The first question asked by the novice to microwave networking is always: "How fast does it go?" followed immediately by "What distance does it cover?" The answers are not always simple, because you can gain extra distance by giving up speed, and vice versa. In addition, the performance of the system depends heavily on the choice of antennas. Finally, the local environment (topography, vegetation, weather) will influence your network performance.
This article is an attempt to explain the relationships between these parameters, in order to enable you to properly compare different equipment.
How Far?The quality of a radio-frequency communication link is a function of five parameters:
Power and SensitivityTransmit power and receiver sensitivity are expressed relative to a reference level of 1 milli-Watt (mW) and abbreviated dBm. In the unlicensed ISM bands, the maximum power we are allowed to feed the antenna in the USA is 1 W or 30 dBm. In Europe, it is 250 mW or 24 dBm. The sensitivity of a good ISM band receiver ranges from -75 dBm to -90 dBm. (-90 dBm means the receiver can decode a signal at 1 nanoWatt !)
Transmit power is limited by the regulatory authority. Under the US rules (FCC) you can feed up to 27 dBm of power into a 9 dBi omnidirectional antenna (4 W EIRP or 36 dBm), or 24 dBm of power into a 24 dBi directional antenna for point-to-point links (48 dBm or 16W EIRP). Under the European rules (ETSI) the maximum effective radiated power (transmit power plus antenna gain) is 100 mW EIRP (20 dBm).
Receiver sensitivity is generally measured by reducing the input power until the error level exceeds a defined threshold. It is common to indicate the sensitivity as the level when the error rate has increased to 10E-6 (one bit error per 1 million bits of data). With a lower data rate, the connection will be more robust. Typically, the sensitivity decreases by 3dB when the data rate is doubled.
Antenna SystemsThe ability of the antenna to shape the signal and focus it in a particular direction is called "antenna gain" and is expressed in terms of how much stronger the signal in the desired direction is, compared to an "isotropic radiator" which is an hypotetical antenna that distributes the signal evenly in all directions. To express the relationship to the isotropic reference, this is abbreviated dBi. The typical omni-directional "stick" antenna is rated at 6-8 dBi, indicating that by redirecting the signal that would have gone straight up or down to the horizontal level, 4 times as much signal is available horizontally. A parabolic reflector design can easily achieve 24 dBi.
For more information, read our Antenna Selection Guide.
Under the antenna system, we also need to account for losses in the cables between the radio and the antenna. Count on 0.25 dB of loss for each connector and the following losses per 100 feet of feed cable (the figure in parenthesis is how many feet of cable it takes to lose 10 dB):
Free-Space LossAs the radio signal travels through space, it deteriorates for two reasons:
-L = C + 20 * log(D) + 20 * log(F)
where D is the distance, and F is the frequency in MHz. The constant C is 36.6 if D is measured in miles, and 32.5 if D is in kilometers. The following are some examples of free space losses:
These figures do not take into account deterioration due to weather. Typically, we recommend allowing 15 dB of "fade margin" to accommodate for weather, antenna alignment, and other miscellaneous losses.
Putting it Together
Assume that you have a 2.4GHz multipoint radio system consisting of
The maximum allowable loss would be 123 dB. If we want a 15 dB link margin to protect against weather, then we are at 108 dB allowance for distance, which would be 1.6 miles.
If the radio system allows you to improve the sensitivity by dropping the link speed, you can typically gain 3dB of sensitivity by dropping to half speed. This would allow you to increase the distance to 2.3 miles.
If this system is used in point-to-point mode, the 6 dB omni antenna can be replaced with a 24dBi directional antenna, which would allow you to run 12.4 miles at full speed.
If your radio had a sensitivity of -90 dB instead of -80 dB, your multipoint system can serve an area out to 5 miles instead of 1.6 miles at full speed.
The RF Link Budget Calculator which is very helpful in making these calculations.
Evaluating AFAR Radios
The above discussion is strictly factual and generic, and should be of interest for anyone contemplating installation of a wireless system. In the following, we apply these principles to a specific radio, namely the PulsAR-24027 Wireless Ethernet Bridge from AFAR Communications.
At its full link speed of 2.75Mbps, the receiver sensitivity is -90 dBm. In a typical point-to-point configuration, with 24dBi antennas, the link budget looks like this:
As you can see, the European rules are much less favorable for outdoor deployment of unlicensed wireless equipment. The reports one reads on the Internet about very successful installations would lead a knowledgeable observer to conclude that many people are cheating on the power limits and getting away with it. Still, the numbers are not as bad as you would have expected from comparing the EIRP limit (16 W versus 100 mW) because you can back off a bit on the fade margin with the shorter distances: The amount of rain drops in the path will be less.
AFAR Communications, Inc | 81 David Love Place | Santa Barbara, CA | USA 93117
T: +1 805 681-1993 | F: +1 805 681-1994
T: +1 805 681-1993 | F: +1 805 681-1994