@Article{berthold2007ofdm,
	author = {Berthold, Ulrich and Jondral, Friedrich K and Brandes, Sinja and Schnell, Michael},
	title = {OFDM-based overlay systems: A promising approach for enhancing spectral efficiency},
	journal = {IEEE Commun. Mag.},
	volume = {45},
	number = {12},
	pages = {52–58},
	year = {2007},
}