  Broadcasting systems have to deal with channel variability in order to offer
the best spectral efficiency to the receivers. However, the transmission
parameters that maximise the spectral efficiency generally leads to a large
link unavailability. In this paper, we study analytically the trade-off between
spectral efficiency and coverage for various channel resource allocation
strategies when broadcasting two services. More precisely, we consider the
following strategies: time sharing, hierarchical modulation and bit division
multiplexing. Our main contribution is the combination of bit division
multiplexing with non-uniform QAM to improve the performance of broadcasting
systems. The results show that this scheme outperforms all the previous channel
resource allocation strategies.
