[Discuss-gnuradio] A basic USRP2 + BBN receiver + Samples_per_symbol question + An observation

[Bishal Thapa]

Hello list,
  I have been fiddling around with USRP2 version of BBN code for a week now. First, one observation I found is: if and only if I set decimation to 4 (i.e., 25Msps), I see almost all of the 1Mbps packets sent by my 802.11 sender. This includes data packets (1400 bytes payload), and
IEEE802.11 association/reassociation request packets. I wonder why?

My Specification:

• Ubuntu Specification: Ubuntu 10.04 LTS (Lucid Lynx)
• Kernel Version: 2.6.32-25-generic
• GCC version: 4.4.3
• BBN Code Location: https://www.cgran.org/cgran/projects/bbn_80211/branches/usrp2_version
• CPU: (Bo) Intel(R) Core(TM) 2 CPU E8400 @ 3GHz
• Memory: 4GB
• Wireless Card: Dlink WDA-1320 Atheros Chipset (using madwifi driver)

My setup:
  - Infrastructure mode
 -  AP has a fixed data rate set to 1Mbps so that BBN code receives *ALMOST* all the packets

Now, onto my MAIN question:
 - If the sender at 1Mbps is using DSSS (Barker code), and I am receiving with decimation factor 4 using USRP2,
   - How many samples per symbol should I expect (I am assuming 1 symbol per bit, thus samples per symbol =  samples per bit)?  I collect samples using usrp2_rx_cfile.py [rx-file=b.dat] and analyze in Matlab.
   - So, 25MSps (complex float symbol) sampling rate, how many samples per bit ought I expect. I do b = read_complex_binary('b.dat') in matlab , and look at plot(abs(b))?

Thank you in advance for your time.