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On Tue, 27 Apr 2010 21:04:15 -0700, Jeff Liebermann
wrote: Example: Can someone tell me which line number offers the meaning for Diversity? A link to the Wikipedia page would probably have been sufficient: http://en.wikipedia.org/wiki/Diversity_scheme The problem is that NONE of the diversity schemes mentioned in the Wikipedia article apply to the single antenna example under discussion. So, this is an example of a "straw man" argument (not yours, Tom's): a solution to a problem that is undefined. There are, thus, many solutions that none can refute and why Tom's is the sine qua non is built on a foundation of sand. In my never humble opinion, there's no way to provide any form of diversity reception improvement with a single antenna, unless one also has two feeds, going to two different receivers, and ending in either a decision switch, or some form of intelligent combiner. Well, to Tom's credit, there is ample discussion of that - but that discussion does not answer the question, which means there is no way to test for validity. I am not interested in interpretations of Tom, nor abstractions culled together from disjoint statements. How about my definition? Sorry, Jeff, but unless you are the author of the Wikipedia reference, I cannot answer your question. No matter which scheme is used, a diversity reception scheme must demonstrate an improvement in availability, BER, or SNR over a single antenna, or it's not really diversity. I presume the statement above is your definition. Reducing S+N/N satisfies what you call diversity and provides an example of a self-referential definition in that you appeal to with "SNR." Self referential definitions are logical nulls. In other words, does increasing capture area qualify as diversity for a single antenna? If so, diversity means less noise or a better signal in comparison. What is diverse about ordinary directivity? What is the profit in having two words describe the same thing? Even with an informal presumption of the meaning of diversity, we can both agree that diversity is not also directivity. Or perhaps it is that, and with one characteristic more. This returns us to the question with some refinement: what is diversity in the face of directivity? I have a hunch directivity is a distraction, but that returns us to the original question. I want to know where (literally, not figuratively) Tom defines what Diversity is. He doesn't. I didn't think so and I was asking because I didn't consider it worth the effort to search for something so obscured by the baggage of peripheral discussion. I'm rather confused as to his "stereo diversity" which I guess uses the listeners ears and brain as the decision switch or decoder. I think he might be referring to a direct conversion receiver where one channel is quadrature leading and the other is quadrature lagging, resulting in a stereo-like effect. I will admit this was my interpretation too. Strange how you have to sift the diamonds out of the horse-****. I had worked in this field and built quadrature detectors 40 years ago to the same ends as you describe. Analog TV color detection had been doing it for at least 20 years before that. I suppose there is a metaphor of diversity there, but it came with the subject of quadrature detection as a solution, not as a recent invention. The quad detector is a direct conversion receiver as you say. For other readers: The signal is split through two channels each mixed with the same base-band source, with one feed of the source shifted 90 degrees for one channel. I suppose here we could drop the input splitter and simply feed in two antenna drives. The separate mixer outputs feed separate headphone elements (the classic application way back then) and the brain perceives the signal as existing in a literal 2D (binaural) space. The consequence of this perception is a heightened ability to discriminate one signal from the rest within the bandpass of reception. The bandpass is perceived as a physical left-to-right space and because the classic application was through headphones, this space was also between the ears. For the modern reader, this was like having a spectrum analyzer in your head. This is the classic situation of being able to listen to one conversation in a crowded room full of speakers (the cocktail party problem) without becoming overwhelmed by overlapping dialog. A simple test is when you tune to the signal of interest, any off-frequency signals are perceived as inhabiting this 2D space at a literal off-center. As no two transmissions occupy the exact same frequency (Tom explicitly mentions errors as small as a quarter Hertz), then they lose being at the center of attention. The brain supplies a huge computational engine that computers have yet to match. The topic of Quad detection is cool in its own right, but I don't see how tarting it up with the discussion of Diversity (especially when the term is one of dim provenance) really adds anything. Quad detection read more like window dressing than the clincher to the topic at hand. 73's Richard Clark, KB7QHC |
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