Wireless technology has become an essential part of modern lives, almost as much as electricity. High-frequency wireless signals are applied in many different forms, typically for communications. Signals may travel as far as from the Earth to an orbiting satellite, and back again, to as nearby as inches from a wireless transceiver in an office building.
Such communications systems operate by means of long- and short-haul links, requiring such critical components as antennas, receivers, and transmitters. But, while the block diagrams for these links may appear quite similar, the performance requirements for wireless communications links can be quite different, as typified by the many different standards that capture the operating parameters specific to each type of communications system.
For a wireless communications link, the maximum possible distance while maintaining normal radio operation will be determined by a number of factors, including transmit power, receive sensitivity, signal frequency/wavelength, and interference from other radio waves and the surrounding environment. Radio links are usually considered under ideal conditions, which is the line-of-sight (LOS) distance between the transmitter and the receiver. As a condition of electromagnetic (EM) propagation, the radio waves will suffer some amount of path loss even with optimum LOS operating conditions and no obstructions. Path loss refers to the decrease in strength of radio waves over distance.
Transmit power is one of the main factors in achieving an RF/microwave link of any considerable distance. This is due to the fact that radio waves follow an inverse square law in terms of distance traveled. When the distance of a radio link is doubled, there will be one-fourth the amount of transmitted power at the receiver.
From the point of view of the transmitter, if the transmit power for an RF/microwave link is quadrupled, or raised by 6 dB from a starting point, it will double the distance that can be covered by that radio link, assuming the same sensitivity at the receiver for both transmitter power levels. Wireless communications systems designers typically use a link budget to account for the various gains and losses in a link to estimate the amount of signal power that’s expected to be available at the receiver.
The Long and the Short of It
Although there’s no absolute definition as to what constitutes a short-haul wireless link versus a long-haul link, a short-haul link is considered anything covering a few kilometers or less while a long-haul link is usually 20 to 50 km or longer. The relatively long wireless links between cellular communications systems, such as 4G LTE, can usually be identified, at least at one end, by their fairly large parabolic dish antennas (Fig. 1).