  In this paper, we consider the problem of joint user scheduling and dynamic
pilot allocation in a Time-Division Duplex (TDD) based Massive MIMO network
under varying traffic condition. One of the main problems with Massive MIMO
systems is that the number of available orthogonal pilot signals is limited,
and the dynamic allocation of these signals to different users is crucially
needed to utilize the full benefit of these systems. In addition, pilot signals
are radio resource control (RRC) configured in practice, and hence the frequent
reconfiguration causes high signaling overhead and is costly. Using Lyapunov
optimization framework, we develop an optimal algorithm that first assigns
pilots dynamically based on queue sizes and the channel conditions of users as
well as the reconfiguration cost at large time-scale. Then, it schedules users
on a small-time scale. Numerical results show the efficacy of our algorithm and
demonstrate that pilots do not need to be configured frequently at the expense
of increased queue delay.
