Wireless Impairments

Wireless impairments come from many sources, including the following:

• Path loss Radio signals lose power at a rate that increases with the square of the distance from the transmitter. The path loss, in decibels (dB), represents the ratio of the strength of the transmitted signal to the received strength. Path obstacles are materials such as water, metal, concrete, or masses of dirt that increase path loss.
• Multipath Multipath is the artifact of reflections and echoes. For example, with antenna televisions, when an image has a ghost figure around it, that is an echo. Multipath can create secondary and tertiary signals that compete with the primary signal. Poor connections and cabling can also induce multipath distortion in cable TV signals.
• Fading There are a number of propagation characteristics, and they vary with the different frequencies. As a mobile station moves through a cell, the multipath signals abruptly and rapidly add to and subtract from each other. As a result, very good signals are interspersed with very poor signals. This effect is referred to as a Rayleigh fade (named after the physicist Lord Rayleigh). The multipath delays can be predicted on a statistical basis, and components can be designed to handle the problem.
• Doppler effects As a mobile station transmitter is moving toward or away from the base station, there can be frequency shifts, called Doppler effects, in the received signal.
• Co-channel interference Although frequencies are not to be reused in adjacent cells, if a clear path exists, a transmission on the same channel in a non-adjacent cell might reach a user in another part of the service area and cause interference. Wireless systems operating in unlicensed spectrum are also subject to interference that may come from other wireless devices or networks.

Mobile Phones and Health Issues

Health issues related to mobile phone use are of legitimate concern. One important finding is that mobile phones may cause the bloodbrain barriera network of tiny capillaries surrounding the brain that protects the brain from toxinsto break down. Another finding is that mobile phone radiation has shown effects on blood proteins. While there is no conclusive proof, many studies point to the need to continue research. You can watch an interesting video on the radiation effects of mobile phones and find summaries of many of the research studies conducted over the years at www.globalchange.com/radiationtv.html.

• Interference and noise Interference and noise are by-products of precipitation in the air, metals in the environment, and a variety of other anomalies. Error correction techniques are needed to fix these problems.
• Foliage Foliage can be a source of interference because the water in leaves absorbs radio signals.
• Weather effects Weather can cause interference, particularly in the higher frequencies, where each radio wave is smaller than a drop of rain.
• Environmental obstacles Radio signals cannot penetrate various materials, so walls, desks, buildings, hills, vehicles, and other environmental obstacles can affect the performance of radio.
• Range and electrical power Range and electrical power are considerations because more radio power is required to increase the range or to compensate for poor path quality.

Antennas