nRF9160 maximum transmit duty cycle

[Gmiddles]

I am curious if there is an inherit maximum transmit duty cycle of either M1 or the nRF9160.  I am asking for this because we need these numbers for SAR test.  Our product will be near the body and the duty cycle will be an important parameter for this test.

[Andreas]

Hi,

 

The highest possible duty-cycle with the current modem FW (v1.1.0) for Cat-M1 and CE mode A is 32 ms TX out of 37 ms duty cycle period. This is a very theoretical figure, but it is possible, constituting of maximum amount of repetitions for data channel (32ms) and no repetitions on control channel (5ms including 2ms for retuning gaps) . For Category-NB1, maximum amount of transmissions is 256 ms TX out of 296ms duty cycle with long data channel repetitions and with TX gaps available (40ms).

 

Best regards,

Andreas

[Gmiddles]

Andreas,

Thank you for the detailed response. The duty cycle was much higher than I was expecting. Which made me realize I didn't ask the question correctly. The very last sentence of FCC Title 47 Part 2.1093 (d)(2) says: Exposure may be averaged over a time period not to exceed 30 minutes to determine compliance with general population/uncontrolled SAR limits.

With this said, I assume the module couldn't transmit for 32ms out of every 37ms at 23dBm, indefinitely. I imagine the module would start to thermal regulate its transmit power. For example, I did some reading on BLE and referencing "Bluetooth Low Energy, The Developer's Handbook" by Robin Heydon. (specifically page124-125) when transmitting data in one direction on an encrypted link, the maximum duty cycle is just:

(max size packet + acknowledge packet) / (total time to send and acknowledge data)

(328 + 80) / (328 + 150 +80 +150) = 408/708 = 58%

328us is the length of an encrypted connection packet

150us "cool down" time required after a very long packet transmission

80us is the receiver's "acknowledgement" response with zero payload in the packet

With this example does M1 or the nRF9160 have a "cool down" time and/or a non-transmit time to listen for the base station's acknowledgment" or any other parameters that would limit the duty cycle over a 30 minute period?

The situation where I imagine the most amount of transmissions in a 30min period is when we do a firmware update over the air and the device is at a cell edge.  In this situation the device will need to acknowledge the messages it gets from the base station, but will remain in receive mode the majority of the time.

Thank you,

Greg

[Gmiddles]

Andreas,

Have you had a chance to look into what would be the maximum duty cycle over a 30 minute period?

Thank you,

Greg

[Andreas]

Hi,

 

I have not gotten a clear answer to this. I can confirm though that the MFW/protocol does not contain any cooldown time as you describe it, the temperature is only monitored and the module will shut down if the temperature exceeds 85 degC. This is heavily dependent on your HW though, the thermal dissipation capabilities of mainly the ground plane of your product.

I will update you when I have something more.

 

Best regards,

Andreas