  In this paper, we investigate the performance gains of adapting pilot spacing
and power for Carrier Aggregation (CA)-OFDM systems in nonstationary wireless
channels. In current multi-band CA-OFDM wireless networks, all component
carriers use the same pilot density, which is designed for poor channel
environments. This leads to unnecessary pilot overhead in good channel
conditions and performance degradation in the worst channel conditions. We
propose adaptation of pilot spacing and power using a codebook-based approach,
where the transmitter and receiver exchange information about the fading
characteristics of the channel over a short period of time, which are stored as
entries in a channel profile codebook. We present a heuristic algorithm that
maximizes the achievable rate by finding the optimal pilot spacing and power,
from a set of candidate pilot configurations. We also analyze the computational
complexity of our proposed algorithm and the feedback overhead. We describe
methods to minimize the computation and feedback requirements for our algorithm
in multi-band CA scenarios and present simulation results in typical
terrestrial and air-to-ground/air-to-air nonstationary channels. Our results
show that significant performance gains can be achieved when adopting adaptive
pilot spacing and power allocation in nonstationary channels. We also discuss
important practical considerations and provide guidelines to implement adaptive
pilot spacing in CA-OFDM systems.
