Code Tracking Loop Research Articles

A New Code Discrimination Algorithm for C/A Code Tracking Loop of GPS Receiver

该文分析了GPS C/A码相位估计误差与载波频率估计误差对相关函数的影响，并推导出了相关公式。从线性，对信号幅度与载波频率估计误差的敏感性和计算量等角度分析了现有的码鉴别器算法的优缺点。基于上面的分析与研究，提出了一种新的C/A码鉴别器算法。通过理论分析与仿真实验，证明该算法有良好的线性特性，能有效抑制对信号幅度与载波频率估计误差的敏感性，且计算量低，跟踪精度高，抗多经性能好，在性能上优于已有的算法。

A low-complexity adaptive receiver for DS-CDMA systems in unknown code delay environment

In direct-sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay-locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low-complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least-mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean-square error, of the proposed receiver. Copyright © 2010 John Wiley & Sons, Ltd.

The Effect of Intermittent Signal on the Performance of Code Tracking Loop in GNSS Receivers

This paper analyzes the code tracking performance in the presence of signal blanking in Global Navigation Satellite System (GNSS). The blanking effect is usually caused by buildings that obscure the signal in either a periodic or random manner. In some cases, ideal blanking is used to remove random or periodic interference. Nevertheless, the effect of temporary discontinuity of signal often leads to the tracking and position error. To analyze this problem, three types of blanking model are considered: no blanking, periodic blanking, and random blanking of the signals input into the code tracking loop. The mean time to lose lock (MTLL) is to assess the performance of code tracking system under signal blanking. Finally, the effect of steady-state tracking errors on the performance of tracking loop resulting from blanking environment is also discussed.

Selection of the signal synchronization method in software GPS receivers

Introduction This paper presents a critical analysis of the signal processing flow carried out in a software GPS receiver and a critical comparison of different architectures for signal processing within the GPS receiver. A model of software receivers is shown. Based on the displayed model, a receiver has been realized in the MATLAB software package, in which the simulations of signal processing were carried out. The aim of this paper is to demonstrate the advantages and disadvantages of different methods of the synchronization of signals in the receiver, and to propose a solution acceptable for possible implementation. The signal processing flow was observed from the input circuit to the extraction of the bits of the navigation message. The entire signal processing was performed on the L1 signal and the data collected by the input circuit SE4110. A radio signal from the satellite was accepted with the input circuit, filtered and translated into a digital form. The input circuit ends with the hardware of the receiver. A digital signal from the input circuit is brought into the PC Pentium 4 (AMD 3000 +) where the receiver is realized in Matlab. Model of software GPS receiver The first level of processing is signal acquisition. Signal acquisition was realized using the cyclic convolution. The acquisition process was carried out by measuring signals from satellites, and these parameters are passed to the next level of processing. The next level was done by monitoring the synchronization signal and extracting the navigation message bits. On the basis of the detection of the navigation message the receiver calculates the position of a satellite and then, based on the position of the satellite, its own position. Tracking of GPS signal synchronization In order to select the most acceptable method of signal synchronization in the receiver, different methods of signal synchronization are compared. The early-late-DLL (Delay Lock Loop), TDL (Tau Dither Loop) and Costas's PLL (Phase Lock Loop) models of loop tracking of signal synchronization are presented. The analysis is performed by processing the signals from the same satellite and under the same conditions of the initial signal synchronization. The multiple signal processing showed the advantages and disadvantages of the particular methods and the most acceptable solution proved to be the implementation of the DLL tracking loop phase synchronization and the code tracking loop PLL carrier frequency synchronization. The influence of the parameters of the loop itself is shown as well. After the extraction of navigation bits, all bit extracts do not come with an equal phase. This may lead to a wrong decision in rejecting the Decider and bits. Therefore, the paper proposes an additional synchronization of the extracted bits selected by the navigation messages NDA (Non-Data-Aided) algorithm in order to perform their correct detection and to position the receiver much faster. Comparison of methods of signal synchronization Based on the analysis of the methods, the simulation results are compared. The results showed that the TDL loop is not resistant to dynamic disturbances of signals, since the synchronization tracking breaks up. In addition, this loop shows susceptibility to the noise from free space. These are sufficient reasons to propose the implementation of the DLL signal tracking loop as an acceptable solution. In addition to the proposal of the tracking loop, tracking loop coefficient values are determined. Conclusion The performed analysis showed that the TDL loop is not resistant to noise and dynamic disorders of the input signal. Therefore, an appropriate solution for the implementation is the DLL loop with six correlates. The DLL tracking loop coefficients are determined as a compromise solution and they are B = 2Hz and γ = 0.7. Also, the coefficients of the PLL tracking loop of frequency synchronization are B = 20Hz and γ = 0.7. The application of the NDA algorithm results in an additional synchronization of the navigation bit message, thus making the receiver operate faster and more precisely.

Spreading code phase measurement technique for snapshot GNSS receiver

Driven by the expanding demand of location based services (LBS), the integration of satellite navigation receiver into mobile devices has received much attention in the last years. Snapshot positioning technique has turned out to be a very suitable candidate in LBS applications for its predominance of low cost and low power dissipation. However, the limited snapshot length is a major hurdle that limits the availability and accuracy of receivers. A code phase measurement technique is developed and used for snapshot receivers. By utilizing the features of the modernized global navigation satellite system (GNSS) signals and the characteristic of snapshot positioning, it uses a novel dual update-rate tracking loop with a non-causal smoothing estimator to obtain a relatively accurate estimation from a second or less snapshot data. Experiments with simulated data show that the proposed method has better estimation accuracy than the conventional code tracking loop in confined data length snapshot receivers.

Performance Analysis of a Complex Multicode Tracking Loop in Bandlimited Rayleigh Fading Channels

This paper presents the root-mean-squared tracking error performance analysis of a class of coherent digital delay-locked loops for multicode direct sequence spread spectrum signals in bandlimited correlated Rayleigh fading channels. In the transmit side, multiple independent PSK-modulated data streams in the in-phase and qudrature phase branches are spread by short mutually orthogonal codes before being further complex spread by a long complex PN code. We assume that the system employs a pilot code channel to assist the receiver's synchronization and channel estimation. The proposed code tracking loop incorporates the pilot-aided channel estimator and derives the timing error from all short-code-spread subchannels. Our analysis takes into account the effects of imperfect channel estimate, correlated frequency selective Rayleigh fading and band-limiting. Numerical results are presented to quantify the impact of the resulting multipath interference within the same code channel and amongst different code channels.

Generalized Theory of Code Tracking with an Early-Late Discriminator Part II: Noncoherent Processing and Numerical Results

Code tracking is an important attribute of receivers for Global Positioning System (GPS) and other global navigation satellite systems (GNSS). This paper and its antecedent provide analytical expressions for performance of code-tracking loops using early-late discriminators, under small-error conditions. Expressions are provided for output signal-to-noise-plus-interference ratio (SNIR) and code-tracking error, for arbitrary signal spectra, and Gaussian noise and interference having arbitrary spectral shapes. This second paper addresses noncoherent early-late processing (NELP) for given receiver precorrelation bandwidth and given early-late spacing, comparing the results to results for coherent early-late processing (CELP) and to a lower bound (LB) on code-tracking error. Theoretical expressions are derived and compared, and numerical results are provided to examine the effect of different modulation designs and interference conditions.

Generalized Theory of Code Tracking with an Early-Late Discriminator Part I: Lower Bound and Coherent Processing

Code tracking is an important attribute of receivers for Global Positioning System (GPS) and other global navigation satellite systems (GNSS). This paper and its sequel provide analytical expressions for performance of code-tracking loops using early-late discriminators, under small-error conditions. Expressions are provided for output signal-to-noise-plus-interference ratio (SNIR) and code-tracking error for arbitrary signal spectra, and Gaussian noise and interference having arbitrary spectral shapes. This first paper addresses coherent early-late processing (ELP) for given receiver precorrelation bandwidth and given early-late spacing, also providing a tight lower bound on code-tracking error independent of discriminator design. Theoretical expressions are derived, showing that code-tracking accuracy depends on more than merely signal-to-noise ratio and early-late spacing - the shape of signal and interference spectra are important, as is the receiver precorrelation bandwidth.

Design and implementation of an adaptive code discriminator in a DSP/FPGA-based Galileo receiver

In the design of a global navigation satellite system receiver, the tracking performance depends on the code tracking loop and the associated discriminator. An adaptive code discriminator under a multi-correlator architecture has been designed based on a multi-objective principle to achieve a balance among various, sometimes conflicting, design objectives. With the proposed optimization approach and adaptive logic, concerns of large pull-in region and small steady-state error can be addressed. The proposed scheme is implemented in a digital signal processor/field programmable gate array board and an experiment is conducted to process GIOVE-A signals. The test results reveal the advantages of the proposed code tracking architecture and discriminator design.

A constrained minimum mean square error code tracking loop for synchronous DS-CDMA systems

A constrained minimum mean square error code tracking loop (CMMSE-CTL) is proposed in this paper for synchronous direct sequence code division multiple access (DS-CDMA) systems in a multiuser environment. By introducing assistant weighted despreading signals into the local reference signals of the loop, we exploit the resultant extra degrees of freedom to improve the tracking performance under multiuser environment. The derived mean square tracking error is taken as the cost function and minimized with respect to the weight parameters, under the constraints which ensure an odd-symmetric bias-free S -curve with enhanced tracking range. Simulations based on Gold codes compare the S -curve, root-mean-square (rms) tracking error, mean-time-to-lose-lock (MTLL) performance of CMMSE-CTL and the traditional modified code tracking loop (T-MCTL). It is shown that CMMSE-CTL outperforms T-MCTL greatly in that it provides unbiased stable code tracking with reduced mean square tracking error, an enhanced tracking range, and near–far resistance.

Performance Analysis of a De-correlated Modified Code Tracking Loop for Synchronous DS-CDMA System under Multiuser Environment

This paper presents the performance analysis of a De-correlated Modified Code Tracking Loop (D-MCTL) for synchronous direct-sequence code-division multiple-access (DS-CDMA) systems under multiuser environment. Previous studies have shown that the imbalance of multiple access interference (MAI) in the time lead and time lag portions of the signal causes tracking bias or instability problem in the traditional correlating tracking loop like delay lock loop (DLL) or modified code tracking loop (MCTL). In this paper, we exploit the de-correlating technique to combat the MAI at the on-time code position of the MCTL. Unlike applying the same technique to DLL which requires an extensive search algorithm to compensate the noise imbalance which may introduce small tracking bias under low signal-to-noise ratio (SNR), the proposed D-MCTL has much lower computational complexity and exhibits zero tracking bias for the whole range of SNR, regardless of the number of interfering users. Furthermore, performance analysis and simulations based on Gold codes show that the proposed scheme has better mean square tracking error, mean-time-to-lose-lock and near-far resistance than the other tracking schemes, including traditional DLL (T-DLL), traditional MCTL (T-MCTL) and modified de-correlated DLL (MD-DLL).

Anti-Interference Receiver Structures for Direct Sequence Spread Spectrum Signals

Conventional narrowband interference (NBI) rejection algorithms often assumed perfect pseudo-noise (PN) code synchronization. The functions of NBI rejection and code tracking are performed separately and independently by an adaptive filter and a code tracking loop, respectively. This paper presents two new receiver structures for direct sequence spread spectrum (DS/SS) systems, one operates in coherent mode and the other operates in noncoherent mode. Both receivers are designed to suppress NBI and minimize tracking jitter. Numerical results show that the proposed coherent receiver performs as good as the conventional receiver that uses an LMS NBI rejection filter with zero tracking jitter. The noncoherent receiver, when compared with the coherent one, suffers less than 3dB degradation for bit error probability smaller than 10-3.

Multi-objective approach in GNSS code discriminator design

In spread spectrum ranging system receivers such as the Global Navigation Satellite System (GNSS) receivers, a discriminator is used in the code tracking loop to provide an error signal for the tracking of incoming signals. Typically, the discriminator manipulates several correlator outputs to form the correction signal. A multi-objective approach is adopted here to design the coefficients associated with a multi-correlator-based discriminator to lead to a better performance tradeoff in large tracking range, acceptable multipath-induced bias, high sensitivity, free of ambiguity, and small variance. It is shown that the multiple-objective problem can be tackled via a quadratic/linear programming method, rendering the optimal coefficient vector for the implementation of the discriminator. In addition, as the discriminator design is often subject to conflicting requirements, it is shown that the proposed design approach is capable of establishing bounds on achievable performance. Examples with respect to BPSK (binary phase-shift keying) NRZ (nonreturn-to-zero), and BOC (binary-offset carrier) signals are given to illustrate the design approach.

A robust sequence synchronization unit for multi-user DS-CDMA chaos-based communication systems

This paper demonstrates a way of achieving and maintaining sequence synchronization in multi-user direct sequence code division multiple access (DS-CDMA) based chaotic communication systems. Synchronization is achieved and maintained through the code acquisition and the code tracking phase, respectively. The performance of the proposed system is evaluated in the presence of additive white Gaussian noise and interuser interferences. Throughout, a pseudo-random binary sequence (PRBS) is used as the synchronizing pilot signal within the multi-user chaotic communication system. In addition, the Logistic and Bernoulli chaotic maps are also used as the pilot signals in the investigation of the code acquisition performance. The code acquisition circuit is evaluated in terms of the probability of detection and probability of false alarm. The corresponding results demonstrate an ability to achieve initial synchronization. Furthermore, it is shown that in terms of code acquisition, the PRBS outperforms the Logistic and Bernoulli chaotic maps. A mathematical model of the code tracking loop is then presented. From the model, a control law for the generation of time offset estimates is derived. The robustness of the synchronization unit is then demonstrated in terms of the bit error rate. It has been shown that for the case of 1, 2, 3, 4, and 5 users, the bit error rate goes below the maximum acceptable limit of 10 −3 at the bit energy to noise power spectral density ratio of approximately 8, 9, 9.5, 11, and 12 dB, respectively. Furthermore, a gradual degradation in performance, above the maximum acceptable bit error rate limit, is demonstrated for the increasing number of users.

Effects of Signal Deformations on Modernized GNSS Signals

Satellite-based navigation requires precise knowledge of the structure of the transmitted signals. For GPS, accurate knowledge of the shape of the code correlation peaks is required to ensure no biases are introduced into the position solution. It is generally presumed that all GPS-like satellite signals are virtually identical. However, in 1993 a satellite malfunction introduced significant distortion onto one of the satellite C/A codes. That distortion caused range errors to vary with receiver filter characteristics and code tracking loop implementation. As a result, high-integrity systems such as the Wide Area Augmentation System (WAAS) must implement signal deformation monitors to detect and remove signals that become anomalously distorted. In the future, WAAS will rely on modernized signals from both GPS (L5) and Galileo (E1/L1/E2 and E5A/E5B). This should increase performance for users; however they must still protect against potential signal deformations. Although the International Civil Aviation Organization (ICAO) has agreed on a threat scenario for GPS L1 signals, no such agreement exists for modernized signals. In addition, each of these signals will have different chipping rates and their correlation peak structures will be quite different from that of the GPS C/A code. Their code tracking loop implantations are as yet not well- defined, but may differ somewhat from traditional architectures. An additional complication is the unknown receiver filter characteristics that the new receivers will employ. Each of these factors may render a given signal and/or receiver configuration more or less sensitive to signal deformations. This paper analyzes the range error sensitivity of several modernized signals subjected to distortions of the type considered in the ICAO threat model for signal deformations. To isolate the effects of the signal-in-space deformation errors, it assumes an ideal, wideband receiver filter and basic early-minus-late code tracking implementations for the new codes. It also compares the distortion-induced range errors for the new codes to those currently modeled for the C/A code. Finally, these results are used to motivate threat model refinements and receiver tracking loop constraints that minimize the affects of this error source for the modernized GNSS signals.

PN code tracking using noncommensurate sampling

The use of a noninteger symbol to sample time ratio, noncommensurate sampling, in pseudonoise (PN) code-tracking loops is investigated. A noncoherent PN code-tracking loop using noncommensurate sampling and single-bit quantization is presented. An analysis method is developed to characterize the system and determine the steady-state error variance of the timing estimate. The performance of an equivalent analog code-tracking loop is derived and compared with the digital loop to investigate the degradation incurred through sampling and quantization

PN Code Tracking Using Noncommensurate Sampling

The use of a noninteger symbol to sample time ratio, noncommensurate sampling, in pseudonoise (PN) code-tracking loops is investigated. A noncoherent PN code-tracking loop using noncommensurate sampling and single-bit quantization is presented. An analysis method is developed to characterize the system and determine the steady-state error variance of the timing estimate. The performance of an equivalent analog code-tracking loop is derived and compared with the digital loop to investigate the degradation incurred through sampling and quantization.

Performance Analysis of a Decision-Feedback Coherent Code Tracking Loop for Pilot-Symbol-Aided DS/SS Systems

In this paper, the performance of a decision-feedback coherent code tracking loop for pilot-symbol-aided (PSA) direct-sequence spread-spectrum systems, considering the effect of both symbol and channel estimation errors, is analyzed. An analytical closed-form expression on the steady-state jitter variance in additive white Gaussian noise (AWGN) environments using the linear analysis model is first derived. Then, the upper and lower bounds of the jitter variance in fading environments are obtained based on the analysis of the loop bandwidth variation according to the input signal level. Finally, a second-order coherent tracking loop is designed, and its performance in both AWGN and fading environments is evaluated, along with verification by computer simulations. It is demonstrated that analytical results are very close to the simulation results in AWGN environments and that the jitter variance in fading environments is located between analytical upper and lower bounds with its position depending on the fading rate.

Code tracking loop for M-ary/SS systems using outputs addition of correlators

A correlator output addition method is proposed for the code tracking of a communications system based on M-ary/SS scheme in which one of several sequences prepared on the transmitter side is transmitted depending on several bits of data. In the correlator output addition method, M advance correlators and M delay correlators are used in addition to M correlators for signal demodulation. A DLL is configured by using the sum of the M correlator outputs. In the code tracking system using the conventional locking PN sequence, there occurs a fixed deterioration of the SN ratio due to splitting of the signal power for the locking PN sequence. In the proposed method, this deterioration does not occur. However, this method is more likely to be affected by the output noise of each correlator if the number of sequences M is increased. Also, by comparison with another conventional method DFL scheme, it is found that the present method has better code tracking characteristics under a large noise. Hence, the method is suitable for cases where locking must be maintained even if the number of sequences M is small and the noise is large. © 2006 Wiley Periodicals, Inc. Electron Comm Jpn Pt 3, 89(8): 30–38, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.20171

GPS Signal-to-Noise Measurement in Weak Signal and High-Interference Environments

In GPS receivers, measurement of the carrier power-to-noise density, C/N0, is important for determining whether the code and carrier tracking loops are in lock, controlling the response of the receiver to low signal-to-noise environments, and determining the signal-to-noise environment in order to assess or predict receiver performance. However, in a weak signal or high-interference environment, the C/N0 measurements can be very noisy. To mitigate this problem, an investigation was conducted into methods of signal-to-noise measurement. Three C/N0 measurement techniques were investigated by theoretical analysis and simulation study, and performance was compared with a range of different smoothing times. These were the established narrow-to-wideband power ratio method, a correlator comparison method, and a discriminator output statistics method. All were found to be suitable in low signal-to-noise environments provided long averaging times on the order of 25 s or more are used.