I think that some clarifications about BLE and ANT+ hen working with HRV data are required.
First, the HR monitor calculates RR intervals in the same way regardless the communication protocol you want to use. Quality of the RR measurements depends only on the hardware you wear.
Then, transmission of this information from the HRM to the paired device happens in a different way depending on the selected protocol:
- BLE: the HRM transmits every second the RR-int corresponding to all beats (up to 6) happened in the last second. So it could send information corresponding to HR around 360 bpm
- ANT+: only RRint from lst beat is sent in each message. Frequency of these messages is set when you start the communication between the HRM and the device. It can go from 1 to 4 Hz. Setting the frequency at 4 Hz allows to send information up to HR around 240 bpm without missing beats (enough for human beings)
Bad reputation of ANT+ is due to Garmin using low frequency for transmission when using this protocol. In this case it is enough for HRV analysis at resting condition (purpose of all HRV metrics provided by Garmin), but it will miss lots of beats when exercising. So, RRints natively recorded by Garmin under ANT+ are not valid at all for further DFA and you have to go to BLE.
In case of alphaHRV, comunication is directly managed by the app and frequency is set to 4 Hz to avoid missing beats. Comparing results from both BLE and ANT+ in that way has shown same results.
So, there is no issue related to ANT+ itself, but to native use of ANT+ by Garmin.
Any app (Garmin, android, iOS desktop) recording RR intervals under ANT+ can get same results as using BLe if frequency is correctly set.