Diversity Combiners
Diversity Combiners
(OP)
Can anyone help me to understand the differences between pre and post detection diversity combiners.
I'll describe the application.
I've a digital PCM signal that is being transmitted at S Band (say 2GHz). It is transmitted using left and right hand circular polarisation.
The receiving antenna then passes the two received signals to two receivers and they are connected to a diverstity combiner. The combiner can either be a pre detect or post detect. WHich is better and why?
I'll describe the application.
I've a digital PCM signal that is being transmitted at S Band (say 2GHz). It is transmitted using left and right hand circular polarisation.
The receiving antenna then passes the two received signals to two receivers and they are connected to a diverstity combiner. The combiner can either be a pre detect or post detect. WHich is better and why?





RE: Diversity Combiners
I'm curious about how you send the same (?) signal with two opposite circular polarities. Are they on the same RF channel? If so, then you might just have made slant linear polarization the hard way.
RE: Diversity Combiners
I don't know much about the application yet so I don't know if they are transmitting on different frequencies (I guess they are).
RE: Diversity Combiners
RE: Diversity Combiners
RE: Diversity Combiners
http://citeseer.ist.psu.edu/rahman05pre.html
While in the USAF, I worked on troposcatter (2GHz) systems with analog post-detection quad diversity combining receivers (tubes). These were upgraded to pre-detection quad diversity combining receivers (solid-state).
The modernization from tube to solid-state was the significant improvement.
Check library for books on radio communications, especially tropospheric scatter radio.
"The Lenkurt Demodulator" was a trade magazine that covered microwave/troposcatter communications. You might be able to find issues in a library or a used bookstore.
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RE: Diversity Combiners
BTW - The transmit string was baseband feeding 1 IF modulator (70MHz) feeding two up-converters (2.1 & 2.4 GHz, for example) feeding RF Power Amps (1KW). The receive string was 4 receivers (2.1V, 2.1H, 2.4V, 2.4H), each down-converted to 70 MHZ, combined to a single 70MHz channel, and detected to baseband.
The baseband was analog FDM (frequency division multiplexing).
The same system "supposedly" could be used for digital TDM (time division multiplexing) with filter changes.
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RE: Diversity Combiners
www.google.com/search?q=ViaSat+QDC-100
One simple explanation is that with multiple antennas being actively combined, you could (for example) steer a null onto a noise source based on the antennas and wavelengths at the RF carrier frequency. But once you detect the signals and noise, then the noise (now baseband) cannot be canceled without also canceling the signal.
The fundamental advantage is the differences in the several signals at RF. Many of those differences disappear once you detect the baseband signals. By detecting, you're tossing away critical information that exists only at RF.
That's the explanation programmed into my gut. But I suspect that it's only one aspect of a larger truth.
RE: Diversity Combiners
Design Handbook for Line of Sight Microwave Communication Systems
www.every
para 4.4.25.2 - "...the earlier the signals are combined the better is the ultimate performance."
sec 4.5.34 Diversity - more explanation of diversity and combining with digital/analog system.
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RE: Diversity Combiners
I knew there were newer references. This is from a computer science paper.
www.t
The paper concludes pre-detection maximal ratio combining is optimal to maximize SNR of the received signal.
Post-detection combining may perform better under some conditions, especially impulse noise.
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RE: Diversity Combiners