Coverage predictions define the mathematical
likelihood that a randomly-selected location will
have a signal strength equal to, or greater than your
specified threshold. But there are limits. Because it is
averaged out across your entire service area you can
not be 100% confident that a specific signal level will
occur at a given time and place. No matter how good
the data, it cannot perfectly represent the real world.
Once your network is in place, you will want to confirm that you
are getting what you paid for–effectively a coverage guarantee.
Coverage Verification Testing (CVT) can give a specified
confidence (usually 99%) that your network is delivering the area
reliability you specified, by randomly sampling coverage across
your network service area.
To verify that your installed network meets coverage
requirements, your coverage must be specified as either:
• Covered area reliability – the proportion of randomly-
selected locations within the predicted coverage boundary
where service can be expected, or
• Service area reliability – the proportion of all locations within
the service area where service can be expected.
The level of service you require also needs to be defined. Where
possible, the service threshold should be a single, measurable,
objective value. Common coverage design thresholds are signal
strength (RSSI) and Bit Error Rate (BER), which may have been
derived from a specified Delivered Audio Quality
CVT physically measures area reliability in a robust, repeatable
Where to sample
and affordable way. In this situation, reliability refers to the
proportion of locations that meet or exceed the coverage
Statistical sampling requires each sample to be randomly and
independently selected. Obviously, if all samples were taken right
next to radio sites, the test would not be valid. Nor would taking
all samples in deep valleys at the edge of coverage. Neither
would provide an accurate measure of reliability.
If time and money were no object, every possible location would
be tested, and a very precise reliability measure achieved.
Clearly this is impractical; another approach is needed, to
balance precision and affordability. This requires a controlled
randomization approach, that balances random sampling and
even distribution, by spreading sufficient samples evenly across
the service area.
To distribute samples across the service area in an unbiased way,
coverage engineers create a test grid, which divides the service
area into evenly-sized test tiles (often one-to-two kilometres
square). A random sample is taken within each test tile. So, while
not random in the strictest sense, sampling is randomized within
each test tile. When designing the coverage verification test, the
coverage engineer can adjust the tile size, to ensure that enough
samples are taken to meet specified confidence levels, while
keeping the sampling as evenly-spread as possible. The tiles
themselves are not tested: they are simply a device to distribute
(As the actual sampling is performed by vehicles on public roads
– which conveniently replicates actual mobile radio use – any
tiles that do not have at least partial road access are excluded.)
Randomized Test Points