Alternative Binary Offset Carrier Research Articles

A double sideband combined tracking method for Galileo E5 AltBOC signals

The Galileo navigation satellite system (Galileo) E5 Alternative Binary Offset Carrier (AltBOC) signal brings various challenges due to its complex modulation, wide bandwidth, and multi-peaked auto-correlation function. While wideband tracking needs to solve the ambiguity problem and design dedicated baseband channels, the single-sideband cannot have the outstanding performance of the AltBOC signal. We propose a new tracking method called “Double Sideband Combined Tracking” (DSCT), which can fully exploit the AltBOC signal’s code tracking accuracy without ambiguity and ensure compatibility with Binary Phase Shift Keying (BPSK) processing channels, easily implemented in hardware. The DSCT employs one phase locked loop and one delay locked loop to track the carrier and code, respectively. The double-sideband correlation results used by the two loops are recovered by coherently combining the single-sideband correlation results of the two BPSK channels. Meanwhile, the combined model, the loop discriminator, and the ambiguity detection of the DSCT are discussed. Furthermore, the code tracking error caused by thermal noise is modeled and analyzed. The test results based on real Galileo E5 signals show that the DSCT exhibits better or comparable code tracking accuracy to the AltBOC wideband tracking method. When the loop falsely locks onto a side-peak, the DSCT can quickly detect and re-lock on the main peak.

Theoretical assessment and application of modern GNSS signals acquisition performance

The application of the Global Navigation Satellite System (GNSS) is extensive and has widely infiltrated the national economy and social development in various fields. As critical part of the satellite navigation system design process, GNSS signals performance assessment has received tremendous attention for a long period of time. The mathematical model of GNSS signal detection is built based on single point detection test. An improved acquisition performance assessment method that considers the modulation type, code search step-size, and acquisition algorithms is proposed herein. This study investigates the performance of the overall detection and main/side peak detection for Binary Offset Carrier (BOC), Multiplexed Binary Offset Carrier (MBOC), and Alternative BOC (AltBOC) signals using matched and unambiguous acquisition methods, and obtains quantitative results. With a fixed code search step-size, the multi-peaks of the autocorrelation functions of BOC, MBOC, and AltBOC signals will degrade the acquisition performance. The higher the sub-carrier rate, the more serious is the degradation. Although signal-power loss occurs to some extent, Binary Phase-Shift Keying-like (BPSK-like) and Sub Carrier Phase Cancelation (SCPC) methods significantly improve the acquisition performance.

Thermal noise performance analysis for dual binary phase‐shift keying tracking of standard BOC signals

Among the existing binary offset carrier (BOC) signal receiving technologies, the dual binary phase-shift keying tracking (DBT), originally designed for alternate BOC (AltBOC) modulated Galileo E5 signal and then extended to standard BOC signals, has its unique characteristics. It incorporates both single-band and dual-band processing schemes into a unified and simple structure to meet both requirements of improving ranging precision and anti-jamming. However, in this study, the authors propose that it is problematic to directly extend the DBT technique from the AltBOC signal to the standard BOC signal. The root cause of this problem lies in the fact that the cross-correlation functions between the received standard BOC signal and the local replicas in the upper and lower sidebands are not an ideal correlation triangle as the AltBOC case, but with some ripples. This leads to more complex behaviours when the DBT technique is applied to standard BOC signals. The rigorous theoretical analysis of this technique is given in respect of thermal noise performance in this study, which can be used not only to optimise the tracking loop parameters of DBT but also to compare the tracking performance of DBT and other tracking technologies.

Expressions for the Autocorrelation Function and Power Spectral Density of BOC Modulation Based on Convolution Operation

We present universal expressions for the autocorrelation functions (ACFs) of the Binary Offset Carrier (BOC), Multiplexed BOC (MBOC), and Alternative BOC (AltBOC) modulations based on convolution operations. We also derive the expressions for the power spectrum densities (PSDs) of these modulations using the Fourier transform of their ACFs. The results obtained in this contribution are useful for Global Navigation Satellite System (GNSS) signal simulation, performance evaluation, and high-performance acquisition and tracking algorithm design. The derivation methods of the expressions for the ACFs are common and can be used to derive expressions for the ACFs of other BOC-based modulations.

Research on multipath suppression performance of multiple BOC signals of navigation satellite

BOC(Binary Offset Carrier) signal is a kind of navigation satellite downlink signal. Compared with BPSK(Binary Phase Shift Keying) modulation signal, it can improve the multipath suppression performance of navigation system. BOC signal modulation includes a variety of extended signal modulation modes, including TMBOC(time-multiplexed BOC), AltBOC(Alternative BOC) signals, etc. It is an important subject to study the performance of multipath suppression of various BOC signals. In this paper, the multipath error model is constructed by using the noncoherent early-late processing track loop, the performance of multipath suppression of TD-AltBOC(Time Division Alternative BOC) and AltBOC signals for different front-end bandwidth of the receiver is analyzed. By studying the average multipath error and envelope value of AltBOC (15,10), BOC (1,1) and TMBOC (6,1,4/33) signals, the performance comparison results of three kinds of BOC modulation are given. In order to verify the research results of multipath suppression performance, the receiver is used to receive the signals of different BOC modulation systems transmitted by navigation satellite, The multipath error is obtained by the combination of dual frequency pseudo range and carrier phase. The experimental results show that AltBOC (15,10) has the best multi-path suppression performance, followed by TMBOC (6,1,4/33), and BOC (1,1) has the worst multi-path suppression performance compared with the others, which proves the correctness of the research results of multi-path suppression performance of various BOC signals.

Analysis on the Potential Performance of GPS and Galileo Precise Point Positioning using Simulated Real-Time Products

With the evolving Global Navigation Satellite System (GNSS) landscape, the International GNSS Service (IGS) has started the Multi-GNSS Experiment (MGEX) to produce precise products for new generation systems. Various analysis centres are working on the estimation of precise orbits, clocks and bias for Galileo, Beidou and Quasi-Zenith Satellite System (QZSS) satellites. However, at the moment these products can only be used for post-processing applications. Indeed, the IGS Real-Time service only broadcasts Global Positioning System (GPS) and Globalnaya Navigatsionnaya Sputnikovaya Sistema (GLONASS) corrections. In this research, a simulator of multi-GNSS observations and real-time precise products has been developed to analyse the performance of GPS-only, Galileo-only and GPS plus Galileo Precise Point Positioning (PPP). The error models in the simulated orbits and clocks were based on the difference between the GPS Real-Time and the Final products. Multiple scenarios were analysed, considering different signals combined in the Ionosphere Free linear combination. Results in a simulated open area environment show better performance of the Galileo-only case over the GPS-only case. Indeed, up 33% and 29% of improvement, respectively, in the accuracy level and convergence time can be observed when using the full Galileo constellation compared to GPS. The dual constellation case provides good improvements, in particular in the convergence time (47% faster than GPS). This paper will also consider the impact of different linear combinations of the Galileo signals, and the potential of the E5 Alternative Binary Offset Carrier (AltBOC) signal. Even though it is significantly more precise than E5a, the PPP performance obtained with the Galileo E1-E5a combination is either better or similar to the one with Galileo E1-E5. The reason for this inconsistency was found in the use of the ionosphere free combination with E1. Finally, alternative methods of ionosphere error mitigation are considered in order to ensure the best possible positioning performance from the Galileo E5 signal in multi-frequency PPP.

Non‐orthogonal AltBOC modulation for BeiDou B2 signals

New three-level binary offset carrier (BOC) subcarriers are constructed, based on which the non-orthogonal alternative BOC (AltBOC) modulation and its unequal power generalised form are achieved. The new constant envelope modulations have achieved a lower hardware complexity, which provide a solution to dual-frequency multiplexing problem confronted by BeiDou B2 frequency band.

A Unified Dual-frequency Constant Envelope Multiplexing Design Framework for Modernised GNSS Signals

In the field of modernised Global Navigation Satellite System (GNSS) signal design, several Dual-frequency Constant Envelope Multiplexing (DCEM) methods have been recently proposed. However, the existing DCEM methods, such as Alternative Binary Offset Carrier (AltBOC), generalised AltBOC and Asymmetric Constant Envelope Binary Offset Carrier (ACE-BOC), are only applied in some special cases. In this paper, we present a unified DCEM design framework for GNSS signals. The existing DCEM methods can be unified in this framework. First, the signal components at two carrier frequencies are combined into two single-frequency constant envelope signals. Then, the linear sum of dual-frequency signals with non-constant envelopes is obtained. Finally, the linear sum is converted into the corresponding DCEM signal by solving an optimisation problem. The proposed design framework has no strict constraints on the number, power ratio and phase relationship of the signal components. Moreover, some special design cases under this framework are also analysed in detail. The analytical results show that the proposed design method can reach higher multiplexing efficiency compared with the existing methods. Based on the proposed method, we suggest a scheme to multiplex the BeiDou regional signals and global signals at the B2 frequency. The simulation results of correlation functions and Power Spectrum Density (PSD) verify the correctness and effectiveness of the proposed design method.

Weighted Discriminator Function Based Unambiguous Tracking Method for Dual-Frequency Constant Envelope Modulated Signals

The dual-frequency constant envelope modulated (DCEM) signals, represented by alternative binary offset carrier signal, have obvious advantages in code tracking accuracy and anti-multipath performance compared with traditional binary phase shift keying signal, and are attracting more and more attention in the field of global navigation satellite system signal design. However, DCEM signal has a multiple-peaked auto-correlation function, which would lead to the tracking ambiguity problem. To deal with this problem, this paper proposes an unambiguous tracking method based on weighted discriminator function (WDF). This method uses the discriminator functions of noncoherent early minus late power (NELP) and sub carrier phase cancellation (SCPC) to generate a linear WDF without no false-lock points. In addition, a simplified version of the proposed WDF method is also provided for interoperable application. The theoretical code tracking error in thermal noise is derived out and verified. The performance of the proposed WDF is evaluated and compared with SCPC and pseudo correlation function methods with the help of simulation. Results show that our method has a significant improvement in code tracking accuracy. Not only the ambiguity threat is efficiently removed, but also the outstanding performance of DCEM signal is maintained.

Phase-aligned Dual-frequency Constant Envelope Multiplexing Technique for GNSS Signals

In the new generation of Global Navigation Satellite Systems (GNSS), dual-frequency constant envelope multiplexing is widely desired and is becoming an important subject in signal design. Considerable work has been devoted to multiplexing for the Alternative Binary Offset Carrier (AltBOC)-like signal model, for which each sideband consists of two or fewer signal components. In this paper, a phase-aligned dual-frequency constant envelope multiplexing technique is proposed for a general dual-frequency signal model. This multiplexing technique can be used to combine two constant-envelope-modulated signals in two sidebands into a composite signal with a constant envelope, where the constant-envelope-modulated signal in each sideband consists of an arbitrary number of signal components with an arbitrary power ratio and phase relationship among the signal components. A Lookup Table (LUT)-based signal generation method is also proposed, for which the required driving clock rate of the signal generator can be flexibly adjusted to meet the requirements of the satellite payload. Applications for the AltBOC-like signal model and a general dual-frequency signal model in the Beidou B1 band validate the flexibility and high multiplexing efficiency of our method. Specifically, AltBOC is a special case of the proposed method.

Asynchronous Multilevel Search Strategy for Fast Acquisition of AltBOC Signals

Alternative binary offset carrier (AltBOC) signals can be approximated by four synchronized direct sequence spread spectrum (DSSS) signals, each pair of which is a quadrature phase shift keyed (QPSK) signal at a different frequency. Therefore, depending on the strength of an incoming AltBOC signal, an acquisition technique can reduce the mean acquisition time (MAT) by searching the four DSSS signals asynchronously; the search for each of the four DSSS signals can start at one of the evenly separated hypotheses on the two-dimensional hypothesis space. And detection sensitivity can be improved by multiple levels when different numbers of search results for the same hypothesis are combined. In this paper, we propose a fast AltBOC acquisition technique that has an asynchronous search strategy and efficiently utilizes the output of the four search results to increase the sensitivity level when sensitivity improvement is needed. We provide a complete theoretical analysis and demonstrate with numerous Monte Carlo simulations that the MAT of the proposed technique is much smaller than conventional AltBOC acquisition techniques.

General AltBOC Modulation with Adjustable Power Allocation Ratio for GNSS

Alternative Binary Offset Carrier (AltBOC) modulation signal has the constraint of an equal power allocation scheme. Thus, it is not flexible enough to meet different requirements. To solve this problem, we propose a General AltBOC (GAltBOC) modulation. The proposed technique can achieve the same function as AltBOC. Meanwhile, its power allocation ratio can be adjusted when required. The detailed derivation of the GAltBOC modulation is presented, and three representative cases as well as the signal properties are discussed. To further improve the combination efficiency, we develop the Interlacing GAltBOC (IGAltBOC) modulation based on the GAltBOC modulation. The Power Spectrum Density (PSD) and correlation functions of the proposed signals are verified by simulation. The code tracking error and implementation complexities are analysed and compared with existing methods. Results show the proposed signals indeed enhance the flexibility of power allocation ratio with the same level code tracking accuracy. Compared with AltBOC modulation, the proposed signal can reach a higher combination efficiency at the expense of relatively higher implementation complexity. Compared with Asymmetric Constant Envelope Double-sideband (ACED) modulation, the proposed signals have significantly lower complexity of signal generation, which is helpful in simplifying the signal generation in the payload transmitter.

Precise acquisition of global navigation satellite system signals in the presence of multipath and influence on tracking capability

New Galileo and enhanced GPS signals use binary offset carrier (BOC) waveforms namely composite BOC (CBOC) and alternate BOC (AltBOC). For good tracking, these signals require precise acquisition. The more sidelobes the autocorrelation function has, the tighter the pull-in range region is, and the more precise the acquisition must be. Further, when multipath propagation is encountered, the received waveform is so distorted that a precise acquisition becomes difficult. To solve this problem, two approaches are considered by enhancing either the acquisition stage or the tracking one. At the acquisition stage, the authors focus on Bayesian estimation where Bernoulli–Gaussian and Laplacian (Lp) priors for channel distribution are compared. On one hand, the BG prior leads to rather complex optimisation criterion that can be solved by a Markov Chain Monte Carlo algorithm. This is a very efficient but time consuming method. On the other hand, the Lp prior results in exact maximum a posteriori calculation. In this case, the Bayesian estimation involves an ℓ1-norm penalisation and provides quite good performance for reasonable acquisition complexity. At the tracking stage, many works have dealt with false lock positions, such as the sidelobe cancellation method (SCM) or multiple gate delay (MGD) technique. In this article, the authors highlight the need for accurate acquisition for better detection and estimation at the tracking output. The SCM and MGD methods are taken as examples to illustrate this influence. The analysed signals are GPS BPSK(1), BPSK(10), and Galileo CBOC(6, 1, 1/11) and AltBOC(15, 10) modulated open service signals. The impact of multipath is studied for line of sight (LOS) signal, LOS plus multipath replicas, and multipath alone. The authors compare the results of classical acquisition followed by precise tracking to those of Bayesian acquisition followed by classical tracking. The results of Bayesian acquisition followed by precise tracking are also provided.

Unambiguous Tracking Method for Alternative Binary Offset Carrier Modulated Signals Based on Pseudo Correlation Function Technique

Based on the concept of pseudo correlation function (PCF), we propose an unambiguous tracking method for alternative binary offset carrier (AltBOC) modulated signal, which is employed in global navigation satellite system.The proposed method employs two specifically designed chip spreading symbols for local reference signals to generate a no side-peak function instead of the original multiple-peak auto correlation function (ACF) for tracking. Results demonstrate that the code tracking and multipath mitigation performances of the proposed method are better than the BPSK Like method on the whole.

Correlation functions of complete AltBOC signals with a constant envelope

The structure of the modulating function (MF) of a complete alternative binary offset carrier modulated signal (AltBOC signal) with a constant envelope (eight-component complete AltBOC signal) is considered. Analytical expressions for the correlation functions (CFs) of single elements of the MFs of complete AltBOC signals with a constant envelope are derived at typical values of multiplicity coefficient Nm of meander pulses (Nm = 2 and 3). Plots of the CFs of single elements of the MFs of complete AltBOC signals with a constant envelope are obtained for Nm = 2 and 3. The properties of these CFs are discussed. With the help of the Fourier transform applied to the CFs, the energy spectra (spectral power densities) of single elements of the MFs of complete AltBOC signals with a constant envelope are calculated at Nm = 2 and 3. In illustrating examples, most attention is focused on complete AltBOC signals with a constant envelope at the carrier frequency fcar = 1191.795 MHz for the AltBOC (15, 10) modulation and the repetition rate of the elements of ranging-code pseudorandom sequences fs = 10fref = 10.23 MHz, where the reference (base) frequency is fref = 1.023 MHz. These signals are applied in the Galileo European satellite radio navigation system (SRNS) and, as is supposed, in the Chinese BeiDou-2/Compass SRNS.

The correlation functions of single elements of the modulating functions of double complex meander noise-like navigation signals (alternative binary offset carrier modulated signals)

The correlation characteristics of elements of complex meander pseudo-random sequences (PRSs) are analyzed. These sequences are the basis for forming double complex meander (DCM) noise-like signals (alternative binary offset carrier modulated signals—AltBOC signals) applied in new-generation satellite radio navigation systems (in particular, the European Galileo and Chinese Bei-Dou/Compass systems). Analytical expressions are obtained for the correlation functions (CFs) of single complex meander symbols (MSs) and for the cross CFs (CCFs) of the sine and cosine components of complex MSs. Formulas for the CFs of single elements of the modulating functions (MFs) of an elementary (two-component) AltBOC signal and a four-component AltBOC signal are derived for typical values of multiplicity coefficient Nm (Nm = 2 and 3). Plots of CFs and CCFs are depicted, and the properties of these functions are discussed. The Fourier transform is applied to obtain the energy spectra of single MSs of an elementary AltBOC signal and a complete four-component AltBOC signal. In the examples, the attention is focused on AltBOC signals with the AltBOC (15, 10) modulation and the repetition rate of PRS elements fs = 10 fref = 10.23 MHz, where the reference frequency is fref = 1.023 MHz.

Time-Multiplexed Offset-Carrier QPSK for GNSS

A new method of time-multiplexing quadrature phase shift keying (QPSK) signals modulated by a complex subcarrier is proposed. In its simplest form the proposed time-multiplexed off-set carrier quadrature phase shift keying (TMOC-QPSK) can replace the constant-envelope alternate binary offset carrier (AltBOC) currently used for some Global Navigation Satellite System (GNSS) signals. The signal structure is described focusing on the signal generation methodology, power spectral density (PSD), and correlation function in comparison with the alternate binary offset carrier (AltBOC) modulation first used for the Galileo E5 signal. It is shown that the PSD and correlation functions match exactly to those of an AltBOC modulated signal. A method to realize the correlator for the proposed TMOC-QPSK signal is described, and the complexities compared with AltBOC are presented. A field-programmable gate array (FPGA)-based implementation shows that the core TMOC-QPSK correlator requires 32% less hardware resources and consumes 23% less power compared with an AltBOC correlator.

Unambiguous Tracking Method for Alternative Binary Offset Carrier Modulated Signals Based on Dual Estimate Loop

We propose an unambiguous tracking method for Alternative Binary Offset Carrier (AltBOC) modulated signals based on the Dual Estimate Tracking (DET) loops. The proposed method employs a combined PRN code and a pure cosine signal which can be easily implemented in the hardware to make DET loops available to the complicated AltBOC signal structure. The two dimensional correlation function for AltBOC is derived, and an unambiguous delay estimate is deduced according to the correlation function. Simulation results reveal that the proposed method performed better in tracking performance than the traditional unambiguous tracking methods.

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.

The spectral characteristics of navigation alternative binary offset carrier modulated signals (AltBOC signals)

Analytical expressions are obtained for spectral densities and energy spectra of single elements and single periods of modulating functions of alternative binary offset carrier modulated signals (AltBOC signals) for new-generation satellite radio navigation systems (SRNSs) (in particular, the Galileo system). The spectral characteristics are presented and analyzed for an elementary (two-component) AltBOC signal, a complete AltBOC with a time-variable envelope (a four-component complete AltBOC signal), and a complete AltBOC with a constant envelope (an eight-component complete AltBOC signal) in the case when the multiplicity coefficient of meander pulses has an arbitrary value. Plots are depicted, and the properties of energy spectra of single elements of modulating functions of all groups of AltBOC signals are discussed in the cases of the following modulation types: AltBOC(10, 10), AltBOC(15, 10), AltBOC(20, 10), and AltBOC(25, 10).