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Official Position on Adaptive Predistortion

Gerald-K5SDR
Gerald-K5SDR FlexRadio Employee ✭✭
edited June 2020 in New Ideas

Periodically the topic of Adaptive Predistortion comes up for discussion on this community.  Our team has been consistent in our position on this subject but sometimes it is useful to restate what has been said many times before in the interest of clarity.  Here is a restatement of the facts:

  1. The FLEX-6000 Series hardware has the necessary connections and internal RF port to port isolation to support adaptive predistortion software.  We have verified that transmitter output to receiver input isolation (e.g. ANT1 to RXA) is >80 dB on all bands including 6m, which is more than ample to accommodate adaptive predistortion using an external amplifier in the loop.

  2. FlexRadio remains interested in adaptive predistortion technology but we have not in the past nor will we now make a time frame commitment.  Our internal engineering resources are 100% allocated to complete our published road map for v1.x releases through v2.0.  We believe that the features committed in those releases are currently of higher priority to the vast majority of our customers than is adaptive predistortion.  

  3. We periodically review and rank our software backlog and feature requests.  We look at the ranked priorities and compare the development magnitude for each feature to the available engineering resources within a release cycle.  Based on those factors, we make decisions on what features make the cut in the release planning process.  So far adaptive predistortion has not made the cut.  That doesn't mean it won’t make the cut in a future release.
73 and thanks for your support,
Gerald
«13

Comments

  • Ken - NM9P
    Ken - NM9P Member ✭✭
    edited June 2020
    Thanks, Gerald.  I will be glad to install APD when you get to it, but for now....First things first.  
  • KT0AM  - Mark
    KT0AM - Mark Member ✭✭
    edited June 2020
    Dang... my background is geophysics and data processing and I don't have a clue about adaptive predistortion. I've often thought that some of the techniques used in geophysical data processing, like FK filtering (as opposed to time domain filtering) my be interesting to play with but this is a new concept for me. Anyone have a quick explanation?
  • Bob G   W1GLV
    Bob G W1GLV Member ✭✭
    edited June 2020
    Gerald, we stand behind you on that decision. Your leadership abilities have made what FRS is today. Continue with the fine job your team provides for the Flex community.
  • Jay -- N0FB
    Jay -- N0FB Member ✭✭
    edited December 2019
    I agree with Bob.   It sound like you have the correct priorities.  Full steam ahead.  As always, please thank your staff for their hard work.  We do appreciate them.

  • WA6FXT Mike
    WA6FXT Mike Member ✭✭
    edited June 2017
    Gerald, Like Bob says... "We stand behind you ..." In fact, I'm sure others, like myself, stand in front and help try and respond to those who want to interfere with the current roadmap. We ALL want everything (hihi); but, most of us are perfectly happy with the path FRS has chosen. That is not to say some would not have preferred a different sequence (hihi); however, I'm enjoying each and every update. (Bugs included... Heck, I love a challenge. 73's and keep up the great work! Mike
  • np2g
    np2g Member
    edited November 2015
    Guys
    It is just a positive event  for a absolutely state of the art  transmitter .One that exceeds the present possibilities .  And smart companies  Like Flex will implement it .

    The statement has always been  sooner would be better.

    So the next time someone asks if you would like that million dollar signal. Try saying NO.  

    When Flex does have the time to implement  Pure signal or whatever they call it  It will be the best .

    Just remember the fastest and shortest distance between 2 points  isn't a straight line ,
  • Steve K9ZW
    Steve K9ZW Member ✭✭✭
    edited November 2016
    http://bit.ly/1whDgYV Third down is the wiki ... 73 Steve K9ZW
  • KT0AM  - Mark
    KT0AM - Mark Member ✭✭
    edited December 2016
    Thanks for the link, Steve, very interesting.
  • Bill N5TU
    Bill N5TU Member ✭✭
    edited April 2016
    Hi, Gerald:

    Thanks for the communication.  I look forward to the implementation of adaptive predistortion, but you have plenty to do right now.

    73, Bill, N5TU
    Dit-Dit!

  • Rob N4GA
    Rob N4GA Member
    edited January 2015
    Basic equivalent in the analog world of negative feedback in a differential amplifier?

    But in this case, done digitally?


  • Ken - NM9P
    Ken - NM9P Member ✭✭
    edited December 2016
    I would say it is more like forward error correction. Though the analogy is not totally accurate. Or like what Bose does on some of its speakers .... It modifies the input into the amp in order to account for the imperfections in the speakers.
  • k3Tim
    k3Tim Member ✭✭
    edited December 2016
    It is more more complicated than that...

    Also, 100% agree with the OP also (if that means anything...) 

    k3Tim

  • Rob N4GA
    Rob N4GA Member
    edited January 2015
    Tim,

    I doubt it's a ton more complicated. What's normally so "complicated" about things like these, are the terms used (adaptive pre-distortion in this case) and the convoluted explanations.

    Sounds to me simply that you characterize the PA at a given frequency, mode, load, etc., then modify the input to the PA to optimize for the PA characteristics.

    All done digitally.

    If that's accurate, then it should be called something like digital error correction, or digital negative feedback, etc.

    Not something that sounds like you are introducing distortion.

    :)

    Last, the **** for the buck is probably minimal, thus the low priority being given by FLEX.



    N4GA


  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited January 2017
    Here are some screen shots from a talk at Friedrichshafen 2014 that may explain ADP better than words

    HOWEVER. Flex Need's to finish 1.4, 1.5 and V2.0 BEFORE they start with ADP which is still clearly a "Science Project"

    imageimageimage
  • k3Tim
    k3Tim Member ✭✭
    edited December 2016
    Sometime ago I worked on a design of an HDTV transmitter that would use the pre-distortion.  This was for 16-QAM and it saved a lot of grief compared to using conventional tuned filters to get sharp skirts.  A PhD from local University did the algorithm design (the University was/is in top 5 Engineering schools).  I recall the h/w design had about 20 FIR taps running at high speed.  I believe the problem is the conditions are changing dynamically (load / PA current / Temps / etc) therefore one has to correct dynamically.

    I did see an IEEE paper about using some phase inversion of the 3'rd and higher order products, feeding those back and that would also seem to work.  A  quick review seemed to show it was an easier route but not sure if it would apply to this setup.

    I don't particularly care the term Pre-Distortion either.  In the case of the 16-QAM one would know where in the constellation the symbol should be and where it ended up in the PA and then 'offset' it in the input stream to the PA so it landed exactly as expected.  That would seem an easier problem than complex voice signals.  Since the PhD handled the algorithm not sure.

    If I come across an interesting / readable paper I'll report back.  Problem is the day job keeps me busy and at night / early morning the 6500 has my full attention.  
    What a beast!

    Best Regards,

    k3Tim

  • Rob N4GA
    Rob N4GA Member
    edited January 2017
    Very nice, thank you for posting that.

    What would be nice is if APD could eliminate ALL the high energy harmonics so you could eliminate post PA filters all together.

    I wonder if this could be accomplished with a different PA topology (vs push-pull) along with APD. Imagine maybe parallel amps with one side used primarily to negate harmonics, all controlled with APD. Execution gets very complex very quickly...

    Going form -16 to -53 on IMD3 is pretty darn impressive though!
  • k3Tim
    k3Tim Member ✭✭
    edited May 2020
    The complete PPT is at:

    http://www.n3sh.org/Tech%20Section/PureSignal_final_selection.pdf

    slides 20&21 shows the correction algorithm - doesn't look complicated but the transcendentals take time to compute.  Converting to FPGA code I won't comment on.

    In this blog:
    https://sdrzone.com/index.php?option=com_easyblog&view=entry&id=34&Itemid=518

    that author states only IMD is corrected so it seems harmonics are not.  Also he states:
    "badly tuned linear amp stages" can't be corrected.  To me that's the crux of the problem in the worse offenders.

    Bye

    _..--
    k3Tim




     
  • Rob N4GA
    Rob N4GA Member
    edited May 2020
    Good stuff Tim!

    To convert an algorithm to FPGA code, you use EDA tools from Synopsis, Mentor or Cadence.

    Mentor's is called Catapult C and costs about $200k fully configured! 

    You write Algorithms in C and it converts to VHDL or Verilog for your FPGA designing pleasures.


  • k3Tim
    k3Tim Member ✭✭
    edited December 2016
    Ouch !  $200k.  You would think one could write the C code and have a consultant / contract company convert a project for you to VHDL.  The design / engr / production gets hugely expensive.  I am plugged into this owing to the day job.

    I wasn't aware they were converting C to VHDL - thanks for the info - good stuff.

    VHDL is a big step up from the old Mead-Conway method:
    http://en.wikipedia.org/wiki/Mead_%26_Conway_revolution

    I also see we have JTAG on the PWB, a big plus for factory testing.

    Best Regards,

    Tim


  • np2g
    np2g Member
    edited August 2018
    Wait a minute guys 
    If we are looking at the present implementation of "Pre   or clear signal " the code is already out there  Yes In open source  .
     
    The hardware is already in the Flex to accomplish this event .
    According to Flex the flex was built with the ability to do this.


    So     there is a relatively easy to follow  Open source  programming " call it a guide " that if anyone wanted to do this  Anyone!!!!    it could be utilized  . If just to start things off. 

    And Now putting the entire package into the FPGA  isn't a real smart idea.

    I think it would benefit anyone thinking about  this , a simple You Tube on a present system using the reduction would explain far more than any   techno papers that were written that started this whole thing . 

    What you would find that there is USER necessities  presently to set this up. You would find that there is ,When adapted this way, secondary programming  which can reside in the processor that can accomplish this A/B comparison. This gives a relative easy way to continually improve or modify it . 

    Simple statement . And I can give you the techno analysis . 

    As it has been named It compares the wave form of a transmitted signal in the band pass (PRE) and compares it with the output of the transmitted signal (Both of equal strength) the difference in this wave form  is sees and measures it corrects /  shifts to match the template . 

    This goes for any and all  difference it sees (In that band pass) compared to the algorithm it uses as a template. 

    The result in the real world is (Seen and heard every day) a CLEAN IMD reduced  SIGNAL Which can be displayed( using the most harsh of tests the 2 tone  test)  . The test  just about every manufacturer stays away from .

    And 200K Well  will take time (Programming time)   Someone else did it with volunteers .(A true ham spirit) 

    I am glad we are actually understanding what benefits we would get. And how significant utilization of absolutely any system that reduces IMD  is fantastic .

    And I am glad some day Flex will Implement it . 





  • Gene - K3GC
    Gene - K3GC Member ✭✭
    edited July 2018
    I just read this thread and now I have a head ache.

    Could someone explain adaptive pre distortion or pure signal or whatever it is; in baby talk.  I love my 6500 and I really like talking on the radio but I don't begin to understand pure signal (sounds great,- pure anything sounds great :) )

    Please tell me what it is all about and why I care.

    Color me confused
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited April 2019
    Simple Explanation;

    All transmitters are non-linear and as a result they put out spurious signals that are not on the transmit frequency.   Adaptive Pre Distortion (ADP) measures this non linearity and corrects the input signal to the transmitter so as to minimize the spurious output signals.

    Should you care?

    For most applications - it probably wont make that much of a difference as modern transmitters and amplifiers already do a pretty good job of suppressing the unwanted signals.  It is nice to have if you live near other hams and want to reduce your interference to them.

    In theory it should really help with digital modes such as PSK where IMD can make receiving difficult
    However, the non-linear nature of the correction can distort the digital transmit signal so it is recommended NOT to use ADP for digital modes.

    OTOH.. if you are a fanatic who wants to put your best foot forward.  ADP will make your signal better.

    Pure Signal is a Brand Name of ANAN for ADP.. so always use ADP when referring to the function.
  • Gene - K3GC
    Gene - K3GC Member ✭✭
    edited July 2018
    Many thanks, Howard
    I think I have a grasp on it now.
  • WQ2H - Jim Poulette
    edited March 2020
    Gerald - Many thanks for the update; sounds like a plan. Sticking to the technology roadmap shows discipline, focus, and a definite slant for the long haul. Aside that, you really don't want to many irons in the fire at the same time anyway.

    73 Jim, WQ2H


  • Burch - K4QXX
    Burch - K4QXX Member ✭✭
    edited July 2018
    Just to point out, this update was 4 years ago.
  • WQ2H - Jim Poulette
    edited August 2018
    My point exactly. I've been in product development for 30 years and I know how hard it is to muster concentration and stay focused on a plan. It's understandable and inevitable when priorities shift - but chasing every ambulance that drives by is not the way to do it. From the behavior of my 6600M so far, it looks like whatever plan they have in place is working pretty well.

    73 Jim, WQ2H
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited August 2018

    There are still so many other more impactful things on the TO DO LIST such as Multi Client that ADP is not yet at the top of the list.
  • Lee - N2LEE
    Lee - N2LEE Member ✭✭
    edited August 2018
    While there is possibly some users who see APD as a must have, I believe that 95% of the people  buying radios either don't know what it is or could careless. I don't think I am going out on a limb by saying most of us are looking for radios with better receiver performance and features such as better noise reduction. 
  • KY6LA_Howard
    KY6LA_Howard La Jolla, CA. Paris and Sablet FranceMember ✭✭✭
    edited August 2018

    I agree with Lee... but to clarify.. better TRANSMIT Spurious Noise Reduction which unless you have a very good spectrum analyzer - you will never know its is being reduced.
  • Ernest - W4EG
    Ernest - W4EG Member ✭✭
    edited October 2019
    Living in a CC&R HOA community with a vertical surrounded by bamboo trees: I could care less about APD!

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