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Deploying Point-to-Point Wireless Links: Page 3 of 8

That Old Math Magic

For several years, we have referred to the "magic of physics" associated with wireless systems. Obviously, there's nothing magical about it, but the underlying complexity of radio engineering makes it difficult for nonexperts to grasp how hundreds of megabits of data can be pushed down an invisible pipe every second. Regardless, you'll need to understand some basic principles to install one of these systems.

First, there's frequency. Radio-wave behavior varies with changes in frequency. As you move to higher frequencies, the wavelengths become shorter, affecting their propagation characteristics. For example, low frequency AM radio waves (500 KHz to 1,500 KHz) can propagate for hundreds or thousands of miles, and adverse weather and physical obstructions have little effect. But at frequencies greater than 40 GHz, radio waves are readily absorbed by rain and fog. Most point-to-point systems are deployed at microwave frequencies, with operations in the unlicensed 2.4- and 5-GHz bands most common.

Closely related to frequency is the amount of bandwidth available for a specific link. This is a product of both the size of the RF channel (for example, 10 MHz) and the analog-to-digital conversion techniques employed. One key issue facing system designers is spectral efficiency, or the amount of data that can be packed into an RF channel. Since available spectrum is limited, it's obviously beneficial to use advanced signal modulation techniques to pack as many bits into a channel as possible. However, as spectral efficiency increases, so too does the cost of underlying components and the possibility of errors. It's a delicate balancing act. Most point-to-point systems can operate at multiple alternate channels within a specific frequency band, which can be important in mitigating interference.

While it's common to refer to the power of RF systems in watts, designers of point-to-point systems more often use decibels (dB), a logarithmic transformation that lets us measure the ratio between input and output signals while avoiding numbers with many zeros. One of the primary purposes of a radio system is to amplify a signal, thereby generating gain, measured in decibels. Without getting into the underlying math, you can understand a lot just by knowing that a 3 dB increase in gain means the signal is 2 times bigger. Radio power is often measured in watts or milliwatts, so when decibels are used to represent the ratio between input and output signals, using milliwatts as the reference (indicated by dBm), 0 dBm is equivalent to 1 milliwatt, 10 dBm to 10 milliwatts, 20 dBm to 100 milliwatts, and 30 dBm to 1 watt