A survey on downlink–uplink decoupled access: Advances, challenges, and open problems

Abstract

The emergence of new uplink (UL) intensive applications, coupled with the ambitious vision of the Internet-of-Things (IoT), will soon unleash unprecedented increases in UL wireless data traffic. However, current network designs are mainly concerned with optimizing the downlink (DL) performance, hence cannot give full play to UL communications. Exacerbated by high path-loss and low UL to DL duration ratio in the 5G new radio time division duplex (TDD-NR) spectrum, the UL capacity is much smaller than that of the DL one. To overcome such a problem, a DL–UL decoupled access (DUDe) approach has been proposed, in which the users are allowed to connect to different base-stations (BSs) at different frequency bands in each link direction. Moreover, DUDe can be an add-on to many 5G (and beyond) technologies, such as millimeter-wave communication, multi-connectivity, device-to-device (D2D) communication, cellular-enabled unmanned aerial vehicle (UAV) communication, and mobile edge computing (MEC). However, despite the apparent consensus on its advantages, the realization of DUDe in 5G networks is in its infancy and still requires more efforts from both academia and industry. This survey provides a holistic overview on the DUDe approach, including operating bands DUDe (i.e. supplementary UL) and serving BS DUDe, reviews its fundamentals and explores potential use-cases. Other 5G UL coverage enhancement technologies, such as UL carrier aggregation, evolved universal terrestrial radio access-new radio (EUTRA-NR) dual connectivity (EN-DC), and UL Tx Switching are also compared with DUDe. Lastly, cutting edge research works in this area are summarized along with a discussion on the potential challenges and open research problems.