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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
