17. Wireless Connectivity

17.2 Operation

Similar to any transmission system, a wireless system needs a transmitter, a receiver and a transmission medium. In a wireless system, the transmission medium is air rather than the cables used by conventional wired systems. The use of air as a transmission medium utilizes two major spectra: infrared and radio frequency.

The key difference between the use of infrared and radio frequency is the support of roaming. Infrared is a line-of-sight technology. There has to be a direct line of sight or at least a surface to bounce the waves from the transmitter to the receiver. On the other hand, radio frequency systems can penetrate through objects such as walls and doors in most office buildings; hence their popularity in present wireless systems. FCC rules allow only small sections of the electromagnetic spectrum (figure below) to be used for wireless data networks; thus techniques are needed to avoid interference from other devices that share the space or perhaps multiple stations using the same frequency.

A technique developed by the military in the 1970s to help secure transmissions offers a way around this problem. This technique is called Spread Spectrum Technology (SST). It involves spreading transmissions across a range of frequencies, rather than transmitting on one frequency all the time.

One approach known as Frequency-Hopping Spread Spectrum (FHSS) involves dividing a range of the radio spectrum into individual channels, each on a specific frequency. A transmitter can hop from one channel to the next and if the receiver is aware of the hopping pattern of the transmitter, it can follow the pattern and receive the information. The second method of spread spectrum is Direct Sequence Spread Spectrum (DSSS). The source data to be transmitted is first exclusive ORed with a pseudorandom binary sequence. The bits making up the sequence are random but the same sequence is made much larger than the source data rate. When this data is modulated and transmitted it occupies a wider frequency band than the original source data bandwidth. This would make the signal appear as noise to any other devices using the same frequency spectrum. All the members of this wireless system know the binary sequence being used .(Halsall, 1996) Thus, all receivers first search for the known preamble sequence, once it has been recognized, the receivers start to interpret the bit stream.

FCC rules for DSSS transmission requires 10 or more redundant data bits to be added to each signal. This limits the maximum throughput of DSSS transmitters to approximately 2 Mbps when using the 902-MHz band, and approximately 8 Mbps in the 2.4-GHz band.

Copyright © 1996 Farhood Moslehi, All Rights Reserved

Farhood Moslehi <moslehi@vt.edu>
Last modified: Dec 10, 1996