Link Availability – why is this concept so important for the design of a Microwave Radio link?
Radio link design: Using ‘Link Availability’ to predict link annual outage time (as determined by frequency, link distance, terrain characteristics, climate, diversity methods etc.)
A question that often arises relates to achieving the maximum possible data throughput speed between buildings, towers or masts. Yes – this objective is important. However, one ALSO needs to specify the time-related availability of this connection (normally specified per year).
While the data throughput speed requirement is often considered to be a final objective for a radio link, ‘Link Availability’ is actually the primary consideration when doing a radio link design.
It does not help to have a microwave radio link achieving the required data throughput speed for a fraction of the year. When there are link outages, there are drops in receive signal level that result in reduced data throughput speed or result in link ‘bit’ or ‘frame’ errors – the link will then be considered to be ‘Unavailable’.
The consequence of poor or no link design (i.e. not considering link availability) can affect mission- and life-critical systems.
Design choices of location, antenna size, antenna height, link distance, the terrain, the frequency and climate affect the calculation of Link Availability.
For rural deployments, the minimum distances between masts or towers can often be in excess of 50km – so, how does one go about achieving maximum data throughput speed, while at the same time ensuring that the Link Availability time is maximized.
Another way to think about this Link Availability objective is as follows – how can we minimize link the link outage probability (Pw)? Link Outage Probability and Link Availability time are closely related using the formula (Link Availability = 1 – Pw).
Firstly, what is ‘link availability’? What does it mean and how important is it? A criterion for link design of carrier-grade microwave links is often achieving 99.999% (or ‘5-nines’) availability. The current United Kingdom availability objective for bidirectional transmission is 99.994% per 100 km (Freeman, 2006).
There are 365 x 24 x 60 minutes or 365 x 24 x 60 x 60 seconds in a year. 99.999% availability means the annual outage time will be (1-0.99999) x 365 x 24 x 60 = 5.3 minutes. The table below shows the ‘Outage time’ for different link-availabilities.
So, if the microwave radio link designer does not pay attention, there could be an ‘unwitting link design’ with say 99.990% availability, leading to ~1 hour of outage over a year. ‘Outages’ arise over the course of a year when there are link bit or frame errors or the link speed drops due to periodic reduced receive signal level.
Why does the receive signal drop (fade)? Answer: depending upon link location, there are statistically predicable Atmospheric Refraction changes that affect the amount of energy that propagates over the earth surface as an electromagnetic wave and finally reaches the intended receiver.
The equations below are helpful to consider the practical implications and effects of link distance on ‘Link Availability’. Please note that ‘Link Availability’ is proportional to distance cubed. So, while one may not need a high link fade margin for a 5km link, the link availability’s cubic-sensitivity to distance means that a much higher link margin is needed for a 20, 30, 40, 50 or 60km link (so, much larger antenna sizes are needed for longer distance links!
The requirement for larger diameter antennas has impacts on product, installation and maintenance costs; logistics; tower-loading (twisting and swaying) practicalities etc.).
The two equations below show which variables are important when trying to minimize link outage time. Link distance (range) is the most important because Link Availability is proportional to d3.
As an example, using the Barnett-Vigants [B-V] method, let’s consider average terrain and climate factors, 5.8GHz and a Link Margin of 10dB. The link availability is 99.999% when the link distance is 3.1km. Put another way, again using the Barnett-Vigants formula, the 5.8GHz link margin must be 25.6dB for 10km, 34.5dB for 20km, 39.8dB for 30km, 43.4dB for 40km and 46.5dB for 50km.
While there are differences between the B-V and ITU methods, one gets an idea of the effects on link availability due to distance: to achieve the same link availability, the link’s receive power must be increased by 20.9dB (more than 100 times) for a 5-fold increase in link distance (i.e. from 10km to 50km).
Link Availability Formula Summary
(Basile L. Agba, March 22–26, 2010) Outage Probability Barnett-Vigants
C=0.25 – good propagation conditions (mountains/dry climates) (Garrison, 2001-11-15)
C=1 – average propagation conditions (average terrain/climate)
C=4 – difficult propagation conditions (over water coast)
ITU P.530 Model
ITU P.530 Formula for use in the calculation of link availability
Pw is a % (ITU, 09/2013) ITU P.530 Model (Equation 8, page 8 of Rec. ITU-R P.530-15)
Pw= the percentage of time (for quick planning applications) that fade depth FFM (dB) is exceeded in the average worst month.
f = Frequency [GHz]
d = Link Distance [km]
FFM= Flat Fade Margin [dB] = PRx-PSensitivity
hL = The height above sea level [m] of the lowest antenna
 An older availability calculation method, but still succeeds in showing the effect of link distance on required link margin.
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