Soft Output MIMO detection

Started by pankajb August 12, 2007
    I simulated a soft output MIMO detection scheme in which a mimo
detector produces LLRs which are fed to a soft input viterbi decoder(using
vitdec matlab function). LLRs (using max-log approx.) are computed based on
the euclidean distances of the received point wrt to the lattice points. I
noticed that the LLRs have large magnitudes ( 40-50 or even more) at
higher SNR. I observed that the BER performance is more or less same as
that of hard decoder.
1) Is it because the magnitudes of LLRs are so large that the soft input
viterbi essentially treats them as though it received hard inputs?
2) Do I have to normalize the euclidean distances(or LLRs) to take in to
account for the "blowing up" of the constellation (or lattice) due to
if so with what quantity?
   I will elaborate more on this. Suppose consider a BPSK scheme in fading
channel to compute LLRs I need to compute the distance of received point
from 1*h and -1*h, where "h" is an instantaneous fading sample. These
distances will obviously depend on the value of h (even if I am generating
fading process which has unit average energy). if h is very large the LLR
will be large too. How is this problem overcome? does all this play
out in two or higher dimensional lattices (in a 4x4 MIMO with 16-QAM
modulation it has 8 real dimensions).
 system model for 4x4 16-QAM..rate 1/2 conv. code

random bits--->conv. encoder-->QAM symbol mapping(gray coding)-->
channel+noise--->soft mimo detection and QAM demapping---> soft in
viterbi(vitdec function matlab).
for sake of brevity I have not shown the interleaver, but in the actual
simulation I have included interleaver.

Thanks for your time