Precoding for OFDM under disguised jamming

This paper considers jamming-resistant OFDM system design under full-band disguised jamming, where the jamming symbols are taken from the same constellation as the information symbols over each subcarrier. First, we analyze the impact of disguised jamming on OFDM systems. It is shown that due to the symmetricity between the authorized signal and jamming, the BER of OFDM systems without symbol-level precoding or only with repeated symbol-level coding is lower bounded by a modulation specific constant, which cannot be improved by increasing SNR. Second, we develop an optimal precoding scheme which minimizes the BER of OFDM systems under full-band disguised jamming. It is shown that the most efficient way to combat full-band disguised jamming in OFDM systems is to concentrate the total available power and distribute it uniformly over a particular number of subcarriers instead of the entire spectrum. The underlying argument is that for a particular subcarrier, when the signal-to-jamming ratio is large enough, then the receiver can distinguish the authorized signal from disguised jamming under the presence of noise. Both theoretical analysis and numerical results demonstrate that the BER performance of OFDM systems under full-band disguised jamming can be improved significantly with the proposed precoding scheme.