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CC8520: Hopping average time of occupancy

Arthur Thompson27
Expert 1800 points
Part Number: CC8520

Hello,

Our certification lab is looking into compliance with FCC 15.247, a(1)(iii), it says:

Frequency hopping systems in the 2400-2483.5 MHz band shall use at least 15 channels. The average time of occupancy on any channel shall not be greater than 0.4 seconds within a period of 0.4 seconds multiplied by the number of hopping channels employed. Frequency hopping systems may avoid or suppress transmissions on a particular hopping frequency provided that a minimum of 15 channels are used. 

Here is what we know about our system:

  1. The unit has 18 hopping channels, that means the average time of occupancy shall not greater than 0.4s within 7.2s period. 
  2. We also know that the transmissions sent by the Master and Slave both are variable in length between a minimum and maximum duration.  We also know that the maximum duration transmission happens immediately after the Slave joins the network.  We have measured the maximum duration to be 1.14ms.
  3. We know that in a given 100ms time period, the unit will transmit on a given frequency a maximum of 8 times.  (There are 40 timeslots in a 100ms period, 1/5 of these timeslots are devoted to each Active Channel)
  4. Combining #2 & #3, we can calculate a worst case duty cycle of 9.12% (8*1.14ms)/100ms
  5. If we hold the worst case duty cycle over the entire 7.2s period from #1, the time of occupancy over that period would be 7.2*9.12%=0.66s which is over the 0.4second limit

I am confident that the units do not actually transmit at the maximum 1.14ms duration over the entire 7.2sec period, but I'm not sure how to actually demonstrate compliance.  Has TI looked at this before?

Thanks,

Arthur

  • 0
    TI__Mastermind 27145 points

    Hi,

    Strictly speaking, it should meat the spec mentioned in your Point 1.  (The average time of occupancy on any channel shall not be greater than 0.4s within 7.2s period. ).

    But if you are confident (it occupies <0.4s) then you can show the Probability of Channel Slot Occupancy time calculation to your test lab. We have shown this Probability Calculation on our 15.4 Stack Frequency Hopping Scheme.

    Attached is the SWRA529. Please refer to section 4.2 (Table 2).

    Also please make sure that it will pass (it occupies <0.4s) in the Lab tests.

    swra529a.pdf

    Thanks,

    PM

  • 0 in reply to PM
    Expert 1800 points
    PM,

    I'm not sure I understand it as well as you. Could you tell me what variables you would fill in for Equation 1 in the linked paper to prove the point?

    How do you suggest we test the (occupies <0.4s) requirement in the lab?

    Thanks,
    Arthur
  • 0 in reply to Arthur Thompson27
    TI__Mastermind 27145 points
    Hi,

    Please refer to "Binomial Distribution" link under References Section of the App note SWRA529.

    You mentioned " I am confident that the units do not actually transmit at the maximum 1.14ms duration over the entire 7.2sec period, but I'm not sure how to actually demonstrate compliance. Has TI looked at this before?" in your post.

    Did you test it in your lab about the above statement?

    You should test and make sure that the channel occupies <0.4Sec in 7.2 Sec in most of the times and then you can provide the Probability theory based on your values.

    Thanks,
    PM