A method for operating a transmitter, the method comprising: generating a sequence of frames at a given framing rate, wherein each of the frames includes a plurality of OFDM symbols, wherein each of the OFDM symbols generated by: applying modulation to data sub-carriers, decoy sub-carriers and mask sub-carriers to obtain respectively modulated data sub-carriers, modulated decoy sub-carriers and modulated mask sub-carriers, wherein the decoy sub-carriers and mask sub-carriers are randomly modulated, wherein the data sub-carriers are modulated based on data associated with one or more users; applying a security transformation that maps input sub-carriers to intermediate sub-carriers based on a keyed cryptographic sequence, wherein the input sub-carriers include a) the modulated data sub-carriers and b) the modulated decoy sub-carriers; performing an inverse Fourier transform on a transform dataset to obtain a time-domain composite signal, wherein the transform dataset includes the intermediate sub-carriers and the modulated mask sub-carriers; transmitting the composite signal via one or more antennas; wherein the decoy sub-carriers and the mask sub-carriers are modulated at a different rate than a user modulation rate of the data sub-carriers to assist in suppressing spectral components of the composite signal at the user modulation rate. generating a sequence of frames at a given framing rate, wherein each of the frames includes a plurality of OFDM symbols, wherein each of the OFDM symbols generated by: applying modulation to data sub-carriers, decoy sub-carriers and mask sub-carriers to obtain respectively modulated data sub-carriers, modulated decoy sub-carriers and modulated mask sub-carriers, wherein the decoy sub-carriers and mask sub-carriers are randomly modulated, wherein the data sub-carriers are modulated based on data associated with one or more users; applying a security transformation that maps input sub-carriers to intermediate sub-carriers based on a keyed cryptographic sequence, wherein the input sub-carriers include a) the modulated data sub-carriers and b) the modulated decoy sub-carriers; performing an inverse Fourier transform on a transform dataset to obtain a time-domain composite signal, wherein the transform dataset includes the intermediate sub-carriers and the modulated mask sub-carriers; transmitting the composite signal via one or more antennas; wherein the decoy sub-carriers and the mask sub-carriers are modulated at a different rate than a user modulation rate of the data sub-carriers to assist in suppressing spectral components of the composite signal at the user modulation rate. applying modulation to data sub-carriers, decoy sub-carriers and mask sub-carriers to obtain respectively modulated data sub-carriers, modulated decoy sub-carriers and modulated mask sub-carriers, wherein the decoy sub-carriers and mask sub-carriers are randomly modulated, wherein the data sub-carriers are modulated based on data associated with one or more users; applying a security transformation that maps input sub-carriers to intermediate sub-carriers based on a keyed cryptographic sequence, wherein the input sub-carriers include a) the modulated data sub-carriers and b) the modulated decoy sub-carriers; performing an inverse Fourier transform on a transform dataset to obtain a time-domain composite signal, wherein the transform dataset includes the intermediate sub-carriers and the modulated mask sub-carriers; transmitting the composite signal via one or more antennas; wherein the decoy sub-carriers and the mask sub-carriers are modulated at a different rate than a user modulation rate of the data sub-carriers to assist in suppressing spectral components of the composite signal at the user modulation rate.