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[Discuss-gnuradio] RSSI value in inband code

From: Stefan Brüns
Subject: [Discuss-gnuradio] RSSI value in inband code
Date: Fri, 14 Nov 2008 18:48:44 +0100
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after looking at the verilog code for some time, these are my results:

in module usrp_inband_usb, for each of the four aux adcs, the signal level is 
read from the adc and an gliding average of the rssi and an saturation 
indicator is is calculated. The rssi value is put but into the lower 16 bits 
of an 32 bit bus, which is passed on through the module rx_buffer_inband and 
reaches the module packet_builder.

In packet_builder, the rssi bus of the adc corresponding with the channel 
which is transmitted is selected. Until here, everything is fine.

As said before, the rssi value is in the 16 lower bits of the rssi bus. When 
looking at the calculation of the rssi value, it is apparent that it is a IIR 
filter with output value in the range [0..0x7ff] (max value is determined by 
the max absolute value of the aux adc, which is 0x7ff).

Now, the packet_builder takes the lower 6 bits of the 16 bit register with an 
value range corresponding to 11 bits and puts it into the usb packet. So as 
soon as the average value of the adc is higher than 0x3f (63), the rssi value 
in the usb packet will be just noise.

Change the rssi value to a logarithmic encoding. With a 3.3 split, 10 bits can 
be encoded, which is almost the range of the aux adc (11bit). Step size is 
about 0.1dB.

A verilog module for encoding the rssi value is attached. I have no tools for 
verilog, so I did not test it. Dont know if it works, and if the the changes 
in the packet_builder for getting the log value from the linear value is 

Would be nice if anyone could test this and pass an fpga firmware on to me.


Stefan Brüns  /  Bergstraße 21  /  52062 Aachen
mailto:lurch at gmx.li  http://www.kawo1.rwth-aachen.de/~lurchi/
   phone: +49 241 53809034     mobile: +49 151 50412019

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