Cwa-website: FAQ: Add equivalent distances to dB levels in FAQs

[MikeMcC399]

Where to find the issue

https://www.coronawarn.app/en/faq/#encounter_count_19
https://www.coronawarn.app/de/faq/#encounter_count_19
https://www.coronawarn.app/en/faq/#encounter_19_calc
https://www.coronawarn.app/de/faq/#encounter_19_calc

Describe the issue

Some new FAQ articles explain risk calculations referring to dB levels. There is no display of dB levels in the CWA so these figures have no meaning to the majority of app users. I imagine also that most readers will not know that dB is a logarithmic scale.

Suggested change

Add explanatory distance levels to the dB levels referred to in the FAQs.
According to https://github.com/corona-warn-app/cwa-documentation/blob/master/cwa-risk-assessment.md "each dB value is associated with a particular distance".

Attenuation | Distance
--- | ---
55 dB | 1.5m
63 dB | 3m
73 dB | 8m

[dsarkar]

OK, sounds good to me. Will be included.

[vaggelisdouros]

2 questions from my side:

a) can you provide the formula that translates the bluetooth signal attenuation (measured in dB) into distance (measured in meters)? In multiple cases, it is mentioned that a 73 dB attenuation corresponds to a distance of 8 meters, whereas 55 dB are translated into 1.5 meters, but i have not found the formula used or at least a reference to a paper/textbook.

b) In the updated diagram for risk calculation with enf v2, it is unclear to me how the time where the signal attenuation is between 63 dB and 73 dB is taken into account. Unless there is a typo, It seems that this signal is considered harmless and does not take part in the calculation of the weighted exposure time (even with a 0.5 coefficient), therefore i do not really understand whether the threshold is set to 73 db or 63 dB https://github.com/corona-warn-app/cwa-documentation/blob/master/images/risk_calculation/risk_calculation_enf_v2_overview.pdf

[MikeMcC399]

@vaggelisdouros
These are good questions. I suggest you submit them as two separate questions through documentation question.

Note that Google states in https://developers.google.com/android/exposure-notifications/ble-attenuation-overview#distance-proxy:

Attenuations as distance proxy
"Attenuation is a very noisy proxy of distance. A very low attenuation indicates a very high probability of a short distance, but a high attenuation can be caused by many phenomena and is not always indicative of a long distance."

You may also find some interesting background reading in https://developers.google.com/android/exposure-notifications/ble-attenuation-overview concerning the calibration procedure for Bluetooth Low Energy (BLE).