Hello,
appended is another patch for fixing a bug in the calculation of the
frequency lists. This time the patch is for the "Range 256 format".
The problem is that the operand for the "smod" operation might be
negative, in this case the simplified version won't work as expected.
In the patch I introduced a separate function for "smod" which takes
care of the sign. I have not yet checked if the other formats are also
affected, this would be the case if the "smod" operand can be negative.
There might be other solutions to fix the problem without the need
for a separate function, however I have not thought further about it.
A test vector is the following frequency list ("Range 256 format",
first byte is the length):
09 8b 1c 83 8c 15 ef 02 2d 30
The correct ARFCNs are
569 571 576 578 586 608 712 715 719
The uncorrected version would instead return:
444 457 460 464 569 576 578 586 608
This means four ARFCNs are wrong which will cause problems if for
example the frequency list contains the ARFCNs for hopping.
Best regards,
Dieter
Dieter Spaar, Germany spaar(a)mirider.augusta.de
On Thu, Sep 1, 2011 at 7:30 PM,
<baseband-devel-request(a)lists.osmocom.org> wrote:
Hello,
Thank you Dieter and Scott for your fast responses.
> ---------- Forwarded message ----------
> From: "Dieter Spaar" <spaar(a)mirider.augusta.de>
> To: baseband-devel(a)lists.osmocom.org
> Date: Thu, 01 Sep 2011 07:59:31 CEST
> Subject: Re: 900MHz packet radio?
> Hello Paul,
>
> On Thu, 1 Sep 2011 13:41:11 +0930, "Paul Gardner-Stephen" <paul(a)servalproject.org> wrote:
>>
>> So, as the open-source group with the most experience reprogramming
>> baseband radios, what is the feasibility of creating a
>> proof-of-concept using the types of phones you already work with to
>> send and receive arbitrary data packets without reliance on a cell
>> tower (even for time synchronisation)?
>
> It depends a lot on what you want to use on the lower layers
> (e.g. Modulation). The TI Calypso based phones have a built-in
> GMSK hardware modulator, so it is GMSK. Also the symbolrate is
> fixed to the one used by GSM.
I had assumed that the modulation would be fixed in this way, and it
is not an issue for what I am wanting to do. In fact, it makes a lot
of sense to keep the GSM modulation for simplicity.
> Then there is the DSP which does the signal processing work. Its
> again tied to the GSM standard (format of the burst and length).
> It might be possible to put something else into the relativ small
> RAM inside the DSP, but this could be a huge effort.
This seems to me to be the hardest part of the whole process.
When you say a huge effort, do you think it would be possible for
someone to be able to get something working if they were able to work
on it full-time for a few months?
> Besides that you have to modify the hardware of the phone (remove
> a filter) so that the phone can receive another phone. But this
> is probably the smallest problem of all.
Indeed, this is the smallest problem I think. In fact, if we use the
part of the ISM915 band that is right next to the GSM band, then my
understanding is the filter will only attenuate by a few db --
although I am happy to be corrected on this point.
> Best regards,
> Dieter
> --
> Dieter Spaar, Germany spaar(a)mirider.augusta.de
>
>
>
>
> ---------- Forwarded message ----------
> From: Scott Weisman <sweisman(a)pobox.com>
> To: baseband-devel <baseband-devel(a)lists.osmocom.org>
> Date: Thu, 1 Sep 2011 10:18:21 +0300
> Subject: Re: 900MHz packet radio?
> I think you would be better taking your existing hardware, which must be somewhat custom to begin with (since it works over wifi) and use something like a HopeRF module (http://www.hoperf.com/) or similar. HopeRF has transceivers with power output up 100mW, data rates up to 256kbps, and a choice of 4 different bands (315, 433, 868, 915). This sounds like it would be a far easier choice to incorporate into your already-working system than hacking OsmocomBB-capable phones.
> There is a lot of development from many different companies in this area and quite a bit of hacker activity as well.
We are exploring that path simultaneously, and have a batch of the
exactly the HopeRF modules that we will use, to demonstrate the range
and utility of using the ISM band for long-range mesh telephony.
But it is important also that we prove that it can be done using the
baseband radio in a phone.
We all know that it can if we have the freedom to design the baseband
radio from scratch, but that it can also be done it is also important
that I show that for handsets where the baseband radio can be
reprogrammed without changing the hardware.
It all boils down to being able to deflect any straw arguments that
might be raised against the feasibility of long-range mesh telephony
using baseband radio.
Paul.
> Scott
>
> On Thu, Sep 1, 2011 at 7:11 AM, Paul Gardner-Stephen <paul(a)servalproject.org> wrote:
>>
>> Greetings all,
>>
>> At the Serval Project we have created a mobile mesh telephony system
>> that currently works over wifi.
>>
>> >From the outset, we have wanted to get it working on the ISM915 and/or
>> ISM868 bands that are located adjacent to the GSM 850/900 frequency
>> allocations.
>>
>> My initial investigations and enquiries indicate that this should be
>> possible by creative programming of the baseband processor in many
>> models of phones. The trick, as I suspect you well know, is the
>> difficulty in getting the information and tools required to reprogram
>> these radios.
>>
>> I am now in a position to potentially fund further work on this.
>>
>> So, as the open-source group with the most experience reprogramming
>> baseband radios, what is the feasibility of creating a
>> proof-of-concept using the types of phones you already work with to
>> send and receive arbitrary data packets without reliance on a cell
>> tower (even for time synchronisation)?
>>
>> I know there are a lot of constraints and problems, but I am most
>> interested in creative solutions that can get us to a working
>> prototype, however crude, that can be used to demonstrate the
>> feasibility of what I am proposing.
>>
>> If this discussion is off-topic here, I am happy to hold the
>> conversation at the serval-project-developers google group, but I am
>> equally comfortable with it continuing here.
>>
>> Thanks in advance,
>> Paul Gardner-Stephen.
>> Shuttleworth Telecommunications Fellow at Flinders University.
>>
>
>
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> baseband-devel(a)lists.osmocom.org
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>
>
Hello Paul,
On Thu, 1 Sep 2011 13:41:11 +0930, "Paul Gardner-Stephen" <paul(a)servalproject.org> wrote:
>
> So, as the open-source group with the most experience reprogramming
> baseband radios, what is the feasibility of creating a
> proof-of-concept using the types of phones you already work with to
> send and receive arbitrary data packets without reliance on a cell
> tower (even for time synchronisation)?
It depends a lot on what you want to use on the lower layers
(e.g. Modulation). The TI Calypso based phones have a built-in
GMSK hardware modulator, so it is GMSK. Also the symbolrate is
fixed to the one used by GSM.
Then there is the DSP which does the signal processing work. Its
again tied to the GSM standard (format of the burst and length).
It might be possible to put something else into the relativ small
RAM inside the DSP, but this could be a huge effort.
Besides that you have to modify the hardware of the phone (remove
a filter) so that the phone can receive another phone. But this
is probably the smallest problem of all.
Best regards,
Dieter
--
Dieter Spaar, Germany spaar(a)mirider.augusta.de
