 bands, as long as they conform with specified regulations that govern use, such as observing the power lim- its. One of the complexities of building products for use in unlicensed spectrum is that not all countries have allocated the same bands for unlicensed operation. One of the biggest questions, and opportunities, in securing more spectrum for wireless sys- tems lies in the spectrum currently allocated to the broadcast television industry. Depending on the country, a single TV channel occupies either a 6MHz or 8MHz transmission band, which is equal to 300 or more digital cellular channels. 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), repre- sents the ratio of the strength of the transmitted signal to the received Wireless Impairments 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 Ray- leigh). 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 effec