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.
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