  Traditionally, wireless cellular systems have been designed to operate in
Frequency Division Duplexing (FDD) paired bands that allocates the same amount
of spectrum for both downlink (DL) and uplink (UL) communication. Such design
is very convenient under symmetric DL/UL traffic conditions, as it used to be
the case when the voice transmission was the predominant service. However, with
the overwhelming advent of data services, bringing along large asymmetries
between DL and UL, the conventional FDD solution becomes inefficient. In this
regard, flexible duplexing concepts aim to derive procedures for improving the
spectrum utilization, by adjusting resources to the actual traffic demand. In
this work we review these concepts and propose the use of unpaired Time
Division Duplexing (TDD) spectrum on the unused resources for small eNBs
(SeNB), so that user equipment (UEs) associated to those SeNB could be served
either in DL or UL. This proposal alleviates the saturated DL in FDD-based
system through user offloading towards the TDD-based system. The flexible
duplexing concept is analyzed from three points of view: a) regulation, b) Long
Term Evolution (LTE)standardization, and c) technical solutions.
