Hack 81 Calculating the Link Budget
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Figure out whether a long distance link is even possible before you buy any equipment. How far will it go? That's a very good question. It depends on all sorts of factors, including the power output and sensitivity of your card, quality of your cable, connectors and antenna, intervening clutter and noise, and even weather patterns (on long distance links). While it's impossible to precisely take all of these variables into account, you can make a good estimate before buying any hardware. The following describes a simple way to build an estimate, referred to as the link budget. First, figure out how much loss the signal will incur in the space between the two sites. This is called the path loss. One common formula for estimating path loss at 2.4 GHz is: L = 20 log(d) + 20 log(f) + 36.6 where L is the loss in dB, d is the distance in miles, and f is the frequency in Megahertz. So, suppose you wanted to set up a five-mile link between two points, using channel 6 (2.437 GHz): L = 20 log(5) + 20 log(2437) + 36.6 L = (20 * 0.69) + (20 * 3.38) + 36.6 L = 13.8 + 67.6 + 36.6 L = 118 At five miles, with no obstacles in between, you will lose 118 db of signal between the two points. Our link must tolerate that much loss (plus a bit extra to account for weather and miscellaneous interference) or it will be unreliable. If you don't want to bother calculating path loss on your own, you can use Table 6-1 to get a rough estimate. This table was computed with the above formula and rounded up.
Now that you have the free space path loss, add up all of your gains (radios + antennas + amplifiers) and subtract your losses (cable length, connectors, lightning arrestors, and miscellaneous other losses). Let's assume you are using Orinoco Silver cards (15 dBm) and no amplifiers, with a 12 dBi sector on one side, and a 15 dBi yagi on the other. I assume you're using 1 meter of LMR 400 and a lightning arrestor on each side, allowing a 0.25 dB loss for each connector, and 1 dB for each pigtail. Since all of the units are in dB, we can use simple addition and subtraction: Site A: Radio - Pigtail - Arrestor - Connector - Cable - Connector + Antenna 15 - 1 - 1.25 - .25 - .22 - .25 + 12 = 24.03 Plus Site B: 15 - 1 - 1.25 - .25 - .22 - .25 + 15 = 27.03 Equals: 51.06 total gain Now, subtract the path loss from that total: 51.06 - 118 = -66.94 There you have the perceived signal level at either end of the link: -66.94 dBm. But is it enough for communications? Look up the receiver sensitivity specs for the Orinoco Silver card to see how much signal it needs. You can look it up in the radio card documentation, or consult the handy table in [Hack #65]. Consulting the table, it appears that the Orinoco Silver has a receive sensitivity of -82 dBm at 11 Mbps. As we are providing a signal of -66.94 dBm, we have a "fudge factor" of 15.06 db (82 - 66.94 = 15.06). Theoretically, this usually works at 11 Mbps (in good weather), and should have no problem at all syncing at 5.5 Mbps. The radios should automatically sense when the link becomes unreliable, and re-sync at the fastest possible speed. Typically, a margin of error of 20 db or so is safe enough to account for normal intervening weather patterns. Using more powerful radios (such as the Cisco 350 at 20 dBm, or the EnGenius/Senao at 23 dBm), more sensitive radios (again, like the Cisco 350 or EnGenius/Senao), or higher-gain antennas would help shore up this connection to 11 Mbps. Using higher-gain cards in conjunction with high-gain dishes makes it possible to extend your range well beyond 25 miles, but be sure to observe the FCC limits on power and gain. Online tools like Green Bay Professional Packet Radio's Wireless Network Link Analysis can give you a good ballpark estimate on what it will take to make your link possible. Check out their excellent resources at http://www.gbonline.com/~multiplx/cgi-bin/wireless.main.cgi. |
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