[Replicant] The (!)Baseband/Public Cellular Telephony Network Path

[Josh Branning]

Hi,

Recently I attempted a 9km communication via 4W CB radios and a homemade 
1/2 wave dipole, from a rural village, to a town. I figured with more 
power and a low frequency, they might fare better for unlicensed 
communication. (I am based in the UK.) Unfortunately, I didn't manage a 
voice signal over that distance. Someone other, who is into amateur 
radio told me it may be to do with being at the peak of a solar cycle. I 
have only made contact via CB within the village, and major road where 
truckers pass. I have tried (thus far unsuccessfully) to make contacts 
using digital modes such as BPSK31 and JT-65 over FM. I use a CB radio 
with built in VOX, as it is easier than building the VOX circuitry or 
PPT switch. I am yet to set up an realtek SDR dongle at the other 
location so I can try *really* slow PSK to see if I can make a digital 
link by myself (in voice tests, the other radio did not have VOX). [1]

I am also looking into LoRa transceivers, which claim up to 15km 
distance. Unfortunately, most actual/web reports only give a 2-5km range.

I visited the local amateur radio club, and they seem to think a 
extremely efficient and small narrow band signal will tend to travel 
further, though personally I have doubts if that advantage can be 
produced in software (eg. the transmitter has to probably be designed 
for the narrow bandwidth).

I have found a potential method of communicating from/to satellites, but 
(?legally?) you'd probably need to ask someone for permission to use the 
network [2] ... tehehe.

The good:

1) satellites
2) *many* ground nodes
3) AX.25 unnumbered frames, meaning truly distributed routing, as 
opposed to IP.
4) runs on different modulation schemes
5) has bidirectional internet gateways
6) anyone can run a gateway/network
7) can send text messages and other (text messages tested, other not)
8) can run on microcontrollers
9) free as in freedom
10) only cost of setup, else, free as in beer (no connection fee)
11) email servers/gateways exist
12) IP can run atop of AX.25

The bad:

1) Generally uses amateur radio rather than unlicenced frequencies. 
(though could use other.)
2) Generally uses FM. (though could use other.)
3) Possibility of getting a hobbyist into trouble and worse, getting 
complete lock-down/regulation.
4) Not generally spread spectrum.
5) easy-to-spoof delivery mechanism

Unknown:

1) voice/images
2) cellular phone network integration


That said, I was also looking at the frequency/distance relationship. 
Problem is the lower the frequency, the bigger the antenna you need. 
Obviously you can transmit with a small antenna if the impedance is 
close (using chokes etc.) but that requires *major* custom hardware, 
which is somewhat impractical.

I would be interested to see if a ICE40 FPGA could be used as a PWM 
transmitter/receiver, similar to what was done with the Raspberry Pi. I 
asked on the free calypso mailing list, and generally speaking, the 
advice was a GSM modem would be difficult but not impossible with an 
FPGA. Worth perhaps also noting that the freecalypso project uses TI 
proprietary software (though the source is available).

I am pretty surprised that the unlicensed link you used in the talk went 
39km. I would be interested to know the make and model of the USB 
transceiver you used. Other people always seem to be able to go real far 
on little power [7][8], I myself am jealous and have had little success.

Thats me done,


Josh


References:

[1] 
https://www.maplin.co.uk/p/intek-m-899-vox-multi-standard-cb-transceiver-r52av
[2] https://en.wikipedia.org/wiki/Automatic_Packet_Reporting_System
[3] https://packages.debian.org/jessie/soundmodem
[4] http://xastir.org/index.php/Main_Page
[5] https://en.wikipedia.org/wiki/Fldigi
[6] http://wsjtx.net/
[7] http://wsprnet.org/drupal/wsprnet/spots
[8] https://www.pskreporter.info/pskmap.html

On 05/08/17 13:22, Denver Gingerich wrote:
> On Fri, Aug 04, 2017 at 08:02:00PM -0400, 宇都 大輝 wrote:
>> Recently, I have watched the Libreplanet talk by Mr. Gingerich, "A
>> fully-free cell phone experience, no baseband required"
>
> In case it helps for context, here are the slides and video of the talk:
>
> https://ossguy.com/talks/20170326_libreplanet/
> https://media.libreplanet.org/u/libreplanet/m/a-fully-free-cell-phone-experience-no-baseband-required/
>
>> My question is what are your thoughts on using devices like GTA04, Neo
>> Free Runner, Nokia N900, (and hopefully) Neo900/DragonBox Pyra and a
>> GNU+Linux distro (eg. Debian/Parabola)? And what might be the issues
>> that we need to resolve?
>
> I think this might be referring to my comments about preferring non-Android OSes around https://ossguy.com/talks/20170326_libreplanet/#slide-25 in the talk (I believe that is close to 10 minutes in on the video IIRC).
>
>
> I'm happy to discuss my talk further if it helps with this conversation. :)
>
> Denver
> https://jmp.chat/
> https://ossguy.com/
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