shapes

Only some of the satellites have circular orbits.

Others have elliptical orbits. These orbits have further classifiers: Perigee: point on orbit

when satellite is closest to earth.

Apogee: point on orbit when satellite is farthest from earth.


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Putting a Satellite in OrbitPutting a Satellite in Orbit

A rocket must accelerate to at least 25,039 mph to completely escape Earth's gravity and fly off into space.

Earth's escape velocity is much greater than what's required to place an Earth satellite in orbit.

With satellites, the objective is not to escape Earth's gravity, but to balance it.


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Different Roles for SatellitesDifferent Roles for Satellites

Weather satellites help meteorologists predict the weather or see what's happening at the moment. The satellites generally contain cameras that can return

photos of Earth's weather.

Communications satellites allow telephone and data conversations to be relayed through the satellite. The most important feature of a communications satellite

is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency.


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Different Satellites (Cont’d)Different Satellites (Cont’d)

Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).

Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.

Navigational satellites help ships and planes navigate, e.g., GPS.


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Different Satellites (Cont’d)Different Satellites (Cont’d)

Rescue satellites respond to radio distress signals. Earth observation satellites observe the planet for

changes in everything from temperature to forestation to ice-sheet coverage.

Military satellites are up there, but much of the actual application information remains secret.


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TransponderTransponder

Some satellites have (hundreds of) transponders for communication purposes.

A transponder1) receives transmissions from earth

(uplink);

2) changes signal frequency;

3) amplifies the signal; and

4) transmits the signal to earth (downlink).


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Satellite DishSatellite Dish

Ground stations feature large parabolic dish antennas with high gain and directivity for receiving the weak satellite signal.

Satellite signals

The larger the dish isthe higher the receivedsignal power.


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Orbits of Different SatellitesOrbits of Different Satellites

Earth

1000 km

35,768 km

10,000 km

LEO (Iridium) GEO (Inmarsat)

HEO

MEO (ICO)

Not drawn to scale


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Satellite CostsSatellite Costs

Satellite launches don't always go well; there is a great deal at stake. The cost of satellites and launches to name one.

For example, a recent hurricane-watch satellite mission cost $290 million. A missile-warning satellite cost $682 million.

A satellite launch can cost anywhere between $50 million and $400 million. Russian launches are generally the cheapest and the French launches are the most expensive.

A shuttle mission pushes toward half a billion dollars (a shuttle mission could easily carry several satellites into orbit).


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How can I see an Overhead How can I see an Overhead Satellite?Satellite?

This satellite tracking Web site (http://www.heavens-above.com/) shows how you can see a satellite overhead, thanks to the German Space Operations Center.

You will then need your coordinates for longitude and latitude, available from the USGS Mapping Information Web site (http://geonames.usgs.gov/).


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Locating an Overhead SatelliteLocating an Overhead Satellite

Satellite-tracking software is available for predicting orbit passes. The above websites will help with this. Note the exact times for the satellites.

Use binoculars on a clear night when there is not a bright moon.

Ensure that your watch is set to exactly match a known time standard.

A north-south orbit often indicates a spy satellite!


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GPS


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Recall: What it isRecall: What it is

GPS: Global Positioning System is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations.

Uses the principle of triangulation and time- of-arrival of signals to determine the location of a GPS receiver.


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Typical GPS ApplicationsTypical GPS Applications

Location - determining a basic position

Navigation - getting from one location to another

Tracking - monitoring the movement of people and things.

Mapping - creating maps of the world

Timing - bringing precise timing to the world


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Triangulation RequirementsTriangulation Requirements

To triangulate, a GPS receiver measures distance using the travel time of radio signals.

To measure travel time, GPS receiver needs very accurate timing.

Along with distance, receiver need accurate data on where satellites are in space.

System will also need to correct for any delays the signal experiences as it travels through atmosphere.


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Components of GPS SystemComponents of GPS System

Control Segment: five ground stations located on earth.

Space Segment: satellite constellation (24 active satellites in space).

User Segment: GPS receiver units that receive satellite signals and determine receiver location from them.


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Ground Monitor StationsGround Monitor Stations

Falcon AFBColorado Springs, CO

Master Control Monitor Station

HawaiiMonitor Station

Ascension IslandMonitor Station

Diego GarciaMonitor Station

KwajaleinMonitor Station


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Important TerminologyImportant Terminology

Satellite transmits Ephemeris and Almanac Data to GPS receivers.

Ephemeris data contains important information about status of satellite (healthy or unhealthy), current date and time. This part of signal is essential for determining a position.

Almanac data tells GPS receiver where each GPS satellite should be at any time throughout day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system.


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TOA ConceptTOA Concept

GPS uses concept of time of arrival (TOA) of signals to determine user position.

This involves measuring time it takes for a signal transmitted by an emitter (satellite) at a known location to reach a user receiver.

Time interval is basically signal propagation time.


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TOA Concept (Cont’d)TOA Concept (Cont’d)

Time interval (signal propagation time) is multiplied by speed of signal (speed of light) to obtain satellite to receiver distance.

By measuring propagation time of signals broadcast from multiple satellites at known locations, receiver can determine its position.


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Measuring Distance using a Measuring Distance using a PRC SignalPRC Signal

At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code (PRC).

The receiver begins running the same digital pattern also exactly at midnight.

When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.


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Measuring DistanceMeasuring Distance

The length of the delay is equal to the signal's travel time.

The receiver multiplies this time by the speed of light to determine how far the signal traveled.

Assuming the signal traveled

in a straight line, this is the

distance from receiver to

satellite.


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Differential GPSDifferential GPS

Technique called differential correction can yield accuracies within 1-5 meters, or even better, with advanced equipment.

Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point.

Because physical location of base station is known, a correction factor can be computed by comparing known location with GPS location determined by using satellites.


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Using GPS DataUsing GPS Data

A GPS receiver essentially determines the receiver's position on Earth.

Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude of its current position. To make the

navigation more user-

friendly, most receivers

plug this raw data into

map files stored in

memory.


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Using GPS Data (Cont’d)Using GPS Data (Cont’d)

You can use maps stored in the receiver's memory, connect the receiver to a computer that can hold more

detailed maps in its memory, or simply buy a detailed map of your area and find your

way using the receiver's latitude and longitude readouts.

Some receivers let you download detailed maps into memory or supply detailed maps with plug-in map cartridges.


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Using GPS Data (Cont’d)Using GPS Data (Cont’d)

A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move.

If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing.

This is what happens in cars equipped with GPS.


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Using GPS DataUsing GPS Data

With this information and its built-in clock, the receiver can give you several pieces of valuable information: How far you've traveled (odometer) How long you've been traveling Your current speed (speedometer) Your average speed A "bread crumb" trail showing you exactly where you

have traveled on the map The estimated time of arrival at your destination if you

maintain your current speed


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WiFi NetworkingWiFi Networking


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WiFiWiFi

WiFi is the wireless way to handle networking. It is also known as 802.11 networking. The big advantage of WiFi is its simplicity.

You can connect computers anywhere in your home or office without the need for wires. The computers connect to the network using radio signals, and computers can be up to 100 feet or so apart.


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Wireless Networking Wireless Networking StandardsStandards

WiFi refers to the protocols that allow wireless networking.

These protocols are codified in standards.

Standards are mutually agreed upon rules adopted by the industry on how the wireless networks operate.

There are several standards that enable wireless local area networks (WLANs).


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Understanding Wireless Understanding Wireless NetworkingNetworking


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Walkie-Talkie NetworkWalkie-Talkie Network

If you want to understand wireless networking at its simplest level, think about a pair of walkie-talkies.

These are small radios that can transmit and receive radio signals.

Recall, when you talk into a Walkie-Talkie, your voice is picked up by a microphone, encoded onto a radio frequency and transmitted with the antenna.


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Walkie-Talkie Network Walkie-Talkie Network (Cont’d)(Cont’d)

Another walkie-talkie can receive the transmission with its antenna, decode your voice from the radio signal and drive a speaker.

Simple walkie-talkies like this transmit at a signal strength of about 0.25 watts, and they can transmit about 500 to 1,000 feet.


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In order to do this, we require

Each computer is equipped with a walkie-talkie. We would give each computer a way to set whether it

wants to transmit or receive. And we would give the computer a way to turn its binary

1s and 0s into two different beeps that the walkie-talkie could transmit and receive and convert back and forth between beeps and 1s/0s.


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This would actually work.

The only problem would be that the data rate would be very slow. A walkie-talkie is designed to handle the human voice (and it's a pretty scratchy rendition at that), so you would not be able to send very much data this way. Maybe 1,000 bits per second.

Another problem: the walkie-talkies could not be used to connect to the internet.


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WiFi’s Radio TechnologyWiFi’s Radio Technology

The radios used in WiFi are not so different from the radios used in walkie-talkies.

They have the ability to transmit and receive.

They have the ability to convert 1s and 0s into radio waves and then back into 1s and 0s.

There are major differences, of course.


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WiFi’s Radio Technology WiFi’s Radio Technology (Cont’d)(Cont’d)

WiFi radios that work with the 802.11b and 802.11g standards transmit at 2.4 GHz, while those that comply with the 802.11a standard transmit at 5 GHz.

Normal walkie-talkies normally operate at 49 MHz. The higher frequency allows higher data rates.

WiFi radios use much more efficient coding techniques (process of converting 0’s and 1’s into efficient radio signals) that also contribute to the much higher data rates.


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WiFi’s Radio Technology WiFi’s Radio Technology (Cont’d)(Cont’d)

The radios used for WiFi have the ability to change frequencies.

For example, 802.11b cards can transmit directly on any of three bands, or they can split the available radio bandwidth into dozens of channels and frequency hop rapidly between them.

The advantage of frequency hopping is that it is much more immune to interference and can allow dozens of WiFi cards to talk simultaneously without interfering with each other.


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WiFi RangeWiFi Range

Regardless of which setup you use, once you turn your Wireless Access Point on, you will have a WiFi hotspot in your house.

In a typical home, this hotspot will provide coverage for about 100 feet (30.5 meters) in all directions, although walls and floors do cut down on the range.

Even so, you should get good coverage throughout a typical home. For a large home, you can buy inexpensive signal boosters to increase the range of the Hotspot.


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Another Way to Amplify WiFi Another Way to Amplify WiFi SignalsSignals

A WiFi repeateris installed to extend coverage.

WirelessAccess Point


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Infrastructure versus Ad HocInfrastructure versus Ad Hoc

All the connections that we have talked about today require a connection from a device equipped with a wireless network interface card (NIC) to a wireless access point.

Generally, all such connections are operating in what is known as the infrastructure mode. Here the wireless network resembles a cellular architecture.

Wireless devices can also communicate directly with each other, i.e., it is not required that they communicate with an access point first.


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Infrastructure versus Ad Infrastructure versus Ad HocHoc

When devices with NIC cards communicate directly with each other, the wireless network operates in ad hoc mode.

Essentially peer-to-peer communication is enabled.


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Ad Hoc ModeAd Hoc Mode

Ad Hoc connections can be used to share information directly between devices. This mode is also useful for establishing a network where wireless infrastructure does not exist.

Some uses, Synchronize data between devices. Retrieve multimedia files from one device and “play”

them on another device. Print from a computer to a printer without wires.

There are many applications of ad hoc networking in the military and in specialized networks.


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How are Multiple Transmitters How are Multiple Transmitters Supported?Supported?

Recall the method for supporting multiple transmitter is called the multiple access method.

In 802.11 systems, only one user is allowed to communicate with a receiver at a time (cannot use another frequency channel support a second or third additional user).

The way the one user is selected depends on the carrier sense multiple access with collision avoidance (CSMA/CA) random access method.


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CSMACSMA

To help illustrate the operation of CSMA, we will use an analogy of a dinner table conversation.

Let’s represent our wireless medium as a dinner table, and let several people engaged in polite conversation at the table represent the wireless nodes.


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CSMA (Cont’d)CSMA (Cont’d)

The term multiple access covers what we already discussed above: When one wireless device transmits, all other devices using the wireless medium hear the transmission, just as when one person at the table talks, everyone present is able to hear him or her.

Now let's imagine that you are at the table and you have something you would like to say.

At the moment, however, I am talking.


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CSMA (Cont’d)CSMA (Cont’d)

Since this is a polite conversation, rather than immediately speak up and interrupt, you would wait until I finished talking before making your statement.

This is the same concept described in the CSMA protocol as carrier sense.

Before a station transmits, it "listens" to the medium to determine if another station is transmitting. If the medium is quiet, the station recognizes that this is an appropriate time to transmit.


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CSMA/CACSMA/CA

Carrier-sense multiple access gives us a good start in regulating our conversation, but there is one scenario we still need to address.

Let’s go back to our dinner table analogy and imagine that there is a momentary lull in the conversation.

You and I both have something we would like to add, and we both "sense the carrier" based on the silence, so we begin speaking at approximately the same time. In 802.11 terminology, a collision occurs when we both spoke at once.


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CSMA/CA (Cont’d)CSMA/CA (Cont’d)

The collision will result in an undecipherable message to the intended receivers (listeners).

What we need is a polite contention method to get access to the medium; this is the collision avoidance part of CSMA/CA.

802.11 has come up with two ways to deal with this kind of collision.

One uses a two-way handshake when initiating a transmission.

The other uses a four-way handshake.


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2 Way Handshake2 Way Handshake

Node with packet to send monitors channel.

If channel idle for specified time interval called DIFS, then node transmits.

If channel busy, then node continues to monitor until channel idle for

DIFS. At this point, terminal backs-off for random time

(collision avoidance) and attempts transmitting after waiting this random amount of time.


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If the node does not back-off the random time, then it will definitely collide with another node that has something to send.

Reason for random back-off time is that if I choose a random time and you choose a random time, the probability that we choose the same random time is slim.

This way we both back-off transmitting and will therefore will probably not interfere with each other when we are ready to transmit.


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2 Way Handshake (Cont’d)2 Way Handshake (Cont’d)

First way of the 2 way handshake was for the transmitter to send its information packet to the destination node, after following the collision avoidance method described above.

If the packet reaches the destination without problems, the destination sends a short packet over the wireless medium acknowledging the correct reception.

This packet is typically called an ACK packet. ACK is the second way of the 2 way handshake.


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“Listen before you talk”

If medium is busy, node backs-off for a random amount of time after waiting DIFS, just as before.

But now, instead of packet, sends a short message: Ready to Send (RTS). This message is basically attempting to inform others that “I have something to send.”


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RTS contains destination address and duration of message.

RTS tells everyone else to back-off for the duration.

If RTS reaches the destination okay (no one else collides with this message), the destination sends a Clear to Send (CTS) message after waiting a prescribed amount of time, called SIFS.


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After getting the CTS, the original transmitter sends the information packet to its destination.

In these systems, the transmitter cannot detect collisions. The receiver uses the CRC to determine if the packet reached correctly. If it does then, it sends out an ACK packet.

If the information packet not ACKed, then the source starts again and tries to retransmit the packet.


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Access Point Laptop

RTS

CTS

Data

ACK


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What is WiMax?What is WiMax?

WiMax is a radio technology that promises to deliver two-way Internet access at speeds of up to 75 Mbps at long range.

Its backers claim that WiMax can transmit data up to 30 miles between broadcast towers and can blanket areas more than a mile in radius with bandwidth that exceeds current DSL and cable broadband capabilities.

So, some believe that it could slash the cost of bringing broadband to remote areas.


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WiMax (Cont’d)WiMax (Cont’d)

WiMax, short for Worldwide Interoperability for Microwave Access, is the latest of the wireless "last mile" broadband technologies.

ISP see WiMax as a means of connecting rural or remote areas with broadband service, something that would be technically, physically or economically difficult to do by burying wire for DSL or cable connections.

Laying wires is especially difficult in hilly areas like Susquehanna.


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Benefits over SatelliteBenefits over Satellite

In rural areas, the real competition to WiMax would be satellite data services.

The benefit that WiMax offers over satellite is that satellite offers limited uplink bandwidth (upload data rates are not as high as download data rates).

Further, satellite suffers with high latency.


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WiMax (Cont’d)WiMax (Cont’d)

In congested cities, WiMax products could shift traffic to help relieve heavy demand on broadband networks.

WiMax will work with other shorter-range wireless standards, including Wi-Fi, which has taken off as an easy way to provide Internet access throughout a home or business.

Eventually, advocates hope to see the standard evolve into a mobile wireless Internet service similar to cellular data technologies. It may not ever be as wide-area as cellular but will offer higher data rates.


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WiMax ProtocolsWiMax Protocols

The protocols that govern WiMax have been standardized. They are collectively referred to as 802.16.

Like Wi-Fi = 802.11, WiMax = 802.16.

Overall vision for 802.16 is that carriers (e.g., ISP) would set up base stations connected to a public (wired) network. This is like cellular.

Each base station would support hundreds of fixed subscriber stations. Fixed means that subscriber stations do not move. Plans to expand the standard to include mobile stations is in the working.


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More on WiMaxMore on WiMax

Base stations will use the 802.16 protocols to dynamically allocated uplink/downlink bandwidth to subscriber stations based on their demand.

802.16 has been developed for several frequency bands (various licensed frequencies in 10-66 GHz, also licensed and unlicensed frequencies in 2-11 GHz).

In the unlicensed bands, 802.16 can be used as a backhaul for wi-fi systems or a longer-range alternative, i.e., replacing hotspots with hotzones.


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Some Technical Specs on Some Technical Specs on WiMaxWiMax

The radio technology is based on OFDM.

802.16 standards incorporate use of adaptive antenna arrays, which can be used to create dynamic beams in desired directions.

Standards offer option for a mesh mode network topology.


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Mesh Networking in WiMaxMesh Networking in WiMax

When a subscriber unit is not in line of sight with the base station (does not have a good signal strength), then it may be able to make a peer-to-peer connection to a neighbor, i.e., hop to a neighbor’s subscriber unit.

The neighbor’s unit may be in line of sight with the base station, in which case this neighbor would serve as a relay station (a repeater).

If the neighbor’s unit is not in line-of-sight then another hop can be made.


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Mesh ModeMesh Mode

Trunk(Wired)Network

Trunk

Residential Business


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WiMax CostsWiMax Costs

Analysts estimate that subscriber stations for home access will initially cost up to $300.

Base stations will cost as little as $5,000 but will reach $100,000, depending on their range. Each base station may be able to support up to 60 T1 class subscriber lines.

In some cases, consumers would lease subscriber stations from carriers the way they do with cable set-top boxes as part of their service plans.


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RFID


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Radio-Frequency Identification and Near-Field Communications

Another growing wireless technique is radio frequency identification (RFID).

RFID uses thin, inexpensive tags or labels containing passive radio circuits that can be queried by a remote wireless interrogation unit.

The tags are attached to any item that is to be monitored, tracked, accessed, located, or otherwise identified.


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The tag is a very thin label-like device into which is embedded a simple passive single-chip radio transceiver and antenna.

The chip also contains a memory that stores a digital ID code unique to the tagged item.

For the item to be identified, it must pass by the interrogation or reader unit, or the reader must physically go to a location near the item.


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The reader unit sends out a radio signal that may travel from a few inches up to no more than a hundred feet or so.

The radio signal is strong enough to activate the tag.

The tag rectifies and filters the RF signal into direct current that operates the transceiver.


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This activates a low-power transmitter that sends a signal back to the interrogator unit along with its embedded ID code.

The reader checks its attached computer, where it notes the presence of the item and may perform other processing tasks associated with the application.


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Basic concept and components of an RFID system.


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The standard is under the auspices of EPCGlobal, the organization that also standardizes the Electronic Product Code (EPC) used on all tagged items.

A key benefit of the new standard is that it is designed to read multiple tags faster. Tag read rates as high as 1500 tags per second are possible.

The tags can operate reliably in an environment with multiple readers transmitting and receiving simultaneously.


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Near-Field Communications One of the newest forms of wireless is a version of

RFID called near-field communications (NFC).

It is an ultrashort-range wireless whose range is rarely more than a few inches.

It is a technology used in smart cards and cell phones to pay for purchases or gain admittance to some facilities.


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ReferenceReference

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

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satellite communicatio n

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