Qiaosheng Eric Zhang Matthieu R. Bloch Mayank Bakshi Sidharth Jaggi Undetectable Radios: Covert Communication under Spectral Mask Constraints. 992-996 2019 ISIT https://doi.org/10.1109/ISIT.2019.8849818 conf/isit/2019 db/conf/isit/isit2019.html#ZhangBBJ19