rf engineering
The microwave engineering of highly reliable and deterministic terrestrial links is
a complex and detailed undertaking. MCIS is an industry expert in this field.
The reliability, diffraction loss, and rain modeling algorithms used by MCIS are
very conservative and have been used to design US military communication systems
for years.
The microwave communication system is designed under 3 conditions:
Standard, Extreme and Survivability.
Typical Standard conditions are 50 mph winds
and heavy rains (tropical storm). Reliabilities quoted by MCIS are always under
those standard conditions (under normal weather conditions reliabilities are
typically much higher).
Extreme conditions are very low probability events where
system degradation is acceptable but system failure is not (e.g. a reduction in
throughput is expected but the link is maintained). These are typically 90 mph
winds and heavy rains.
Survivability is typically defined by the local building
code or 120 mph, whichever is greater. All of these conditions are customer
defined and cost-performance trades are developed to help the customer make
informed decisions.
The RF path and propagation analyses are a critical piece of a microwave
communication system and must include terrain and climate modeling, diffraction
analyses caused by obstructions in the Fresnel zone (Radio Line of Sight) of the
RF wavefront, multipath and reflectivity analyses, and wind loading (causing
angular deflection or mispointing of the antennas) to name a few.
Climate modeling
includes the loss of reliability due to climate conditions typical to the area
(cloud density, water droplet size, storm intensity, etc. are taken into
consideration), as well as the regions refractivity gradient data swings causing
sub-refraction or super-refraction of the RF wavefront.
Terrain modeling in
conjunction with reflectivity analyses is very important in regions that are flat
or over water. In cases where the RF path traverses water regions that are
affected by tidal variations, tidal analyses must be performed (the tides rise
and lower the effective height of the antennas thus changing the reflection point).
The above elements are a very important part of any microwave communication system
because these conditions can cause severe diffraction loss from obstructions in the
path that may not cause any signal degradation under standard atmospheric refraction
conditions (in the Southeast US and coastal areas these conditions are very common).
Other changes in atmospheric conditions move the reflection point or specular
reflection region which can cause severe multipath fading under those conditions
and very little fading under other conditions.
For systems involving multiple radios (especially in the un-licensed spectrums),
self-interference tends to be the industry’s worst problem. To avoid this problem
noise floor testing and analyses, as well as self-generated noise source testing
and analysis must be performed at each site in order to optimize system performance
and achieve high reliability under all conditions.
An RF interference/isolation
analysis must be performed at each site to verify a co-channel C/I of at least +12
dB or to whatever is required by the radio (this is required to obtain the
published BER, typically 10^-6, used in the reliability calculations). Based on
this analysis, frequency reuse plans, polarization diversity plans, and spatial
filtering requirements are developed.
Calculated interference from all transmitters
at each receiver will be analyzed to verify the C/I requirements are being met.
Any co-located receivers will also be analyzed with respect to near-field coupling
of the co-located antennas and the required isolation will be defined. These
analyses drive the antenna selection with respect to required main beam roll-off,
side-lobe suppression, back-lobe suppression, and polarization isolation.
In order for the system to work, a detailed path survey must be performed. This
path survey must be physically surveyed. This means that the height of every
building or tree in the path must be measured. Although this is time consuming,
it is imperative for the success of the system. Using radios without space
diversity (all un-licensed radios and many licensed radios do not have space
diversity) usually requires the lowest possible heights to avoid severe multipath
fades. However, 60% of the first order Fresnel zone must be cleared under all
climate conditions.
To optimize system performance in all climate conditions, the
height of all obstructions must be known. The terrain and what is on it in the
specular reflective region (a region that surrounds the reflection point) is very
important to determining the effect of multipath (e.g. if there is a forest in the
specular reflection region, multipath interference would be expected to be minimal,
however if there is a lake, multipath interference would be very large).
MCIS is very well versed in all aspects of engineering a highly reliable microwave
communication system. It is our opinion and the opinion of our clients that having
a communication system that works all the time is worth the extra effort it takes
to do it right. Regardless of whether the system is utilizing unlicensed or
licensed spectrum, the laws of physics do not change and the same level of
engineering is required to obtain the highly reliable system our clients demand
and MCIS delivers.