Physical layer techniques for wireless communication security

In this dissertation, we consider physical layer techniques in wireless communication security. We study how to crack an adversary's communication which uses direct-sequence spread-spectrum (DS-SS), and how to protect wireless communication systems against interference. These security measures are classified as electronic protection (EP) and electronic attack (EA) among electronic warfare (EW), respectively. EW is the use of the electromagnetic spectrum to effectively deny the use of this medium by an adversary, while optimizing its use by friendly forces.;We investigate the performance of the physical layer of wireless communication systems considered in this dissertation. On the topic of eavesdropping on an adversary's secure communication, we discuss how to crack a DS-SS system. The DS-SS is a covert technique resistant to interference, interception and multipath fading. Identifying spread-spectrum signals or cracking DS-SS systems by an unintended receiver (or eavesdropper) without a priori knowledge is a challenging problem. To address this problem, we first search for the start position of data symbols in the spread signal (for symbol synchronization). After synchronization, we remove a spread sequence by a less-expensive and more accurate cross-correlation based method, and identify the spread sequence by using a matched filter. We also propose a zigzag searching method to identify a generator polynomial that reduces memory requirement and is capable of correcting polarity errors existing in the previous methods. In addition, we analyze the bit error performance of our proposed method.;With regard to protecting wireless communication systems against interference, we propose an enhanced transform domain communication system (ETDCS) with narrow band interference (NBI) avoidance capability as a countermeasure for a single-carrier single-input single-output (SC-SISO) communication system, a vertical-Bell Laboratories layered space-time (V-BLAST) architecture with non-stationary interference avoidance capability as a countermeasure for a single-carrier multi-input multi-output (SC-MIMO), and a multi-carrier transform domain communication system (MC-TDCS) as a countermeasure for multi-carrier multi-input single-output (MC-MISO) systems, respectively.;The TDCS is a viable solution for interference avoidance. An interference avoiding fundamental modulation waveform is synthesized at the transmitter to avoid intentional interference, and the receiver adapts its matched filter to match the transmitted fundamental modulation waveform in the frequency domain. By doing so, spectrally interfered regions are avoided altogether via adaptive spectral notching.;The basic idea for the ETDCS is to synthesize an adaptive fundamental modulation waveform by a non-parametric spectral estimator, called Capon's method. The advantages of the ETDCS and a V-BLAST with the minimum mean square error (MMSE) detector are integrated to enhance bit error performance in narrow band interference (NBI) environment. The concepts of ETDCS and multi-carrier modulation are combined together in the MC-TDCS to combat multipath fading and to avoid intentional interference.;In code division multiple access (CDMA) or multi-carrier CDMA (MC-CDMA) systems, because users do not use completely orthogonal spreading codes or orthogonality of the spreading codes are destroyed by multipath fading, there is residual multiple-access interference (MAI) present at the output of a matched filter. Transmitter pre-filtering techniques can be employed to mitigate the MAI and channel distortions in the downlink of the MC-CDMA. We analyze the bit error rate performance of a downlink time division duplex MC-CDMA with a pre-filtering transmitter antenna array at the base station (BS), rather than at the mobile terminals (MTs). We also incorporate a pre-filtering approach at the MC-TDCS to mitigate the MAI.;In summary, this research studies physical layer measures in wireless communication security. We provide countermeasures for SISO and MIMO communication systems against jamming and unintentional interference, while we also study vulnerabilities of DS-SS systems as well as how to crack the DS-SS system.