Building a microwave radio network can be a challanging task. However, if
planned properly, enterprises can avoid a lot of headaches and also problems
that may arise once the network is being used.
Clearing fresnel zone: Irrespective of whether the radio link planned
is point-to-point or point-to-multipoint, the first thing to do is to verify
that it will have not only a clear line of sight, but at least 60 percent of the
first fresnel zone be clear of obstructions as well. The longer the distance,
the more important it is for this verification. If the fresnel zone is blocked,
then you will get a lower signal level on the distant end than expected-even
if you can literally 'see' the other antenna in the distance.
Perform RF-path analysis: Even if the fresnel zone is partially
blocked, it is still possible to get a link, provided that the radio system is
designed to have a strong signal at the other end of the link. In planning
long-range microwave links where unobstructed line-of-sight and clear fresnel
zone are not certain, an RF path analysis should be done. There are many
software packages available that have terrain data and can create a path
profiles from latitude/longitude coordinates.
line of sight: The software programs for RF path analysis can only indicate for
certain if a link will not work due to terrain obstruction. A clear path on
paper is not a guarantee that your link will work, since it does not show trees
or buildings. So, even if there is a 'clear' link on paper, RF analysis may
show if there are 80-feet trees that can block the signal.
SOM calculation: In case of both clear line of sight and 60 percent of
the first fresnel zone clear (or nearly clear), how can you know if you will
have a good link or not? How much gain do your antennas need to have? How much
coax cable loss is too much? If your link is at 2.4 GHz, should external
amplifiers be used? Or, given your fixed base-station antenna with a pre-set
gain, how far can you reach with the different types of client antennas? And,
which clients will need amplification? By doing a system operating margin (SOM)
calculation, you can test various system designs to see how much fade margin (or
safety cushion) your link will theoretically have.
The SOM is determined by computing the difference between the received signal
and the radio's receiver sensitivity. The typical formula used is:
RX signal = EIRP — FSL + RX antenna gain — coax cable loss
Regarding the minimum SOM needed, there is no absolute answer to this
question, but the higher it is, the better. Most engineers agree that 20 dB or
more is quite adequate. Some think as low as 14 dB is still good. Others operate
systems down to 10 dB or less.
The problem with accepting a lower SOM is that you have a smaller safety
margin. You run the risk of your link going down in case of interference, an
antenna off its aim, atmospheric conditions, moisture in your coax,
ice/rainwater on the radome, or a host of other factors.
Determining interference: SOM is not the only determining factor. It
is the actual SNR at the receiver that makes a link reliable. If there is noise
or interference on the channel, the SNR will deteriorate. This could be an issue
if you are co-locating at a site with other radios operating in the same band.
You need to find out what frequency spectrum these radios are occupying. If
these transmitters have energy or sideband noise on your receive channel and
their antennas are close to your's, you will likely get interference from
them, perhaps to the point where your link will not work.
Using an amplifier: With coax cable at the receiver and no amplifier at the
receiver antenna, the SNR at the antenna does not survive when it actually
reaches the radio itself. In this case, the noise generated in the RF front end
of the radio is a factor. If an amplifier is used on the receive end, the SNR
(as it appears at the antenna) is preserved all the way down the coax to the
radio. This phenomenon largely occurs because the low-noise amplifier mounted on
the pole sets the noise floor for the system.