[By Subject] [By Date] [By Sender [By Thread]
Previous In Time

Next in Time

From: karn@qualcomm.com
SS & Real-time comms
Date: Fri, 6 Dec 1996 19:26:38 -0800 (PST)
Harry,

You're right, I'm not on the VHF reflector. I got draw into the discussion by Greg Jones, who is on it. So I included the address in my responses, as I have in this one.

Let me answer one specific question of yours.


>not transmitting the SS signals in the weak signal portions of a band. Does
>this mean that a SS transmitter would spread it's signal with gaps covering
>perhaps the satellite 2 meter downlinks used by Oscar satellites and also
>have gaps in it's output spectrum so that it does not transmit in 144.2 MHz
>range? Doesn't seem like an easy thing to do to me. How do you create an
>algorithm that accomplishes this in real like? Can someone that knows more
>about real SS signals tell us more on how this is accomplished?

This is an excellent question. In both direct sequence and frequency hopping, it is possible to carve "holes" in the transmitted spectrum if necessary.

In direct sequence, you can theoretically do this with a notch filter on the transmitter output. If the notch is narrow, the receiver will just deal with it as it would any other frequency- selective channel impairment.

But "carving holes" is much easier with frequency hopping -- you just drop the appropriate "channels" from your hopping list. And if you do hit an occupied channel, the QRM takes the form of an occasional burst rather than a continuous small increase in the background noise.

Conversely, a direct sequence spread spectrum system can resist a strong narrowband jammer with a notch filter, but a frequency hopped system with the proper burst error correction techniques is a more practical approach. If the hopper lands on a narrowband interferer, a burst of errors results until the hopper goes somewhere else. The right code (e.g., an interleaved Reed-Solomon code) can easily handle this as long as the fraction of occupied channels is below the code's design limit.

If the number of hopping channels is large, then the expected number of busy channels (narrowband jammers) per hopping cycle becomes quite predictable; this is the "law of large numbers" at work. As long as the coding is designed to withstand at least this many error bursts per hopping cycle, then the system will work reliably.

It is certainly possible for a FHSS system to detect that certain narrowband channels are in continuous, long-term use and to agree to drop them from the hopping schedule. This saves transmitter power that would otherwise be jammed and reduces interference. In the limit, it automates what hams have long done manually, though at a much slower limit: if you get QRMed, move around until you find a clear spot. In the short term, though, you have to be able to deal with transient QRM, because you don't want to spend *too8 much overhead negotiating hopping lists.

I feel that such an adaptive FHSS system is probably the best overall approach for a local utility spread spectrum overlay on existing amateur VHF/UHF FM repeater, simplex and packet channels. Because the weak signal segments and satellite segments are relatively small, I see no problem with "hard wiring" them out of the hopping list.

Phil ------ Submissions: vhf@w6yx.stanford.edu Subscription/removal requests: vhf-request@w6yx.stanford.edu Human list administrator: vhf-approval@w6yx.stanford.edu