Re: [Discuss-gnuradio] Problems attenuating in software - solution?

[Eric Blossom]

On Sat, Feb 03, 2007 at 04:01:02PM -0800, Dan Halperin wrote:
> Hi,
> 
> The groups in our networking class have each built an ad-hoc wireless
> networks using a modified version of tunnel.py. For the next step we
> want them to develop routing in a multi-hop environment. In the code
> that we started them, we pretty much used the same transmit_ and
> receive_paths.
> 
> I figured the easiest way to simulate a larger-scale wireless deployment
> would be to decrease the TX amplitude, similar to putting an attenuator
> in between the antenna and the USRP. The two places that volume control
> is directly used are in setting the receiver gain and setting the
> transmitter amplifier. The receiver gain is simply set to its midpoint
> by default, and the amplifier is controlled by an application parameter
> (12000 by default).
> 
> I measured packet reception rates transmitting 1000 small packets across
> the air, and I am not able to generate the kind of environment we need.
> For amplitudes above 800 or so, it's basically the case that around
> 950/1000 packets will be received, and then if I go much lower than that
> what happens is the majority of the first k packets are received and
> then after that another packet is never seen. K can vary as low at 10
> and as high as 1000, but is typically in the 150-700 range. Something in
> the tracking or synchronization is going wrong, I think.
> 
> Also, these problems seem to be very dependent on frequency in a strange
> way; if I send on 2.462G (802.11 Channel 11), I can near-zero loss rates
> around amplitude 500 even, but at 2.437G it's much higher. Since part of
> the class is enabling the students design their radios to flexibly
> change their behavior as a function of the environment, it's hard to
> design system parameters to make this an interesting task; it seems that
> some frequencies are just unusable.
> 
> I'm using more or less the same code as
> tunnel.py/transmit_path.py/receive_path.py with GMSK modulation and
> bitrates varying between 125k, 250k, and 500k.
> 
> Does anyone have suggestions as to how to fix this, and/or whether there
> is a better way to set up this environment? Sadly, I don't have enough
> lab space to e.g. actually distribute the machines around a large building.
> 
> Thanks
> 
> Dan

Dan, how are you controlling the receiver h/w gain?

Right now we don't implement closed-loop AGC (that is, there's no code
that reads the RSSI and uses it to control the receiver gain).  This
is a known problem, ticket:66, http://www.gnuradio.org/trac/ticket/66

Feel free to fix it ;)

Eric