Why does MAXPWR differ across LTE bands?

Hello,

Testing with the MC7411 across different bands shows that MAXPWR is not 23 dBm uniformly across all bands. E.g., on b41, MAXPWR shows a limit at 22 dBm. Is there a reason that some bands have a different limit from the 23 dBm max tx power permitted per the 3gpp spec?

Also: do all Sierra Wireless IoT modules (7411, 7455, …) share the MAXPWR configuration, or are some shipping with all bands at 23 dBm?

To reproduce:

AT!ENTERCND=“A710”
AT!MAXPWR?41,2

Thanks!

@philipp

So for the class of device we are the 3GPP spec’s state that UE’s must transmit at +23dBm +/- 2dBm in any of the bands we operate. In our spec we have defined tighter limits for each of the bands within this 3GPP spec limit to give better predictability to our customers based on calculations and measurements from our design and production calibration procedure (each unit is calibrate individually to make sure it meets the spec). We could just state the 3GPP limits and still be fully in line with the specs.

This margin (particularly the wide 3GPP limit) is because performance across bands will vary depending on the silicon (variation within the actual components) and design of the unit.

Regards

Matt

Thanks, @mlw, for the background. I had a couple follow-up questions:

In our spec we have defined tighter limits for each of the bands to give better predictability

That’s interesting! Could you provide some additional information what the predictability is with respect to? What would be some potential (negative) effects of upping MAXPWR for those bands that are currently at 22.0 or 22.5 dBm in the modem?

We could just state the 3GPP limits and still be fully in line with the specs.

Does that mean I could adjust MAXPWR to 23dBm for all bands and still (technically) remain within the 3GPP spec?

Thanks again!

@philipp

For the record here are my responses to the commands, not sure why some of them have responded with error, I did not look too closely.
EM7565 maxpwr responses.txt (745 Bytes)

So the ‘predictability’ is around knowing more precisely the actual level the unit will transmit when commanded to transmit at full power based on our factory calibration. This is going to be important when doing things like Verizon OTA testing as they have very tight RF budget limits.

As to the original question, why the difference, I don’t know, but I suspect, that some harmonics were probably closer to the limit that the team liked and as a result they had the margin to reduce the power transmitted and meet the specs more comfortably.

You can increase the power, but as the warnings all over the command tell you, you might make it transmit outside of the mask, the devices are calibrated in production to make sure they meet all of the required specs. The units do not explicitly ‘know’ they are transmitting at 23dBm, they only know what the calibration process tells them. For instance in the past for some really special projects I have manually modified the calibration point so that when the unit was told to transmit max power, which it thinks is +23dBm, it was actually +26dBm (as I say they were special projects).

Regards

Matt

Thanks, @mlw, for the additional info, very helpful. Curious: I assume the calibration is shared for all devices of a type, i.e., all EM7565 will share your configuration? Or could calibration vary between one EM7565 and another? Thanks again.

And: Could the Verizon firmware (say) differ from the Generic firmware with respect to their MAXPWR settings for a band?

@philipp

Configuration will be the same across all devices of the same type as they will be the same RF design but if it changes then the team will do what is best for it i.e. it is likely that EM7565 and EM7511 are the same but since the RF is different it might not be.

Regards

Matt