One key component for developing a wireless network of any type, is the calculation of the link budget.

The link budget is nothing more than a relatively simple mathematical calculation of the system signal gains and losses relative to the signal source(s) and the receiver(s) signal strength necessary for reliable communication.

While the actual Link Budget calculation is simple addition and subtraction, the preliminary figures for the calculation are determined by the gains or losses in decibels (db). The formula used to determine the decibel gain or loss is 10 times the Logarithm of (P{one} (Power in Watts or milliwatts) divided by P{two}). Note that gains or losses are in "db" or "dB" (deci Bel or a tenth of a Bel - in honor of Alexander Graham Bell), while power levels may be referenced in dBm (milliwatt reference) or dBw (watt reference).

A typical link budget may look like this:

Transmitter

signal: 1w =  30 dbm

cable loss    -10 db

antenna gain + 6 db

total avail = +26 dbm

Receiver required signal = -80 dbm

Receiver antenna gain + 3 db

total signal needed at receiver location = -77 dbm

Path losses

92 db per mile (4 miles) - this value varies by frequency and inverse square law must be applied to calculate total free space path loss, in this case -104 db

 source: 30dbm losses: -114db gains: + 9db l&g -105db total -75dbm

In this case the total signal available at the receiver terminal is calculated to be -75 dBm, which is 2 db greater than necessary. But this is a very simple example, as there are no entries for attenuation from foliage, building materials, and other environmental factors. Also this example is for specific parameters and should not be used without understanding the other factors that affect received signal strength and quality.

Link budgets should be done as preliminary work before a wireless network is designed. Link budgets can be validated and adjusted by on site RF survey equipment. No contracts for a complex RF network project should be signed without at least a network performance clause based upon a link budget acceptable to all parties involved in the project.

 Home last update: July 22, 2005 17:42