Hi,
I am working on soft bit maximal ratio combiner (SBMRC). It's basically a MRC but instead of combining the actual signals according to their SNR, we combine the LLRs from separate branches and send them to Soft Decision Viterbi Decoder (SDVD). For this, I have modified gr-ieee 80211 which generates soft bits and integrated a SDVD with it. It works good when I use either single branch or both branch separately.
In order to implement SBMRC, for every OFDM symbol decoded, a
vector of LLR (size 48 X 1) is generated from both the receiver
branches. These two vectors of LLR are further added and sent to
SDVD. I configured the ADD block to take 48 floats as input.
First I made a simulator for SBMRC, but even after increasing the
output buffer size to 500000, warnings of buffer over flow kept
coming.
Then I used USRP, it simply refuses to work. I am using NI 2901 Tx/Rx A and Tx/Rx B ports as my receive branches. The LEDs go green for a second then stop. No error no warning.
Looks like the ADD block is the cause. I have never seen
this, so I am clueless where to debug. Am I missing something
fundamental here ?
The attached picture usrp_sbmrc says details of my schematic and the results when I use USRP.
The attached picture sbmrc_sim says details of my schematic and the results when I use simulations.
