  Small cell networks (SCNs) are envisioned to embrace dynamic time division
duplexing (TDD) in order to tailor downlink (DL)/uplink (UL) subframe resources
to quick variations and burstiness of DL/UL traffic. The study of dynamic TDD
is particularly important because it provides valuable insights on the full
duplex transmission technology, which has been identified as one of the
candidate technologies for the 5th-generation (5G) networks. Up to now, the
existing works on dynamic TDD have shown that the UL of dynamic TDD suffers
from severe performance degradation due to the strong DL-to-UL interference in
the physical (PHY) layer. This conclusion raises a fundamental question:
Despite such obvious technology disadvantage, what is the true value of dynamic
TDD? In this paper, we answer this question from a media access control (MAC)
layer viewpoint and present analytical results on the DL/UL time resource
utilization (TRU) of synchronous dynamic TDD, which has been widely adopted in
the existing 4th-generation (4G) systems. Our analytical results shed new light
on the dynamic TDD in future synchronous 5G networks.
