Wen Long Chin

ML Estimation of Timing and Frequency Offsets Using Distinctive Correlation Characteristics of OFDM Signals Over Dispersive Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising technology for communication systems. However, the synchronization of OFDM over dispersive fading channels remains an important and challenging issue. In this paper, a synchronization algorithm for determining the symbol timing offset and the carrier frequency offset (CFO) in OFDM systems, based on the maximum-likelihood (ML) criterion, is described. The new ML approach considers time-dispersive fading channels and employs distinctive correlation characteristics of the cyclic prefix at each sampling time. The proposed symbol timing estimation is found to be a 2-D function of the symbol timing offset and channel length. When compared with previous ML approaches, the proposed likelihood function is optimized at each sampling time without requiring additional pilot symbols. To practically realize the proposed method, a suboptimum approach to the ML estimation is adopted, and an approximate but closed-form solution is presented. Nonlinear operations of the approximate solution can be implemented using a conventional lookup table to reduce the computational complexity. The proposed CFO estimation is also found to depend on the channel length. Unlike conventional schemes, the proposed method fully utilizes the delay spread of dispersive fading channels (which usually reduces the accuracy of estimations). Furthermore, the Cramér-Rao lower bound (CRLB) on the CFO estimate is analyzed, and simulations confirm the advantages of the proposed estimator.

A Blind Synchronizer for OFDM Systems Based on SINR Maximization in Multipath Fading Channels

This paper presents a blind synchronizer for orthogonal frequency-division multiplexing (OFDM) systems based on signal-to-interference-and-noise ratio (SINR) maximization. Due to the incurred losses from intersymbol interference (ISI) and intercarrier interference (ICI) introduced by synchronization errors, the SINR of the received data drastically drops. By taking advantage of this characteristic, both the symbol time and carrier frequency offsets are intuitively estimated by maximizing the SINR metric. For the SINR metric, two blind SINR estimations are investigated. The estimations do not need prior knowledge of the channel profiles and transmitted data. As such, the proposed maximum SINR (MSINR) synchronization algorithm is nondata aided so that the transmission efficiency can be improved. Moreover, to reduce the computational complexity, the early-late gate technique is proposed for the implementation of the synchronizer. Simulation results exhibit better performance for the MSINR algorithm than conventional techniques in multipath fading channels.

Blind Symbol Synchronization for OFDM Systems Using Cyclic Prefix in Time-Variant and Long-Echo Fading Channels

In this paper, blind symbol synchronization for orthogonal frequency-division multiplexing (OFDM) systems based on the cyclic prefix (CP), considering time-variant long-echo fading channels, is introduced. The basic idea of our contribution is to obtain an estimate of channel-tap powers using the correlation characteristics of the CP. The maximum-likelihood (ML) estimation of the correlation characteristics is derived and employed for channel-tap power estimation. An intersymbol-interference (ISI) metric is minimized using the estimate of channel-tap powers. The metric, which is based on the statistical properties of received samples, turns the mobility factor into a coefficient, which can be simply compensated. The metric is suitable for both short- and long-echo channels. The performance is analyzed. The proposed algorithm has moderate complexity. Simulations confirm the advantages of the proposed estimator.

IEEE 1588 clock synchronization using dual slave clocks in a slave

IEEE 1588 is a standard used in the synchronization of independent clocks that run on separate nodes of a distributed measurement and control system. This work presents a method that employs dual slave clocks in a slave to measure the link propagation delay, clock skew and offset. By accurately deriving these parameters, the proposed approach can reduce the deviation from the master clock to several orders of magnitude better than the required specification. The proposed technique fully conforms to the IEEE 1588, and can be used in the environments of symmetric and asymmetric communication links, such as xDSL.

Low-complexity energy detection for spectrum sensing with random arrivals of primary users

Due to random arrivals of primary-user signals, the timing misalignment issue should be considered for spectrum sensing in cognitive radio (CR) systems, such as CR-based femtocell networks. To deal with this issue, two approaches were recommended in the literature, including Bayesian and generalized likelihood ratio test (GLRT) detectors. However, the Bayesian test requires perfect knowledge of the distribution of unknown parameters. Therefore, it is impractical due to its implementation complexity. To design a low-complexity energy detector (ED), this work proposes an ED scheme based on the GLRT algorithm. As a result, maximum-likelihood estimation for the timing misalignment is devised, and the performance of the proposed scheme is analyzed. The results show that the proposed GLRT detector features a low-complexity and satisfactory performance.

Channel-Based Detection of Primary User Emulation Attacks in Cognitive Radios

Recently, cognitive radio (CR) has recently emerged as a useful technology to improve the efficiency of spectrum utilization. However, wireless networks are accompanied with an important security flaw that they are much easier to be attacked than any wired network. Many security issues are discovered. Within which, the most important one is primary user emulation attack (PUEA). In this work, we propose a method by using the characteristics of wireless channels to identify the PUEA. In the wireless environment, the statistical property of the wireless channel between the receiver and transmitter is unique; therefore, we can use this feature as a radio fingerprint. By employing the capability of spectrum sensing in CR, we can identify primary user emulation attackers via the uniqueness of wireless channels. Compared with conventional security schemes based on higher layer protocols, whose information must be passed to the upper layers, the proposed scheme using physical layer is more efficient in terms of the detection time. Simulations confirm the advantages of the proposed scheme.

Iterative Synchronization-Assisted Detection of OFDM Signals in Cognitive Radio Systems

Despite many attractive features of an orthogonal frequency-division multiplexing (OFDM) system, the signal detection in an OFDM system over multipath fading channels remains a challenging issue, particularly in a relatively low signal-to-noise ratio (SNR) scenario. This paper presents an iterative synchronization-assisted OFDM signal detection scheme for cognitive radio (CR) applications over multipath channels in low-SNR regions. To detect an OFDM signal, a log-likelihood ratio (LLR) test is employed without additional pilot symbols using a cyclic prefix (CP). Analytical results indicate that the LLR of received samples at a low SNR can be approximated by their log-likelihood (LL) functions, thus allowing us to estimate synchronization parameters for signal detection. The LL function is complex and depends on various parameters, including correlation coefficient, carrier frequency offset (CFO), symbol timing offset, and channel length. Decomposing a synchronization problem into several relatively simple parameter estimation subproblems eliminates a multidimensional grid search. An iterative scheme is also devised to implement a synchronization process. Simulation results confirm the effectiveness of the proposed detector.

Standardization and Security for Smart Grid Communications Based on Cognitive Radio Technologies—A Comprehensive Survey

Today’s electric power grids have been ageing and are ill-suited to meet fast-growing demands for electricity energy generation, delivery, and supply. Global climate change and greenhouse gas emissions on the Earth caused by power industries put a high pressure on the existing power grids. Consequently, smart grid (SG) has emerged to address these challenges. The SG can achieve improved load balancing through accessing instantaneous electricity demand information via two-way communication and power flows, which help power plants match their output to the demand precisely. To this end, SG works based on the exchanges of a large amount of information generated from metering, sensing, and monitoring. Hence, the choice of communication infrastructure for SG is critical to provide secure, reliable, and efficient data delivery between various SG components. Cognitive radio (CR) network has been recognized as a promising technology to address communication requirements, standardization, and security problems of SG. Moreover, possible solutions in CR-based SG communications are also identified. In particular, we identify the major challenges of communication architecture, standardization, and security issues to implement CR-based SG communications. The aim of this paper is to offer a comprehensive review on the state-of-the-art researches on CR-based SG communications, to highlight what have been investigated and what still remain to be addressed, particularly, in standardization and security aspects.

On the Analysis of Combined Synchronization Error Effects in OFDM Systems

This work analyzes combined effects of major syn- chronization errors, including the symbol time offset (STO), car- rier frequency offset (CFO) and sampling clock frequency offset (SCFO) of orthogonal frequency-division multiplexing (OFDM) systems. Such errors degrade the performance of an OFDM re- ceiver by introducing inter-carrier interference (ICI) and in- ter-symbol interference (ISI) into the systems. Traditionally, de- signing an OFDM receiver needs plenty of Monte Carlo simula- tions because the synchronization errors are simultaneously in- evitable in practical environments. Therefore, we formulate the theoretical signal-to-interference-and-noise ratio (SINR) to assist the design of OFDM receivers. By knowing the required SINR of specific application, all combinations of allowable errors can be derived. Then, cost-effective algorithms could be easily designed.

Joint effects of synchronization errors of OFDM systems in doubly-selective fading channels

The majority of existing analyses on synchronization errors consider only partial synchronization error factors. In contrast, this work simultaneously analyzes joint effects of major synchronization errors, including the symbol time offset (STO), carrier frequency offset (CFO), and sampling clock frequency offset (SCFO) of orthogonal frequency-division multiplexing (OFDM) systems in doubly-selective fading channels. Those errors are generally coexisting so that the combined error will seriously degrade the performance of an OFDM receiver by introducing intercarrier interference (ICI) and intersymbol interference (ISI). To assist the design of OFDM receivers, we formulate the theoretical signal-to-interference-and-noise ratio (SINR) due to the combined error effect. As such, by knowing the required SINR of a specific application, all combinations of allowable errors can be derived, and cost-effective algorithms can be easily characterized. By doing so, it is unnecessary to run the time-consuming Monte Carlo simulations, commonly adopted by many conventional designs of synchronization algorithms, in order to know those combined error effects.