Wireless Communication and the Electromagnetic Spectrum

The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. As the frequency of an electromagnetic wave increases its wavelength decreases. In any frequency, electromagnetic waves travel through a vacuum at the speed of light. When traveling through a medium the speed of electromagnetic waves decrease.

Data communication frequencies

1. Twisted Pair (0 - 100Mhz)
   
2. Coaxial Cable (1Khz - 1Ghz)
   
3. AM Radio (530Khz - 1600Khz)
   
4. FM Radio (88Mhz - 108hz)
   
5. Terrestrial and Satellite / Microwave (1Ghz - 1Thz)
6. Infrared (1Thz - 100Thz)
   
7. Optical Fiber (100Thz - 1Phz)

Unguided Media

Unguided Media transmit electromagnetic waves without using a solid conductor. Some authors say that air or water is unguided media's media. However it should be noted that Electromagnetic waves do not require any media to propagate and can travel even through vacuum. Wireless Communication would be a better term.


Wireless Data Communication frequencies

1. AM Radio (530Khz - 1600Khz)
   
2. FM Radio (88Mhz - 108hz)
   
3. Terrestrial and Satellite (1Ghz - 100Ghz)
4. Infrared (1Thz - 100Thz)

Propagation Methods

1. Ground Propagation - radio waves travel through the lowest portion of the atmosphere following the curvature of the planet.
2. Sky Propagation - high frequency radio waves radiate upward into the ionosphere where they are reflected back to the earth.
3. Line-of-sight Propagation - very high frequency signals are transmitted in straight lines directly from antenna to antenna.

Wireless Transmission Frequencies

Radio Wave

Although there is no clear cut division between radio waves and microwaves, electromagnetic frequencies between 3Khz and 1Ghz. Behavior of the waves, rather than frequencies, is a better criterion for classification. Radio waves are mostly omnidirectional and propagated using an omnidirectional antenna. When an antenna transmits radio waves, they are propagated in all directions. This means that sending and receiving antennas do not need to be aligned. Radio waves are used for multi-casting, in which there is one sender but many receivers such as AM and RM radio, television, maritime radio, cordless phones and paging. Radio waves can penetrate walls. Radio waves travel to long distances. The disadvantage of omnidirectionality is that the signal is susceptible to interference by another signal of the same frequency. The entire band is regulated by authorities and any part of the band requires permission.

Microwaves

Microwaves have frequencies between 1 and 300 GHz. Microwaves are unidirectional. When an antenna transmits microwaves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. This means that even if two pairs of antennas transmit microwaves of the same frequency, they will not interfere with each other as long as the pairs are not aligned. Microwave propagation needs to be line-of-sight. The microwave band is relatively wide. Therefore wider sub-bands can be assigned and a high data rate for each sub-band is possible. Though microwaves travel shorter distances relative to radio waves. Microwaves use unidirectional antennas such as a parabolic dish and a horn antenna. They are used in cellular phones, satellite networks and wireless LANs.

Infrared

Infrared signals, with frequencies from 300Ghz to 4Thz, can be used for short-range communication. Infrared signals, having high frequencies never pass through walls. This is advantageous when communication systems are separated by physical walls. However we can not use infrared outside a building because the sun's rays contain infrared waves that interfere with the communication.