Speech coding for security and defense apps, Why do we need very low bitrate? Options

[kwwong]

If anyone can enlighten me as to why we need such low bitrates (< 800 bps) for security and defense, that would be splendid.

When using low bitrates you can use spread-spectrum algorithms to distribute the signal power over a very wide range - even to a degree where the signal power will be lower than the background noise - and thus "hide" the signal in the background noise and make it practically impossible for any other party to even detect that you are transmitting something.

Can you enlightened me further in this area? How about some detail equations presentations?

http://en.wikipedia.org/wiki/Spread_Spectr...ecommunications

Opps, my data communication knowledge is ancient as the egyptian mummies...

[Woodinville]

How you can recieve something when the signal is below the noise level? Doesn't make sense to me.

Perhaps you need to consider the meaning of "below the noise level"?

With a narrowband signal, as you narrow the bandwidth, the noise level drops (we're assuming wideband gaussian, for now), does it not?

With a wideband signal, one needs to consider what happens when one compacts (demodulates) the signal, without compacting the noise (which is noise, after all, yes?).

[kwwong]

How you can recieve something when the signal is below the noise level? Doesn't make sense to me.

Perhaps you need to consider the meaning of "below the noise level"?

With a narrowband signal, as you narrow the bandwidth, the noise level drops (we're assuming wideband gaussian, for now), does it not?

With a wideband signal, one needs to consider what happens when one compacts (demodulates) the signal, without compacting the noise (which is noise, after all, yes?).

Sounds like a Wiener filtering problem to me?

[cabbagerat]

I think Garf's theory that it is to avoid direction finding is closest to the target. Although PN spread spectrum technologies are making this less critical than it once was, transmitting less data makes it much easier to avoid Huff-Duff type detection and tracking. Passive detection is still very useful and can detect transmitters of even very low level signals (even ones using spread spectrum like technologies).

Also, people often assume that military equipment is the best and the fastest. This isn't really true for two reasons. The first is most obvious - that equipment needs to be extremely rugged, which is hard to do with cutting edge technology. Another is that there is a lot of military equipment that is very old, stuff that was bought in the seventies (or earlier) and for a number of reasons is still in service. This isn't always a bad thing (it works, so why replace it with something possibly less reliable) but is mostly due to budgetary constraints. Militaries generally either have very nice new equipment or really old stuff.

[DonP]

Another consideration, and I really don't know the answer, is how many channels do you want to have available in a war zone if the terrain is such that it has to go through a satellite or high altitude aircraft based repeater?

On first read of the question, I was thinking about NRO sort of stuff where they may be archiving all phone calls in a region.