  This paper considers the uplink pilot overhead in a time division duplexing
(TDD) massive Multiple Input Multiple Output (MIMO) mobile systems. A common
scenario of conventional massive MIMO systems is a Base Station (BS) serving
all user terminals (UTs) in the cell with the same TDD frame format that fits
the coherence interval of the worst-case scenario of user mobility (e.g. a
moving train with velocity 300 Km/s). Furthermore, the BS has to estimate all
the channels each time-slot for all users even for those with long coherence
intervals. In fact, within the same cell, sensors or pedestrian with low
mobility UTs (e.g. moving 1.38 m/s) share the same short TDD frame and thus are
obliged to upload their pilots each time-slot. The channel coherence interval
of the pedestrian UTs with a carrier frequency of 1.9 GHz can be as long as 60
times that of the train passenger users. In other words, conventional
techniques waste 59-uploaded pilot sequences for channel estimation. In this
paper, we are aware of the resources waste due to various coherence intervals
among different user mobility. We classify users based on their coherence
interval length, and we propose to skip uploading pilots of UTs with large
coherence intervals. Then, we shift frames with the same pilot reused sequence
toward an empty pilot time-slot. Simulation results had proved that the
proposed technique overcome the performance of conventional massive MIMO
systems in both energy and spectral efficiency.
