Radio Overhead impact in FCC Duty-Cycle calculation

RE: Radio Overhead impact in FCC Duty-Cycle calculation

p.martins — Tue, 26 Sep 2023 17:02:30 GMT

Understood, thank you for clarifying helsing !

RE: Radio Overhead impact in FCC Duty-Cycle calculation

helsing — Fri, 22 Sep 2023 12:29:00 GMT

Measuring the maximum duty cycle is usually simpler than calculating it. Another straightforward method is to simply assume a 100% duty cycle. This duty cycle needs to be calculated over a 100ms timeframe.

The distinction between a 100% and an 85% duty cycle is minimal, and by assuming 100%, you can ensure that you never exceed it. When you increase the duty cycle, you get a logarithmic increase. For instance, going from 50% to 100% corresponds to a 6dB, while going from 85% to 100% only results in a 1.4dB.

RE: Radio Overhead impact in FCC Duty-Cycle calculation

helsing — Thu, 21 Sep 2023 13:40:35 GMT

Hi Pedro, sorry again for the delay. I will get back to you later tonight,

Best regards,

Håkon

RE: Radio Overhead impact in FCC Duty-Cycle calculation

p.martins — Wed, 20 Sep 2023 17:00:19 GMT

Hi helsing ,

Did you have the time to look into my message above?

RE: Radio Overhead impact in FCC Duty-Cycle calculation

p.martins — Thu, 14 Sep 2023 13:53:21 GMT

Hi helsing ,

Thank you for your reply.

[quote userid="90815" url="~/f/nordic-q-a/103385/radio-overhead-impact-in-fcc-duty-cycle-calculation/445415"]

For certification, you may use a 100% duty cycle to avoid any surprises.

[/quote]

I actually want to estimate a worst case scenario that is not 100%. If my understanding is correct, according to the BLE protocol, the peripheral device can never be transmitting 100% of the time, so I would argue that using a 100% Duty-Cycle is too restrictive and not realistic. Of course it is the safest, but doing that doesn't come without tradeoffs on the maximum radiated power.

Onthis ticket, it is argued that the maximum theoretical duty-cycle possible is 86.40%, but it doesn't take in consideration the overhead of the Radio that occurs every connection interval, as you have confirmed in your answer above. Assuming that that overhead is realistic, wouldn't it make sense to estimate the worst possible Duty-cycle as the maximum amount of TX active it can occur in a connection interval? Do you consider this to be an incorrect way of approaching the problem?

To compute that duty-cycle, I am factoring in:

the connection interval time;
maximum packet length;
Radio overhead;
maximum number of packets in a connection interval (considering the max packet length, the ACK response and IFS)

For instance, for a connection interval of 45 ms, at 1Mbps PHY, with the LE Data Packet Length Extension, one would get:

MTU (bytes)	247
ATT max packet (bytes)	251
LL max packet (bytes)	265
Time per LL packet (us)	2500
Connection Interval overhead caused by Radio start-up (us)	2012
Time left for TX+RX+IFS in a connection interval (us)	42988
LL Packets per Connection Interval	17
TX ON time per LL packet (us)	2120
TX On time per connection interval (us)	36040
Theoretical max Duty-cycle	80.09%

For a 15ms connection interval, for instance, the Theoretical max Duty-cycle of 70.67%. Since the overhead is constant per connection interval, the greater the connection interval, the greater the duty-cycle.

If this way of approaching the problem is reasonable to you, them I would argue that the maximum Duty-cycle possible for the BLE could be computed, similarly to the above, by using the maximum supported connection interval by the peripheral. Does any of the above make sense to you?

Do you see any reason why we should stick with the 100% instead of the 80.09%, assuming the maximum supported connection interval of our peripheral device is 45ms?

Thank you in advance!

All the best,

Pedro

RE: Radio Overhead impact in FCC Duty-Cycle calculation

helsing — Mon, 11 Sep 2023 12:37:07 GMT

Hi Pedro, sorry for the delay.

[quote user=""]I am trying to calculate the maximum duty-cycle for the FCC certification.[/quote]

For certification, you may use a 100% duty cycle to avoid any surprises.

[quote user=""]My question is: is this overhead present in all connection intervals, effectively lowering the maximum Radio Duty-cycle possible?[/quote]

This overhead is necessary, however, you may adjust the length of the connection interval and then send and receive multiple packets per connection interval.

RE: Radio Overhead impact in FCC Duty-Cycle calculation

p.martins — Mon, 04 Sep 2023 16:10:51 GMT

Hi Håkon, thank you in advance!

Best regards,

Pedro

RE: Radio Overhead impact in FCC Duty-Cycle calculation

helsing — Mon, 04 Sep 2023 12:39:37 GMT

Dear Pedro, thank you for the request. I will look into this and get back to you.,

Best regard,

Håkon