[barbegal]

The big problem with all radio based contact tracing solutions is the receiver. Receivers use at a minimum several mW of power. A typical Bluetooth low energy receiver will use about 50mW so will drain a coin cell battery in a day. Bluetooth low energy saves energy by transmitting for very short periods of time so transmit power is low but receiver power can be high. Even asking most consumers to leave their phone's Bluetooth recovery on all day is a big ask because of the impact on battery life.

Schemes could be designed to better synchronise receivers and transmitters so the receiver can be switched off most of the time but this currently can't be done with Bluetooth.

[dmitrygr]

Your phone's Bluetooth scanning IS always on if it's an iPhone or Android. Always. By default.

[Taniwha]

Yes, and it has a honking big battery that you charge every day or so.

A card with a proposed 6 month battery life with receiver PLLs that are always spun up probably needs more battery than you can fit in a tiny thin card

[barbegal]

That may be true when the screen is on, but from my experience scanning is not switched on in sleep modes on iPhones and most Android phones. I don't have definitive proof of this but I have never seen a modern phone be able to reliably scan whilst in sleep mode for any long period of time.

[dmitrygr]

it is on just at a lower duty cycle.

source: worked on this for a few years at googl, both chromeos and android