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[aprssig] RE: [OZAPRS] Spread Spectrum APRS

Gerry Creager N5JXS gerry.creager at tamu.edu
Sat Oct 16 01:28:23 UTC 2004


Er... actually 802.11b *IS* direct sequence spreading.  802.11a/g are 
orthogonal frequency division multiplexing, with a 64-carrier 
constellation.

In the case of spread spectrum, the idea is, talking "over" someone 
isn't a problem because most spread spectrum methods tend to use forward 
error correction and can detect and often correct single (or small 
numbers of) bit error.  The issue of simultaneous site arrival is rarely 
at issue.  Carrier detect is hardly in the PHY of 802.11b.  It resides 
higher up the stack.

If everyone is using the same pseudorandom sequence for spreading... and 
is identically locked to the same timesource, THEN you might see the 
same hidden transmitter issues one sees with ALOHA and CSMA (+/- CD). 
However, simply using either DSSS or FHSS doesn't mean you'll see 
capture effect and blocking of all but the strongest (not necessarily 
nearest) stations.

Gerry

Darryl Smith wrote:
> Well... Yes... And it is called "GPS tracking over CDMA 1x"
> 
> Seriously, CDMA and the APRS philosophy of no acknowledgements does not work
> too well. Neither does collision avoidance.
> 
> The reason why 802.11 actually works is that mostly it is not Spread
> Spectrum. Therefore you can to some extent carrier detect. And it has
> RTS/CTS, but that is another matter.
> 
> When you have Spread Spectrum it is difficult to detect other users and
> therefore you are likely to transmit over someone. Also you will only hear
> the closest stations thanks to the way Spread Spectrum works. [You can
> ignore the fact that NBFM which APRS uses normally also has the same issue
> with only hearing the closest stations in this argument].
> 
> Having said all that... Do you have a proposal to put to us with some
> hardware :-)
> 
> Darryl
> 
> ---------
> Darryl Smith, VK2TDS   POBox 169 Ingleburn NSW 2565 Australia
> Mobile Number 0412 929 634 [+61 4 12 929 634 International] 
> www.radio-active.net.au\blog\ - www.radio-active.net.au\web\tracking
>  
> 
> -----Original Message-----
> From: ozaprs-bounces at marconi.ics.mq.edu.au
> [mailto:ozaprs-bounces at marconi.ics.mq.edu.au] On Behalf Of Andrew Rich
> Sent: Saturday, 16 October 2004 12:12 AM
> To: TAPR APRS Special Interest Group; Ozaprs at Marconi.Ics.Mq.Edu.Au
> Subject: [OZAPRS] Spread Spectrum APRS
> 
> Has someone addressed this issue before ?
> 
> Has it been tested ?
> 
> 
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> 
> 
> 
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
Page: 979.228.0173
Office: 903A Eller Bldg, TAMU, College Station, TX 77843




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