  Next generation wireless systems will need higher spectral efficiency as the
expected traffic volumes per unit bandwidth and dimension will inevitably grow.
As a consequence, it is necessary to design coding schemes with performances
close to the theoretical limits, having high flexibility and low complexity
requirements at transmitter and receiver. In this paper, we point out some of
the limitations of the Bit Interleaved Code Modulation (BICM) technique which
is the state of the art adopted in several standards and then propose some new
lower complexity alternatives. These low complexity alternatives are obtained
by applying the recently introduced Analog Digital Belief Propagation (ADBP)
algorithm to a two stage encoding scheme embedding a hard decoding stage. First
we show that for PAM$^2$ type constellations over the AWGN channel, the
performance loss caused by using a hard decoded stage for all modulation bits
except the two least protected is negligible. Next, we consider the application
of two stage decoders to more challenging Rician channels, showing that in this
case the number of bits needed to be soft decoded depends on the Rician factor
and increases to a maximum of three bits per dimension for the Rayleigh
channel. Finally, we apply the ADBP algorithm to further reduce the detection
and decoding complexity.
