[Discuss-gnuradio] Sounding signal output power and spectrum

[Qi Chen]

Hi all,

I have been doing some channel sounding measurements with the USRP,  
and there is something that I am not sure about.

1. The gr-sounder default transmit amplitude is 4096, is there a  
particular reason why this number is chosen? My guess is because a 12- 
bit DAC is used.
2. All my measurements were conducted with RFX 2400 d'board, and the  
d'board is connected directly to a spectrum analyzer, the measured  
output power levels are listed below:

128:    -28 dBm
256:    -23 dBm
512:    -17 dBm
1024:  -11 dBm
2048:  -6 dBm
4096:  1 dBm
so far so good(roughly 6dB step when I double the tx amplitude), and  
all signals cover 32MHz bandwidth,  but when I change --tx-amplitude  
to 8192, it gave me a spike at the center frequency with very narrow  
bandwidth, the power measured is about 11 dBm, any tx amplitude value  
greater than 8192 yields a small power (-61dBm) with 32MHz bandwidth.  
I am sure this has something to do with the DAC and PA, but not sure  
exactly how to explain it.

3. I did a indoor measurement with TX-RX separation of 50 meters (w/  
LOS), the received channel impulse response has 5 chunks of CIRs  
instead of one, and the number of samples between each chunk is  
always 800 chips, I am sure those CIRs are not multipath delays since  
in an indoor environment the corresponding delays can't be 800-chips  
apart from each other(way too long). Am I missing something here?

Thanks in advance for any help.

Qi Chen