Received November 26, 2015, accepted December 18, 2015. Date of publication xxxx 00, 0000, date of current version xxxx 00, 0000. Digital Object Identifier 10.1109/ACCESS.2015.2510702 Five Decades of Hierarchical Modulation and Its Benefits in Relay-Aided Networking AQ:1 HUA SUN1, CHEN DONG2, SOON XIN NG3, (Senior Member, IEEE), AND LAJOS HANZO3, (Fellow, IEEE) 1Research Group of Communications, Signal Processing and Control, School of Electronics and Computer Science, AQ:2 University of Southampton, Southampton SO17 1BJ, U.K. 2University of Southampton, Southampton SO17 1BJ, U.K. 3School of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, U.K. Corresponding author: L. Hanzo (
[email protected]) AQ:3 This work was supported in part by the European Research Council Advanced Fellow Grant and in part by the Engineering and Physical Sciences Research Council U.K. under Grant EP/L018659/1. 1 ABSTRACT Hierarchical modulation (HM), which is also known as layered modulation, has been widely 2 adopted across the telecommunication industry. Its strict backward compatibility with single-layer modems 3 and its low complexity facilitate the seamless upgrading of wireless communication services. The specific 4 features of HM may be conveniently exploited for improving the throughput/information-rate of the system 5 without requiring any extra bandwidth, while its complexity may even be lower than that of the equivalent 6 system relying on conventional modulation schemes. As a recent research trend, the potential employment of 7 HM in the context of cooperative communications has also attracted substantial research interests. Motivated 8 by the lower complexity and higher flexibility of HM, we provide a comprehensive survey and conclude with 9 a range of promising future research directions.