Re: Direct Sequence Spread spectrum modulation
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Hi Reg ¨C ? Yes ¨C also the data rate will be of great consequence when you consider the effects you mention, as well as the accuracy of the timing source. ?For real high data rates even a good TCXO may work fine. For low data rates, GPS is almost needed for sure. 73 ¨C Mike ? Mike B. Feher, N4FS 89 Arnold Blvd. Howell NJ 07731 848-245-9115 ?
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From: [email protected] < [email protected]> On Behalf Of Reginald Beardsley via groups.io Sent: Tuesday, November 3, 2020 4:51 PM To: [email protected]Subject: Re: [qex] Direct Sequence Spread spectrum modulation ? Mike,
Yes, GPS time is key. You've also got to determine the propagation and multipath delays so they can be summed properly. You *can* synchronize by using a special PRN. With a 0.5 ppm TCXO you could stay close enough to determine the phase and frequency difference, but lots more trouble.
It appears I was so blinded by the need for GPS that I failed to notice the lack of Rx. But my RSP2 should take care of those chores with the 2nd RFzero providing the clock. But more work too :-(
Have Fun!
Reg On Tuesday, November 3, 2020, 02:28:17 PM CST, Mike Feher <n4fs@...> wrote: It seems they would not only have to have the same seed, or key, but also the same timing. 73 - Mike
Mike B. Feher, N4FS
89 Arnold Blvd.
Howell NJ 07731
848-245-9115
-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Craig Johnson, AA0ZZ
Sent: Tuesday, November 3, 2020 3:03 PM
To: [email protected]
Subject: Re: [qex] Direct Sequence Spread spectrum modulation
Reg,
Fascinating!? I've wanted to play with this for many years also.
Quick question that I've never understood about PRN's.? Does everyone (ham community) use the same formula for generating PRNs such that if a station's call is used as a seed then both stations would have the same sequences for encoding and decoding?
73,
-Craig, AA0ZZ
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Reginald Beardsley via groups.io
Sent: Tuesday, November 03, 2020 1:34 PM
To: [email protected]
Subject: [qex] Direct Sequence Spread spectrum modulation
This is a simplified outline of a digital mode I have wanted to test for almost 40 years.? It's a direct application of the basic theory of probability and random processes.? I later learned it had been done in the 1950's.? I'll skip the math for now.
Generate a pair of Pseudo Random Number (PRN) sequences using the station call sign forward and backwards as seeds.
Clock the PRNs using a GPSDO time base and fixed set of frame epochs.? Spread the signal out over most of the band allocation and use a set of binary window lengths so each step in frame length is 3 dB of power gain.
One of the two station PRNs is used for MARK and the other for SPACE.? So at the receiver the signal is digitized and crosscorrelated with the 2 PRNs. The one with the largest peak energy at the expected arrival times is the symbol.? This simplifies additively including multipath as useful signal.? This also permits easily listening to other stations on a selective basis.
CQ is done with the same PRN generator system using "CQ" and "QC".
Once contact is established shift to the communicating stations unique PRNs.
For net operation,? use the net name as the seed value when joining or leaving the net.? This allows stations in the net to track what stations to listen for.
Identification is by sending the station call using a standard PRN pair (e.g. "FCC").
The effective signal power at the receiver is the time-bandwidth-power product.? So the power level and data rate can be adjusted as conditions permit.? Mathematically, the system? should get as much power gain relative to the noise level as desired? at the price of reducing the data rate.? There should not be significant mutual interference despite stations occupying the same BW.? For a 30 second/bit data rate one should get around 55 dB of gain.
Technically there is nothing new in? the concept outlined above.? It simply was too expensive and complex to implement in the past.? ? The economics have changed.? A Raspberry Pi 400 and an RFzero? should be able to implement this with ease and drive a 4K display.
Have Fun!
Reg
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