  Several previous works have addressed the inherent trade-off between
allocating resources in the power and time domains to pilot and data signals in
multiple input multiple output systems over block-fading channels. In
particular, when the channel changes rapidly in time, channel aging degrades
the performance in terms of spectral efficiency without proper pilot spacing
and power control. Despite recognizing non-stationary stochastic processes as
more accurate models for time-varying wireless channels, the problem of pilot
spacing and power control in multi-antenna systems operating over
non-stationary channels is not addressed in the literature. In this paper, we
address this gap by introducing a refined first-order autoregressive model that
exploits the inherent temporal correlations over non-stationary Rician aging
channels. We design a multi-frame structure for data transmission that better
reflects the non-stationary fading environment than previously developed
single-frame structures. Subsequently, to determine optimal pilot spacing and
power control within this multi-frame structure, we develop an optimization
framework and an efficient algorithm based on maximizing a deterministic
equivalent expression for the spectral efficiency, demonstrating its generality
by encompassing previous channel aging results. Our numerical results indicate
the efficacy of the proposed method in terms of spectral efficiency gains over
the single frame structure.
