Does Bluetooth Mesh have frequency jumping?

[Fridtjof]

Greetings!

I am looking into Bluetooth Mesh, and the allowed radiated power according to Norwegian law. The general limit for 2.4GHz is 10mW, but if we utilize FHSS the limit is 100mW. If we use other frequency-hopping strategies (DSSS?) the limit is 10mW/MHz. So we can use higher power if we do frequency hopping.

This brings me to my question: what frequency hopping techniques are in Nordic's implementation of Bluetooh Mesh? And (if applicable) what is the bandwidth that I should plug into the 10mW/MHz formula?

I have seen that frequency hopping was not recommended for "swarms" or "meshes" 5 years ago (devzone.nordicsemi.com/.../229217), but I don't think Torbjørn is talking about Bluetooth Mesh in his answer.

I have also seen that DSSS is implemented in the "Wireless UART" sample (https://devzone.nordicsemi.com/f/nordic-q-a/48486/2-4ghz-models-support-dss-or-fhss), but this is not Bluetooth Mesh, which might be different.

-Fridtjof

[Edvin]

Hello Fridtjof,

Bluetooth Mesh uses the standard Bluetooth LE advertising channels, 37, 38 and 39. As far as I understand, the FHSS requirement is 5 channels, so Bluetooth Mesh does not meet this requirement.

Best regards,

Edvin

[Fridtjof]

Thank you for the answer, Edvin! Do you know how BT Mesh switches between these channels (e.g. is it DSSS?), and what number I can plug into the 10mW/MHz formula?

[Edvin]

No, it is not DSSS. The channels are spread out through the spectrum (to avoid the WiFi channels). See this figure:

(source: https://academy.nordicsemi.com/courses/bluetooth-low-energy-fundamentals/lessons/lesson-2-bluetooth-le-advertising/topic/advertising-process/ )

The Mesh stack is built on top of the softdevice controller, so it behaves the same as an advertiser and scanner would do in normal BLE applications.

So whenever it sends out a message, it will send it as a packet on all 3 channels, in a burst. The devices that are not transmitting are always listening. I am not sure exactly what parameters they use (scan window and scan interval), but it will have a scan window = scan interval, to get the max RX time, and it will increment the channel on every scan interval. 

Fridtjof said:

and what number I can plug into the 10mW/MHz formula?

I am not sure exactly what formula we are talking about here.

Best regards,

Edvin

[Fridtjof]

I think I was able to figure it out using your answer and the official EN 300 328 specification which Norwegian law references.

1. Bluetooth Mesh does not fulfill the requirements in `EN 300 328`. This spec does not actually define what Frequency Hopping is, but does as you say include a provision that IF you use FHSS THEN you must have at least 5 channels.
2. The 10mW/MHz is a Spectral Power Density limit, which is relevant for equipment which does NOT jump between frequency channels, but uses some other spread-spectrum method.

So in conclusion we Bluetooth Mesh falls under the general 10mW e.i.r.p. limit for 2.4GHz equipment, and cannot use a higher transmit power despite using several channels.

Thanks again, Edvin!