🚀 go-pugleaf

In article <1990Dec20.231903@envy.bellcore.com> karn@thumper.bellcore.com writes:
]Where processing gain *does* buy you something is in the presence of
]narrowband interference in your signal passband ("narrowband" == much
]narrower than the spreading bandwidth). An interfering narrowband
]signal will be *spread* by the despreading process at the receiver to
]the full spreading bandwith of the system, so for our example its
]effect on the wanted signal will be reduced by the 20-dB processing
]gain of the system.
]
]So "processing gain" would be better described as spread spectrum's
]"unwanted signal rejection ratio", analogous to the "adjacent channel
]rejection" figures given for conventional narrowband receivers. Since
]the latter figures are often 60 dB or more, you can see that spread
]systems are actually at a disadvantage when the signals are not all
]closely matched in level. This is the well-known "near-far" problem,
]and solving it requires careful transmitter power control.

Try this on for size. This stuff is subtle enough that I hope I
got it right:

Some of this depends on whether it is best to think of it in the
"time" domain or "frequency" domain. For example, if we achieved
our "spread spectrum" by relatively slow frequency hopping, we would
only have degradation when the signals coincided - which could be
very rare. Consider a system where the frequency is swept in a sawtooth
manner. If the the sawtooths of two stations are anti-synchronized
(synchronized out of phase), you would only experience interference
as a click when the other guy reset his ramp (and thus generated a
broadband signal). This would be a system best looked at in the
time domain (and perhaps it shouldn't really be called spread "spectrum").

On the other hand, if an interfering transmitter has essentially a white noise
modulation, I agree that the processing gain does nothing for you over
a narrow band system.


Specifically - assume a 10kHz information bandwidth spread over
1 MHz. Sure enough, the receiver sees a 20dB better SNR than
a receiver using the 1 MHz bandwidth for 1 MHz of information.
However, so would a 10kHz narrow band receiver. No processing
gain here!

Assume that there are multiple systems operating in the same
frequency band. If they are narrowband, sometimes there will be collisions
that completely disable the colliding systems. If those same systems
were spread spectrum, and the relative signals strengths were
right (VERY IMPORTANT!), they could co-operate. I think this is the
intent behind the 900 MHz part 15 devices. The Japanese, however,
have taken an alternative approach in their 900 MHz personal radio
service: they detect interference and simply shift frequencies.

Where you really gain bandwidth by using SS, I suspect, is in situations
like the satellite business described by the previous poster. In this
case, you have controlled signal strengths (everyone is at the
same distance from the satellite, and presumably transmitting the
same power). I believe you also have more or less random usage 
patterns - bursty data, etc. In such a situation, you have an allocation
problem - how do you give the bandwidth to the guy that needs it. An
old solution uses some form of TDMA - such as CSMA or CSMA/CD. These
are not very efficient at high usages - they tend to start thrashing.
The SS solution in that case should be more elegant - increased interference
yields linear decreases in SNR, rather than collisions and retries.
Up to some level of usage, the system should be well behaved.

I suspect this is why the VSAT folks are pushing it. Their needs fit
this very specialized use.

For those in love with SS, let me offer a challenge:

  Develop a VHF/UHF repeater technology for Spread Spectrum that
would actually yield better spectrum usage than currently existing
narrow band systems (such as trunked systems). Have fun!

-- 
John Moore HAM:NJ7E/CAP:T-Bird 381 {ames!ncar!noao!asuvax,mcdphx}!anasaz!john 
USnail: 7525 Clearwater Pkwy, Scottsdale,AZ 85253 anasaz!john@asuvax.eas.asu.edu
Voice: (602) 951-9326        Wishful Thinking: Long palladium, Short Petroleum
Opinion: Support ALL of the bill of rights, INCLUDING the 2nd amendment!
Disclaimer: The opinions expressed here are all my fault, and no one elses.