Non‐orthogonal AltBOC modulation for BeiDou B2 signals

The global navigation satellite system (GNSS) signal modulation type plays a crucial role in determining the performances of positioning, navigation and timing (PNT) services. Currently, the binary offset carrier (BOC) modulation signal and binary coded symbol (BCS) modulation signal are both bipolar signals, which greatly restrict the room of improving the GNSS signal performance. Therefore, multi-level coded symbol (MCS) modulation has received great attention in the field of GNSS signal design. The MCS modulation is the most extensive step-coded symbol modulation mode, where BOC modulation and BCS modulation are its special cases. Since the waveform symbol of the MCS modulation signal can be arbitrarily valued, the optimal GNSS signal can be designed. However, like the BOC modulation signal, the MCS modulation signal also has the problem of ambiguous tracking, and then results in a large pseudo range measurement error, which is unacceptable for the new generation GNSS with high accuracy. In recent years, the unambiguous tracking of GNSS signals has become a hot research subject in the navigation signal processing domain and many methods are presented, and those methods can be divided into three categories:BPSK-like method, bump jump (BJ) method, and side-peak cancellation (SC) method. However, these methods are designed for BOC signal, and they are not suitable for MCS signal. Therefore, in this paper we propose a general unambiguous tracking algorithm based on the pseudo correlation function (PCF), which is suitable for MCS modulated signals. Firstly, the unitary expression of MCS modulated signal based on waveform value vector is given, then the unitary formula of cross-correlation function for MCS signal is derived and the definition of PCF is given. Then the constraint condition which should be satisfied to realize unambiguous tracking is analyzed in depth, and the universal constructing method of two reference signals and the relationship between each other are derived according to this constraint condition, which brings great convenience for solving the specific MCS signal. The code tracking loop model of GNSS receiver based on the proposed method is illustrated. It is observed that the proposed method can receive different MCS signals under the same receiver loop framework, and can simplify the design of the receiver while eliminating the tracking ambiguous problem. Finally, as a special case of MCS signal, the applications of the proposed method in four kinds of BOC signals are discussed respectively, and then the waveform value vector of the reference signal and the unitary expression of code discriminator are derived. Simulation results show that the proposed method can effectively solve the ambiguous tracking problem of MCS signal, which has good performance and broad application prospect.

The optimized design of the spreading modulation technique is considered to be the major way to achieve both spectrum compatibility and performance improvement. To allow a variety of signals to share the limited frequency band of GNSS and further improve the signals’ ranging accuracy and anti-interference performance, several new spreading modulation techniques have been proposed in recent years. In this chapter, we will introduce a variety of typical spreading modulation techniques such as BPSK with rectangular chips (BPSK-R), binary offset carrier (BOC), binary coded symbol (BCS), composite BCS (CBCS), time-multiplexed BOC (TMBOC), composite BOC (CBOC), quadrature multiplexed BOC (QMBOC), as well as alternative BOC (AltBOC). The processing ambiguity threat of BOC and MBOC signals and possible solutions are also discussed. Considering that in the next-generation GNSS implementations, new requirements and new constraints will constantly emerge and new modulation methods will be needed, this chapter is not meant to be regarded as a review of the development of navigation signal modulation technology, or an interpretation of the signal formats being used by several major satellite navigation systems. Instead, we regard the spreading modulations used in satellite navigation signals as general techniques and focus our discussion on the principles of these modulation techniques and the design ideas behind their generation processes.

Binary offset carrier (BOC) modulation signals have been widely adopted in global navigation satellite systems (GNSSs) such as Galileo and modernized global positioning system [1] [2]. Although this kind of modulation offers excellent performances, a main drawback of BOC modulation is that ambiguous acquisition and biased tracking for its multi-peak autocorrelation function (ACF). This article presents herein a novel acquisition technique allows removal of side peaks thoroughly and acquire better acquisition performance. This scheme is applicable to BOC (n, n) modulation signals. Theoretical analysis and system simulation demonstrate the algorithm proposed has better performance in acquisition aspect compared with other typical algorithms under the influence of additive white Gaussian noise.

Satellite navigation and positioning have become an indispensable component of smart devices and applications. In order to avoid signal interference in L1 frequency and provide novel services, Global Positioning System III (GPS III) updated a modern signal named L1C and tested in three Quasi-Zenith Satellite System (QZSS) satellites launched in 2017. In December 2018, the first GPS III satellite was launched, which implies the joint positioning using QZSS and GPS L1C signal is improved. L1C signal offers a series of advanced designs in signal modulation, message structure and coding, etc. In this regard, this paper presents complete methodologies for L1C signal joint receiving processing. For the on-air signals, this paper presents a methodology and results from collecting and assessing binary offset carrier (BOC) modulation and time-multiplexed BOC (TMBOC) modulation used in L1C signal. Using the same omnidirectional antenna and the test-equipment, the L1C signal was collected in Xi’an and Sanya, China, respectively. Experiments in Xi’an verified the joint positioning method to complement the GPS III and QZSS satellite constellations. The methodology in this paper evaluates the positioning error of BOC(1,1) and TMBOC(6,1,4/33) under the same environment and satellite constellation in Sanya. It is also verified that the joint positioning error is less than the QZSS-only positioning due to the optimization of the satellite constellation.

With the successful application in Galileo system, alternate binary offset carrier (AltBOC) has aroused great interests in academia and engineering. Compared to the traditional GNSS signals, AltBOC has significant advantages in performance. First, it can combine four candidate signals from two frequencies into a constant-envelope signal, which would save transmitters on the satellite. Second, it can be processed in two different processing modes, i.e. the single-sideband processing mode and ultra-wideband processing mode, to satisfy different navigation requirements. With excellent ranging performance and considerable flexibility for different processing modes, other GNSS systems has already planned to apply the dual-frequency constant-envelope modulation (DCEM) in the next generation system or the update of existing systems. However, along with further scientific research, it is found that AltBOC also has some imperfections. First, the generation and processing complexity of AltBOC is higher than traditional GNSS signals. Second, it is not flexible and can only be used in the situation where each candidate signal has the same power. Under this requirement and technique background, several different DCEMs are proposed, inclusive of ACEBOC, TMOC-QPSK, TD-AltBOC and BS-ACEBOC. Each DCEM has its advantages and disadvantages in light of the priorities of the designers. In this paper, a comprehensive comparison of the existing DCEMs is presented. As a new DCEM, BS-ACEBOC is introduced briefly in this paper. Next, all these existing DCEMs will be reviewed and analyzed briefly. Then, the comparisons of these DCEMs are presented, from the ranging performance, complexity and multiplexing efficiency. At last, the comparison result will be concluded and comprehensive conclusions will be addressed.

Binary Offset Carrier (BOC) modulation technology is gradually replacing the traditional BPSK modulation signal as the main modulation method of the new generation satellite navigation signal due to its superiority in multipath suppression performance and code tracking accuracy. However, the tracking and mis-locking problem of BOC signals has become a key point limiting the widespread application of BOC signals. In this paper, we use code reference waveform (CCRW) technology and optimized Bump-jump (BJ) algorithm to analyze the multipath performance of low-order BOC signals under five code reference waveforms. Aiming at the high peak BOC signal with many peaks and high peak-to-peak ratio, we propose a new code reference waveform ensures the high-order BOC signal has good anti-multipath performance and solves the problem of extremely easy mis-locking of its receiver.

Navigation receivers of global positioning systems that are operated in complex electromagnetic environment must have an increased noise immunity characteristic. One way of such noise immunity rising is to use incoherent operating mode of navigation receiver. There are known processing algorithms for navigation signals with Binary Phase Shift Keying (BPSK) modulation. But in the near-term outlook Global Navigation Satellite Systems (GLONASS), Global Positioning Systems (GPS) and Galileo known navigation signals with Binary Offset Carrier (BOC) modulation will be radiated. Processing algorithms for such signals are not described in scientific literature. So it is interesting to work out processing algorithms for BOC modulated signals in incoherent operating mode of navigation receiver. The article is devoted to synthesis and analysis of incoherent subcarrier tracking system under receiving BOC (1,1) navigation signals. To synthesize an optimal tracking system, optimal estimation theory and dual estimation method are used that allow taking into account multi-peak a posteriori distribution of delay that caused by the periodic process (digital sinusoid) in the radio signal modulation function. When analyzing the synthesized estimation algorithm, the procedure for describing delay discriminators and dual variable discriminators were used in the form of a statistical equivalent, including discrimination and fluctuation characteristics. New equations have been obtained that describe optimal tracking system for incoherent mode of a navigation receiver. New expressions for delay and dual variable discriminators are derived. The article gives a scheme for these discriminators and formulas for their statistical characteristics. Simulated results are provided that illustrate accuracy characteristics enhancement. The obtained tracking algorithms may be used in the navigation consumer equipment of GLONASS, GPS, Galileo and other radio-navigation satellite systems operating with BOC(1.1) modulated signals.

Binary Offset Carrier (BOC) modulation signals have been applied in Global Navigation Satellite Systems (GNSS) because they offer a higher positioning accuracy and higher multipath rejection. However, there is a drawback in that the autocorrelation functions have multiple side peaks, meaning that this technique also leads to the large main peak estimation error problem and a low correlation decision efficiency problem. In this paper, we propose a new Main Peak Extraction (MPE) method for high-order BOC signals to solve these problems. In the new method, the synthesis cross-correlation function is established, and the geometry graph is formatted to calculate the estimation main peak. We eliminate all side peaks and improve the main peak phase estimation precision under the condition that the sub-carrier phase is offset. The results of the simulation demonstrate that the new method can achieve better main peak decision efficiency, side peak cancellation ability and phase estimation performance than traditional methods.

The recent researches on the tracking of binary offset carrier (BOC) modulated signals have been studied focusing onresolving the ambiguity problem caused by the multiple side-peaks in BOC autocorrelation. In this paper, we proposea novel unambiguous BOC tracking scheme with an improved tracking performance by using partial-pulses of BOCsignals. Firstly, we observe that a sub-carrier consists of two partial rectangular pulses referred to as partial-pulses,and then, generate multiple partial-correlations composing the BOC autocorrelation. Finally, a correlation function withno side-peak is constructed by combining the partial-correlations, and then, a delay lock loop employing the proposedunambiguous correlation function adjusts the phase of the local BOC signals. From numerical results, it is confirmedthat the proposed scheme provides a better tracking performance than the conventional schemes in terms of thetracking error standard deviation.

Navigation signals are the basis of satellite navigation systems, which determine the navigation systems' performance (such as tracking accuracy, anti-interference capability etc.), and are determined by some constraints (such as radio frequency resources, construction cost, technology level etc.). The aim of signal design is to select "proper" signals considering these above factors and make tradeoffs between performance and constraints. This paper will summarize the signal structure evolution of present satellite navigation and conclude some principles and issues which will be greatly valuable to our 2nd satellite navigation system. There's four main satellite positioning systems presently, the U.S. Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), the Europe Galileo satellite navigation system and the Chinese BD-1 satellite positioning system. Among these systems, the signal characteristics of GPS and Galileo are good reference to our 2nd satellite navigation system. The improvement of GPS signals is a main aspect of GPS modernization. The aim of GPS modernization is mainly to modernize its signal structure so as to improve the whole systems' performance. The signal structure of Galileo has been revised for several times, and its present signal is the results of long time signal design. From GPS modernization and Galileo signals status, we can summarize some key specifications of navigation signals and some technologies to support these specifications. Based on this, we'll discuss some problems which should be attentive in our country's 2nd satellite navigation system and give some suggestions. This paper is organized into five sections. The first section will give a brief introduction of present satellite navigation system and their main features. Section two will review the GPS and Galileo signals structure evolution, including GPS modernization, Galileo signals status and the comparison of two systems' signals. In the third section, the relationship between signal parameters and system performance will be analyzed, including RF signal characteristics, modulation type, ranging code (such as code type, length and rate) and navigation data. In section four we will discuss some inspiration and technologies concerning our 2nd satellite navigation system, such as the use of multiple frequency signals (ionospheric delay estimation and carrier ambiguity resolution), pilot tones & dataless channel, binary offset carrier (BOC) modulation, and so on. The last section is conclusion, in which the whole paper will be retrospected and some useful conclusion will be given.

The binary offset carrier (BOC) modulation is used in many GNSS signals, not only for its facility for spectrum sharing but also for improvement of ranging performance due to a larger Gabor bandwidth. However, BOC signal acquisition and tracking encounters the ambiguity of multiple peaks and many methods have been proposed to address this problem. A simple method to avoid such an ambiguity is to extract the split spectra, treat them separately as BPSK signal each, and then combine the results. To recover the reduced sideband signal strength, the individual sideband correlations are combined non-coherently or coherently. Coherent combining is more gainful than the non-coherent counterpart particularly when the incoming signal is weak or when a jamming signal is present. Direct long coherent integration (LCI) requires a fine frequency search grid, which would be prohibitive if the frequency search were implemented by brute force. Alternatively, a two-stage approach seems efficient and practical. It starts with a first stage of short correlation for coarse search, followed by a second stage of coherent integration with refined search. In addition to dealing with unknown navigation data bits that change sign during the long integration interval, phase-alignment is required to compensate for code migration and frequency divergence, which are two detrimental effects unique to split-spectrum signals integrated over a long period of time. In this paper, a two-stage method for standalone direct acquisition of weak multi-band split-spectrum signals is described. By standalone, we mean absence of any external aiding, say, from a network or an inertial navigation system (INS). By direct, we mean the acquisition procedure without going through legacy codes. The proposed method makes judicious use of coherent and non-coherent integrations with time and frequency adjustments so as to maximize the combining of data-modulated and dataless (pilot) chips, upper and lower sideband signals, and signals on different frequencies. Issues and enabling techniques are described. Extensive computer simulation results are presented to illustrate the functionality and performance of the method.

The long spreading code and the binary offset carrier (BOC) modulation technique are used in the next-generation Global Navigation Satellite System (GNSS) to improve positioning performance and to reduce inter-GNSS interference; however, the signal acquisition process of a GNSS receiver can take more time and requires additional hardware resources than legacy Global Positioning System (GPS) receivers. This paper presents the BOC 2-D compressed correlator (TDCC) technique for the fast acquisition of the next-generation GNSS signals. In the proposed BOC-TDCC, signal power in neighboring code-phase hypotheses and Doppler frequency hypotheses can be coherently combined and tested by a single compressed correlator in the first stage, and the conventional correlator-based serial search technique is employed in the second stage to test each hypothesis combined in the first stage. The performance of the BOC-TDCC is demonstrated with numerous Monte Carlo simulations and tested with real data. The BOC-TDCC has much lower mean acquisition time (MAT) than other conventional search schemes, which demonstrates that the BOC-TDCC is an effective search scheme for next-generation GNSS signals.

A multitude of GNSS signals are transmitted at the same frequency band, and even the same signal may have several components with different codes. For examples, different signals are modulated onto the in-phase and quadrature components (e.g., L1C-I and L1C-Q, I5 and Q5) and are time-division multiplexed (e.g., odd and even chips of M-code). Most of these signals adopt the binary offset carrier (BOC) modulation and thus have a split-spectrum as compared to the legacy signals that use binary phase shift keying (BPSK) and have a dominant lobe at the band center. BOC is used not only for the ease of spectrum sharing but also for its improved ranging performance due to a larger Gabor bandwidth. Both weak signals and in particular the presence of strong interference lead to an effective signal to noise ratio (SNR) that is very low. To pull such a signal out of noise and interference, it is necessary to combine multiple signal components available at the same time and then integrate them over time so as to add up the signal strength while averaging the noise and interference out in a burn-through process. However, it is not straightforward to apply the linear additive coherent operation due to the difference in phase and frequency between signal components as well as the presence of unknown data bits, which may become counterproductive if not treated properly. In this paper, we investigate several nonlinear multiplicative schemes including non-coherent, semi-coherent, and cross-conjugate-multiply for signal accumulation (combining and integration), and compare them to the coherent scheme. The study shows that according to the SNR definition at the output of nonlinear operations, some of the schemes may have the same SNR level per analysis of second-order statistics, yet they exhibit different Pd vs. Pfa characteristics, which can be attributed to different distribution tail behaviors (higher order statistics). Understanding of the detection performance is thus important in trading off combining and integration schemes for implementation. Extensive computer simulation results are presented to illustrate the functionality and performance of the schemes.

Frequency-hopping binary offset carrier modulation improves the anti-interference performance and mitigates the autocorrelation function (ACF) ambiguity problem of binary offset carrier modulation. To save payload resources and make high-power amplifiers on satellites operate at the nonlinear saturation region, there is further demand for finding an efficient constant-envelope frequency-hopping binary offset carrier multiplexing technique to combine several signal components. Thus, we propose a dual-sideband constant-envelope multiplexing modulation, named asymmetric constant-envelope frequency-hopping binary offset carrier multiplexing (ACE-FHBOC), which is also a multicarrier constant-envelope multiplexing modulation. ACE-FHBOC provides higher design flexibility in the number of subcarrier frequencies than ACE-BOC while maintaining the same flexibility of signal design as ACE-BOC in the number of signal components and power ratio among components. We first establish the theory and give implementation methods of ACE-FHBOC. Then, we develop a software-defined receiver to simulate and analyze the performance for several specific ACE-FHBOC and ACE-BOC signals. The results show that the recommended ACE-FHBOC signals have lower ACF ambiguity, better anti-narrowband interference, and multipath performance than ACE-BOC under the same conditions. With these advantages, ACE-FHBOC is a promising solution for the signal design of new generation global navigation satellite systems.

The long PN code in a Binary Offset Carrier (BOC) modulated signal has a higher rate than the GPS C/A code, which makes the acquisition process in the new global navigation satellite system (GNSS) much more complex.To achieve a fast acquisition of the GNSS signal with a reduced number of correlators and low computational complexity, a two-stage deterministic compressed (DS) GNSS acquisition technique has been proposed.Nevertheless, the ambiguous threat of BOC signal isn't taken into account in the DS technique.To eliminate the ambiguity threat in the acquisition process of BOC (m, n) signal, an unambiguous deterministic compressed acquisition is proposed in this paper.The main idea of the proposed technique is to introduce a simplified GRASS algorithm in the second stage measurement.Theoretical analysis and simulation results show that, at the expense of some performance degradation the proposed technique can completely remove the undesired positive side peaks of BOC (m, n) signal, and reduce the chance of false peak acquisition compared with the DS technique.