  We investigate the network stability problem when two users are scheduled
simultaneously. The key idea is to simultaneously transmit to more than one
users experiencing different channel conditions by employing hierarchical
modulation. For two-user scheduling problem, we develop a throughput-optimal
algorithm which can stabilize the network whenever this is possible. In
addition, we analytically prove that the proposed algorithm achieves larger
achievable rate region compared to the conventional Max-Weight algorithm which
employs uniform modulation and transmits a single user. We demonstrate the
efficacy of the algorithm on a realistic simulation environment using the
parameters of High Data Rate protocol in a Code Division Multiple Access
system. Simulation results show that with the proposed algorithm, the network
can carry higher user traffic with lower delays.
