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.
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