Radio Overhead impact in FCC Duty-Cycle calculation

[p.martins]

Dear Nordic support,

I am trying to calculate the maximum duty-cycle for the FCC certification.

To do so, I started by finding the ratio between the time the TX is On during each Link Layer packet transmission and reception. I am calculating the TX time by computing the time to transmit the 265 bytes of a packet with the LE Data Packet Length Extension, which is 265 bytes*8/1Mbps = 2120 us. The time per LL packet includes this, plus the IFS, the ACK and next IFS, yielding a total 2500us.

This yields a theoretical maximum duty-cycle of 84.80% (2120/2500), without taking into consideration any connection interval value nor packets fully fitting on the connection interval. According to this thread, the value is actually 86.40%, due to 40us on the IFS to prepare the TX radio. Can you confirm that?

However, after further investigation with the Online Power Profiler, one can see that, during a connection interval, there's at least 2012 us before and after the communication occurs where the antenna is not in TX, RX or waiting during the IFS.

My question is: is this overhead present in all connection intervals, effectively lowering the maximum Radio Duty-cycle possible?

All the best, 

Pedro

[helsing]

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.

[p.martins]

Understood, thank you for clarifying helsing !