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Re: [Discuss-gnuradio] Precise freq hopping w/ inband signaling

From: George Nychis
Subject: Re: [Discuss-gnuradio] Precise freq hopping w/ inband signaling
Date: Wed, 14 May 2008 16:14:34 +0100

On May 14, 2008, at 2:01 PM, "Brian Padalino" <address@hidden> wrote:

On Wed, May 14, 2008 at 7:40 AM, George Nychis <address@hidden> wrote:
Thanks Brian, this is definitely helpful.

No problem.

I think I'm going to have to pass on implementing this functionality.

Well hold on now - there may still be things you can do!

1. according to Matt it takes ~200us to lock to a new frequency. I still can't find exact Bluetooth specs in terms of guard times (which would be the max time to hop), but I suspect its tens of microseconds. Do you know the
GSM frequency hopping times?

I believe they are in the 2ms range.

2. there's no way I'm making a USRP hardware modification. However, could something be devised where the FPGA relay's something encapsulated back to the FX2 to get around this? Something such as encapsulating an I2C write in a in-band command packet, wait for the timestamp to reach, and then strip the in-band packet out and pass it from the FPGA to the FX2? I don't know
how possible that is.

Sounds a bit tricky and a little too complicated.  I've got an idea to
think about (further down).

3. I _think_ that the dboard tuning and control for USRP2 happens in the FPGA, according to something Jonathan mentioned to me off list. Given that I already have a 95% full FPGA with all of the MAC-enhancements.... I don't even know if I'd be able to fit some sort of relaying functionality in if its even possible.... which will be much easier to do in USRP2 if tuning is done in the FPGA. So if that's the case, this functionality will wait for

What about bandpass sampling?  Nyquist just requires that we sample 2x
the bandwidth.  We have that criterion met with the 64Msps ADC, and
the CIC/halfband filtering that happens takes out the high frequency
images that will occur during the mixing stage (done by the CORDIC in
the FPGA).

If we know a) the frequency range it needs to cover and b) the
frequencies it's going to hop to, you can pick a centralized frequency
that has the most image rejection, and just twiddle the phase
increment value connected to the CORDIC and let the CIC/halfband do
the rest.  Only one tuned frequency, and something that is
controllable within the FPGA can give you some significant frequency
hopping capabilities.

So what do you think?  Do we know (a) and (b) listed above for
Bluetooth?  For GSM?


PS - I realize this works for RX only, but there might be the option
for a separate daughterboard for the TX portion of a frequency hopping
scheme to be figured out as well without hardware mods.

I think this is a good RX hack in an attempt to decode some specific protocol. But I guess what I was hoping for was a general technique that provided flexible frequency hopping for any protocol. So I think if someone really wanted a full GSM or Bluetooth decoder this is a good idea.

I do know some people on campus trying to write a full Bluetooth decoder so i will pass this on.

Thanks :)

- George

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