“Optimal Combining” Diversity ?

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I ran across a couple of interesting discussions started by W5UN and K1UO on diversity reception.   Below are the links and excerpts from Steve’s (N5AC) replies:

https://community.flexradio.com/flexradio/topics/6700_vs_k3_still_no_contest

 “The situation is this -- not only is the FLEX-6700 capable of what I would call "antenna diversity with human decoder" where you stick two receivers that are locked, one in each ear (a.k.a. poor man's diversity), the FLEX-6000 has the necessary hardware to support MRC/optimal combining.  Poor man's diversity is essentially a form of what is called selection diversity where two ears are used to actively select the best of the signal at any point in time. This is how classic voting receiver systems worked on repeaters as implemented by GE and Motorola in the day. With selective diversity, two signals with different SNRs are presented to a voting system (electrical or your ear) for the selection of best SNR at any point in time.” 

“In contrast, MRC/optimal combining is different in that DSP is used to look at the correlated signals (the ones you want) and non-correlated signals (noise) and then subtract out the noise. This nominally provides a 3dB SNR improvement on the signal you give to your brain. This is really a big difference -- it can essentially lower the noise floor by 3dB at your location. Again, this can be 3dB better than the BEST of the two signals you get off the two antennas which is not possible with selection diversity.”

“Additionally, we have a mechanism in the FLEX-6700 to take a single antenna and run it to two data converters.  This is also another form of diversity, but it aims to lower the noise floor of the data converters.“ 

https://community.flexradio.com/flexradio/topics/diversity_reception?topic-reply-list%5Bsettings%5D%5Bfilter_by%5D=all

 “Two of the diversity forms we discussed -- the linked slice or simple diversity method and the optimal combining method -- made the cut. The first method we plan to do in v1.3 and the second is more of what we call a science experiment. In other words, we want to do some research and testing to find out what works best before we commit to a date. When we do this, we generally do not put the item on the roadmap because we do not want to have to come back and say "well it doesn't look like it will make xyz release after all." Having said this, we have already started work on optimal combining and the quicker that work comes to conclusion, the quicker you will see it in the software.”

Assuming that the current implementation is the “simple diversity” version, is MRC/optimal combining, and/or routing a single antenna to 2 data converters to lower SNR, still in the works, even if in the science project stage? 

Thanks,

Howard 

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HCampbell WB4IVF

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Posted 3 years ago

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Michael Coslo

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In an aside to this, I did experiments a few years back on different frequencies comparing a vertical and horizontal antenna to answer a few questions about "which antenna is better - fodder for a lot of conversations. The setup included an antenna switch, and a decade attenuator pad. Antennas were a 96 foot coublet 55 feet above ground, and a Butternut Vertical antenna. While the results were expected for skip zones, esp on 20 meters with well known skip zones, there was one surprise.


When the band exhibited fade, and people complained of the changing band, I used that to switch between antennas. I might be on the dipole, and switched to the vertical. Bam! Op came in much better. Then they might fade again. Switch back to horizontal, and there they are again. This would continue until the band closed for the evening.

Did not do transmit because those little attenuator don't care much for that, releasing the magic smoke that makes them work. But it might be a nice application for what you speak of. One horizontal, one vertical antenna.
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HCampbell WB4IVF

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I noticed the same thing sometimes switching between antennas before getting the 6700.  Since getting the 6700 I’ve tried pairing up my tx antennas with rx antennas for diversity with often good results.   I just got a 2x6 remote tx antenna switch to replace a 1x6 switch so I can pair up my tx dipoles with my tx verticals too.  My next antenna project will be a 160M inverted L to complement the current 160M dipole.  Finding a good location that keeps some spacing between them while keeping the L away from the neighbors might be a problem though.  So this will be my last antenna project for a while, since I don’t want to push my luck with the neighbors any further! (-:

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Mystery Ham

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I've been thinking over some similar thoughts regarding diversity on the 6700.  For the radio amateur with close in neighbors and, thus, sources of RF noise out of his or her control, a diversity mode that allows the 2nd receiver to use a smaller antenna to receive the localized noise and use processing to null out the noise.  It may require a slider controls to adjust phase and depth.

Mystery Ham
(Edited)
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Steve - N5AC, VP Engineering / CTO

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Official Response
Howard, we continue to work on a number of different projects like this in the background.  Discerning the best SNR in a digital signal is fairly easy since you have a quality indicator, the bit error rate (BER) and you can adjust for lowest BER.  The problem is more complex, for example, if you have a 500Hz bandwidth and there are several CW signals in that bandwidth.  Which should be optimized?  What if the optimization results in a null on one of the signals?  Having said this, we are still experimenting with algorithms in between the other research we are doing.

One thing I'd like to add to the discussion is that using the two ear method (called simple diversity before) has a unique property of spreading signals around in the sound stage.  We have heard from avid contesters that in a pileup this allows the brain to better sort through the signals, just like focusing on one person at a cocktail party in a noisy room.  You would lose this capability in a situation where the radio did the diversity combining for you, but in some cases this would be better.

I mat a cognitive researcher this last weekend and I got a chance to ask some ham radio relevant questions. I asked about the block that some folks have around CW and the inability to go past a certain speed.  I suggested that I believed there was a translation occurring in the cognitive process -- going from a sound to mental pictures of dots and dashes to a lookup table for CW letters to the letter.  She made a couple of very interesting comments.  First, she said that the brain pathways for sound are almost immediate and require very little in the way of cognitive horsepower.  If you can go from sound to meaning, you can do things much faster.  The second thing she said that caught my attention was that visual memories are more deeply lodged.  She said once you learn a visual memory, it's more difficult to shake or overwrite that.  The real lesson here is that if you want to teach someone the code, NEVER show them a chart of dots and dashes!  But in relation to this topic, it made me realize that the brain is so good at sorting out sounds in a sound stage that while simplistic, the simple diversity takes advantage of lots of horsepower in your noodle.

So we're still pursuing the optimal combining technique and hope to have something in the future, but I think based on contester experience and what I've learned about cognitive processes that the simple diversity method is useful by itself!
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HCampbell WB4IVF

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Steve –

I can relate to your comments on sound and meaning.  When I was learning code, it wasn’t until I started to recognize the “sound” of letters and then common terms instead of focusing on the dots and dashes that I started to really improve.  After retirement several years ago I got back into hamming, and although my speed is not where it was when I left off cw is still my favorite mode ....    

Actually, the currently implemented “two ear” diversity works pretty well for me, and I’m in the process of improving my remote antenna switching so I can use more of my antennas in that mode.   And when I saw your posts my reaction was wow, this could really be the icing on the cake!  I hope your research pays off.  Thanks much for the update!

Howard

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Ken - NM9P

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Steve, that explains exactly why I had a four-year long plateau at 10 wpm....which I have been told is the maximum speed possible for the "look-up table" method of CW reception. (Which is one reason the old General CW requirement was set at 13 wpm, to force people to learn it correctly as an "auditory alphabet" instead of a "code.")

I learned the code in 6th grade when they had an exercise in my spelling book with the dots & dashes. I passed 5 wpm easily as a freshman in High school, but choked on the 13 wpm. It took four years and lots of practice with Wayne Green's "butt buster" CW tape to get over the hurdle. And it took another 8 years to get over the 20 wpm hurdle.. and that only happened because I was writing a training program in GWBasic for my wife so that she could learn the code. Spending so much time testing at high speeds to perfect the program helped me get in the "zone."

The human brain is indeed a very powerful tool. For many types of sound processing it is still better than many complex computer programs. Now the trick it to combine the two for maximum effect.
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EA4GLI - 8P9EH - Salvador

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Ted, NX6C

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Salvador
   My FlexRadio 5000 with the dual receiver and the PowerSDR diversity adjustments have been providing me that exact kind of noise reduction shown on the YouTube video.  Those of us with the 5000 and dual receiver have been using this technique for a long long time.  I check the bands with the 6700 and it's many slices, but for real difficult noise problems that can benefit from the diversity of the 5000 I keep and use the 5000.  The two radios, the 5000 and the 6700 are the best combo I've ever seen or used.
Ted
NX6C
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EA4GLI - 8P9EH - Salvador

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It is a big thing I am still missing on the 6700... the current diversity is just...  "basic"? As demonstrated in that video Phase Noise Reduction is possible and it is being done (can't be called a experiment anymore). I really want to get the "WOW" features at some point. But I will leave the "complaining" for a later date as we are all now enthralled with 1.10 and redoing our shacks! :) Maybe after Christmas we can get back into Diversity for Noise Mitigation, Adaptive Pre Distortion, and a new one for me, now that I have 75w on 144MHz.... decent FM modulation. But lets enjoy the new milestone 1.10 version and we will keep nagging these guys to get the very best they can create after Christmas. :)
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ka7gzr

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I like the people above have used the diversity function with the 6700. In my case I have two magnetic loops phased 180 degrees from each other going into the two RX ports. I transmit on a vertical and it has worked very well for me.

The only issue I have had is with the software retaining my settings in memory and controlling the audio between the two RX only channels. I have outstanding software "bug" reports concerning this and hope they get addressed in a future release.