Topband: Re: diversity.

[Tom McDermott]

Lots of good discussion on diversity, some comments:

In general, an HF channel experiences Rayleigh fading (while an opening
exists!). Rayleigh fading means that two multipath signals of almost equal
strength are present, and the result is that they occasionally cancel one
another resulting in a fade.
	
 You may be familiar with the 'waterfall' chart showing the bit error rate
performance of a digial signal on a non-fading channel. A Rayleigh channel
degrades that waterfall into a straight line (bad, in that an extremely
strong signal does not result in as good performance as you would desire).
Diversity improves the slope of the fading curve. Dual-diversity doubles the
slope (good), quad-diversity quadruples it (even beter), etc. up to the
limit imposed on a non-fading channel. Basically, what this tells us is that
(from a probability point of view) diversity is simply selecting the
strongest of the two (or four) channels.

A good and more available current reference is:

"Fading Channel Issues in System Engineering", Stein, Seymour, IEEE Journal
on Selected
	Areas in Communications (JSAC), VOL SAC-5, No 2, February 1987, pg
68-89.

Diversity can be accomplished in a number of different ways, or combinations
of those ways: polarization, space, time, frequency, code, etc. On a
Rayleigh channel, diversity is worth a lot.

	-- Tom, N5EG