Data/pilot combination strategies for Compass B1C signal acquisition

The new Compass B1C signal, with a power ratio of the data channel to the pilot channel of 1:3, adopts BOC(1,1) in its data channel; a new modulation, namely, a quadrature multiplexed binary offset carrier (QMBOC) in its pilot channel modulates the narrowband BOC(1,1) and the wideband BOC(6,1) components in phase quadrature with each other. To fully exploit the B1C signal and to improve the acquisition sensitivity in a degraded signal or an indoor environment, this paper proposes to combine the data and the pilot channels for B1C signal acquisition. The noncoherent, coherent, and differential coherent strategies are considered for application in the B1C signal. Considering the power inequality between the data and the pilot channels, the maximum signal‐to‐noise ratio combination (MRC) is utilized in these strategies to obtain an optimal combination. This paper also proposes to simplify the differential coherent strategy to obtain a better anti‐noise performance under stable conditions. The computer simulation results are given to verify the theoretical expressions, and some discussions with respect to the proposed strategies are provided.     

同阶二元偏移载波调制信号跟踪技术

针对同阶二元偏移载波调制卫星导航信号,提出一种高效新算法解决信号跟踪中的模糊性。分析了同阶二元偏移载波调制原理、信号跟踪模糊性及解决方法,利用本地扩频码与接收信号的相关函数特点,构建了码跟踪环路鉴相曲线,选择了合适的相关间距作为符号判决,联合构成鉴相器,推导了算法设计的正确性,给出了实现架构。仿真结果表明,码相位捕获误差±0.9个码片内均可正确跟踪。该方法可进一步应用于全球卫星导航系统新体制信号的接收处理。     

BOC调制信号频谱特性及仿真分析

BOC调制技术是一种广泛应用于GPS的现代化以及伽利略系统中新型信号调制技术。为研究其频谱特性,分析了BOC调制技术的基本原理及信号的产生过程,应用Matlab软件对其频谱特性进行了仿真,阐述了这种信号调制方式的优点;此外对其扩展技术MBOC,CBOC,TMBOC以及AltBOC做了简要介绍,对进一步研究导航信号现代化具有一定意义。     

Code phase discrimination function shaping method for Compass B1C QMBOC (6,1,4/33) signal

The discrimination function shaping method has been proposed to improve the anti‐multipath performance for a global navigation satellite system receiver. However, the existing methods fail to fully exploit the potential anti‐multipath performance of multiplexed binary offset carrier (MBOC)–modulated signals. In this paper, a code phase discrimination function shaping method is proposed for a Compass B1C quadrature MBOC‐modulated signal. The in‐phase binary offset carrier (BOC) (1,1) signal and quadrature‐phase BOC (6,1) signal replicas are designed according to the least squares estimation. Then a shifted cross‐correlation function (CCF) between the in‐phase BOC (1,1) signal and its pseudorandom noise code is multiplied with the obtained CCFs to ensure an unambiguous tracking. Finally, a discrimination function with a small pull‐in range can be obtained by a weighed combination of the in‐phase and quadrature‐phase CCFs. A computer simulation is conducted to assess the performances under multipath and thermal noise conditions.     

Unambiguous S‐curve shaping technique for multipath mitigation in Global Navigation Satellite Systems alternative binary offset carrier signals

Global Navigation Satellite Systems positioning accuracy indoor and urban canyons environments are greatly affected by multipath because of distortions in its autocorrelation function. In this paper, we propose a new multipath mitigation technique based on the concept of S‐curve shaping for the new alternative binary offset carrier (AltBOC)‐modulated signals, which will most likely be used in both European Galileo system and Chinese Compass system. The definition of an optimum S‐curve is employed to reduce the false lock points and to improve the multipath mitigation capability via determining the shape of the local code tracking reference function. The structure of the proposed code tracking loop for AltBOC signals is quite simple and only requires one complex correlator. Results demonstrate that the proposed technique provides superior multipath mitigation performance compared with the conventional receiver correlation techniques and removes the ambiguity completely. Copyright © 2012 John Wiley & Sons, Ltd.     

四相鉴频器辅助的高动态BOC信号载波跟踪

叉积鉴频器的输出频率范围比较窄,捕获信号以后的多普勒频偏可能不在其跟踪范围内。针对此问题,提出了使用四相鉴频器( FQFD )算法辅助已经成型的二阶锁频环加三阶锁相环模型。首先,利用四相鉴频器的非线性特性将接收信号频偏大步长牵引到较低范围,然后使用锁频环消除其大部分动态性,最后利用锁相环跟踪精度高的特点实现高动态二进制偏移载波( Binary Offset Car-rier,BOC)信号载波的快速准确跟踪。在分析各跟踪模块算法的基础上,讨论了其本身的热噪声误差、动态适应力以及最优带宽等相关问题,理论分析和仿真结果验证了该方法比原有跟踪算法提高了300 Hz左右的鉴频范围,并且跟踪效果良好。     

多频段高动态信号载波频偏捕获方法研究

研究了多频段高动态信号载波频偏问题,分析了多频段给频偏捕获带来的困难,针对低数据率、大载波频偏的情况,讨论了抽取倍数对捕获范围和精度的影响,并提出对于不同频段,通过改变相应的抽取倍数来满足不同的频偏和精度,再进行最大功率搜索法估计频偏的捕获方案。仿真结果表明,与传统方法进行比较,该方案在低信噪比下条件下估计性能更好,能够适应UHF和S频段,在数据率9.6 kb/s的条件下,估计精度在UHF频段下为300Hz,在S频段下为600Hz。     

A dual-frequency GNSS sensor for space applications

High-performance laboratory breadboards of a dual-frequency global navigation satellite system (GNSS) receiver have been developed at the Institute of Satellite Navigation at the University of Leeds (ISN) for a number of years under a European Space Agency (ESA) programme. The main objective of these breadboards is to serve as development tools for receiver signal-processing research in view of scientific applications of GNSSs. Some of these applications will be realized by means of receivers embarked on satellites in low earth orbit (LEO) as part of on-going and future ESA programmes and will include precise orbit determination, radio occultation measurements and measurements of the parameters characterizing GNSS signals reflected at the sea surface. The principal application for which the signal-processing developments described in this paper have been targeted is radio occultation. During an occultation event the GNSS signal encounters a challenging environment. Substantial Doppler dynamics occur during a measurement, due to the effect of sharp gradients of the refraction index. Signal attenuation and deep fading caused by multipath propagation are also encountered. The areas of concern are the acquisition and tracking of signals in high dynamics and at low carrier-to-noise ratios (C/N₀), and also the performance of tracking the GPS P code without full knowledge of the spreading code, as dual-frequency atmospheric measurements are crucial to this application. © 1998 John Wiley & Sons, Ltd.     

OFDM系统中基于导符号和判决数据的频率跟踪环路

在相干正交频分复用(OFDM)系统中为跟踪多径信道的实时变化通常在时频网格上插入导符号来进行信道估计.对利用这些导符号结合判决数据进行频率跟踪的算法作了进一步的研究.在时域和频域都重新推导了频偏的估计结果,经过合理近似,得到了时域最大似然估计的近似解;在频域推导了另一种形式的似然函数,得出了最大似然估计的解析表达式.新的频率跟踪算法降低了计算量,提高了频率同步的精确度.     

微弱扩频信号捕获的积分时间延拓技术

相对于普通扩频接收机,微弱扩频信号捕获需要更长的积分时间以获得处理增益。然而,伪码多普勒频偏导致能量不能在一个码相位上持续积累,限制了积分时间的进一步增加。为此,提出了一种扩频信号捕获时间延拓技术,增加了扩展积分单元,通过对扩展积分结果进行路径搜索,从而有效地减少了码多普勒引起的相位滑动对相关运算积分时间的限制。仿真和实测结果表明所提技术突破了传统全球导航卫星系统( GNSS)捕获算法的积分时间上限,提高了系统灵敏度。针对带内信噪比低于-50 dB的GPS L1 P码捕获,给定硬件资源约束和5 kHz多普勒范围,算法的捕获概率优于传统算法约3 dB。     

Localization under anchor node uncertainty for underwater acoustic sensor networks

This paper deals with the problem of sensor node localization in the presence of uncertainty in anchor node location. Aqueous environments are prone to adverse effects of underwater currents. This adversity causes non‐negligible mobility to the anchor nodes deployed under water. Localization in the presence of uncertainty in the anchor node location is quite challenging. Also, the authors consider the ray‐bending property of underwater medium due to depth dependent sound speed, to furnish the accurate position estimate of the target node. Standard ray equations are used to model the path followed by acoustic rays in water. Maximum likelihood estimation is proposed to estimate the location of target node with uncertainty in anchor node positions and is compared with the scheme with exact knowledge of anchor node positions, and the results are reported. Monte Carlo simulation is used to assess the performance of the proposed method. Also, the Cramer‐Rao lower bound with uncertainty in anchor nodes is derived and described. Simulation results of the proposed algorithm outperform the existing algorithm with known anchor location by up to 49.4%, and hence, accuracy is improved in the proposed method.     

一种改善 GNSS 弱信号捕获性能的伪码优化设计

强多址干扰下的弱信号捕获是高灵敏度GNSS接收机比较常见又难以解决的问题。本文提出一种非等长的伪码设计方法,通过使各路导航信号调制伪码码长互质,从时域上降低了伪码的互相关值,从频域上降低了目标信号和干扰信号的谱分离系数。理论分析和仿真表明,基于非等长的伪码优化方法,能够改善强多址干扰环境中弱信号的捕获性能。     

Cramer‐Rao lower bound and parameter estimation for multibeam satellite links

Data throughput and availability of multibeam satellite links are limited by interference problems, in particular when the design is determined by an aggressive reuse of frequency bands. Usually, this is mitigated by appropriately selected techniques like precoding or multi‐user detection. Before such methods are applicable, however, the most important transmission parameters have to be recovered successfully even under very challenging conditions caused by (full) frequency reuse. The detailed analysis of the log‐likelihood function characterizing the multibeam scenario shows that a beamwise decoupling of the estimation task is possible, if the following three conditions are met: (i) alignment of all frequency offsets; (ii) symbol synchronicity; (iii) orthogonal synchronization sequences. Based on these assumptions, it turns out that the modified Cramer‐Rao lower bound (MCRLB) for carrier frequency and phase, symbol timing and signal amplitude is the same, no matter whether we are dealing with a single or a multiple beam situation. Furthermore, because a maximum likelihood framework for parameter estimation is not available in closed form, we introduce a sub‐optimal concept of low‐complex algorithms. Finally, it is verified by simulation results that the jitter variance of each recovery scheme is close to the related MCRLB, when the decoupling conditions are satisfied. Copyright © 2016 John Wiley & Sons, Ltd.     

Theoretical Limits Analysis of Indoor Positioning System Using Visible Light and Image Sensor

To solve the problem of parameter optimization in image sensor‐based visible light positioning systems, theoretical limits for both the location and the azimuth angle of the image sensor receiver (ISR) are calculated. In the case of a typical indoor scenario, maximum likelihood estimations for both the location and the azimuth angle of the ISR are first deduced. The Cramer‐Rao Lower Bound (CRLB) is then derived, under the condition that the observation values of the image points are affected by white Gaussian noise. For typical parameters of LEDs and image sensors, simulation results show that accurate estimates for both the location and azimuth angle can be achieved, with positioning errors usually on the order of centimeters and azimuth angle errors being less than 1°. The estimation accuracy depends on the focal length of the lens and on the pixel size and frame rate of the ISR, as well as on the number of transmitters used.     

扩频体制低轨卫星通信信号捕获与跟踪系统设计

针对扩频体制下低轨卫星信号的捕获及跟踪等系统设计实现问题,根据卫星信号模型进行了捕获及跟踪算法设计,重点对基于快速傅里叶变换的快速捕获算法、锁频环、锁相环和码跟踪环路进行了设计,并进行了工程系统实现。试验验证表明,该系统可以实现码分多址体制低轨卫星信号捕获、跟踪处理,工作性能参数满足系统需求。     

一种“北斗”B1C信号无模糊捕获算法

针对"北斗"B1C信号同步过程中存在的捕获模糊性问题,根据"北斗"B1C信号的信号特点,基于伪相关函数方法,设计了一种适用于"北斗"B1C信号的无模糊捕获算法。该算法采用数据分量和导频分量非相干组合的捕获策略,实现了B1C信号数据分量和导频分量的无模糊捕获,提高了捕获灵敏度。利用导航终端接收的卫星信号数据,对所提捕获算法进行了仿真验证,并分析了其无模糊捕获性能和捕获灵敏度。仿真结果表明,所提算法能准确且快速地捕获"北斗"B1C信号。     

Cramer–Rao bound and minimum variance unbiased estimator for joint sampling clock offset and channel taps in MC‐CDMA systems

Exact closed‐form expressions of the Cramer–Rao bound (CRB) for joint sampling clock offset and channel taps are obtained in multi‐carrier code division multiple access systems. CRB is undoubtedly the most well known variance's bound to determine. It provides a benchmark against which we can compare the performance of any unbiased estimator. Furthermore, minimum variance unbiased (MVU) estimator for these parameters is proposed. Moreover, maximum likelihood (ML) and least‐squares estimators for joint sampling clock offset and channel taps are presented. Best linear unbiased estimator is also introduced just for channel taps. The performances of the estimators are compared through simulation results with the proposed CRB. Our results show the better performances of MVU and ML estimators with more computational complexity compared with the others. Copyright © 2008 John Wiley & Sons, Ltd.     

Mobile GPS carrier phase tracking using a novel intelligent dual‐loop receiver

Carrier phase information is necessary for accurate measurements in global positioning system (GPS) applications. This paper presents a novel intelligent GPS carrier tracking loop with variable‐bandwidth characteristics for fast acquisition and better tracking capability in the presence of dynamic environments. Our dual‐loop receiver is composed of a frequency‐locked loop‐assisted phase‐locked loop structure, the fuzzy controllers (FCs), and the ATAN discriminator functions. The soft‐computing FCs provide the time‐varying loop gains to perform accurate and reliable control of the dual‐loop paradigm. Once the phase dynamic errors become large under kinematic conditions, the fuzzy loop gains increase adaptively and achieve rapid acquisition. On the other hand, when the tracking errors approach zero in the steady state, the loop gains decrease and the corresponding dual‐loop receiver returns to a narrowband system. Four types of carrier phase signals, i.e. phase offset, decaying sinusoidal phase jitter, frequency offset, and frequency ramp offset, are considered to emulate realistic mobile circumstances. Simulation results show that our proposed receiver does achieve a superior performance over conventional tracking loops in terms of faster settling time and wider acquisition range while preventing the occurrence of cycle slips. Copyright © 2008 John Wiley & Sons, Ltd.     

A hybrid joint ML technique for CFO,timing offset,and channel estimations in an OFDMA uplink

In this paper, we propose a novel hybrid joint maximum‐likelihood estimator for carrier frequency offset (CFO), timing offset, and channel response of all users in the uplink of an orthogonal frequency division multiple access (OFDMA) system. The proposed estimation method significantly reduces complexity of this multiparameter, multidimensional optimization problem, using the concept of separation of the different user signals by means of newly defined projection operators. This projection technique is combined with the alternating projection method available in the literature to arrive at a new hybrid algorithm that offers significant performance advantages in terms of computational complexity and estimator performance. The joint estimation of the CFOs, timing offsets, and channel coefficients for all active users together at the base station of the OFDMA uplink is a rarely addressed task. The proposed method also offers the flexibility of application to any subcarrier assignment scheme used in OFDMA systems. Extensive simulation studies corroborate the advantages of the new hybrid method for all three estimation requirements in the multiuser OFDMA uplink. Copyright © 2012 John Wiley & Sons, Ltd.