Establishing the Link Budget

The final step in the whole process is establishing what are known as link budgets for the
individual airlinks. This term describes a series of calculations for determining a reliable
connection between two points, and it is just that—a budget encompassing the signal gain
provided by the antenna and the external RF amplifier, if one is used; the losses imposed by the
normal attenuation of the signal over distance; the losses from varying atmospheric effects;
and the losses from various equipment-related issues at the receiving end such as ohmic losses
in the connector cables. Also figuring in the calculations are the gain of the receiver’s antenna
and the sensitivity of the receiver, that is, the threshold signal level required for the receiver to
achieve a specified output level at a specified signal-to-noise ratio. Normally the network operator
will want to establish a margin of at least 40dB above the sensitivity threshold of the
receiver; this is termed fade margin. This margin ensures that the transmission will not be
interrupted by temporary increases in interference or by unusually adverse atmospheric
conditions.
Note that fade margin is directly related to availability (the percentage of time the network
is down). Conventionally, each 10dB of margin translates into an order of magnitude increase
in availability. Thus, a radio link with a fade margin of 50dB will have five nines (99.999 percent)
availability, which is the standard metric for wireline systems.
Link budget computations differ considerably depending on the band in which the network
is operating. Atmospheric attenuation, for instance, is relatively unimportant in the lower microwave region but hugely important in the millimeter microwave region. In the first
instance it can generally be left out of the calculations, and in the second it is ignored at the
operator’s peril.
In calculating the link, budget network operators have one main variable with which to
work—namely, the transmit power of the nodes. By increasing power they also increase the
fade margin and rise above the background interference. But they also increase the interference
seen in other links, and they soon face legal limits as well. Broadband access radios are
generally limited to a few watts maximum just about everywhere, and for the unlicensed bands
the figure commonly is 1 watt.
Given the diversity of terminal designs in the marketplace and the changing parameters
over frequency, a complete listing of all relevant calculations is impractical. As in many other
areas of network design, manufacturer support is essential here.
Again, formal link budgets are generally established only for the most critical airlinks,
those serving multiple high-value customers or providing base station backhaul. The cost in
technician time of optimizing the link for each subscriber to basic access services is just too
great. The following is an example of a link budget table, courtesy of Alvarion:

    
        

    
    

where the following is true:
SP: Spare (available margin) over sensitivity
PT: Transmitted power
GB: Base station antenna gain
GT: Subscriber unit antenna gain
PL: Path loss (using a chose propagation model)
FM: Fade margin (as required for quality of service)
RA: Rain attenuation
LS: System losses (cables attenuation, implementation losses, antenna gain reduction)