Time Synchronization

The FPGA technology has been playing a considerable role in portable and mobile communication. This is due to the features of flexibility, accuracy and configurability in designing and implementation. The paper presents a complete design for a 16-QAM transmitter and receiver based on the Virtex4 FPGA Kit. The implemented system can be applied in particle. Based on the principles of carrier synchronization, time synchronization, core tools for phase-different detecting as well as adaptive equalization processing in System Generator (a software of Xilinx), the authors have designed a complete baseband IF 16-QAM system, in which the baseband signal is upconverted into IF frequency (up to 12MHz) at the transmitter and then is downconverted at the receiver. After timing synchronizing, the adaptive equalizing and phase recovering, the received baseband signal is displayed in the oscilloscope's screen. These accurate experiments conducted in Virtex 4 FPGA board kit have shown a promising foundation for developing coding, algorithms in 16-QAM modulation scheme.

Satellite-based navigation systems uses one-way ranging measurements for system orbit estimation and time-keeping, due to its operational advantage when compared with two-way ranging technique, in terms of complexity of ground monitoring stations (completely passive and requiring a simple omnidirectional antenna to track all the satellites in view). However, a sufficient number of simultaneous independent measurements is required to solve the system unknowns: in particular simultaneous visibility of multiple stations by an individual satellite (allowing to separate the ground stations clocks contributions since the SVs clocks disappear), as well as simultaneity of observations from the same monitor stations of a large number of satellites (allowing to recover the SVs clocks parameters, since the GSS clocks drop out) is the key to an effective separation in the solution of the clock contributions from the pseudo-ranges. In the Galileo IOV phase (consisting of 4 satellites on two orbital planes and a ground network of 20 Sensor Stations), the first condition is clearly fulfilled, however the second condition is not met for a considerable part of the time. If two GSSs do not see simultaneously a single Galileo satellite, they will not be able to estimate their clocks time and frequency drifts, i.e. they will not be synchronized. The free running clocks will essentially enter a holdover mode, were the relative time between the two stations will be slowly drifting as a function of the initial conditions and the stability of the clocks. The ground stations synchronization will gradually degrade with time and when a satellite will rise on the horizon they will be essentially not synchronized to the extent required to carry on a one-way-based Orbit Determination & Time Synchronization (OD&TS). During the IOV phase, the limited number of satellites available and the peculiar characteristics of the Galileo orbits will make difficult for the Orbit Determination and Time Synchronization to start producing meaningful data, therefore some form of intermediate operational configuration must be sought to help in the OD&TS process initialization. The paper will address the proposed solution to overcome the problem of Galileo system initialization starting from the intermediate configuration with first 2 satellites (first IOV Launch) up to final IOV Configuration after second IOV launch. The proposed solution will be based on a limited use of GPS to insure the synchronization of the Galileo Sensor Stations, relying on the exploitation of the Linked Common View Time Transfer (LCVTT) Technique, while the Galileo Orbit Determination and SVs clocks characterization will be carried on autonomously and independently by GPS, in a two step process, up to the achievement of the IOV Configuration with 4 satellites, when the nominal Orbit Determination and Time Synchronization process will be operated. Moreover the paper will address the development of the LCVTT Algorithm, carried on as part of development of the infrastructure aiming to support the Galileo Verification Phase currently under definition as part of Galileo Phase C/D/E1 contract. The algorithm design and implementation will be presented together with the validation carried out (both for LCVTT and MLCVTT) to verify that the synchronization accuracy is adequate to support the Galileo System Initialization.

Media Access Control (MAC) must be carefully considered in multi-hop Underwater Acoustic Sensor Networks (UASNs) due to limited bandwidth and large propagation delay. In particular, variable propagation delays in UASNs cause inaccurate time synchronization and therefore make reservation-based protocols less favorable. Large propagation delays limit the performance of carrier sense in predicting the status of the intended recipients, and therefore CSMA protocols show bad performance in UASNs too. Therefore, simple protocols, such as Aloha, show promise for UASNs. In this paper we consider the performance of a multi-hop ppersistent ALOHA protocol.

In the automation world the intelligent electrical devices need to be accurately synchronized for time stamping of data and motion control. In this paper we discuss how to achieve precise time synchronization over switched Ethernet networks in a generic way. The addressed synchronization requirements relate to substation automation, which is associated with five levels of synchronization accuracy defined as the IEC 61850 classes T1 to T5. The paper challenges the hardest requirements and present general solutions for IEC classes T4/T5 (4µ µ µ µs/1µ µ µ µs) and

The contribution of embedded generation to the connected capacity of the power system is increasing. In order to make best use of this resource, splitting the distribution network, also known as islanding, is being considered by some utilities. Operating the ...

The field of Remote health monitoring now includes technologies such as home and mobile health monitoring, teleretinal imaging, tele-radiology, remote cardiac monitoring, video conferencing and sensors for remote diagnosis and treatment to patients. In this regard, implantable wireless body sensor networks (IWBSNs) have recently emerged as an important and growing research area. These implantable sensors are required to be reliable, very small, battery-operated, and capable of collecting data, processing it, and transmitting it wirelessly and efficiently. Since these devices are required to run with limited resources (energy, processing, and memory), their utility protocols (collecting, processing, and communication) should be designed carefully, not only to work reliably but, more importantly, to be resource-efficient. The life time of the embedded batteries associated with these sensor nodes varies from a few days to a few weeks as was described in a previous work by the authors. In this paper, we propose a novel technique which allows the implanted sensor nodes to communicate with a base station located outside the body efficiently by consuming the minimum amount of energy. Our proposed protocol allows the battery to last significantly longer even for years with a gain of up to 100's times of power saving. This will improve the quality of patient life, and reduce risk of infection resulting from frequent chirurgical operations needed to replace such implantable batteries. Also, a new time synchronization algorithm is briefly introduced in this work that is especially applicable to our proposed communication protocol.

— The main objective of developing this system is to present an automated way to generate the subtitles for audio and video. By replacing the tedious method of the current system will save time, reduce the amount of work the administration has to do and will generate the subtitles automatically with electronic apparatus. This system will first extract the audio, then recognise the extracted audio with the available speech recognition API. Later the recognized audio is converted to the text and saved in text file having extension “.srt”. Later on, this “.srt ” file can be opened in a media player to view the subtitles along with video.

Time synchronization is a critical piece of infrastructure for any distributed system. For years, protocols such as NTP (the Network Time Protocol) have kept the Internet's clocks ticking in phase. However, a new class of networks is emerging. Advances in miniaturization and low-cost, low-power design have led to active research in large-scale networks of small, wireless, low-power sensors and actuators. These systems are closely coupled to the physical world and have strict energy constraints; this leads to stronger accuracy and precision requirements while limiting the resources that can be used to achieve them. Is NTP the right choice for these new networks?

This paper presents a distributed implementation of RAND, a randomized time slot scheduling algorithm, called DRAND. DRAND runs in O(δ) time and message complexity where δ is the maximum size of a two-hop neighborhood in a wireless network while message complexity remains O(δ), assuming that message delays can be bounded by an unknown constant. DRAND is the first fully distributed version of RAND. The algorithm is suitable for a wireless network where most nodes do not move, such as wireless mesh networks and wireless sensor networks. We implement the algorithm in TinyOS and demonstrate its performance in a real testbed of Mica2 nodes. The algorithm does not require any time synchronization and is shown to be effective in adapting to local topology changes without incurring global overhead in the scheduling. Because of these features, it can also be used even for other scheduling problems such as frequency or code scheduling (for FDMA or CDMA) or local identifier assignment for wireless networks where time synchronization is not enforced.

This paper introduces a new IEEE standard, IEEE 802.15.5,which provides mesh capability for wireless personal area network (WPAN) devices. The standard provides an architectural framework enabling WPAN devices to promote interoperable, stable, and scalable wireless mesh topologies. It is composed of two parts: low-rate WPAN mesh and high-rate WPAN mesh. In this paper, we present only low-rate WPAN mesh because it is designed to support wireless sensor networks. IEEE 802.15.5 low-rate part is a lightweight scalable mesh routing protocol that caters well to the requirements of resource-constrained wireless sensor networks. By binding logical addresses to the network topology, IEEE 802.15.5 obviates the need for route discovery. This eliminates the initial route discovery latency, saves storage space and reduces the communication overhead and energy consumption. A distributed link state scheme is further built atop the block addressing scheme to improve the quality of routes, robustness, and load balancing. The routing scheme scales well with regard to various performance metrics. The standard also provides enhanced functions such as multicast, reliable broadcast, power saving, time synchronization, route tracing and portability. We also present the performance evaluation of major functions performed with a 50-nodes tested deployed over a whole floor (100 x 140 ft2) at CUNY Engineering building. The results testify that the IEEE 802.15.5 will serve well for wireless personal area networks and wireless sensor networks.

Recent advances in micro-electromechanical (MEMS) technology have led to the development of small, low-cost, and low-power sensors. Wireless sensor networks (WSNs) are large-scale networks of such sensors, dedicated to observing and monitoring various aspects of the physical world. In such networks, data from each sensor is agglomerated using data fusion to form a single meaningful result, which makes time synchronization between sensors highly desirable. This paper surveys and evaluates existing clock synchronization protocols based on a palette of factors like precision, accuracy, cost, and complexity. The design considerations presented here can help developers either in choosing an existing synchronization protocol or in defining a new protocol that is best suited to the specific needs of a sensornetwork application. Finally, the survey provides a valuable framework by which designers can compare new and existing synchronization protocols.

Data-aided multi-user burst frequency synchronization under interference limited conditions is analyzed. It is assumed that a unique-word based synchronization mechanism is used to achieve synchronization. In a first approach this unique-word is assumed to be located at the beginning of the burst structure. The interference represents frequency-adjacent bursts that are coarsely time synchronous. This corresponds to a typical multi frequency (MF) TDMA system with adjacent channel interference (ACI) limited minimal frequency separation. A novel approach is analyzed to reduce the interference impact of the carrier offset estimation, applying interference cancellation iteratively to increase the estimation accuracy. We demonstrate improved frequency synchronization with this novel approach of iterative burst synchronization.

This paper addresses issues with time synchronization using the IEEE 1588-2008 for distributed measurement and control systems. A practical implementation of the transparent clock is presented with the overall system architecture and detailed operation of each building block. To verify the submicrosecond accuracy using the implemented devices, an experimental setup that was analogous to a practical distributed system has been built. Measured results from the experiment show that the time error is limited below 30 ns for nodes that were connected by three switches. It is remarkable that the results are observed in spite of large packet queuing delays that were introduced by a traffic generator. The discussion on sources of time error that was outlined here provides technical considerations to designing IEEE 1588 systems.

The general theory of relativity was developed by Einstein a century ago. Since then, it has become the standard theory of gravity, especially important to the fields of fundamental astronomy, astrophysics, cosmology, and experimental gravitational physics. Today, the application of general relativity is also essential for many practical purposes involving astrometry, navigation, geodesy, and time synchronization. Numerous experiments have successfully

Software defined radios ( S D R ) are highly configurable hardware platforms that provide the technology for realizing the rapidly expanding third (and future) generation digital wireless communication infrastructure. Many sophisticated signal processing tasks are performed in a SDR, including advanced compression algorithms, power control, channel estimation, equalization, forward error control and protocol management. While there is a plethora of silicon alternatives available for implementing the various functions in a S D R , field programmable gate arrays (FPGAs) are a n attractive option for m a n y of these tasks for reasons of performance, power consumption and configurability. Amongst the more complex tasks performed in a high data rate wireless system is synchronization. This paper is about carrier and timing synchronization in SDRs using FPGA based signal processors. W e describe and examine a QPSK Costas loop for performing coherent demodulation, and report o n the implications of a n FPGA mechanization. Symbol timing recovery is addressed using a differential matched filter control system. A tutorial style approach is adopted to describe the operation of the timing recovery loop and considerations for FPGA implementation are outlined. 195 0-7695-0871-5/00 $10.00 0 2000 IEEE

Wireless mesh networks (WMNs) consist of mesh routers and mesh clients, where mesh routers have minimal mobility and form the backbone of WMNs. They provide network access for both mesh and conventional clients. The integration of WMNs with other networks such as the Internet, cellular, IEEE 802.11, IEEE 802.15, IEEE 802.16, sensor networks, etc., can be accomplished through the gateway and bridging functions in the mesh routers. Mesh clients can be either stationary or mobile, and can form a client mesh network among themselves and with mesh routers. WMNs are anticipated to resolve the limitations and to significantly improve the performance of ad hoc networks, wireless local area networks (WLANs), wireless personal area networks (WPANs), and wireless metropolitan area networks (WMANs). They are undergoing rapid progress and inspiring numerous deployments. WMNs will deliver wireless services for a large variety of applications in personal, local, campus, and metropolitan areas. Despite recent advances in wireless mesh networking, many research challenges remain in all protocol layers. This paper presents a detailed study on recent advances and open research issues in WMNs. System architectures and applications of WMNs are described, followed by discussing the critical factors influencing protocol design. Theoretical network capacity and the state-of-the-art protocols for WMNs are explored with an objective to point out a number of open research issues. Finally, testbeds, industrial practice, and current standard activities related to WMNs are highlighted.

Two experiments were conducted in ewes in order to develop an oestrus-ovulation short time synchronization protocol based on combined FGA, PGF 2␣ , GnRH, eCG treatments, for use in dairy sheep before natural service (Experiment 1) or for fixed-time artificial insemination (Experiment 2), during the breeding season. In Experiment 1 seventy-five non-lactating dairy ewes were subdivided into 5 treatment groups (N = 15): (1) Group Fe -control, which received FGA vaginal sponges (14 days) + eCG (Day 14); (2) Group FPe, FGA (5 days) + PGF 2␣ (Day 5) + eCG (Day 5); (3) Group PFe, PGF 2␣ (Day 0) + FGA (5 days) + eCG (Day 5); (4) Group PFG, PGF 2␣ (Day 0) + FGA (5 days) + GnRH (30 h after sponge removal, s.r.); (5) Group GPe, GnRH (Day 0) + PGF 2␣ (Day 5) + eCG (Day 5). Ewes were checked for oestrus and hand-mated. Time of ovulation was recorded by laparoscopy for 10 animals from each treatment.

The necessity for precise time synchronisation of measurement data from multiple sensors is widely recognised in the field of GPS/INS integration. Having precise time synchronisation is critical for achieving high data fusion performance. The limitations and advantages of various time synchronisation scenarios and existing solutions are investigated in this paper. A criterion for evaluating synchronisation accuracy requirements is derived based on the comparison of the Kalman filter innovation series and the platform dynamics. An innovative time synchronisation solution using a counter and two latching registers is proposed. The proposed solution has been implemented with off-the-shelf components and tested. The resolution and accuracy analysis shows that the proposed solution has the capability to reach a time synchronisation accuracy at 0.1ms level if INS could provide hard-wired timing signal. A synchronisation accuracy of 2 ms has been achieved when the test system was used to synchronise a low-grade MEMS IMU which has only an RS-232 data output interface.

Tight synchronization timing is expected to play a crucial role for the realization of emerging Internet of Things (IoT) high value real-time applications such as smart transportation and smart grid. Most packet-based synchronization protocols require two-way communications delay symmetry for precise accuracy. The Precision Time Protocol (PTP) provides mechanisms to measure and take into account the delay asymmetry problem, such as the residence time measurements, but recommend  PTP support at all nodes. In this paper, we consider partial on-path PTP support, where a subset of the nodes are PTP unaware. We propose probing-based mechanisms in order to estimate asymmetry and improve the synchronization performance. The extensive simulation results show that the proposed lightweight per-packet asymmetry mitigation (PPAM) mechanism, aiming at estimating the per-packet delay components, outperforms the other considered protocols mainly at low to mid network loads, while under certain conditions the regular PTP with QoS support provides more stable synchronization accuracy.

Ethernet networks. This paper describes a PTP GrandMaster/Slave network which maintains synchronization of internal (stratum-0) NTP reference clocks to within tens of nanoseconds. The configuration of a PTP network and the effects of various PTP configuration options are demonstrated.

Time synchronization is very important for a Time Division Multiple Access (TDMA) Ad Hoc Network. In this paper, we proposed Slot Time Synchronization protocol to synchronize all the nodes in a TDMA network. We compute the receive time from Absolute Slot Number (ASN) and use linear regression to eliminate the clock drift. We have carried out our method in real

In many industrial and trading activities such as navigation, mapping, power distribution, telecommunication, weather station, digital radio, industrial control systems and research centers,… one of the main exchanged information which has been transferred between different parts is time tag, indicats the precise time of action taking place. Synchronization between these tags is the main portion of these systems. With transmitting the time synchronizer signal in determined timing intervals to all of these sections by one common reference, synchronization will be done. This common reference could be accessed by GPS Satellites. In this approach GPS satellite, timing signals, Golden Code, methods of coding and decoding of GPS satellite signals will be introdused.

This paper deals with gear condition monitoring based on vibration analysis techniques. The detection and diagnostic capability of some of the most effective techniques are discussed and compared on the basis of experimental results, concerning a gear pair affected by a fatigue crack. In particular, the results of new approaches based on time-frequency and cyclostationarity analysis are compared against those obtained by means of the well accepted cepstrum analysis and amplitude and phase demodulation of meshing harmonics. Moreover, the sensitivity to fault severity is assessed by considering two different depths of the crack. The effect of choosing different transducer locations and different processing options are also shown. In the case of the experimental results considered in this paper, the power cepstrum is practically insensitive to the crack evolution. Conversely, the Spectral Correlation Density function is able to monitor the fault development and does not seem to be significantly influenced by the transducer position. The demodulation techniques are able to localise the damaged tooth; however, their sensitivity is strongly dependent on the proper choice of the filtering band and is affected by the transducer location. The Wavelet transform seems to be a good tool for crack detection; it is particularly effective if the residual part of the time synchronous averaged signal is processed.

One of the main challenges of securing broadcast communication is source authentication, or enabling receivers of broadcast data to verify that the received data really originates from the claimed source and was not modified en route. This problem is complicated by ...

The aim of this thesis was to study the time synchronization in wireless sensor networks which are based on standards NTP and IEEE1588. Time synchronization is timekeeping which requires the coordination of events to operate a system in unison. This can be compared to the conductor of an orchestra keeping the orchestra in time. Another purpose was then to compare the differences of three synchronization protocols.

Time synchronization is a fundamental middleware service for any distributed system. Wireless sensor networks make extensive use of synchronized time in many contexts (e.g., data fusion, TDMA schedules, synchronized sleep periods, etc.). We propose a time synchronization method relevant for wireless sensor networks. The solution features minimal complexity in network bandwidth, storage as well as processing, and can achieve good accuracy. Especially relevant for sensor networks, it also provides tight, deterministic bounds on offset and clock drift. A method for synchronizing the entire network is presented. The performance of the algorithm is analyzed theoretically and validated on a realistic testbed. The results show that the proposed algorithm outperforms existing algorithms in terms of precision and resource requirements.

Object The operating room is rich in digital data that must be rapidly gathered and integrated by caregivers, potentially distracting them from direct patient care. We hypothesized that current desktop computers could integrate enough electronically accessible perioperative data to present a unified, contextually appropriate snapshot of the patient to the operating room team without requiring any user intervention. Materials and methods We implemented a system that integrates data from surgical and anesthesia devices and information systems, as well as an active radiofrequency identification location tracking system, to create a comprehensive, unified, time-synchronized database of all digital Electronic supplementary material The online version of this article (data produced by these systems. Next, a human factors engineering approach was used to identify selected data to show on a large format display during surgery. Results A prototype system has been in daily use in a clinical operating room since August 2005. The system functions automatically without any user input, as the display system self-configures based on cues from the primary data. The system is vendor agnostic with respect to input data sources and display options. Conclusion Automatic integration and display of teamsynchronizing data from medical devices and hospital information systems is now possible using software that runs on a personal computer.

An increasing number of distributed measurement and control applications requires that network nodes are able to estimate the position and the time measured by other devices on a common spatio-temporal reference frame. Accurate packet timestamping is a necessary condition to achieve this goal. However, in the case of wireless networks, timestamping accuracy is generally quite poor due to the combination of multiple uncertainty sources (e.g., variable signal attenuation and interferences) affecting wireless message propagation. Various communication schemes have been proposed to improve performance. One of the most recent and promising is the Chirp Spread Spectrum (CSS) modulation. In this paper, the timestamping accuracy of CSS signals similar to those defined in the standard IEEE 802.15.4a is analyzed experimentally using two different platforms based on a Software Defined Radio (SDR). Some results confirm that CSS can be successfully employed for accurate time synchronization and ranging.

Many potential applications such as ocean sampling network, environment monitoring, undersea exploration, disaster prevention, assisted navigation, and mine reconnaissance can be provided by deploying the Underwater Acoustic Sensor Networks. Because of the peculiarities of acoustic communication in underwater, the MAC protocol which play a role of managing and controlling the channels, must overcome the required of energy consumption, propagation delay and time synchronize as well as other factors. In this paper, we summarize and classify some current proposed MAC protocols as well as make a comparison table in order to bring out the current development of a very interesting research area. Beside, we briefly introduce our suggestion of MAC mechanisms for Underwater Acoustic Sensor Networks (UWASNs) named Gain-time and Guard-time TDMA mechanism and UWA-NAV mechanism.

The current paper addresses the issue of estimating the sampling instant in turbo receivers. The proposed synchronizer is based on the expectation-maximization (EM) algorithm and takes benefit from the soft information delivered by the turbo system. Performance of the proposed synchronizer is illustrated by simulation results. In particular, the mean and the variance of the estimator as well as the bit error rate reached by the synchronized system are reported.

This paper presents a robust model-based technique for the detection and diagnosis of gear faults under varying load conditions using the gear motion residual signal. Since the majority of energy in the healthy state of the gear of interest is concentrated at the meshing fundamental and its harmonics, the gear motion residual signal, obtained by the removal of the regular gear meshing fundamental and its harmonics from the time-synchronous average (TSA) signal, will be much less sensitive to the varying load conditions than the TSA signal. Also, the gear motion residual signal usually shows evidence of faults earlier and more clearly than the TSA signal. Therefore, less load dependence and an earlier indication of incipient faults can be obtained simultaneously. A noise-adaptive Kalman filter-based auto-regressive (AR) model is fitted to the gear motion residual signals in the healthy state of the gear of interest. By the selection of a proper model order which provides a compromised model for the healthy gear under various load conditions with the aid of a specific AR model order selection method proposed in this study, further enhancement in load independence can be obtained in the AR model residuals. Consequently, a robust statistical measure, which takes the percentage of outliers exceeding the three standard deviation limits of the baseline AR model residuals, is applied to evaluate the state of the gear of interest. The proposed diagnostic model is validated using full lifetime vibration signals of gearbox operating from new to breakdown under a variety of failure modes and demonstrates remarkable advantages over other newly proposed techniques.

This paper describes the Network Time Protocol (NTP), which is designed to distribute time information in a large, diverse internet system operating at speeds from mundane to lightwave. It uses a symmetric architecture in which a distributed subnet of time servers operating in a self-organizing, hierarchical configuration synchronizes local clocks within the subnet and to national time standards via wire, radio or calibrated atomic clock. The servers can also redistribute time information within a network via local routing algorithms and time daemons. This paper also discusses the architecture, protocol and algorithms, which were developed over several years of implementation refinement and resulted in the designation of NTP as an Internet Standard protocol. The NTP synchronization system, which has been in regular operation in the Internet for the last several years, is described along with performance data which shows that timekeeping accuracy throughout most portions of the Internet can be ordinarily maintained to within a few milliseconds, even in cases of failure or disruption of clocks, time servers or networks.

This paper presents a new online voltage stability index (VSI) that predicts the power system steady-state voltage stability limit. Starting with deriving a method to predict three types of maximum transferable power (real power, reactive power, and apparent power) of a single-source power system, a new VSI based on the calculated load margins is devised. In order to apply the VSI to large power systems, a method is developed to simplify the large network behind a load bus into a single source and a single transmission line using time-synchronized phasor measurements and network parameters. The simplified system model, to which the devised VSI can be applied, preserves power flow and voltage information of the particular load bus under study. The proposed VSI combined with the network simplification method provides the voltage stability margin of each individual load bus in an informative format and identifies the load bus that is the most vulnerable to voltage collapse. Test results from applying the VSI on two test systems validate its applicability for online applications.

The advent of satellite-based time-keeping systems and advances in computer technology have made possible protective relay sampling synchronization within 1 μs. These relays can now provide synchronized phasor measurements that eliminate the need to have different devices for protection, control, and electric power system analysis for system-wide applications and traditional protection applications. System-wide applications have different sampling and signal processing requirements than do traditional protection applications. These different requirements normally are addressed with different devices, one device for each specific function. This paper proposes combining the aforementioned applications in a single device with a flexible signal processing system. The addition of synchrophasor measurement in a protective relay results in increased power system reliability and provides easier disturbance analysis, protection, and control capabilities than do approaches with different information sources.

The new terrestrial digital video broadcasting standard DVB-T2 provides a specific symbol -called P1-symbolfor the initial time and frequency synchronization. In this paper the maximum likelihood (ML) time and carrier frequency offset (CFO) synchronization scheme, which exploits the structure of the P1 symbol in both time and frequency domains is derived. Two lower-complexity solutions are then proposed: 1) a ML estimator that only exploits the time structure of the P1 symbol and 2) a pseudo ML (PML) scheme that resorts to a suboptimal CFO estimator while still performing ML time synchronization. The proposed schemes are compared in terms of both synchronization capabilities and implementation complexity. The Cramèr-Rao bounds for the CFO estimators are also evaluated. Simulation results in a typical DVB-T2 scenario show that both ML and PML schemes have a very close performance while significantly outperforming existing schemes.

Wireless mesh networks (WMNs) consist of mesh routers and mesh clients, where mesh routers have minimal mobility and form the backbone of WMNs. They provide network access for both mesh and conventional clients. The integration of WMNs with other networks such as the Internet, cellular, IEEE 802.11, IEEE 802.15, IEEE 802.16, sensor networks, etc., can be accomplished through the gateway and bridging functions in the mesh routers. Mesh clients can be either stationary or mobile, and can form a client mesh network among themselves and with mesh routers. WMNs are anticipated to resolve the limitations and to significantly improve the performance of ad hoc networks, wireless local area networks (WLANs), wireless personal area networks (WPANs), and wireless metropolitan area networks (WMANs). They are undergoing rapid progress and inspiring numerous deployments. WMNs will deliver wireless services for a large variety of applications in personal, local, campus, and metropolitan areas. Despite recent advances in wireless mesh networking, many research challenges remain in all protocol layers. This paper presents a detailed study on recent advances and open research issues in WMNs. System architectures and applications of WMNs are described, followed by discussing the critical factors influencing protocol design. Theoretical network capacity and the state-ofthe-art protocols for WMNs are explored with an objective to point out a number of open research issues. Finally, testbeds, industrial practice, and current standard activities related to WMNs are highlighted.

Directional antennas offer many potential advantages for wireless networks such as increased network capacity, extended transmission range and reduced energy consumption. Exploiting these advantages, however, requires new protocols and mechanisms at various communication layers to intelligently control the directional antenna system.With directional antennas, many trivial mechanisms, such as neighbor discovery, become more challenging since communicating parties must agree on where and when to point their directional beams to enable communication.

In this paper, we propose a fully directional neighbor discovery protocol called Sectored-Antenna Neighbor Discovery (SAND) protocol. SAND is designed for sectored-antennas, a low-cost and simple realization of directional antennas, that utilize multiple limited beamwidth antennas. Unlike many proposed directional neighbor discovery  protocols, SAND depends neither on omnidirectional antennas nor on time synchronization. In addition, SAND performs neighbor discovery in a serialized fashion allowing individual nodes to discover all potential neighbors within a predetermined time. Moreover, SAND guarantees the  discovery of the best sector combination on both communication ends allowing more robust and higher reliability links. Finally, SAND gathers the neighborhood information in a centralized location, if needed, to be used by centralized networking protocols. The effectiveness of SAND has been assessed via simulation studies and real hardware implementation.

A rapid synchronization method is presented for an orthogonal frequency-division multiplexing (OFDM) system using either a continuous transmission or a burst operation over a frequency-selective channel. The presence of a signal can be detected upon the receipt of just one training sequence of two symbols. The start of the frame and the beginning of the symbol can be found, and carrier frequency offsets of many subchannels spacings can be corrected. The algorithms operate near the Cramér-Rao lower bound for the variance of the frequency offset estimate, and the inherent averaging over many subcarriers allows acquisition at very low signal-to-noise ratios (SNR's).

In Frequency Division Duplex (FDD) systems or Time Division Duplex (TDD) systems with perfect Uplink (UL) /Downlink (DL) alignment, all UL transmissions receive interference only from other UL links and vice versa. However, take into consideration one of the most important benefits that TDD offers, i.e., dynamic UL/DL switching points, the UL and DL transmission can be misaligned. This means UL might be corrupted by DL, and vice versa. The loss of network time-synchronization among different cells makes things even more complicated for TDD systems. Many techniques are developed to achieve network synchronization, e.g., GPS based or IEEE 1588 based techniques. In this paper, the effect that different synchronization and UL/DL switching points introduce to the system performance is studied. The purpose is to find out the benefit that synchronization can offer. Based on the observations, it is suggested to achieve at least loose network synchronization.

Bimanual coordination dynamics have been conceived as the outcome of a global coordinative system, and coordination stability properties and theories of underlying processes have often been generalized over various bimanual tasks. In unimanual timing tasks it has been shown that different timing processes are involved according to tasks, yielding distinctive correlation properties in the within-hand temporal patterns. In this study we compare unimanual with bimanual, tapping with oscillation, and self-paced with externally paced tasks, and we analyze the correlation properties of temporal patterns at both the component level and the coordinative level. Results show that the distinctive signatures of event-based versus emergent, and self-paced versus synchronization timing control known from unimanual tasks persist in the corresponding bimanual coordination tasks. Accordingly, we argue that these different timing processes, and related temporal patterns at the component level, constitute a task-dependent background on which coordination builds. One direct implication of these results is that the bimanual coordination paradigm should be considered multifaceted and not governed by some unitary generic principle. We discuss the need to assess the relationship between temporal patterns at the component level and the collective level, and to integrate serial (long-range) correlation properties into bimanual coordination models. Finally, we test whether the architectures of current bimanual coordination models can account for the experimentally observed serial correlations.