Exlpoiting OFDM for Covert Communication

Abstract

Both LTE and WiMAX (802.16) 4th generation wireless systems (4G) utilize orthogonal frequency division multiplexing (OFDM). These technologies will become more wide spread as time goes on. Therefore, it would be beneficial to study covert communication in the presence of OFDM systems. OFDM is becoming more utilized due to a number of advantages it has over other techniques in wireless communication. OFDM can help to mitigate inter symbol-interference (ISI) resulting from multipath propagation. The lower rate of the sub-carriers is what makes this possible. OFDM takes several narrow-band lower-rate signals, and using the discrete Fourier transform, combines them into one high rate wide-band signal. The orthogonality of the OFDM signal eliminates co-channel interference. OFDM combined with adaptive modulation can be very effective in frequency selective environments. Most OFDM standards call for unused sub-channels for channel spacing and synchronization of transmitter and receiver. The effects we will be discussing in this thesis pertain to 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 analyzing its effects on the OFDM signal. Although this signal is in an unused "slot", it is no longer orthogonal. We will also examine the effect of the OFDM signal on the covert signal. It would also be beneficial to study the ability to recover the information in the covert carrier in the midst of an OFDM signal. It will be important to view the effects of the covert signal on the OFDM signal by varying both its power and bandwidth and noting the results of the bit-error rate of the OFDM signal. This will allow us to deduce the amount of information we can transmit with a relatively low probability of detection (LPD) and the losses incurred by the OFDM user.