Forum: GNU Radio a question about the Analog RSS I measued at the RX end

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Bill S. (Guest)
on 2008-11-09 06:15
(Received via mailing list)
Hello, all!!!

I have a question of measuring RSSI on the USRP board. I searched the
answer throughout the mailing list and I only found out three ways:
1) Analog RSSI (we can read it using AUX ADC)
2) Digital RSSI in FPGA (from output of ADCs)
3) Digital RSSI in host (computed however you like, from the channel zed
signal sent over the bus by the USRP)
Here, I usedv = u.read_aux_adc(0, 0) to read RSSI whenever there is a
packet received, and then I average the whole RSSI values of 3000
packets I received. Below are the data I got: the working condition is
bitrate=2Mbps, modulation = dqpsk, the demo file i was using are
benchmark_tx.py and benchmark_rx.py under
gnuradio-examples/python/digital

--tx-amplitude -s  PER  Rssi
100     100 60.30% 168
250     100 47%   170
500     100 40%   177
1000    100 39%  248
1500    100 40%  819
2000    100 40.2% 19372

50     50  25.09% 169.2084
500     50  25.06% 173.0687
1000    50  22.24% 215.4980
1500    50  22.05% 651.8132
2000    50  23.30% 1138.7028
4000    50  23.40% 1721.1643

250     40  21.30% 168.2404
500     40  19.34% 172.1490
1000    40  18.15% 208.2196
1500    40  18.72% 462.1265

My question is what these digital numbers stand for!!! e.g. if it is
168, how much is the actual RSSI value in dB?
it is a wireless communication with two USRPs standing 0.4 meters apart
from each other. Why does the RSSI increase so nonlinearly when the
tx-amplitude increases linearly or regularly?

Matt told us that the RSSI measures the analog signal level after the
lowpass filters on the board and these filters are about 15-20 MHz wide.
So, how can I narrow the bandwidth of those lowpass filters using
software to get it fit into my intrested area? Is there a way out? Any
help will be very useful to us! Thanks a lot for all!

Bill
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