[aprssig] 6 meter APRS

[Chris Moulding]

Thanks for your comments, Steve.

My other main amateur radio interest beside APRS is the use of 
digi-modes on 30m and I agree with
your comments on symbol rates especially after using modes like OLIVIA 
where you can have 100%
copy on a signal you cannot hear.

The DLL I'm using in the add-on program will also allow operation on 
QPSK. I intend to include all
the PSK and QPSK options in the program to allow experimentation.

As I understand it in the forms of QPSK used by radio amateurs on HF the 
increase in data rate has
been used for forward error correction. The data throughput remains the 
same but with a reduction
in errors.

My motivation to design a PSK/QPSK TNC add-on program comes after trying 
for months to get a
300 bd packet TNC design to work. It will decode a clean signal 
perfectly but feed it with an off-air
HF APRS signal with noise and signal corruption very few packets were 
decoded. An alternative
using PSK or QPSK would be far better for HF operation especially at 
this stage of the sunspot cycle.

73s,

Chris, G4HYG

Chris Moulding
Cross Country Wireless (2009) Ltd
7 Thirlmere Grove, Bolton, Lancs, BL4 0QB, UK
Tel/fax: +44(0)1204 410626
Mobile:  +44(0)7752 391908
Website: http://www.crosscountrywireless.net
Company number 6780346 registered in England and Wales  



Stephen H. Smith wrote:
> Chris Moulding wrote:
>> Hi,
>>
>> One project I'm working on in whatever spare time I have is a PSK-125 
>> TNC add-on for UI-View.
>>
>> To test this I've already written a macro in Fldigi to simulate an 
>> APRS message.
>>
>> I've just spent a few minutes running the message through various 
>> digi modes and CW speeds to
>> measure the time taken. See the results below:
>>
>> The test message is:
>>
>> G4HYG>APRS,GATE,WIDE2-2:=5332.76N/00225.91W&Chris, testing PSK modes 
>> for APRS
>>
>> Time taken to send the message:
>>
>> PSK-31................23.66 s
>>
>> PSK-63................12.30 s
>>
>> PSK-125................6.91 s
>>
>> PSK-250................3.87 s
>>
>> RTTY (45 bd)......18.50 s   (note that > is not in the character set)
>>
>> OLIVIA (8-250)....59.48 s
>>
>> CW (18 wpm).......64.43 s
>>
>> CW (50 wpm).......23.61 s
>>
>> CW (100 wpm).....11.66 s
>>
>
> The biggest problem with data transmission on HF is multipath 
> propagation causing the same bits to arrive at the receiver over 
> several different paths (of different length and time delay) 
> simultaneously.  People liken HF propagation as bouncing signals off 
> the "mirror" of the ionosphere, but it is more like bouncing a 
> searchlight off a fog bank. There is no coherent single surface doing 
> the reflecting. This causes sequential bits to overlap in time and 
> smear each other if they are sent too fast.
>
> The key to reliable HF data transmission is to use relatively low 
> symbol rates (i.e. duration of each symbol substantially longer than 
> the potential spread of propagation times of 10's of milliseconds.)   
> This is why hand-send CW and PSK31 are so effective under conditions 
> of less-than-ideal propagation. Their individual symbols are LONG!
> To up the data rate, one has to increase the information content (i.e. 
> complexity) of each symbol, not increase the symbol RATE.  This means 
> more phases, more amplitudes or more simultaneous carriers but not 
> FASTER symbol rates. 
> Have you considered QPSK rather than the more common BPSK?  It is 
> supported in just about every PSK program, and can double the data 
> throughput without increasing the symbol rate.  
> _______________________________________________
> aprssig mailing list
> aprssig at tapr.org
> https://www.tapr.org/cgi-bin/mailman/listinfo/aprssig
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/x-pkcs7-signature
Size: 3306 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://lists.tapr.org/pipermail/aprssig_lists.tapr.org/attachments/20090826/78542d39/attachment.p7s>