Time Synchronization Scheme Research Articles

Two symbol timing estimation methods using Barker and Kasami sequence as preamble for OFDM-based WLAN systems

The estimation of the exact point for the start of the symbol is significant as the orthogonal frequency division multiplexing (OFDM) systems are very sensitive to timing errors. In the IEEE 802.11a wireless local area network (WLAN), each data packet starts with a preamble consisting of ten short training symbols followed by two long training symbols. The timing metric is computed through autocorrelation of the received samples and their delayed copies. When the preambles are known to the receiver, the timing metric is obtained by crosscorrelation of the received samples with the locally generated samples. The correlation peak of the timing metric indicates the correct symbol time. The symbol timing synchronization schemes in IEEE 802.11a WLAN systems use short training symbols to estimate a coarse symbol time via autocorrelation and then use long symbols to find a fine symbol time via crosscorrelation. In this paper, Barker and Kasami codes are proposed to be used as preambles for timing estimation in OFDM WLAN systems. Timing estimation makes use of correlation of preamble. So preamble with good autocorrelation property has to be chosen. The proposed scheme develops a simple preamble structure and gives more accurate estimate of symbol timing.

Overlay Transmission System on Wireless LAN with RTS/CTS Exchange Taking into Account Timing Synchronization

This paper proposes a new overlay transmission system for wireless LAN with RTS/CTS exchange. Conventional timing synchronization schemes may fail in the presence of inter-system interference, because they have not been designed for overlay transmission. In the proposed system, a transmitter estimates the transmission timing of the next wireless LAN DATA frame, and then sends its DATA frame at almost the same time as the estimated transmission timing to easily establish timing synchronization at the receiver. Moreover, we employ a tapped delay line adaptive array antenna at both transmitter and receiver to effectively suppress interference due to overlay transmission in a rich multipath propagation environment. The frame error rate performances of the proposed system and the IEEE 802.11a wireless LAN are evaluated through computer simulations that assume an exponentially decaying 8-path non-line-of-sight fading channel and include a timing synchronization process. Simulation results demonstrate that the proposed system can achieve overlay transmission while avoiding interference in a rich multipath propagation environment.

Estimating Clock Uncertainty for Efficient Duty-Cycling in Sensor Networks

Radio duty cycling has received significant attention in sensor networking literature, particularly in the form of protocols for medium access control and topology management. While many protocols have claimed to achieve significant duty-cycling benefits in theory and simulation, these benefits have often not translated into practice. The dominant factor that prevents the optimal usage of the radio in real deployment settings is time uncertainty between sensor nodes which results in overhead in the form of long packet preambles, guard bands, and excessive control packets for synchronization. This paper proposes an uncertainty-driven approach to duty-cycling, where a model of long-term clock drift is used to minimize the duty-cycling overhead. First, we use long-term empirical measurements to evaluate and analyze in-depth the interplay between three key parameters that influence long-term synchronization: synchronization rate, history of past synchronization beacons, and the estimation scheme. Second, we use this measurement-based study to design a rate-adaptive, energy-efficient long-term time synchronization algorithm that can adapt to changing clock drift and environmental conditions, while achieving application-specific precision with very high probability. Finally, we integrate our uncertainty-driven time synchronization scheme with the BMAC medium access control protocol, and demonstrate one to two orders of magnitude reduction in transmission energy consumption with negligible impact on packet loss rate.

An Adaptive Timing Synchronization Scheme for Multi-Band Orthogonal Frequency Division Multiplexing Based Ultra-Wideband Communication Systems

This article deals with a new energy based adaptive timing synchronization scheme (ATS) which estimates the symbol timing information within two (2) OFDM symbols and updates the information with different frequency bands (adaptive in sense) in a multi-band orthogonal frequency division multiplexing (MB-OFDM) based system. The new approach provides significant improvement in system performance for high delay spread ultra-wideband (UWB) channel model (CM) environments where fast and low-complexity timing synchronization is a critical issue. This paper also addresses a crucial aspect of UWB channel which is frequency dependent delay characteristics. This effect contributes to different dispersion and timing shift of an UWB signal for different frequency bands. In this work, the wideband channel delay characteristics are studied and delay parameters are found considerably different over frequency bands 3.1---4.6 GHz. Based on this observation, the ATS which estimates and maintains the timing delays of each band separately is presented. The performance of ATS algorithm is measured by mean-squared error (MSE), synchronization probability, signal to interference ratio (SIR) reduction due to synchronization errors and bit error rate (BER) through the computer simulation for several UWB CM environments CM2---CM4. Each of these UWB CMs is simulated for 100,000 noisy channel realizations for both coded and uncoded MB-OFDM system. It is shown that ATS gives signal-to-noise ratio (SNR) improvement of 1.1 dB at BER of 1 × 10?3, 1.2 dB at BER of 2 × 10?4, and 0.7 dB at BER of 2 × 10?4 for CM4, CM3, and CM2 respectively for coded MB-OFDM system over a non-adaptive synchronization scheme [Yak et al., Proceedings of IEEE PIMRC, Berlin, Germany, vol 1, pp 471---475, September 11---14, 2005].

A Maximum Likelihood Fine Timing Estimation for Wireless OFDM Systems

Orthogonal frequency division multiplexing (OFDM) systems are more sensitive to timing synchronization than single carrier systems. This paper presents a fine timing synchronization scheme which utilizes the channel impulse response (CIR) estimated from frequency-domain samples at OFDM receivers. A maximum likelihood estimate (MLE) of timing offset is derived based on the probability distribution of the estimated CIR under time-varying multipath fading. The ML timing scheme is further developed for both integer precision and real-valued precision implementations. In the real-valued timing precision case, a delay locked loop (DLL) structure is devised as an effective way to implement the MLE. Both analysis and simulations of the proposed MLE showed significant improvement over existing schemes under time-varying multipath fading channels.

A New Timing Synchronization Metric for OFDM Based WLAN Systems

A new timing synchronization scheme for Orthogonal Frequency Division Multiplexing (OFDM) based Wireless Local Area Networks (WLANs) is presented. Specifically, use of one long OFDM symbol in the frame format of IEEE802.11a is suggested for coarse timing synchronization. A new timing metric has been proposed which results in significant improvement in performance compared to the results reported.

Maximum Likelihood Multiple Access Timing Synchronization for UWB Communications

Rapid and reliable timing synchronization is a demanding task in the receiver design for ultra-wideband (UWB) communication systems, especially when concurrent multiple access is called for. This paper derives a maximum likelihood based timing synchronization scheme for UWB multiple access. Relying on a set of user-specific training sequences, the novel concept of weighted average template is introduced, leading to a multiple access timing synchronizer that works in the absence of any channel knowledge and is resilient to both multiple access interference and noise. Simulation runs testify the effectiveness of the proposed scheme in terms of mean square timing estimation errors and bit-error-rate detection performance when operating in multiple access and dense multipath scenarios.

A New Approach for Time Synchronization in Wireless Sensor Networks: Pairwise Broadcast Synchronization

This letter proposes an energy-efficient clock synchronization scheme for Wireless Sensor Networks (WSNs) based on a novel time synchronization approach. Within the proposed synchronization approach, a subset of sensor nodes are synchronized by overhearing the timing message exchanges of a pair of sensor nodes. Therefore, a group of sensor nodes can be synchronized without sending any extra messages. This paper brings two main contributions: 1. Development of a novel synchronization approach which can be partially or fully applied for implementation of new synchronization protocols and for improving the performance of existing time synchronization protocols. 2. Design of a time synchronization scheme which significantly reduces the overall network-wide energy consumption without incurring any loss of synchronization accuracy compared to other well-known schemes.

Secure and Efficient Time Synchronization in Heterogeneous Sensor Networks

Due to the collaborative nature of sensor nodes, time synchronization is critical for many sensor network operations. For sensor networks deployed in hostile environments, security is critical to the success of time synchronization. Over the past few years, a number of secure time-synchronization schemes have been proposed for sensor networks. However, these schemes are designed for homogeneous sensor networks and may incur large communication/computation overhead or may cause accumulated synchronization errors (due to multihop relays of time reference messages). Several works have shown that better performance and security can be achieved in heterogeneous sensor networks (HSNs). In this paper, we present a secure and efficient time synchronization scheme for HSNs by utilizing powerful high-end sensors. We implement the time-synchronization scheme in real sensor nodes, and the experiments show that our scheme achieves higher synchronization accuracy than a popular sensor time-synchronization scheme. The analyses demonstrate that our scheme is resilient to various attacks and significantly reduces communication overhead.

A novel time and frequency synchronization scheme for OFDM systems

A new training symbol is designed and a novel time and frequency synchronization scheme for orthogonal frequency division multiplexing systems is proposed. The time synchronization is accomplished by using the symmetric conjugate of the training symbol in time domain and the frequency synchronization is finished by utilizing the good autocorrelation of the training symbol in frequency domain. The simulation results show that compared with conventional schemes, the proposed scheme gives a more accurate estimate of symbol timing and carrier frequency offset and provides a wider acquisition range for the carrier frequency offset.

Vertical and horizontal synchronization services with outlier detection in underwater acoustic networks

AbstractUnderwater Acoustic Networks (UANs) have important applications in ocean exploration and lake pollution monitoring. UANs are however different from terrestrial sensor networks due to their highly variable, long propagation delay, and mobility. Clock synchronization is an important protocol to achieve timing‐based sensor communications. In this paper, we propose a three dimensional, scalable UAN time synchronization scheme that can achieve both horizontal (i.e., in the same water depth) and vertical (i.e., from bottom up to the surface) clock synchronization to overcome the effects of long acoustic delay. To secure UAN clock synchronization services, we also propose a two‐step security UAN synchronization model: (1) correlation test and (2) statistical reputation and trust model. The proposed model can detect outlier timestamp data and identify nodes generating insider attacks. Copyright © 2007 John Wiley & Sons, Ltd.

Efficient Synchronization and Frequency Tracking for Cellular Reuse-I OFDMA Systems

Orthogonal Frequency Division Multiplexing (OFDM) systems are highly sensitive to frequency synchronization errors which cause inter-carrier interference, leading to degradation in the system performance. In this paper, a pragmatic timing and frequency synchronization scheme for reuse-I OFDMA systems that is compatible with IEEE 802.16d/e wireless MAN standard is proposed. The algorithm uses cyclic prefix correlation technique for estimating and tracking the OFDMA frame boundary and fractional frequency offset. Once the preamble is captured, the integral part of the frequency offset and cell-specific preamble code can be estimated. This novel scheme is computationally efficient and robust to co-channel interference and multipath fading. Simulation results show that the proposed algorithm gives excellent performance even at very low signal to interference noise ratios.

Time synchronization methods for wireless sensor networks: A survey

Wireless sensor networks consist of many nodes that collect real-world data, process them, and transmit the data by radio. Wireless sensor networks represent a new, rapidly developing direction in the field of organization of computer networks of free configuration. Sensor networks are used for monitoring a parameter field, where it is often required to fix time of an event with high accuracy. High accuracy of local clocks is also necessary for operation of network protocols (for energy-saving purposes, the nodes spend most of the time in the sleeping mode and communicate only occasionally). In the paper, base techniques used in the existing time synchronization schemes are analyzed; models of local clock behavior and models of interaction of the network devices are described; classification of the synchronization problems is presented; and a survey of the existing approaches to synchronization of time in sensor networks is given.

Effect of imperfect noise estimation in timing synchronization

This paper calculates the performance of an orthogonal frequency division multiplexing (OFDM) system confronted with Rayleigh fading channels in terms of sync detection probability. We adopted a timing synchronization scheme employing simple repeated constant amplitude zero auto-correlation (CAZAC) training symbol with relevant correlation techniques, and analyzed the effect of imperfect noise estimation. Some probability expressions of OFDM timing synchronization for system measurement are evaluated.

Time and Frequency Synchronization for OFDMA Uplink System using the SAGE Algorithm

In this letter, we propose an iterative time and frequency synchronization scheme for orthogonal frequency-division multiple access (OFDMA) uplink system using the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed scheme, the expectation step of the SAGE algorithm is intended to remove the multiple-access interference (MAI) caused by the other asynchronous users. Then, the maximization step is utilized in the timing and frequency offset estimation of each asynchronous user using the MAI-canceled signals. Simulation results present that the proposed scheme shows performance enhancement in the presence of multiple asynchronous users

Network classless time protocol based on clock offset optimization

Time synchronization is critical in distributed environments. A variety of network protocols, middleware and business applications rely on proper time synchronization across the computational infrastructure and depend on the clock accuracy. The Network Time Protocol (NTP) is the current widely accepted standard for synchronizing clocks over the Internet. NTP uses a hierarchical scheme in order to synchronize the clocks in the network. In this paper we present a novel non-hierarchical peer-to-peer approach for time synchronization termed CTP--Classless Time Protocol. This approach exploits convex optimization theory in order to evaluate the impact of each clock offset on the overall objective function. We define the clock offset problem as an optimization problem and derive its optimal solution. Based on the solution we develop a distributed protocol that can be implemented over a communication network, prove its convergence to the optimal clock offsets and show its properties. For compatibility, CTP may use the packet format and number of measurements used by NTP. We also present methodology and numerical results for evaluating and comparing the accuracy of time synchronization schemes. We show that the CTP outperforms hierarchical schemes such as NTP in the sense of clock accuracy with respect to a universal clock.

Attack-resilient time synchronization for wireless sensor networks

The existing time synchronization schemes in sensor networks were not designed with security in mind, thus leaving them vulnerable to security attacks. In this paper, we first identify various attacks that are effective to several representative time synchronization schemes, and then focus on a specific type of attack called delay attack, which cannot be addressed by cryptographic techniques. Next we propose two approaches to detect and accommodate the delay attack. Our first approach uses the generalized extreme studentized deviate (GESD) algorithm to detect multiple outliers introduced by the compromised nodes; our second approach uses a threshold derived using a time transformation technique to filter out the outliers. Finally we show the effectiveness of these two schemes through extensive simulations.

A Robust Timing and Frequency Synchronization Algorithm for DVB-H Receiver

A robust symbol timing and carrier frequency synchronization scheme applicable to orthogonal frequency division multiplexing system is presented. The proposed method is based on correlation and energy detection in time domain. The estimator has a steep timing metric and can provide delay-spread information about the channel. It means that the proposed method can distinguish ISI (inter-symbol interference) free region from ISI region with optimum timing position acquired. The computer simulation results show that the proposed algorithm can track time varying channel condition and has much smaller MSE (mean squares error) than that of ML estimation algorithms.

Asymptotically optimal UWB receivers with noisy templates: design and comparison with RAKE

For pulsed ultra-wideband (UWB) radios, a major challenge in receiver design is to collect sufficient energy from ultrashort pulses exposing to strong multipath scattering. We develop a UWB receiver structure along with low-complexity timing synchronization and data demodulation schemes based on noisy templates (NTs). The NT receiver enjoys full multipath diversity, and achieves asymptotically optimal detection performance with robustness to mistiming. The detection error performance of the NT receiver is analyzed and compared with that of RAKE receivers, under realistic channel and timing estimation errors. Insights on the design tradeoffs of NT versus RAKE reception are provided, using unifying metrics that capture the relative importance of various performance-critical factors of individual receivers. The NT receiver outperforms the RAKE with a limited number of fingers under practical operating conditions.

Robust Time and Frequency Synchronization for OFDM-Based WLANs

A robust time and frequency synchronization scheme is proposed for the high rate OFDM-based wireless local area networks (WLANs). The IEEE 802.11a standardized preamble is efficiently utilized and makes the proposed scheme practical. Simulation results under different channel environments are presented to illustrate the effectiveness of the proposed scheme.