802.11b standard use barker spreading, So if you want to receive data
from a standard 802.11b transmitter you have to use barker. For
decimation it should be as high as possible because 802.11b receiver
needs more more than 22M complex sample per sec to work correctly. But
USB cannot handle that much bit-rate so we have to downsample it to
8McSps. This reduces SNR which reduces receiver performance and range
of activity.
regards,
hamed
Quoting yyzhuang [email protected]:
input_rate = 8000000
Spreading. How should we know what decimation rate and spreading to use?
)
Samples per data bit = 8
250kHz, etc). To get down to baseband, the resultant signal is then
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Mohammad Hamed F.
ECE Department, U of Utah
Salt Lake City
UT, 48112
But when I use one usrp for tx and another for rx, no need to specify
“-d 8”.
Could you tell me why is this? Thanks
Mohammad Hamed F. wrote:
hamed
got a bunch of packets:
gr_fir_ccf: using SSE
parser.add_option(“-b”, “–barker”, action=“store_true”,
Using TX d’board A: Flex 2400 Tx MIMO B
decimation frequency = 16
Mbps
On Mon, Jul 28, 2008 at 12:28 PM, yyzhuang [email protected] wrote:
shifted in frequency down using the digital downconverter:
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802.11b standard uses 11 chip barker code for spreading. It means each
bit is converted to 11 bits and for DBPSK and QBPSK modulation it leads
to a signal with 11MHz bandwidth. When you downsample data with 8 (-d
8) it means you cut the frequency to 4Mhz (instead of 11MHz) and it
means you reduce your signal strength.
BBN transmitter code doesn’t use barker spreading. They use raised
cosine and its spreading factor (or gain) is less than 11. So receiver
doesn’t have to cut the frequency and it works fine.
Did I get your question right?
Quoting yyzhuang [email protected]:
needs more more than 22M complex sample per sec to work correctly. But
Thanks.
input_rate = 8000000
Spreading. How should we know what decimation rate and spreading to use?
32
dxc frequency -4000000.0
For general radio architecture, please see:
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Mohammad Hamed F.
ECE Department, U of Utah
Salt Lake City
UT, 48112
Yes!
“The problem we are overcoming is the USB data rate bottleneck, which is
limited to 32 MBps. Although this bit rate is high enough for many
applications of gnuradio, for an 802.11b receiver it is not sufficient,
because of the signal’s bandwidth. The RF bandwidth of an 802.11b signal
is
11 MHz. The minimum possible sampling rate (due to the Nyquist sampling
criteria) is 22 Msamples/sec. Assuming a resolution of 8 bits for each I
and
Q sample, we require 2*22M = 44 MBps through the USB, which is clearly
over
the limit. Because of these limits, the current gnuradio receiver
implementation of 802.11b (credit to BBN) reduced the signal bandwidth
to 4
MHz prior to sending it through the USB. Effectively, the sub-sampled
signal
arrives at the PC with very low SNR. Typically, 802.11b packets
transmitted
at the 1 Mbps rate can be received, but only at short range.”
This is why the output of the script is all 1Mbps.
If we modify the script a bit, can we tune gnuradio to an 802.11b
channel
and pick up all available access points (and their 1 mbps comm with
wireless
clients)? Do we have 802.11g implementation right now?
Thanks a lot for help
Mohammad Hamed F. wrote:
Mohammad Hamed F. wrote:
hamed
got a bunch of packets:
gr_fir_ccf: using SSE
parser.add_option(“-b”, “–barker”, action=“store_true”,
options:
on the receiving side: ./bbn_80211b_tx.py -f 2.44G
PKT: len=1477, rssi=-10, src=20:61:6e:64:20:73, time=16024824, rate=1
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ECE Department, U of Utah
Salt Lake City
UT, 48112
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arrives at the PC with very low SNR. Typically, 802.11b packets transmitted
at the 1 Mbps rate can be received, but only at short range."
This is not true. Remember when you have complex samples the sample rate
IS the bandwidth. So 802.11b can be covered with 11 Msps 8-bit I and Q
or 22 MByte/s. So in 8-bit mode the 16 Msps standard USRP configuration
can capture the signal.
I had done this previously though I modified my oscillator to be 44 MHz
instead of 64 MHz because (a) the 4x symbol rate interpolation was
easier and (b) to ease the datarate over the USB. I was able to
demodulate all the 802.11b modes including the 11 mbps packets.
-Clark
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