Sharif University of Technology

Physical layer: Wireless

Transmission

The Electromagnetic Spectrum When electrons move, they create electromagnetic

waves that can propagate through space (even in a vacuum).

The fundamental relation between f, , and c (in vacuum) is:

when l is in meters and f is in MHz, lf 300. For example, 100-MHz waves are about 3 meters long, 1000-MHz waves are 0.3-meters long, and 0.1-meter waves have a frequency of 3000 MHz.

The Electromagnetic Spectrum

Wide band

Most transmissions use a narrow frequency band to get the best reception

Wide band Frequency hopping spread spectrum

change frequencies hundreds of times per second security avoids multipath fading Example: 802.11,Bluetooth

Wide band

Direct sequence spread spectrum spread the signal over a wide frequency band used in cell phones: second and third generation mobile

phones

Radio Waves

Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. The properties of radio waves are frequency dependent.

Low frequency penetrate buildings follow curvature of earth low bandwidth omnidirectional

Higher frequencies straight line bounce off of obstacles absorbed by rain

Radio Transmission

Microwave Transmission

microwave communication is widely used for long-distance telephone communication, mobile phones, television distribution.

microwaves travel in nearly straight lines can be narrowly focused

much higher signal-to-noise ratio microwaves do not pass through buildings multipath fading absorption by water

The Politics of the Electromagnetic Spectrum

The ISM (Industrial, Scientific, Medical) bands in the United States

Infrared and Millimeter Waves widely used for short-range they are relatively directional, cheap, and

easy to build they do not pass through solid objects

Lightwave Transmission

Communication Satellites

communication satellite can be thought of as a big microwave repeater in the sky.

transponders each of which listens to some portion of the spectrum,

amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal

Communication Satellites

Geostationary Satellites