An LPI design for secure OFDM systems

Traditional OFDM systems present some embedded features that may be exploited in order to intercept the wireless transmitted signal. However, such a possibility is not accepted in military applications where security is a very important issue. Cyclic prefix is one of the most obvious OFDM parameters that induce cyclostationarity features. Aiming to suppress these features, some classical solutions in literature propose to entirely remove the cyclic extension so that additional processing is needed to eliminate the inter-symbol interference. In this paper we propose two techniques that, when combined, allow to eliminate cyclostationary and spectral features of the OFDM waveform, while maintaining the cyclic prefix advantages. The cyclic-prefix size in each OFDM symbol is varied pseudo-randomly so that cyclostationary features are suppressed. Moreover, a pseudo-random frequency jitter is introduced to the carrier frequency in order to mask subcarrier spectral lines. Once these techniques are applied, the designed system is said to generate a low-probability-of-intercept waveform. Finally, performance in multipath channels is investigated and shown to be effective.