Symbol rates. USRP2 vs USRP1


#1

Hi all,

As some of you know, I am working on porting the BBN 802.11b code to the
USRP2. I understand that the ADC/DAC rates are higher than the USRP1.
What
are the effects, if any, on the rate at which symbol are sent through
the
air? I am suspect that this could be the reason I cannot decode sent
packets.

Colby


#2

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Colby B. wrote:

Hi all,

As some of you know, I am working on porting the BBN 802.11b code to the
USRP2. I understand that the ADC/DAC rates are higher than the USRP1. What
are the effects, if any, on the rate at which symbol are sent through the
air? I am suspect that this could be the reason I cannot decode sent
packets.

Colby

Main change I had to do when I was working on it was the samples/baud. I
only had it recognizing very high SNR 1 and 2Mbps packets.
Doug


Doug G.
Research Assistant
Communications and Signal Processing Lab
Oklahoma State University
http://cspl.okstate.edu
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#3

What changes did you have to make to the sample/baud rate?

How high is high SNR? 10dB? 15dB?

I can only establish communication within simulation. When I try to
transmit
stuff over the air, it fails.

On Wed, Apr 22, 2009 at 3:16 PM, Douglas G. <


#4

Hi Colby!

In the RX path the difference in the adc rate should be compensated by
the fact that you multiplied the decim by a factor of 1.5.
[In your bbn_80211b_rx.py (line 98)]

In the TX path Andrea figured out that:

Rate = 2 * [FPGA_processing_rate / (spb*interp_rate)]

The BBN guys suggested to use spb=4 and interp_rate=32 for communicating
with a wifichipset.
In fact, with USRP1:
2 * [64 MS/s / (4 S/bit * 32)] = 1 Mbps

On the contrary, in USRP2 the FPGA_processing_rate is 100 MS/s.
Therefore, in order to transmit at 1 Mbps, we need to set the
(spb*interp_rate) accordingly:

e.g.
2 * [100 MS/s / (4 S/bit * 48)] = 1Mbps.

We were wondering how it is possible that spb=4 can represent a barker
sequence.
We have tried some different combinations of spb and interp_rate (e.g.
spb=8 and interp=24), so that the result is always 1 Mbps.
But we still have no success.
By “no success”, I mean that the wifi chipset monitoring the air
receives frames that seem to be randomly generated.

Any suggestions?
Anyway, thanks for uploading your branch. :wink:

Danilo


#5

To confirm, you were able to receive 802.11b using the USRP2?


#6

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Colby B. wrote:

To confirm, you were able to receive 802.11b using the USRP2?

Yes, only at 1 and 2Mbps (DSSS w/Barker Code) rates.
Doug


Doug G.
Research Assistant
Communications and Signal Processing Lab
Oklahoma State University
http://cspl.okstate.edu
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