This chapter concerns the usage of in-band full-duplex transceivers in the emerging fifth generation (5G) wireless systems. The specific focus of the treatment of the topic is on the design and analysis of wideband physical layer and link layer data transmission protocols for heterogeneous networks, i.e., how to take advantage of the capability for simultaneous transmission and reception on the same frequency band provided by recent developments in self-interference cancellation.
First of all, we modernize the basic definition of the term ‘full-duplex’ such that it is applicable to 5G systems and consistent with the research community's new conception of the terminology. The technical challenges of the full-duplex technology are the self-interference and co-channel interference that are the price of its main advantage: improved spectral efficiency by frequency reuse. Here, we characterize the fundamental rate–interference trade-off imposed by the choice between full- and half-duplex modes in a primitive three-terminal system.
Thereafter, our discussion proceeds to the design and analysis of full-duplex transmission protocols for 5G communication links. The benefits of spectrum reuse by employing the full-duplex mode in this case are reduced by any kind of asymmetry in the system. In particular, the requested input and output data rates to and from a transceiver are not necessarily equal, and there may be imbalances between corresponding channels or even within them in wideband transmission.
The remainder of this chapter is organized as follows. Section 9.2 serves as an introduction to the basics of in-band full-duplex operation, namely its definition, purpose, challenges, and advantages at large. Section 9.3 discusses the design of full-duplex transmission protocols for heterogeneous 5G networks, identifying the types of communication links for which the technology could be suitable. Section 9.4 analyzes the performance of full-duplex transmission over wideband fading channels in typical asymmetric communication scenarios. Finally, Section 9.5 concludes the discussion with some visions of prospective future research directions and the schedule for adopting full-duplex technology for commercial use.
Basics of Full-Duplex Systems
In-Band Full-Duplex Operation Mode
Over the last few years, the research community working on state-of-the-art wireless concepts has been realigning its conception of some very basic terminology related to bidirectional communication and the operation modes of transceivers. In particular, the notion of duplex(ing) has undergone a complete transformation.