In this paper, we present a generalized framework for active eavesdropping in a frequency hopping spread spectrum passive radio frequency identification system. In our model, there exists an adversarial reader who is able to transmit its own continuous wave signal outside the frequency band of the legitimate reader. Due to the fact that under backscatter modulation, the tag cannot distinguish different frequencies and simply sets the impedance in its circuitry to either low or high to reflect a bit of 1 or 0, and the adversarial reader's received signal is a weighted sum of the response to both its own signal and the legitimate reader's signal. Using this model, we provide a theoretical analysis of the capability of the adversarial reader in terms of the decoding error probability for slow frequency and fast frequency hopping systems. We derive analytic formulas and conduct experiments using software defined radios that act as the legitimate reader, the adversarial reader, and Intel Wireless Identification Sensing Platform tags with parameters as specified in EPC Gen2. Simulations are also used to validate our findings. We find from the theoretical analysis as well the experimental results that the active eavesdropper can achieve a better decoding error rate than a conventional passive eavesdropper, even in the case that the eavesdropper's signal is a low power signal.

In this paper, we propose a new encryption scheme for OFDM systems. The reason for physical layer approach is that it has the least impact on the system and is the fastest among all layers. This scheme is computationally secure against the adversary. It requires less key streams compared with other approaches. The idea comes from the importance of orthogonality in OFDM symbols. Destroying the orthogonality create intercarrier interferences. This in turn cause higher bit and symbol decoding error rate. The encryption is performed on the time domain OFDM symbols, which is equivalent to performing nonlinear masking in the frequency domain. Various attacks are explored in this paper. These include known plaintext and ciphertext attack, frequency domain attack, time domain attack, statistical attack and random guessing attack. We show our scheme is resistant against these attacks. Finally, simulations are conducted to compare the new scheme with the conventional cipher encryption.

Sequences with desirable correlation properties have wide applications in today's communication systems. In this paper, we first extend the known results on zero autocorrelation zone of Golay sequences to 4q-QAM Golay sequences and show three constructions of 4q-QAM Golay sequences with a large zero periodic autocorrelation zone, where q ≥ 2 is an arbitrary integer. We then determine the Golay pairs which have large zero periodic crosscorrelation zone.

Golay sequences have been studied for more than five decades since Golay first discovered those sequences. However, the periodic autocorrelation of a single Golay sequence is unknown. In this paper, for H≥ 2 being an arbitrary even integer, we show there exist three different constructions of H-ary Golay sequences with a zero autocorrelation zone (ZACZ) of length approximately an half, a quarter or one eighth of their period. Those new discoveries on Golay sequences can be explored during synchronization and detection at the receiver end and thus improve the performance of the communication system. We present the application of binary Golay sequences with ZACZ for intersymbol interference (ISI) channel estimation. Compared with m-sequences, Golay-sequence-aided channel estimation has perfect autocorrelations within the zone. Compared with Frank-Zadoff-Chu sequences, Golay-sequence-aided channel estimation requires much lower hardware and computational complexity. We also discuss the performance of Golay-sequence-aided channel estimation in terms of its error variance. Finally, simulations are conducted to show the performance of our proposed scheme against m-sequences and FZC sequences in terms of symbol error rate. The simulations also confirm with our theoretical results.

Sequences with good correlation properties have been widely used in modern communications, radar and sonar applications. In this paper, we consider the autocorrelation property of the single binary and quaternary Golay sequences, with special focus on their zero periodic autocorrelation zone. Some examples along with one case of the proof will be given in order to demonstrate this zero periodic autocorrelation zone. This finding on Golay sequences can be explored during synchronization and detection at the receiver end and thus improve the performance of the orthogonal frequency-division multiplexing (OFDM) system where Golay sequences are employed for the peak-to-average power reduction.

Sequences with good correlation properties have been widely adopted in modern communications, radar and sonar applications. In this paper, we present that a single H-ary Golay sequence or 4q-QAM Golay sequence has a large zone of zero odd periodic autocorrelation, where H ≡ 0 (mod 4) is a positive integer and q ≥ 2 is an arbitrary integer. The conditions on the permutations employed in the boolean functions are the same as those for the sequences with a large zone of zero (even) periodic autocorrelation. More importantly, sequences with large odd periodic autocorrelations centered around the origin could be used to reduce the multipath interference at the receiver end and thus improve the performance of the communication system.