Time-division versus superposition coded modulation schemes for unequal error protection

This paper is concerned with the design problem of coded modulation schemes which provide unequal error protection (UEP) against Gaussian noise. Starting with the case of two bit streams, we consider two classes of UEP schemes: time-division coded modulation (TDCM) and superposition coded modulation (SCM). The early result of Bergmanns and Cover (1974) ensures the asymptotic superiority of SCM. However, accounting for the fact that practical channel codes do not achieve capacity, we show that the validity of this result depends on the prescribed degree of inequality in protection of the streams. To complement the straightforward design and use of TDCM schemes, we propose methods to decode and design SCM schemes. The design examples substantiate the conclusions obtained from performance analysis. We extend the results to the case of UEP of multiple (more than two) bit streams, where a hybrid of SCM and TDCM must also be considered. We show that, for a large range of unequal protection levels, the hybrid scheme outperforms both SCM and TDCM.