Exploiting OFDM systems for covert communication

Both Long Term Evolution (LTE) and WiMAX (802.16) 4th generation wireless systems (4G) utilize orthogonal frequency division multiplexing (OFDM) on the down-link. The advantages of OFDM over other wireless communication techniques are well known. These technologies will become more ubiquitous as time goes on. Here a study is presented of the potential for covert communication within an OFDM waveform. Most OFDM standards call for unused sub-channels for channel spacing, synchronization of transmitter and receiver and to mitigate poor channel response. Here we study the effects of inserting a narrow band signal that will be used for covert communication in one of the unused sub-carrier locations of the OFDM signal and analyze its influence on the target OFDM signal as well as determining the communications capabilities of the covert link. The covert signal inserted in an unused subcarrier locations or frequency "slot", is no longer orthogonal to the target OFDM waveform. Thus, there is potential for the covert signal to impact the target link. Here we examine the effect of the covert signal on the target OFDM system as a function of the location of the covert signal, its power, and bit rate. The communications capability of the covert link is also studied. The amount of information that can be transmitted covertly with minimal impact on the target OFDM user (i.e., relatively low probability of detection) is also presented.