Advanced methods in automatic modulation classification for emerging technologies

Modulation classification (MC) is of large importance in both military and commercial communication applications. It is a challenging problem, especially in non-cooperative wireless environments, where channel fading and no prior knowledge on the incoming signal are major factors that deteriorate the reception performance. Although the average likelihood ratio test method can provide an optimal solution to the MC problem with unknown parameters, it suffers from high computational complexity and in some cases mathematical intractability. Instead, in this research, an array-based quasi-hybrid likelihood ratio test (qHLRT) algorithm is proposed, which depicts two major advantages. First, it is simple yet accurate enough parameter estimation with reduced complexity. Second the incorporation of antenna arrays offers an effective ability to combat fading. Furthermore, a practical array-based qHLRT classifier scheme is implemented, which applies maximal ratio combining (MRC) to increase the accuracy of both carrier frequency offset (CFO) estimation and likelihood function calculation in channel fading. In fact, double CFO estimations are executed in this classifier. With the first the unknown CFO, phase offsets and amplitudes are estimated as prerequisite for MRC operation. Then, MRC is performed using these estimates, followed by a second CFO estimator. Since the input of the second CFO estimator is the output of the MRC, fading effects on the incoming signals are removed significantly and signal-to-noise ratio (SNR) is augmented. As a result, a more accurate CFO estimate is obtained. Consequently, the overall classification performance is improved, especially in low SNR environment. Recently, many state-of-the-arts communication technologies, such as orthogonal frequency division multiplexing (OFDM) modulations, have been emerging. The need for distinguishing OFDM signal from single carrier has become obvious. Besides, some vital parameters of OFDM signals should be extracted for further processing. In comparison to the research on MC for single carrier single antenna transmission, much less attention has been paid to the MC for emerging modulation methods. A comprehensive classification system is proposed for recognizing the OFDM signal and extracting its parameters. An automatic OFDM modulation classifier is proposed, which is based on the goodness-of-fittest. Since OFDM signal is Gaussian, Cramer-von Mises technique, working on the empirical distribution function, has been applied to test the presence of the normality. Numerical results show that such approach can successfully identify OFDM signals from single carrier modulations over a wide SNR range. Moreover, the proposed scheme can provide the acceptable performance when frequency-selective fading is present. Correlation test is then applied to estimate OFDM cyclic prefix duration. A two-phase searching scheme, which is based on Fast Fourier Transform (FFT) as well as Gaussianity test, is devised to detect the number of subcarriers. In the first phase, a coarse search is carried out iteratively. The exact number of subcarriers is determined by the fine tune in the second phase. Both analytical work and numerical results are presented to verify the efficiency of the proposed scheme

Periodogram-based carrier frequency offset estimation for orthogonal frequency division multiplexing applications

In this paper, a novel carrier frequency offset (CFO) estimation algorithm is proposed for OFDM applications. The maximum likelihood estimator (MLE) for the CFO is derived, which reveals the relationship between the CFO and the periodogram of the received signal. Theoretical analysis shows that the proposed MLE is statistically efficient. To realize this MLE in practice, a sub-optimal estimator is also introduced in which zero-padded FFT is invoked for implementation. For the objectives of reducing the implementation complexity and broadening the estimation range, a preamble structure comprising nonuniformly-spaced pilot tones is presented. Based on this preamble, the CFO estimation is split into two phases: the coarse estimation is obtained through the correlation between the received spectrum and the original pattern of the preamble; whereas the fine estimation is obtained by comparing the relative magnitude attenuation in the vicinities of those CFO-shifted pilot tones. Both analytical investigations and computer simulations indicate that the accuracy of this simplified sub-optimal estimator is proportional to the oversize ratio of zero-padded FFT, and its estimation range is equal to the bandwidth of OFDM signal. When the oversize ratio is sufficiently high, the performance of the proposed sub-optimal estimator approaches that of the proposed MLE.published_or_final_versio

GCL Based Synchronization and Time Domain Frequency Offset Correction in OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique that has become the technology of choice in most wireless communication networks of today. Despite the advantages the OFDM system offers, it has some disadvantages like sensitivity to synchronization and high power-to-average-power ratio (PAPR). Any time offset leads to inter-symbol interference (ISI) whereas any frequency offset results in inter-carrier interference (ICI) and, as a result, the system performance degrades. The studies of preamble based time synchronization show that, the standard PN sequence based preamble in IEEE 802.16a is less robust to frequency offset when used in Park’s method of time synchronization - a method that gives more accurate result as compared to other methods. Time domain channel estimation cannot be carried out in the presence of integer frequency offset. This thesis has three specific objectives. Firstly, to design and evaluate a new preamble by making use of a generalized chirp-like (GCL) sequence that has low PAPR and good time and also frequency correlation properties. Secondly, to design a new receiver scheme that estimates and corrects the integer-frequency offset in the time domain and evaluate its performance. And lastly, having corrected the frequency offset in time domain, to estimate the wireless channel in time domain and evaluate its performance. The results show that, the proposed GCL based preamble has better and more robust performance than the standard PN sequence (IEEE 802.16 standard) based preamble in the time and integer frequency synchronization and also in the time domain channel estimation. In the new receiver scheme, the presence of symmetrical correlation shows that received signal is frequency corrected. The results show that the new receiver scheme is able to detect the symmetrical correlation quite accurately. The receiver also works well even in low SNR environment

Secure OFDM System Design for Wireless Communications

Wireless communications is widely employed in modern society and plays an increasingly important role in people\u27s daily life. The broadcast nature of radio propagation, however, causes wireless communications particularly vulnerable to malicious attacks, and leads to critical challenges in securing the wireless transmission. Motivated by the insufficiency of traditional approaches to secure wireless communications, physical layer security that is emerging as a complement to the traditional upper-layer security mechanisms is investigated in this dissertation. Five novel techniques toward the physical layer security of wireless communications are proposed. The first two techniques focus on the security risk assessment in wireless networks to enable a situation-awareness based transmission protection. The third and fourth techniques utilize wireless medium characteristics to enhance the built-in security of wireless communication systems, so as to prevent passive eavesdropping. The last technique provides an embedded confidential signaling link for secure transmitter-receiver interaction in OFDM systems