• State Verified Answer
  • Replies 8 replies
  • Subscribers 70 subscribers
  • Views 1610 views
  • Users 0 members are here
Attachments (0)
Nordic Case Info
  • Case ID: 313953
Options

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

Parents
  • 0

    Hi Pedro, sorry for the delay.

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

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

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

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

  • 0 in reply to helsing

    Hi  ,

    Thank you for your reply.

    helsing said:

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

    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

Reply
  • 0 in reply to helsing

    Hi  ,

    Thank you for your reply.

    helsing said:

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

    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

Children
Related
Terms of service