Funding for this research was provided by:
National Natural Science Foundation of China (61504010, 61376019, 61674020, 61774021)
Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P. R. China (IPOC2017ZT02, IPOC2017ZZ01)
BUPT Excellent Ph. D. Students Foundation (CX2016305)
Text and Data Mining valid from 2018-02-23
Received: 8 January 2018
Accepted: 14 February 2018
First Online: 23 February 2018
: YW (Ph.D.), XY (Ph.D.), XZ (professor), and XR (professor) are from the State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China.
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.