Space-Time Network Codes Utilizing Transform-Based Coding

Traditional cooperative communications can improve communication reliability. However, the simultaneous transmissions from multiple relays are challenging in practice due to the issue of imperfect frequency and timing synchronization. In this work, we propose space-time network codes (STNCs) that overcome the issue of imperfect synchronization, reduce the large transmission delay, and still provide full spatial diversity in multipoint-to-point (M2P) and point-to-multipoint (P2M) transmissions. Relays form single linearly-coded signals from the overheard symbols and transmit them to the destination, where multiuser detection is utilized to detect the desired symbol. For a network of N client nodes, M2P and P2M STNCs result in a diversity order of N with only 2N time slots, a significant reduction from N2 time slots in traditional cooperative communications using TDMA.

This paper provides performance analysis of multiband OFDM UWB system that takes into consideration the effect of intersymbol interference and imperfect frequency and timing synchronization. The system performance is analyzed under UWB multipath fading channels, as specified in the IEEE 802.15.3a channel standard. Average signal-to-noise ratio is first derived for the system. Then an approximation technique is employed to obtain a closed-form average bit error probability that provides a profound understanding of the performance of the multiband OFDM UWB system. Simulation results validate the theoretical analysis.

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.

This paper considers a virtual multiple-input single-output (VMISO) network, where spatially separated single-antenna radios cooperatively transmit the same data to a destination. The existing works for the VMISO system assume perfect synchronization between the nodes. However, in this paper, we investigate the effects of imperfect timing synchronization on the performance of this network. For that purpose, we develop a statistical model for the timing errors and derive the expressions for the bit error probability (BEP) using this model. The results indicate that the BEP performance is degraded due to imperfect timing synchronization and the BEP is dependent upon the number of radios per cluster, signal-to-noise ratio (SNR) and the distance disparities between the nodes.

Automatic modulation classification (AMC) plays a pivotal role for spectrum monitoring in cognitive radio (CR). The objective of this work is to investigate the AMC problem for multiple-input multiple-output (MIMO) systems employing orthogonal frequency division multiplexing (OFDM) under imperfect timing synchronization scenarios. Two major contributions are made in this work: i) specific higher-order cumulants (HOC) are proved to be robust to symbol timing offset (STO). ii) with the feature of multiple HOCs, a random forest based AMC algorithm is proposed for MIMO-OFDM systems, which is robust to timing synchronization error. Numerical simulations are conducted to verify the validation of the algorithm.

In this paper, we investigate cooperative OFDM communications using amplify-and-forward (AF) protocol at the relays, in the presence of imperfect timing synchronization. In most studies on cooperative communications, perfect time synchronization among cooperating nodes is assumed. In practice, however, due to imperfect time synchronization, orthogonality among the subcarriers of the different nodes' signals at the destination receiver can be lost, causing inter-symbol interference (ISI). In this paper, we derive analytical expressions for the average SINR at the DFT output at the destination as a function of timing offset in cooperative OFDM with AF protocol, and illustrate the SINR degradation as a function of the timing offset. We also present an interference canceling (IC) receiver to mitigate the effects of ISI when there is timing offset. We show that the proposed IC receiver achieves improved BER performance even when timing offsets are large.

This paper analyzes the synchronization of a bistatic synthetic aperture radar (SAR) system based upon a geostationary transmitter and small receive-only SAR systems mounted on airplanes or unmanned aerial vehicles (UAVs), namely GEO Spaceborne-Airborne Bistatic SAR. This paper will analyze and present our solution of the space synchronization, time synchronization, frequency synchronization and phase synchronization for the GEO Spaceborne-Airborne bistatic SAR respectively. It will first introduce the GEO Spaceborne-Airborne bistatic SAR system and geometry model briefly. Then possible solutions of the system synchronization will be discussed. It will introduce the synchronization scheme based on the direct path between the transmitter and receiver. It uses synchronization modules to realize the time and frequency synchronization. A mathematical model considering the time and frequency synchronization error is given and a phase error analysis is performed. Range and azimuth compressions of a single target are implemented by simulations.

Raw signal simulation is a useful tool for synthetic aperture radar (SAR) system design, mission planning, processing algorithm testing, and inversion algorithm design. Time and frequency synchronization is the key technique of bistatic SAR (BiSAR) system, and raw data simulation is an effective tool for verifying the time and frequency synchronization techniques. According to the two-dimensional (2-D) frequency spectrum of fixed-receiver BiSAR, a rapid raw data simulation approach with time and frequency synchronization errors is proposed in this paper. Through 2-D inverse Stolt transform in 2-D frequency domain and phase compensation in range-Doppler frequency domain, this method can significantly improve the efficiency of scene raw data simulation. Simulation results of point targets and extended scene are presented to validate the feasibility and efficiency of the proposed simulation approach.

On the basis of analyzing of some existing differential space-time coding (DSTC) schemes, we develop a differential space-time block coding scheme based on multiple levels quadrature amplitude modulation (MQAM) in this paper. By introducing MQAM, the developed scheme can avoid the performance loss of conventional DSTC schemes based on MPSK modulation due to the decrease of minimum distance between constellation symbols in high spectrum efficiency. Compared with the existing DSTC schemes, the developed scheme has lower computational complexity and simple decoder. Moreover, it can achieve higher code rate in the case of more than two transmitter antennas. Theoretic analysis shows that the scheme has higher coding gain than existing differential unitary space-time coding scheme and differential orthogonal space-time coding scheme, and corresponding simulation results also show that our scheme has lower bit error rate (BER) when compared with the above two schemes.

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.

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%.

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.

Space-surface bistatic synthetic aperture radar (SS-BiSAR) for local area monitoring is presented based on BeiDou-2/Compass-2 as illuminator. The time and frequency synchronization and imaging processing are mainly investigated in this paper. In the proposed method, the BiSAR signal model containing time and frequency synchronization errors is firstly set up. In the following, a time and frequency synchronization method is proposed to deal with the absence of posterior ephemeris which is a special issue of BeiDou-2 system. Then, a corresponding back-projection algorithm is deduced to deal with curved trajectory of satellite in long synthetic time. In the time and frequency synchronization method, direct signal from the satellite is used to estimate time delay and phase history and extract navigation message to acquire time and ephemeris information. The synchronization errors are estimated based on the time delay, phase history and the calculated slant range history. In the imaging processing, the signals along sum of bistatic slant ranges are phase compensated and accumulated to form the final focused image. SS-BiSAR experiments using Beidou-2 as illuminators are carried out to validate the proposed methods. The feasibility of SS-BiSAR using BeiDou-2 as transmitter is demonstrated by the imaging results of both a transponder and natural scene.

In the wireless world, the demand for advanced information services is growing. Voice and low-rate data services are insufficient in a world where high-speed internet access is taken for granted. The trend is toward global information networks that offer flexible multimedia information services to users on demand, anywhere, anytime. The third-generation (3G) mobile communication (International Mobile Telecommunications-2000, IMT-2000) systems are being developed to support a wide range of bearer services with low to high data rates Wideband CDMA (W-CDMA), proposed by 3GPP (Third Generation Partnership Project), is one of the major proposals for ITU IMT-2000 RTT (Radio Transmission Technology) standards. In a WCDMA cellular system, the process of the mobile station searching for a cell and achieving code and time synchronization to its downlink scrambling code is referred to as cell search. Cell search is performed in three scenarios: initial cell search when a mobile station is switched on, idle mode search when inactive, and active mode search during a call. The latter two are also called target cell search. This paper presents various issues in time and frequency synchronization for initial cell search scenarios for the Wideband CDMA (W-CDMA) standard.

Generalized frequency division multiplexing (GFDM) is a waveform for the next-generation communication but requires a step dedicated to time and frequency synchronization. The current time and frequency synchronization methods of GFDM do not fully address the requirements on accuracy, robustness to a large frequency offset, and low complexity. This paper develops an accurate synchronization method tailored to large time and frequency offsets. The proposed method jointly synchronizes time and frequency and derives a decision variable from the real part of an inner product. The accuracy of time and frequency synchronization increases because an error in the frequency offset is corrected simultaneously during the process of time synchronization and because the decision variable does not depend on the imaginary part, which we show to be noise. The proposed method can correct a full range of frequency offsets. We compare synchronization accuracy of the proposed method to the state of the art analytically, numerically, and experimentally, in terms of the discriminant probability, the range of correctable frequency offsets, the probability of perfect time synchronization, the root mean square error (RMSE) between estimates and actual offsets, and the bit error ratio. The proposed method outperforms existing methods on all of these metrics. Moreover, it has practical utility and increases the transmission reliability for GFDM-based communication systems.