运动误差对双站SAR相位同步及成像的影响

收、发系统间的相位同步是双站合成孔径雷达的一项关键技术,采用锁相环接收机是实现双站SAR系统相位同步的一种可能方法.本文主要就锁相环接收机在运动误差条件下的相位同步问题进行了研究,并分析了锁相环相位误差对系统成像的影响,最后,给出了计算机仿真结果.     

频率源稳定性对BiSAR成像的影响研究

高稳定度的频率源是实现双站合成孔径雷达(BiSAR)系统相位同步的基础,该文研究分析了 收、发系统独立频率源条件下不同类型的频率误差对BiSAR成像的影响,确定了BiSAR成像的频率源稳定性要求。最后,给出了计算机仿真的结果。     

一种用于双基地雷达的光纤相位同步技术

针对收发分置雷达发射站和接收站之间频率基准的相位同步问题,理论分析了收发站之间的相位误差对雷达的性能影响,并提出一种基于光纤链路的相位同步技术。该技术提取往返光纤链路中引入的相位误差,并通过移相器对输出频率信号进行相位误差预补偿,实现了发射站和接收站的高精度的相位同步。20 km 往返光纤链路的实验结果表明该技术显著抑制了收发站之间的相位误差和噪声,可以有效减少相位误差带来的目标探测偏差。     

Terahertz Bistatic Synthetic Aperture Radar for 1-D Near-Field High-Resolution Imaging

Considering the difficult transceiver-isolation problem of the monostatic synthetic aperture radar (SAR) in the terahertz (THz) band, this paper proposes a compact THz bistatic SAR (BiSAR) geometry. The system allows the separately distributed transmitter and receivers. At the receiving end, there are a direct-wave receiver and an echo receiver, both operating at the heterodyne and in-phase mode. The echo receiver runs along a linear rail to fulfill the scene scanning, while the direct-wave one is fixed as a reference. Furthermore, assuming that the receivers are synchronized, both the problem of synchronization between the separated transmitter and receivers and the problem of timing at the signal acquisition would be solved by utilizing the high coherence between the echo and the direct wave. Based on such a system, the application of THz BiSAR for one-dimensional imaging is taken into consideration. Then, a high-resolution imaging algorithm is proposed benefitting from the total least squares estimating signal parameters via rotational invariance techniques (TLS-ESPRIT) and the spatial smoothing process (SSP). The imaging performance is then demonstrated by both simulations and the experiments in the 0.183 THz.     

Residual Synchronization Error Elimination in OFDM Baseband Receivers

It is well known that an OFDM receiver is vulnerable to synchronization errors. Despite fine estimations used in the initial acquisition, there are still residual synchronization errors. Though these errors are very small, they severely degrade the bit error rate (BER) performance. In this paper, we propose a residual error elimination scheme for the digital OFDM baseband receiver aiming to improve the overall BER performance. Three improvements on existing schemes are made: a pilot‐aided recursive algorithm for joint estimation of the residual carrier frequency and sampling time offsets; a delay‐based timing error correction technique, which smoothly adjusts the incoming data stream without resampling disturbance; and a decision‐directed channel gain update algorithm based on recursive least‐squares criterion, which offers faster convergence and smaller error than the least‐mean‐squares algorithms. Simulation results show that the proposed scheme works well in the multipath channel, and its performance is close to that of an OFDM system with perfect synchronization parameters.     

振动对BiSAR频率基准源影响及其补偿技术研究

双站合成孔径雷达具有高分辨率、隐蔽性强等优点,但其严重依赖于收发系统的频率基准源。晶体振荡器因其突出的性能在合成孔径雷达中作为频率基准源得到广泛使用,但其受振动、冲击等影响较大。该文从分析晶振相位噪声特性开始,讨论了频率源稳定性对双站合成孔径雷达(BiSAR)成像的影响,并重点介绍了一种晶振加速度的全数字补偿技术,在4 g@15 Hz下实现了12 dB的补偿效果。     

双站合成孔径雷达系统同步问题研究

讨论了双站合成孔径雷达 (双站SAR)收、发系统的同步问题 ,详细分析了频率源稳定性对收发系统间相位同步的影响 ,并给出了计算机仿真结果     

高轨星机BiSAR分辨率分析及成像参数优化设计

高轨SAR具有覆盖范围广,重访时间短的优势。但是如果采用高轨SAR卫星同时作为发射机和接收机,不能充分发挥高轨SAR的这些优点。采用飞机或低轨卫星作为接收机平台不但能够更灵活地针对目标区域成像,而且分辨率也将大大提高。但是星机双基SAR(BiSAR)的几何构型复杂,难以直观地获知任意几何构型BiSAR的分辨率特性。该文从BiSAR基平面分辨率出发,根据几何构型得到基平面分辨率与地平面分辨率之间的几何关系,解析地表示出了BiSAR在地平面上的分辨率形状。据此可以评估BiSAR系统的分辨力,并且能够通过优化设计系统带宽和合成孔径时间两个参数使得BiSAR系统能够实现更好的分辨率特性。最后,仿真结果验证了方法的有效性。     

Time and Frequency Synchronization with Channel Estimation for SC-FDMA Systems Over Time-Varying Channels

The constant amplitude zero autocorrelation sequence based synchronization and its usage in the block-type channel estimation for the single carrier frequency division multiple access (SC-FDMA) systems are vulnerable to the time-varying channels. Therefore, the channel estimation errors limit the performance of SC-FDMA systems in fast fading environments and result an irreducible error floor. In this paper, cyclic prefix based maximum likelihood estimator of time/frequency synchronization with comb-type channel estimation for the SC-FDMA systems are proposed to track the channel variations. Moreover, the residual time/frequency offsets calculations are derived for SC-FDMA systems. Simulation results confirm and illustrate that the proposed receiver is capable of tracking fast fading channel parameters and improving the overall performance as compared with the conventional receiver.     

一站固定低频UWB BiSAR时间同步误差分析

时间同步技术是双基地SAR系统的关键技术之一.结合一站固定式低频超宽带双站SAR系统的工作特点,研究了其时间同步误差产生机理.在考虑了不同形式时间同步误差分量的基础上,分别建立了时间同步误差模型,从理论上推导分析了时间同步误差对一站固定式低频超宽带双站SAR成像质量的影响,并给出了量化的同步指标.最后通过仿真验证了理论分析的准确性.工作结果对于一站固定式低频超宽带双站SAR系统设计及同步方法的工程实践提供有力支撑.     

Clock synchronization for packet networks using a weighted least‐squares error filtering technique and enabling circuit emulation service

Circuit emulation service (CES) allows time‐division multiplexing (TDM) services (T1/E1 and T3/E3 circuits) to be transparently extended across a packet network. With circuit emulation over IP, for instance, TDM data received from an external device at the edge of an IP network is converted to IP packets, sent through the IP network, passed out of the IP network to its destination, and reassembled into TDM bit stream. Clock synchronization is very important for CES. This paper presents a clock synchronization scheme based on a double exponential filtering technique and a linear process model. The linear process model is used to describe the behaviour of clock synchronization errors between a transmitter and a receiver. In the clock synchronization scheme, the transmitter periodically sends explicit time indications or timestamps to a receiver to enable the receiver to synchronize its local clock to the transmitter's clock. A phase‐locked loop (PLL) at the receiver processes the transmitted timestamps to generate timing signal for the receiver. The PLL has a simple implementation and provides both fast responsiveness (i.e. fast acquisition of transmitter frequency at a receiver) and significant jitter reduction in the locked state. Copyright © 2006 John Wiley & Sons, Ltd.     

Phase-impairment effects and compensation algorithms for OFDM systems

The simultaneous perturbation of an orthogonal frequency-division multiplexing receiver by phase noise plus a residual frequency offset (due to synchronization errors) is modeled here as a combined phase impairment, whose effect is evaluated analytically for the case of a frequency-selective fading channel. A nonpilot-aided (decision-directed) scheme is proposed, which compensates for the common (over all the subcarriers) phase-impairment effect. By representing the resulting intercarrier interference as an uncorrelated, unequal-variance process in the frequency domain, maximum-likelihood (ML) and approximate ML estimators of the complex-vector and phase-only types are derived and analytically evaluated. The present schemes are also compared with other current methods based on individual phase trackers, one per subcarrier. Finally, two suggestions are introduced for increasing the robustness of the algorithms to tentative-decision errors. It is demonstrated through simulations that the analysis is accurate, and that the proposed schemes achieve error-rate performance close to that of ideal compensation.     

Spatial Channel Sounding Based on Bistatic Synthetic Aperture Radar Principles

In this paper, we propose a simplified spatial channel sounding method by utilizing bistatic synthetic aperture radar (BiSAR) principles. Despite the different deployment geometries compared with a conventional BiSAR system, the feasibility of the approach is established by 1) the proposed method achieves a better spatial resolution than conventional directional channel sounders and 2) reconstruction algorithms based on time-domain back-projection in conjunction with a digital elevation model provide a good imaging performance and are suitable for reconstructing the spatial distribution of scatterers. Simulations of a high-speed rail (HSR) scenario demonstrate that the estimated power delay profiles (PDPs) and power angle profiles (PAPs) are close to the actual values.     

Phased Aligned Passive Bus (PAB) Scheme for ISDN User-Network Interface

This paper proposes an alternative to the passive bus realization of the current CCITT ISDN user-network interface. Our new approach is termed phase aligned passive bus (PAB) scheme. An automatic phase adjusting mechanism using a monitor bit is employed in the terminal equipment. This scheme enables the terminals to operate in complete phase synchronization on a passive bus. The advantages that result from this technique are: 1) Eliminates, in principle, the bus length limitation due to round-trip delay time. If line bit rate of 192 kbit/s is kept, then a practical bus length is about 700 m with no limitation on terminal distribution. 2) Allows various extended bus configurations, such as simple repeatered extention, connection of optical link, and multibranched passive bus. 3) Provides spare bit transmission capability to facilitate intrabus communication and maintenance. As an alternative it keeps the general framework of the current CCITT I-series standard such asB + B + Dchannel structure and LAP-Dprotocol. Experiments realizing long passive buses are shown to confirm the feasibility of the proposed scheme. This concept is also shown to be applicable to a 2-wire full duplex user-network interface, advantageous in the existing office environment, employing time compression transmission.     

一种宽波束机载SAR运动误差的频域补偿方法

对于方位向宽波束机载SAR系统的运动补偿在实际应用中具有重要意义。该文提出了一种对宽波束机载SAR系统运动误差进行频域补偿的方法。利用线性调频信号的时频对应关系,将时域误差转换到频域,基于短时傅里叶变换进行逐块的频域误差校正,得到精确聚焦的图像。文中对算法原理、适用条件、算法流程和运算量进行了详细分析。对P波段机载SAR系统低频运动误差情况进行了点目标和图像的仿真,验证了该方法的有效性。     

Parallel links with current-mode incremental signaling and per-pin skew compensation

This paper proposes a new inter-signal timing skew compensation technique for parallel links with current-mode incremental signaling. Both the transmitter and receiver of the links are current-mode configured to utilize the intrinsic advantages of current-mode signaling. The receiver maps the direction of its channel currents representing the logic state of the incoming data to two voltages whose values are largely different, enabling a convenient recovery of both the logic state and timing information of the received current-mode data in the voltage-mode domain, and suppression of the common-mode disturbances coupled to the channels. Inter-signal timing skews are compensated by inserting a delay line in each channel whose time delay is determined by the phase difference between the transmitted master clock and the output of the recovering comparator. To assess the effectiveness of the proposed inter-signal timing skew compensation technique, a 2-bit 1 Gbytes/s parallel link has been designed in IBM- CMOS technology and analyzed using SpectreRF with BSIM3V3 device models. Simulation results of the parallel link with the proposed deskewing scheme demonstrate that inter-signal timing skews can be effectively compensated using the proposed deskewing scheme.     

基于FPGA的PTP时间同步技术实现

针对现有PTP时间同步技术实现方案系统复杂、结构冗余以及成本高的问题,文中提出了一种基于FPGA的PTP时间同步技术实现方案。通过在确立主从关系的FPGA系统之间交互四种报文的方式,从系统在数据链路层截取Sync和Delay_req报文的时间戳,并计算主从系统之间的链路延迟和时间偏移量,对本地时钟进行补偿以实现时间同步。文中采用Xilinx公司提供的ChipScope Pro工具做板级验证,抓取主从系统的时间信息,并测试同步精度和成功率。实验结果表明,所设计的系统在时间同步精度上可以达到纳秒级,同步的成功率基本维持在99.99%。     

一种基于相位补偿的前向散射阴影逆合成孔径雷达快速成像方法

传统的基于菲涅尔数值积分的前向散射阴影逆合成孔径雷达(SISAR)成像计算复杂且运算量较大。为提高运算速度,该文对SISAR快速成像方法进行了研究。首先提出一种基于快速傅里叶变换(FFT)的SISAR快速成像方法,并给出了FFT引入相位误差的补偿公式;随后通过对运动补偿后信号频谱进行分析给出了SISAR成像的采样准则,其指出成像所需的信号采样率可以远小于奈奎斯特采样率。仿真结果表明,利用FFT和低采样率的快速成像方法可以在精确成像的基础上大大降低运算量,具有实际工程意义。     

Link activation protocols for a mobile communication network withdirective/adaptive antennas

A noncentralized fully distributed mobile communication network with directive/adaptive antennas is proposed. The spatial and temporal synchronization of transmitting and receiving antenna beams is accomplished by employing the global positioning system information and a synchronized spatial search algorithm. The link setup between a communication transmitter (CT) and a communication receiver (CR) depends on the success of both forward and reverse links. A two-way communication link protocol is defined as a basic agreement for the link activation in the network. Two link search algorithms, for additive white Gaussian noise channels, are proposed for use in hostile communication environments as follows: (1) a sequential ascending search power algorithm using an optimal set of link power levels and (2) a sequential Bayesian search algorithm (SBSA) using Bayes rule to update the CR location information. The optimal SBSA can be obtained by using dynamic programming, but the computational complexity is unfeasibly high. The experimental results in which the CT uses a noncoherent matched filter receiver for spread-spectrum code acquisition and the unfriendly interceptor uses a wide-band energy detector for both link search algorithms are presented     

An improved scheme for passive synthetic aperture photonic imaging

An improved scheme for passive synthetic aperture imaging is proposed. With this new technique, the radiation signal from field of view （FOV） is received and its frequency is down-converted to intermediate frequency （IF）. The IF signals are modulated to light-wave by electro-optical phase modulators and transmitted in fiber. At the end of the fiber, a downscaled fiber array is formed according to the receiver array. The image of FOV is obtained with the fiber array. The principles of passive synthetic aperture photonic imaging technique are analyzed deeply and its properties are compared with traditional imaging method of synthetic aperture. By simulation, the spread function of spot source and the image signal-noise-ratio are investigated using the new imaging approach with FOV radiation from different incidence angles. The introduced scheme can reconstruct the target image and perfect the present synthetic aperture imaging systems.