  To transmit a mixture of real-time and non-real-time traffic in a broadcast
system, we impose a basic service rate $r_0$ for real-time traffic and use the
excess rate beyond $r_0$ to transmit non-real-time traffic. Considering the
time-varying nature of wireless channels, the basic service rate is guaranteed
with a service outage constraint, where service outage occurs when the channel
capacity is below the basic service rate. This approach is well suited for
providing growing services like video, real-time TV, etc., in group
transportation systems such as coach, high-speed train, and airplane. We show
that the optimal power allocation policy depends only on the statistics of the
minimum gain of all user channels, and it is a combination of water-filling and
channel inversion. We provide the optimal power allocation policy, which
guarantees that real-time traffic be delivered with quality of service (QoS)
for every user. Moreover, we show that the required minimum average power to
satisfy the service outage constraint increases linearly with the number of
users.
