TECHNOLOGY TRENDS
Free Space Loss: As signals spread out from a radiating source, the energy is spread out over a larger area. As this occurs, the strength of that signal gets weaker.

Free space loss (FSL), measured in dB, specifies how much the signal has weakened over a given distance. The type of antenna used has no effect on FSL since at any appreciable distance all antennas look like a point-source radiator. The difference in FSL between a 2.4 GHz link and a 5.8 GHz link is always about 8 dB, regardless of the distance. This is one of the reasons why 802.11a WLAN devices will have less than half the range of a 2.4 GHz WLAN device (e.g., 802.11b).

l Fresnel Zone: Radio waves travel in a straight line, unless some obstruction refracts or reflects them. But the energy of radio waves is not "pencil thin."

They spread out and get weaker the farther they move from the radiating source—like ripples from a rock thrown into a pond. The area that the signal spreads out into is called the Fresnel zone. If there is an obstacle in the Fresnel zone, part of the radio signal will be diffracted or bent away from the straight-line path.

The practical effect is that on a point-to-point radio link, this refraction will reduce the amount of RF energy reaching the receive antenna. The thickness or radius of the Fresnel zone depends on the frequency of the signal—the higher the frequency, the smaller the Fresnel zone. Therefore, the Fresnel zone is fattest in the center. As with FSL, the antennas used have no effect on the Fresnel zone.

l Receive Signal Level: Receive signal level is the actual received signal level (usually measured in negative dBm) presented to the antenna port of a radio receiver from a remote transmitter.

l Receiver Sensitivity: Receiver sensitivity is the weakest RF signal level (usually measured in negative dBm) that a radio needs receive in order to demodulate and decode a packet of data without errors.

l Antenna Gain: Antenna gain is the ratio of how much an antenna boosts the RF signal over a specified low-gain radiator. Antennas achieve gain simply by focusing RF energy.

If this gain is compared with an isotropic (no gain) radiator, it is measured in dBi. If the gain is measured against a standard dipole antenna, it is measured in dBd. The gain applies to both transmit and receive signals.

l Transmit Power: The transmit power is the RF power coming out of the antenna port of a transmitter. It is measured in dBm, watts or milliwatts and does not include the signal loss of the coax cable or the gain of the antenna.

l Effective Isotropic Radiated Power: Effective isotropic radiated power (EIRP) is the actual RF power as measured in the main lobe (or focal point) of an antenna. It is equal to the sum of the transmit power into the antenna (in dBm) added to the dBi gain of the antenna. Since it is a power level, the result is measured in dBm. Using an amplifier, +24 dBm of power (250 mW) can be "boosted" to +48 dBm or 64 Watts of radiated power.

l System Operating Margin: System operating margin (SOM) is the difference (measured in dB) between the nominal signal level received at one end of a radio link and the signal level required by that radio to assure that a packet of data is decoded without error. In other words, SOM is the difference between the signal received and the radio’s specified receiver sensitivity. SOM is also referred to as link margin or fade margin.

l Multipath Interference: When signals arrive at a remote antenna after being reflected off the ground or refracted back to earth from the sky (sometimes called ducting), they will subtract (or add) to the main signal and cause the received signal to be weaker (or stronger) throughout the day.

l Signal-to-noise Ratio: Signal to noise ratio (SNR) is the ratio (usually measured in dB) between the signal level received and the noise floor level for that particular signal. The SNR is really the only thing receiver demodulators really care about. Unless the noise floor is extremely high, the absolute level of the signal or noise is not critical. The weaker signals have larger negative numbers.