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In article <saitowit.6@underdog.ee.wits.ac.za> saitowit@underdog.ee.wits.ac.za (SAITOWITZ L S) writes:

>Hi,

>Just a couple of quick questions, I've been looking for these 
>answers everywhere, hopefully somebody here can help.

>Using spread spectrum techniques:

>1)What are the trade offs between bandwidth and capacity?
>2)Can one predict a relationship between the capacity used 
>  and probable error rates?

>I realise these are very general and sweeping questions, but 
>all answers will be appreciated. (Please give refs if possible)

>Thanks 
>                                    Lesley Saitowitz
>                                    saitowit@underdog.ee.wits.ac.za 

Leslie:

Your questions are very broad, but here's the general idea. 
The purpose of spread srectrum is to gain immunity to
jamming or interference by using more spectrum than necessary,
and then "collapsing" the bandwidth at the receiver. This tends
to supress the jammer. 

An additional use of spread spectrum is the code division multiple
access (CDMA) scheme in which several users can share the same
bandwidth.

To discuss "capacity" I would have to know more about the
system you have in mind. Generally, capacity is not a concern 
because it is assumed that there is sufficient bandwidth for
all the users. Again, the purpose of SS is not to improve capacity,
but to gain immunity to jamming. BER is improved only in terms
of jammer-to-signal performance.

In a straight-ahead scheme, the spread bandwidth is the message
bandwidth times a large scale factor (the "spreading gain").
Bandwidth expansion may be on the order of 10 to over 10000 (for
military systems).

 Some good references:
"Digital Communications", John G. Proakis, McGraw-Hill
"Spread Spectrum Signal Design", David Nicholson, Computer
Science Press

Let me know if you have more specific questions.

	Laurence Mailaender
	Dept. of Elecrtrical and Computer Engineering
	Univ. of California, Santa Barbara
	lem@synapse.ucsb.ece.edu