Robust asynchronous STBC transceiver for multiple-access frequency-selective channels

In this paper, we propose a robust STBC transmission scheme to combat timing synchronization errors over frequency-selective multiple-access channels. We start by deriving the equivalent channel model in the presence of timing synchronization errors. Based on this correlated equivalent channel model, the statistical channel power gain profile is analyzed and shown to exhibit unequal channel power gains on different subcarriers. Furthermore, a robust statistical bit loading algorithm is proposed to optimize the BER performance in scenarios where link adaptation based on instantaneous channel information is infeasible or undesirable. Simulation results show that our proposed scheme is robust to timing synchronization errors, providing around 7.5 dB SNR advantage at the BER of 10-4 over the conventional scheme in various channel environments.

OFDM based systems are extremely sensitive to time and frequency synchronization errors. This paper addresses the issue of time synchronization. In this paper we are presenting a simple and novel technique for time synchronization. The time synchronization error causes Inter Symbol Interference and hence must be corrected at receiver. In many wireless communication systems, the data packets or frame start with the preamble called training symbol followed by data symbols. The training symbol consists of some specific pattern. The similar frame structure is used for this new algorithm which consists of training symbol followed by OFDM data symbols. This novel technique uses new pattern for training symbol which consists of all identical samples. After applying our new algorithm, a peak is obtained at the end of training symbol. This is used to mark beginning of frame and estimation of timing error. Estimation error graph and BER graph are presented for 4QAM and 16 QAM in this paper. The results are shown for AWGN channel. The technique used is simple and results are encouraging and can be used in any of the OFDM based system.

Requirements of different applications and usage scenarios on a wireless positioning system is very different. A single positioning system is unlikely to meet all the requirements. UWB based positioning systems offer a solution which would meet all the indoor usage requirements. Though, the theoretical positioning accuracy offered by such systems is very good, in actual implementation, their accuracy is dependent on clock accuracy, anchor location, anchor time synchronization and other errors in Time-Difference of Arrival (TDoA) based method. In this paper, we analyze the advantages and problems of the TDoA method. Extensive simulations are given to show the location errors of TDoA method with respect to clock errors, time synchronization errors and anchor locations. Furthermore, we propose two system solutions for enhancing the location accuracy in UWB based positioning systems.

It is very difficult to eliminate the time synchronization error completely. The influence of time synchronization error on direct source position algorithm was researched. The signal model with time synchronization error was given firstly. Then the CRLB of position estimation under the condition that the time synchronization error is exist was derived. In the last, the influence was analyzed by Monte Carlo simulations. The simulation results show that the Cramer-Rao lower bound, under the condition the time synchronization error is exist, is deteriorated obviously compared with the condition the time is synchronized perfectly; when the time synchronization error is shorter than the signal sample period, the CRLBs are same with different time synchronization errors; however the position accuracy of direct source position algorithm drops sharply, if the time synchronization error is not being considered.

Structural damage detection based on wireless sensor networks can be affected significantlyby time synchronization errors among sensors. Precise time synchronization of sensor nodeshas been viewed as crucial for addressing this issue. However, precise time synchronizationover a long period of time is often impractical in large wireless sensor networks due to twoinherent challenges. First, time synchronization needs to be performed periodically,requiring frequent wireless communication among sensors at significant energy cost.Second, significant time synchronization errors may result from node failures which arelikely to occur during long-term deployment over civil infrastructures. In this paper, adamage detection approach is proposed that is robust against time synchronizationerrors in wireless sensor networks. The paper first examines the ways in whichtime synchronization errors distort identified mode shapes, and then proposes astrategy for reducing distortion in the identified mode shapes. Modified values forthese identified mode shapes are then used in conjunction with flexibility-baseddamage detection methods to localize damage. This alternative approach relaxesthe need for frequent sensor synchronization and can tolerate significant timesynchronization errors caused by node failures. The proposed approach is successfullydemonstrated through numerical simulations and experimental tests in a lab.

In all wireless communication systems and especially in OFDM and MIMO-OFDM system synchronization is crucial. The receivers do not know the beginning and the end of the OFDM symbols, so the placement of the FFT window. Moreover, they have their own sampling frequency asynchronous with that of the transmitter. These two points show two synchronization concepts: time and frequency. An error of time synchronization in the reception may cause loss of the orthogonality of subcarriers. This is why it is important to have a robust synchronization algorithm and know its effect on system performance. Several estimation techniques for time and frequency offset have been proposed in the literature, but varied test conditions make comparison difficult. In this paper, we use Schmidl and Cox algorithm as a symbol timing synchronization algorithm for OFDM system. We also see the preamble structure using the equations provided in the IEEE 802.16d. Simulation results show the effect caused by carrier frequency offset (CFO) and timing frequency offset (STO) on the performance of an OFDM system in a WiMAX context.

Global navigation satellite systems (GNSS) are often integrated with inertial measurement units (IMU) for navigation in urban environments. The ability of these integrated systems to achieve precise positioning and navigation is highly dependent on accurate time synchronization. While the current time synchronization algorithms are capable of achieving millisecond-level synchronization in open environments, their performance deteriorates significantly in complex urban environments due to the impact on GNSS measurements of non-line-of-sight (NLOS) signals and multipath effects. Here we propose a loosely coupled GNSS/IMU integration time synchronization error modeling and compensation algorithm to tackle this problem in the context of urban vehicle navigation. In the algorithm, the time synchronization error is augmented as a state variable in the Robust Extended Kalman Filter (REKF), with the robustness factor threshold being dynamically adjusted based on the time synchronization error. Furthermore, we have designed a system time synchronization detection scheme based on two detection factors, with the aim of achieving high-precision GNSS/IMU time synchronization in complex urban environments. Experimental results show that the proposed algorithm synchronizes data time to the millisecond level in urban positioning environments. This represents an improvement of more than 90% in time synchronization precision compared to the traditional EKF-based time synchronization algorithm, and an improvement of more than 60% compared to the REKF-based time synchronization algorithm. Furthermore, compared to the EKF-based GNSS/IMU fusion algorithm, the proposed algorithm enhances the accuracy of the determination of both position and velocity in the horizontal and 3D directions by more than 50% and heading accuracy by 85%.

This paper presents the details of a simulation based study performed to investigate the BER performance degradation (sensitivity) of OFDM systems due to time synchronization error at the demodulator. It is shown that an OFDM system incorporating a cyclic prefix (CP) has an asymmetric effect on the received constellation with synchronization error away from CP introducing intersymbol interference (ISI), while that towards the CP introducing only a phase error. We approximate the time synchronization error by a Gaussian distributed random variable, in order to evaluate the BER sensitivity of OFDM systems to timing error. The BER performance was evaluated for an AWGN channel and a multipath fading channel in terms of the synchronization error variance. The results of this investigation will be useful in developing time synchronization algorithms in OFDM systems to achieve a particular BER performance target for a given channel condition.

Acoustic target tracking is one of the application areas of wireless sensor networks (WSNs) in which time synchronization error influences the results of target tracking. Algebraic numerical methods are used to solve simultaneous equations of target tracking. A more efficient approach is to use algebraic geometry for solving simultaneous equations of target tracking, but the analytic study of error propagation in this approach is difficult. Alternatively, a simulative approach can be used for computing the relation between time synchronization error and target tracking error. This relation helps the middleware layer of WSNs to adjust the resynchronization time of the network more accurately in support of end users' and application programs' requirements for quality of service (QoS). This paper presents two different simulations for extracting the relation between time synchronization error and target tracking error.

Most OFDM analyses assume idealized conditions when it comes to the waveform at the receiver in terms of time and frequency synchronization. In addition, often a cyclic prefix (CP) longer than the channel delay spread is assumed, and thus the impact of the possible inter-symbol interference (ISI) from adjacent OFDM symbols is not a consideration. This work provides a consolidated framework for study and analysis of OFDM systems with imperfections. The imperfections include time and frequency synchronization errors, and channel delay spread beyond the CP duration. Transmit as well as receive filtering operations are explicitly modeled, and different receiver detection criteria are considered. The use of an OFDM waveform for UWB applications is known to be a promising choice to simplify the receiver design and to maximize the bandwidth scalability, as well as the adaptation to various spectrum allocations. We present bit error rate (BER) sensitivity analysis to key system parameters for OFDM-based UWB applications. From these sensitivity analyses, we see graceful performance degradation with decreasing CP length and increasing residual frequency error. The performance degradation can be much more critical for timing synchronization errors.

In this paper parametric tracking of channel and synchronization errors for an adaptive multi-dimensional channel estimator for uplink Orthogonal Frequency Division Multiplex Access (OFDMA) communication systems is proposed. The estimator consists of a two-dimensional LMMSE filter. In real world scenarios, channel estimation should work not only in a wide range of terminal velocities and delay spread but it is also confronted with time and frequency synchronization errors affecting the channel estimation and thus limits the performance of both the coherent receiver and overall OFDMA system. We jointly track channel statistics and synchronization errors so that LMMSE filter coefficients closely reflect the current statistics of the channel. Through simulations it is verified that the proposed channel estimator outperforms the robust LMMSE estimator in the presence of synchronization errors and varying channel conditions.

In this paper, we study the impact of timing synchronization errors on the performance of Multicarrier Differential Chaos Shift Keying systems over wireless channels. Considering the challenges posed by selective-frequency Rayleigh fading and multipath propagation, we analyze the system’s performance under random timing errors and fading effects. Using analytical modeling, including probability density functions for fading coefficients and truncated density functions for timing synchronization errors, the Bit Error Rate (BER) performance is derived. Numerical simulations validate the theoretical findings and highlight critical system parameters that influence robustness. Our results demonstrate that timing synchronization errors significantly degrade BER performance, particularly under harsh wireless channel conditions. To mitigate this degradation, we analyze and evaluate the effect of timing errors and related parameters on system performance, identifying technical solutions such as increasing the receiver sampling frequency within the chip duration and selecting an optimal spreading factor. These findings provide valuable insight into the design of more resilient noncoherent chaos-based communication systems.

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This paper propose a flexible and robust unbalanced multiple description coding and transmission scheme,which is capable of quickly recovering from packet losses and ensuring continuous playback,and further adaptive to both multiple path and single path transmission.Furthermore,this paper discusses the problem of rate allocation between different descriptions when only single path transmission is available and proposes an approximate while efficient rate allocation scheme with the help of two-state Markov link model and a representative rate-distortion model.The experimental results demonstrate the effectiveness of the proposed scheme.

This paper investigates an unmanned aerial vehicle (UAV)-base station (BS) integrated network, where a UAV transmits downlink secrecy data to multiple ground cognitive users while a ground BS utilizes jamming beamforming to help the UAV counter the eavesdropping attack of a ground eavesdropper. In particular, we consider the imperfect eavesdropping and jamming channel state information (CSI) related to the eavesdropper. To maximize the minimum sum secrecy rate of the cognitive users, a robust secure transmission scheme is proposed. The UAV trajectory, UAV transmit power, BS beamforming, and user scheduling are jointly optimized with the constraints of the communication quality of the primary users served by the BS and the UAV available propulsion energy. We formulate a non-convex optimization problem which is challenging to be solved mathematically, and we utilize an alternating optimization technique to divide the original problem into three sub-problems, i.e., UAV trajectory sub-problem, transmit power sub-problem, and user scheduling sub-problem. Besides, they can be solved by the successive convex approximation, semi-definite relaxation and S-procedure, and bivariate relaxation methods, respectively. Moreover, we explore the impact of different parameters of the proposed transmission scheme on the minimum sum secrecy rate of the cognitive users, and verify the superiority of the proposed robust secure transmission scheme design.