Imperfect Timing Synchronization Research Articles

Performance Analysis of Nonlinear Self-Interference Cancellation With Timing Error in Full-Duplex Systems

In this letter, with consideration of the approximation error due to the order of the nonlinear canceller is smaller than that of the nonlinearity model, the performance of the nonlinear self-interference (SI) cancellation is analyzed under imperfect time synchronization. First, the system model of the nonlinear SI cancellation is characterized in the presence of the timing error (TE) between the received SI and the reconstructed one. Next, the closed-form expressions of the residual SI power and the SI cancellation ratio (SICR) are derived for two cases that the order of the nonlinear canceller is smaller than or equal to that of the nonlinear model, which can be used to evaluate the performance degradation of the nonlinear SI cancellation caused by TE. Finally, simulation results reveal that the performance of the nonlinear SI cancellation is sensitive to TE when the nonlinear order of the canceller is sufficient, where the existence of the signal-of-interest (SoI) is also considered.

Moving source localization using TOA and FOA measurements with imperfect synchronization

The clock bias and drift caused by imperfect time synchronization is a practical issue in localization problems. In this paper, we address the moving source localization problem using time-of-arrival and frequency-of-arrival measurements, where imperfect time synchronization is present. We transform the measurement models and formulate a nonlinear and non-convex weighted least squares (WLS) problem to jointly estimate the unknown source position, velocity, clock bias and drift. We then apply semidefinite relaxation (SDR) to the non-convex WLS problem to obtain a convex semidefinite program, which can be solved efficiently. We show through theoretical mean square error analysis that the WLS solution is able to attain the Cramer–Rao lower bound (CRLB) under small Gaussian noise condition. Simulation results show that the proposed SDR method has comparable performance with the maximum likelihood estimation method, and thus achieves the CRLB accuracy, when the noise is not too large.

Analysis of Cooperative Jamming Cancellation With Imperfect Time Synchronization in Physical Layer Security

In physical layer security, imperfect time synchronization results in random mismatch between the received cooperative jamming (CJ) and the reconstructed CJ, leading to performance degradation of CJ cancellation at the authorized receiver. In this letter, the CJ cancellation performance with imperfect time synchronization is analyzed and evaluated by jamming cancellation ratio (JCR). Then, by assuming that the timing error is uniformly distributed within the synchronization accuracy, the expectation and cumulative distribution function (CDF) of JCR are provided. Experimental results confirm the theoretical analysis that imperfect time synchronization causes dramatic performance degradation of CJ cancellation, especially for high jamming-to-noise ratio (JNR).

STBC-Aided Cooperative NOMA With Timing Offsets, Imperfect Successive Interference Cancellation, and Imperfect Channel State Information

The combination of non-orthogonal multiple access (NOMA) and cooperative communications can be a suitable solution for fifth generation (5G) and beyond 5G (B5G) wireless systems with massive connectivity, because it can provide higher spectral efficiency, lower energy consumption, and improved fairness compared to the non-cooperative NOMA. However, the receiver complexity in the conventional cooperative NOMA increases with increasing number of users owing to successive interference cancellation (SIC) at each user. Space time block code-aided cooperative NOMA (STBC-CNOMA) offers less numbers of SIC as compared to that of conventional cooperative NOMA. In this paper, we evaluate the performance of STBC-CNOMA under practical challenges such as imperfect SIC, imperfect timing synchronization between distributed cooperating users, and imperfect channel state information (CSI). We derive closed-form expressions of the received signals in the presence of such realistic impairments and then use them to evaluate outage probability. Further, we provide intuitive insights into the impact of each impairment on the outage performance through asymptotic analysis at high transmit signal-to-noise ratio. We also compare the complexity of STBC-CNOMA with existing cooperative NOMA protocols for a given number of users. In addition, through analysis and simulation, we observe that the impact of the imperfect SIC on the outage performance of STBC-CNOMA is more significant compared to the other two imperfections. Therefore, considering the smaller number of SIC in STBC-CNOMA compared to the other cooperative NOMA protocols, STBC-CNOMA is an effective solution to achieve high reliability for the same SIC imperfection condition.

Cooperative interference power allocation in physical layer security under imperfect time synchronization

Cooperative interference power allocation in physical layer security under imperfect time synchronization

Deep Convolutional Neural Networks for Link Adaptations in MIMO-OFDM Wireless Systems

This letter proposes a deep convolutional neural network (DCNN) approach for adaptive modulation and coding in practical multiple-input, multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Our target is to maximize the throughput and fulfill a packet error rate constraint. We consider practical impairments of MIMO-OFDM receiver, such as imperfect timing synchronization, carrier frequency offset correction, and channel estimation. We treat the estimated channel state information and the noise standard deviation as input features to the DCNN. The main advantages of the proposed approach are: 1) it learns the characteristics of the MIMO-OFDM channel properly and predicts the suitable modulation and coding scheme and 2) it does not need complex features selection.

Discrete time domain analysis of chaos-based wireless communication systems with imperfect sequence synchronization

Abstract Problem of sequence synchronization in chaos-based direct sequence spread-spectrum (DS-SS) systems and code division multiple access systems (CDMA) has been widely investigated. However, no mathematical expressions have been derived in closed form for the bit error probability in these systems when they operate with the imperfect time synchronization (represented by a delay between the received and reference spreading sequence generated in the receiver). Precise derivatives for this bit error probability are necessary to quantify the effect of imperfect synchronization on the overall properties of the system. To implement a random delay between the received and the receiver reference sequence, all signals in this paper are represented in the discrete time domain. To represent finite and random discrete delays between the sequences (which occur in a limited interval) the Gaussian and uniform probability density functions are expressed in discrete form. Furthermore, due to the finite value of possible random discrete delays, the expressions of related truncated density functions are expressed in closed form. Following this approach, the expressions for the bit error probability in closed form for chaotic and random spreading sequences have been derived.

Quadratic constrained weighted least-squares method for TDOA source localization in the presence of clock synchronization bias: Analysis and solution

Time difference of arrival (TDOA) positioning is one of the widely used techniques for locating an emitter. Besides TDOA measurement errors, clock synchronization bias is an important factor that can degrade the localization accuracy. This paper focuses on TDOA localization using a set of receivers, where timing synchronization offsets exist among different receiver groups. A theoretical analysis is conducted and a new localization solution is developed for the case of imperfect time synchronization. The analysis starts with the Cramér–Rao bound (CRB) for the problem and derives the estimation bias and mean square error (MSE) using the classical quadratic constraint weighted least-squares (QCWLS) estimator, which does not consider synchronization errors. Additionally, an alternative performance measure, namely the localization success probability (SP), is introduced to evaluate the location accuracy. An explicit formula for calculating the localization success probability is presented. In addition, an improved quadratic constraint weighted least-squares estimator that accounts for synchronization errors is proposed to reduce the positioning errors. The Lagrange multiplier technique is used to solve this estimator. As a byproduct, a closed-form solution for the estimation of clock bias is also provided. First-order perturbation analysis reveals that the performance of the proposed estimates achieves the Cramér–Rao bound. Simulations corroborate the theoretical results and the good performance of the proposed method.

A Robust State Estimation Framework Considering Measurement Correlations and Imperfect Synchronization

This paper develops a robust power system state estimation framework that accounts for correlations and imperfect time synchronization of the measurements. In this framework, correlations of the measurements obtained from the supervisory control and data acquisition (SCADA) system and the phasor measurement units (PMUs) are separately calculated through the unscented transformation and a vector auto-regression (VAR) model. Specifically, the PMU measurements during the waiting period of two successive SCADA measurement scans are buffered via a VAR model whose parameters are robustly estimated using the projection statistics. The latter take into account their temporal and spatial correlations and provide the needed measurement redundancy to suppress bad data and mitigate imperfect time synchronization. In the case where the SCADA and the PMU measurements do not arrive simultaneously at the control center, yielding imperfect measurement time synchronization, either the forecasted PMU measurements or the prior SCADA measurements from the latest state estimation run are leveraged to restore system observability. Finally, a robust generalized maximum-likelihood (GM)-estimator is extended to integrate the measurement error correlations and to handle the outliers, also known as bad data. Simulation results that stem from a comprehensive comparison with other alternatives under various conditions demonstrate the benefits of the proposed framework.

SiFi

There has been a booming interest in developing WiFi localization using multi-antenna (MIMO) access points (APs). Recent advances have demonstrated promising results that break the meter-accuracy barrier using commodity APs. Yet these state-of-the-art solutions require either multiple APs that are not necessarily available in practice, or multiple-channel measurements that disrupt normal data communication. In this paper, we present SiFi, a single AP-based indoor localization system that for the first time achieves sub-meter accuracy with a single channel only. The SiFi design is based on a key observation: with MIMO, the multiple (typically three) antennas of an AP are frequency-locked; although the accurate Time-of-Arrival (ToA) estimation on commodity APs is fundamentally limited by the imperfect time and frequency synchronization between the transmitter and receiver, there should be only one value for the ToA distortion that can cause three direct-path ToAs of the antennas to intersect at a single point, i.e., the position of the target. We develop the theoretical foundations of SiFi and demonstrate its realworld implementation with off-the-shelf WiFi cards. Our implementation introduces no hardware modification and is fully compatible with concurrent data transmission. It achieves a median accuracy of 0.93 m, which significantly outperforms the best known single AP single channel solution.

Experimental Comparison of FBMC and OFDM for Multiple Access Uplink PON

Filter bank multicarrier (FBMC) is a favourable candidate for next generation optical access networks due to its capability to operate without a cyclic prefix and its high resilience to synchronization errors in the system. In this paper, we analyze the performance of FBMC in upstream passive optical networks (PON), without the employment of inter optical network unit (inter-ONU) frequency guard bands. By using both experimental work and numerical simulations, the impact of relative time offsets on inter-ONU orthogonality is studied. Furthermore, numerical simulations are also used to study the impact of frequency offsets in such systems. Moreover, comparisons are drawn to orthogonal frequency division multiplexing (OFDM)-based upstream PONs. This paper experimentally demonstrates that imperfect time synchronization impacts all subcarriers in an OFDM system, while it affects only the immediately adjacent subcarriers in an FBMC system, which could be corrected for by the restoration of synchronicity between users. A simple feedback mechanism is experimentally demonstrated to automatically restore orthogonality and synchronization between users in order to validate the practical implementation of the scheme.

Impact of Base Station Time Synchronization Mismatch on Almost Blank Subframes

This letter analyzes the impact of imperfect timing synchronization of cellular base stations on feasibility of almost blank subframes, an interference management concept for heterogeneous networks. We claim that the most significant impact of timing synchronization mismatch lays in potential disturbance of a control channel that is located in the beginning of the subframe. We use stochastic geometry to analyze the impact and discover that the timing requirement from time division duplex system suffices also for frequency division duplex system that uses almost blank subframes.

Network-Coded Bi-Directional Relaying for Amplify-and-Forward Cooperative Networks: A Comparative Study

In this paper, a comparative study of network-coded bi-directional amplify-and-forward (BD-AF) relaying is presented. In bi-directional relay networks, communication is performed over two phases: the broadcasting phase, and the cooperation phase. In the broadcasting phase, both source nodes broadcast their signals simultaneously to the N relay nodes, while in the cooperation phase, transmission is based on one of two modes: (1) time-division (TD), or (2) multiple-access (MA). In the TD-BD-AF scheme, each relay node is allocated a time-slot to transmit its processed signal, while in the MA-BD-AF scheme, all the N relay nodes simultaneously transmit network-coded signals to both source nodes, in a single time-slot. Moreover, a suboptimal relay selection (i.e. SRS-BD-AF) that approximately maximizes the sum-of-rates is proposed. Optimal and suboptimal sum-of-rates maximizing power allocations are studied under the TD-BD-AF and MA-BD-AF schemes, respectively, where it is shown that the MA-BD-AF scheme reduces to the SRS-BD-AF scheme. Symbol error rate performance analysis is provided, where it is shown that both the TD-BD-AF and SRS-BD-AF schemes achieve full diversity. Imperfect timing synchronization is analyzed and it is demonstrated that the SRS-BD-AF outperforms the other schemes in terms of the achievable rate. Simulation results are provided to complement the theoretical analysis.

Error Rate for PSK and QAM Modulations for Non-Ideal OFDM Systems with Noisy Channel Estimates and Receive Diversity

This paper characterizes the SER for a non-ideal OFDM system with either M-PSK or M-QAM constellations operating over a frequency-selective, time-varying, Rayleigh fading channel. The imperfections that we consider are inter-carrier interference (ICI) and/or inter-symbol interference (ISI), imperfect time and frequency synchronization, and noisy channel estimates. We consider an MRC receiver with L diversity branches. We present SER characterizations with and without ISI, and show that the effect of ISI, especially for higher order modulation schemes, e.g., 8PSK and above, can be very severe, irrespective of the number of diversity branches.

Space-time network coding

Traditional cooperative communications can improve communication reliability. However, transmissions from multiple relay nodes are challenging in practice. Single transmissions in time-division multiple-access (TDMA) manner cause large transmission delay, but simultaneous transmissions from two or more nodes using frequency-division multiple access (FDMA) and code-division multiple access (CDMA) are associated with the issue of imperfect frequency and timing synchronization. In this work, a novel framework for cooperative communications is proposed to achieve full spatial diversity with low transmission delay and eliminate the issue of imperfect synchronization. This is realized by the use of space-time network codes (STNCs) associated with a novel concept of wireless network cocast. For a network of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> client nodes, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> relay nodes and a base node, the STNCs provide a diversity order of ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> +1) for each symbol with ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> + <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ) time slots, a reduction from 2 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> time slots in traditional FDMA and CDMA cooperative communications for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> being usually greater than <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> and from <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> +1) time slots in traditional TDMA cooperative communications. The STNCs are also applied in networks, where the client nodes located in a cluster act as relays to help one another to improve their transmission performance. The performance in clustering setting is studied to show the improvement in power saving, range extension, and transmission rate.

Energy-Efficient Protocol for Deterministic and Probabilistic Coverage in Sensor Networks

Various sensor types, e.g., temperature, humidity, and acoustic, sense physical phenomena in different ways, and thus, are expected to have different sensing models. Even for the same sensor type, the sensing model may need to be changed in different environments. Designing and testing a different coverage protocol for each sensing model is indeed a costly task. To address this challenging task, we propose a new probabilistic coverage protocol (denoted by PCP) that could employ different sensing models. We show that PCP works with the common disk sensing model as well as probabilistic sensing models, with minimal changes. We analyze the complexity of PCP and prove its correctness. In addition, we conduct an extensive simulation study of large-scale sensor networks to rigorously evaluate PCP and compare it against other deterministic and probabilistic protocols in the literature. Our simulation demonstrates that PCP is robust, and it can function correctly in presence of random node failures, inaccuracies in node locations, and imperfect time synchronization of nodes. Our comparisons with other protocols indicate that PCP outperforms them in several aspects, including number of activated sensors, total energy consumed, and network lifetime.

Performance of equilibrium-based system identification algorithms with incomplete state data

Time-domain parametric system identification algorithms are a main staple of structural health-monitoring applications. In this study, we consider a class of such algorithms whereby a time-invariant system is parameterized through the use of a spatially discretized model, and the optimal values of the system parameters are obtained by minimizing the norm of an equilibrium-error function. The general form of these error functions is the difference between the known external forces and the internal forces of the discrete model predicted through the use of an iterative set of system parameters and the state data measured in the time-domain. The complete set of state data consists of displacement, velocity and acceleration time-histories. For the aforementioned class of time-domain system identification algorithms in existing literature, this complete set of state data is required as input. Therefore, the displacement and velocity time-histories need to be obtained, either by numerical integration of measured acceleration data, or by their direct and independent measurements with appropriate sensors. Integration of acceleration data introduces baseline drifts and numerical errors due to convolution of noise in measurements; whereas, the use of independently measured displacements and velocities are plagued by imperfect time-synchronization of state data as well as the mismatch of the optimal frequency and amplitude ranges of their respective sensors. The aforementioned class of system identification algorithms, which require complete state data, are highly sensitive to such errors, and as a result, their use in practical applications are severely limited. In this study, we present a rational variant of the aforementioned class of algorithms that requires only acceleration data. We investigate the accuracy and the robustness of this novel algorithm, and compare its performance to the complete-state algorithm through numerical simulations.

Optimized Distributed Space-Time Filtering

Distributed space-time filtering (DSTF) facilitates node cooperation and achieves a diversity gain while not requiring node coordination, i.e., cooperating nodes do not have to be aware of their partners. This makes DSTF attractive for application in future sensor, ad hoc, and wireless networks. In this paper, we derive a novel cost function and related practical algorithms for optimization of the signature filter vectors (SFVs) used in DSTF. We extend DSTF to frequency-selective fading channels and compare its performance with that of delay diversity transmission with co-located antennas. For the special case of frequency-nonselective channels we show that SFV optimization is closely related to the signature sequence design for code-division multiple access (CDMA) systems and the design of Grassmannian frames. Numerical and simulation results show that the novel SFV designs perform significantly better than previously proposed designs even if non-ideal effects such as suboptimum equalization, imperfect channel estimation, and imperfect timing synchronization are taken into account

Effect of Imperfect Phase and Timing Synchronization on the Bit-Error Rate Performance of PSK Modulations

A great deal of attention has been devoted in the recent literature to the study of the bit-error rate (BER) performance of phase-shift keyed (PSK) constellations, due to their high spectral efficiency. It is well known that an error in phase or timing synchronization affects the probability of correct decisions at the receiver. This problem becomes much more significant as data rates increase, and the corresponding symbol durations decrease. In this letter, we look into this problem and analyze the exact BER performance of generalized hierarchical PSK constellations under imperfect phase or timing synchronization over additive white Gaussian noise channels.

Radio frequency digital-to-analog converter

Dynamic performance of high-speed high-resolution digital-to-analog converters (DACs) is limited by distortion at the data switching instants. Inter-symbol interference (ISI), imperfect timing synchronization, and clock jitter are all culprits. A DAC output controlled by an oscillating waveform is proposed to mitigate the effects of switching distortion and clock jitter. This architecture has the additional benefit of mixing the DAC impulse response energy to a higher frequency, allowing a high-frequency image of the input to be used as the output. This has the potential for better noise performance and power and hardware savings relative to a conventional DAC+mixer architecture. A narrow-band sigma-delta (/spl Sigma//spl Delta/) DAC with eight unit elements is chosen to demonstrate the radio frequency digital-to-analog converter (RF DAC) concept in a 1.8-V 0.18-/spl mu/m CMOS technology. Measured single-tone SFDR is -75 dBc, SNR is 53 dB, and two-tone IMD3 is -70.8 dBc for a 17.5-MHz band centered at 942 MHz. SNR performance is shown to have the predicted dependence on the phase alignment of the data clock and oscillating pulse.