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ATPC confusion

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Regular Contributor

ATPC confusion

So, ATPC and adaptive TX power for the PTP820 are completely two different things I gather.  Can someone help me understand when I would consider using the ATPC feature and when it may not be a good idea to use it?  I have it enabled on some 820S links but I'm not even really sure if that is advisable. After reading the user guide I'm not sure where it would come in handy.  Does enabling this help to keep the link at a desired TX at the far end so that the RX of the near end doesn't drop -like if a weather event moved across the link that may not affect the TX (of the near side) but would affect the RX (of the near side)?  Confused....

5 REPLIES
Moderator

Re: ATPC confusion

ATPC is Automatic Transmit Power Control.

 

Here are some problems that ATPC tries to solve:

 

  1. If the transmit power is always maximum, the transmitter’s lifetime may be reduced.

 

  1. With a hub site that has more than one link, if one of the links is much shorter than the others, and if the far end transmit power is always maximum, the short link’s far end transmitter may interfere with the other far end transmitters. (This is the “near-far” problem—near transmitters may drown out far transmitters.)

 

  1. Short links with constant reduced transmit power might experience performance degradation during fading.

 

ATPC automatically adjusts the far end transmit power to maintain a reference near end Received Signal Level (RSL). Enable it at both ends.

 

This allows the link to transmit only as high as needed to maintain a given RSL.

 

The transmitter only consumes as much energy as required to meet the far end RSL requirement—ATPC is a "green" feature!

 

This can help the transmitter lifetime, since the transmitter likely won’t transmit at full power all the time.

 

This can assist with the near-far problem and fading problems. Normally, the transmitter operates at lower than maximum power. If a fade occurs, the transmitter increases its power to overcome the fading, provided that the fading isn’t too severe.

 

ATPC is not useful for long links where the transmitters need to operate at full power to meet the throughput and availability requirements. (On a long link, ATPC, if enabled, would never be exercised, since the link always operates at full transmit power.)

 

Does this help?

 

Dave

Moderator

Re: ATPC confusion

Wait.

 

How do you tell whether or not your link is "short" enough to take advantage of ATPC?

 

Use LINKPlanner, of course.

 

You can enable and disable ATPC—watch the "Maximum Power" at each end as you enable and disable ATPC. Does the Maximum Power change? If so, the link may benefit from ATPC.

 

Dave

Regular Contributor

Re: ATPC confusion

Dave, as always you give a great explaination. It does help. Thank you.

 

What is the recommended RSL sweet spot in regards to lifetime with 11Ghz links?  I have some links where we would normally get -26dBm RSL and I turn ATPC on to back off the power and set it to something around -40.  But, that may be too high?


Also, what is considered max power for these?  For example, I have some links that operate at 23dBm per my FCC profile script 1501 in adaptive mode.  But, it will ramp up the power to 26dBM it it needs to modulate at 64QAM or whatever.  So, in this case, is the max power you are referring to the 26 or 23dBM, or both?  Should I set the APTC RSL just above what it needs to obtain the maximum modulation level I'm looking for? 

 

Thanks!

Moderator

Re: ATPC confusion

 

The deployed RSL should never be above -30 dBm. (When it's higher than -30 dBm, it's as if the far end radio is shouting. The near end radio can't discern the message in shouting because it's just too loud. Remember the Tom Petty, Dylan, and the Dead concert of your youth–you should have donned your earplugs when your ears started screaming.)

 

As you know, I'm partial to LINKPlanner.

 

Use LINKPlanner to see how a reduction in transmit power affects availability.

 

If you reduce transmit power and it doesn't affect availability, you should consider using ATPC.

 

If you reduce transmit power and it reduces availability below your requirements, you should not use ATPC.

 

Let's assume your availability is five nines. (The link's 2-way availability, including rain, is 99.999%.)

 

If you reduce the transmit power by 10 dB to reduce the RSL from -30 dBm to -40 dBm, and the availability is still above five nines, then you can use ATPC and set the radio's RSL reference from -30 dBm to -40 dBm.

 

If you reduce the transmit power to reduce the RSL, and the availability goes from five nines to four nines and a five (99.995%), then you probably don't need ATPC. (The link is probably too long for ATPC--there's no transmit power budget to "spend" or "save".)

 

 

I hope this helps!

 

Dave

 

 

Regular Contributor

Re: ATPC confusion

It does help.  Thank you for explaining that.