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Old 06-09-2016, 04:11   #1
zfyoung
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Smile discussion about the technical aspect of DRM implementation

Hi, since modern day telecommunications all apply some form of MIMO, ie. antenna diversity to improve channel capacity or reliability, I was curious why SW broadcast can not deploy the same technique to improve their signal quality.

Having observed the reception data from years of my DRM experiment and other listeners, I found the deep flat fading to be a major problem. when this fading is restricted well within DRM interleaver length of 2sec, it will pose no problem. But due to the nature of ionosphere, this simply is not always the case. And numerous listening experience (including AM) can prove this is quite common esp. when there is only single propagation (skipping) path. On the other hand, I never found RNZI suffer from this kind of problem. That can be explained by its channel profile: long channel impulse response. ie. sufficiently separated multi-path propagation indeed convert the flat-fading into frequency selective fading (which is handled elegantly by DRM design).

Now, if on the transmission site, we deploy antenna polarization diversity: ie. two signals is radiated at orthogonal polarization, and two signals are delayed in time sufficiently large to induce frequency selective fading but short enough not to violate guard interval constraint, wouldn't that solve the problem that has been plaguing the SW listening?

zfyoung
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Old 06-09-2016, 05:52   #2
Aetheradio
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I dont think any kind of diversity can be (practically) done at the transmitter. They would also be wasting half the power, most of the time.
But the serious SWL can implement space diversity antennas
like the ones we used at my first job here.
http://maritimeradio.org/himatangi-makara/makara-radio/
Unfortunately the cost of electricity and the ubiquity of internet (in the eyes of those who control budgets) suggests youd better enjoy it while you can.
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Old 06-09-2016, 06:26   #3
zfyoung
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[quote][wasting half the power/QUOTE]Actually I assume that the total output to 2 Tx antennas remains the same. Also, for ordinary receiver antenna, due to constant random rotation of EM vector, you will not get the max. theoretical response. After all, there is no news that the EM wave could change its polarization randomly when it exit the ionosphere and down to your receiver.

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Old 15-09-2017, 03:39   #4
zfyoung
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Smile Need your help in experimenting DRM diversity reception!

I know this is an idea that has been floating around for quite a long time, but no one seems to bother to try to make some experiment or progress in that respect.

Anyway, since Tx side diversity can't
Quote:
be (practically) done
as some of our forum members observed, I'd like to see if there is any potential (practical) benefit in Rx side diversity.

For that to happen, and to evaluate the performance in real world condition, I need your help in data collecting while I'll do the data crunching. Anyone interested in this project? Not too complicated, just digitally recorded IF samples are needed.

Regards

zfyoung
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Old 16-09-2017, 20:12   #5
F1BJB
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Hi
The digital nature of DRM should allow to merge two recordings from different
locations,aerials, frequency or time to get the best of them.
But as far as I know the recording format doesn't allow this and there is no
merging software.
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Old 17-09-2017, 01:01   #6
AF4MP
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I would combine the two receivers (main & diversity) at analog IF.

Since IF is 12 kHz - it would just need to be a good audio mixer.

To overcome any delay and phase issues, just use an audio phase combiner.

Otherwise use an RF phase combiner just like the AM dx-ers do. MFJ makes a noise cancel unit (MFJ-1026) that could be used - with the diversity signal instead of the noise signal.
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Last edited by AF4MP : 17-09-2017 at 01:35. Reason: clarified the use of RF phase unit.
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Old 17-09-2017, 03:33   #7
zfyoung
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Wink

Quote:
But as far as I know the recording format doesn't allow this and there is no
merging software.
Actually, quite simple: 2 sample channels and recorded in ".WAV" stereo format in left and right channel. That's it!

For optimal performance, of course, more channels, more robust signal. I would challenge the conventional wisdom of AM broadcast and push DRM to its limit by using antenna arrays (polarization and distance diversity). From what I'v calculated, 50 antennas should be OK for DRM Robust mode B in deep flat fading case.


Regards

zfyoung
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