Chapter 8 of the 2010 handbook has a short overview of spread-spectrum
techniques that could be applied to either analog or digital modulation. The
original signal cold be anything (BPSK, FSK, FM...) and is phase or frequency
modulated by a pseudorandom sequence in order to spread the signal over a wider
range of frequencies. In frequency hopping spread spectrum (FHSS) the receiver
and transmitter just shift between a predefined set of frequenies during the
transmission. Direct sequence spread-spectrum (DSSS) applies an additional
level of phase modulation with a pseudorandom sequence to spread the original
signal over a wider range of frequencies. DSSS often just exclusive-ORs
(modulo-2 adds) the data with a spreading sequence at a rate that is a multiple
of the original symbol rate.
Error-correcting codes sometimes increase the bandwidth of a signal, but they
do so by increasing the redundancy in the original signal. This could just be
sending additional copies of the original data or adding parity bits to the
data in block codes or multiplying the current data values with previous data
values in convolutional coding. For example the current data value could be
added to the previous two values and interleaved with the current value added
to the second previous value.
73,
John
KD6OZH
----- Original Message -----
From: Rud Merriam
To: digitalradio@yahoogroups.com
Sent: Sunday, February 28, 2010 00:11 UTC
Subject: [digitalradio] Spectrum Spreading
I avoided most of the discussion in the last week or so but finally decided
to see what the ARRL Handbook had to say. At first I thought it was totally
unhelpful but after it sank in a bit found it some help.
What I gleaned is that many digital modes use spectrum spreading techniques.
The handbook seemed quite clear on this point. I am still trying to understand
what spectrum spreading means. There is an implication in there of using more
spectrum than.. something.
For analog, i.e. voice, this is somewhat clear. If you are sending voice up
to 2.5kz then the spectrum 'something' is around 2.5 kHz SSB, or double that
for AM. Spectrum spreading would utilize some additional spectrum. Consider a
hypothetical mode where you took the voice signal, spread the audio by 4 times
to generate a 10 kHz signal, and used that audio to modulate the RF. That would
be a spectrum spreading technique.
I simply cannot get a handle on what spreading means for a digital signal. Is
the base 'something' CW and PSK31?
From the Handbook, and I gather from the discussion here, there is another
aspect which concerns the way in which the signal is encoded. In my
hypothetical analog mode you might somehow invert or fold the frequency
spectrum. The reverse technique would be required to decode the signal. It is
my sense that some types of encoding are not allowed, while others would be
acceptable.
Not trying to start the entire debate but hoping to get a better
understanding of the meaning of all this.
- 73 -
Rud Merriam K5RUD
ARES AEC Montgomery County, TX
http://mysticlakesoftware.com/
