Friday, June 5, 2015

What Good Is A Coverage Map?

Have you ever gone to a mobile provider's web site or their store to see their coverage map? You'll notice the maps never show speeds. There's a reason: the maps are generally based on signal levels, not speed. Instead, providers say things like "4G LTE coverage," which means nothing to the average user. If you don't have time to read this all the way through, skip to #3 below.

As part of our state-wide mobile testing, we have started to peel back the layer of coverage and ask, "What does coverage really look like when we use actual speed?"

Here is a quick comparison of a single provider's coverage, 1) as advertised, 2) as measured, in terms of average speed, and 3) as adjusted by subtracting one standard deviation from the average (mean) speed.

1. Advertised Speed.
Here is one provider's maximum advertised downstream speed for California. The bright green indicates a downstream speed range of 10-25 megabits per second. The lighter green is 6-10 megabits per second, and the yellow represents a range from 768 kilobits per second to 1.5 megabits per second.

2. Measured Speed.
Here is the same provider's downstream speed based on test results at 1,990 locations across the state. Results from each location were spatially interpolated using a method called "kriging" to create a continuous surface. In this case, the bright green areas are significantly smaller, and more colors have appeared, representing slower speeds: 1.5-3 megabits per second (orange), and 3-6 megabits per second (brown).

3. Mean Minus One Standard Deviation.
Here, we adjusted the measured results at each of the 1,990 locations by subtracting the mean value by the standard deviation for the location where the measurement was performed. We do this, because we have observed that the higher the speed, the larger the standard deviation. In some cases, the standard deviation can be as high as 30% of the mean. This adjusted coverage map may likely be a more realistic picture of your mobile provider's coverage.

4. Higher Speed = Higher Variation
How large is the average standard deviation? Below is a chart showing how large the standard deviation can be when averaged across all measurements for urban, rural, and tribal. Graphs are for both upstream and downstream.



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