Code Division Multiple Access Signals Research Articles

Characterization of the GLONASS-K1+ atomic frequency standard

The fifth GLONASS-K1 satellite with space vehicle number R807 was launched in October 2022. It represents the first spacecraft of the K1+ generation, which offers various technical innovations. Compared to previous K1 satellites, R807 also transmits a code-division multiple access (CDMA) signal in the L2 frequency band in addition to L1 and L2 frequency-division multiple access (FDMA) and L3 CDMA signals. Thus, R807 is the first spacecraft of the K1+ generation. A geometry- and ionosphere-free triple carrier combination is used to analyze the GLONASS R807 clock consistency at different frequencies. Significant inconsistencies were found showing up as variations with a peak-to-peak amplitude of up to 40 cm and periods between 15 min and a few days. Whereas the ultimate explanation for these variations is not known, it is likely that they originate from cross-talk of two oscillators with similar frequency. A short-term clock analysis for integration times up to 100 s based on the one-way carrier phase (OWCP) method shows a superior stability of the R807 clock compared to all other GLONASS satellites including the new K2 generation. The Allan deviation computed from 5 s clock estimates confirms this finding for integration times up to 600 s but shows a significant bump at longer integration times due to the periodic variations mentioned above. Single-frequency OWCP processing confirms consistency of the L1 and L2 FDMA signals whereas the L3 CDMA signal shows a slight phase shift. Although the spurious variations mask the true performance of the K1+ atomic frequency standard, its behavior at short integration times points at a new type of GLONASS satellite clock.

Antenna and attitude modeling of modernized GLONASS satellites

Next to the legacy frequency division multiple access (FDMA) signals in the L1 and L2 band, modernized GLONASS satellites of the M+, K1, and K2 generation also transmit new code division multiple access (CDMA) signals on L3, and, in part, the L1 and L2 frequencies. Depending on the specific satellite platform, either a common antenna or two distinct antennas are used for the individual signals. As a novel feature, the new CDMA navigation messages provide information on the antenna phase center locations as well as the spacecraft orientation during rate-limited noon and midnight turns. We establish a comprehensive set of phase center positions for the individual GLONASS antennas from manufacturer data and information in the CDMA navigation messages, inferred from the comparison of FDMA and CDMA broadcast ephemerides, and obtained from antenna baseline estimates using a triple-frequency carrier phase combination. Based on these, reference values for the signal-specific PCOs of each block of modernized GLONASS spacecraft are derived that may be used to extend the current antenna model of the International GNSS Service (IGS) to all frequency bands. Complementary to the analysis of antenna phase center information, the new CDMA attitude model for GLONASS-K satellites is reviewed. Algorithms for computing relevant parameters of the rate-limited yaw slews including the ramp-up and -down phase based on the Sun/orbit geometry are derived that enable a concise attitude modeling in GLONASS orbit determination and precise point positioning. The model is validated through comparison with yaw angle estimates derived from triple-frequency observations showing consistency at the 1° level for the K2 satellite. On the other hand, unexpected accelerations during noon turns of GLONASS-M+ are identified, which may cause short-term yaw angle deviations of up to 20° relative to attitude models in common use within the IGS.

GLONASS-K2 signal analysis

K2 is a new generation of GLONASS satellites that provides code division multiple access (CDMA) signals in the L1, L2 and L3 frequency bands in addition to legacy L1 and L2 signals based on frequency division multiple access (FDMA) modulation. The first GLONASS-K2 satellite was launched in August 2023 and started signal transmission in early September 2023. Based on measurements with a 30-m high-gain antenna, spectral characteristics of the various signal components are described and relative power levels are identified. A 3 dB (L1) to 4 dB (L2) higher total power is determined for the CDMA signal compared to the legacy FDMA signal and an equal power of the open service and secured CDMA signal components is found. The ranging code of the L2 channel for service information, which has not been publicly disclosed so far, is identified as a Gold code sequence consistent with the data channel of the L1 open service CDMA signal. The high-gain antenna measurements are complemented by tracking data from terrestrial receivers that enable a first assessment of user performance. An up to 50% improvement in terms of noise and multipath performance is demonstrated for the new L1 and L2 CDMA signals in comparison with their legacy counterpart, but no obvious differences between the different binary phase-shift keying and binary offset carrier modulations of the data and pilot components of these signals could be identified for the test stations. Triple-frequency carrier phase observations from L1, L2, and L3 CDMA signals exhibit good consistency at the noise and multipath level, except for small variations that can be attributed to slightly different antenna phase patterns on the individual frequencies. Overall, the new CDMA signals are expected to notably improve and facilitate precise point positioning applications once fully deployed across the GLONASS constellation.

Cognitive Detection of Unknown Beacons of Terrestrial Signals of Opportunity for Localization

A cognitive approach is proposed to detect unknown beacons of terrestrial signals of opportunity (SOPs). Two scenarios are considered in the paper: (i) detection of unknown beacons with integer constraints (IC) and (ii) detection of unknown beacons with no integer constraint (NIC). An example of beacons with IC is the pseudo-noise (PN) sequences in cellular code division multiple access (CDMA) signals. On the other hand, the reference signals (RSs) in orthogonal frequency-division multiplexing (OFDM)-based systems can be considered as beacons signals with NIC. Matched subspace detectors are proposed for both scenarios, and it is shown experimentally that the proposed matched subspace detectors are capable of detecting cellular third-generation (3G) cdma2000 signals and fifth-generation (5G) OFDM signals. A low complexity method is derived to simplify the matched subspace detector with IC for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary phase shift keying ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> PSK) modulation. The effect of symbol errors in the estimated beacon signal on the carrier to noise ratio (CNR) is characterized analytically. Closed-form expressions for the asymptotic probability of detection and false alarm are derived. Experimental results are presented showing an application of the proposed cognitive approach by enabling an unmanned aerial vehicle (UAV) to detect and exploit terrestrial cellular signals for navigation purposes. In one experiment, the UAV achieved submeter-level accurate navigation over a trajectory of 1.72 km, by exploiting signals from four 3G cdma2000 transmitters. In another experiment, the UAV achieves a position root mean-squared error (RMSE) of 4.63 m over a trajectory of 416 m, by exploiting signals from two 5G transmitters.

Mitigating Satellite-Induced Code Pseudorange Variations at GLONASS G3 Frequency Using Periodical Model

With the modernization of GLONASS, four M+ and two K satellites are able to broadcast code-division multiple-access signals at a G3 frequency. The evaluation of the G3 frequency is necessary, among which the satellite-induced code pseudorange variation is one of the most important indicators. Using the code-minus-carrier (CMC) combination, it was found that the magnitude of the code pseudorange variations at the G3 frequency is about 1 m, which is primarily caused by the fact that G3 is transmitted from a different antenna, the same as G1 and G2. However, different from BDS-2 medium Earth orbit and inclined geo-synchronous orbit satellites, the code pseudorange variations at the GLONASS G3 frequency have a very weak relationship with the elevation angle, while a strong correlation exists with the time series, by using wavelet transformation and correlation analysis. Validation is carried out using a single-site model and a continuous multi-site model over 24 h, and the correction performance of these two models is comparable. The systematic deviation of the CMC and Melbourne–Wübbena combinations are significantly corrected, so only random errors remain. With a more concentrated distribution of the pseudorange residuals of single point positioning, the standard deviation of the pseudorange residuals is reduced.

Radiation Mode Simulation for Simultaneous Operation of Spread Spectrum and Phase Modulated Telemetry Downlinks of Co-located GEO Spacecrafts

Transmission of telemetry data to the ground station is critical for all spacecraft missions. The allocated spectrum for Telemetry downlinks of GEO spacecraft has been overcrowded in recent years due to a steady increase in the number of GEO missions. Implementation of Code Division Multiple Access (CDMA) systems in the same bandwidth is proposed as a solution to overcome this problem. To prove the compatibility of the proposed scheme with existing telemetry downlinks in the same bandwidth, a real-time data acquisition-based experiment has been carried out with hardware in radiated mode to simulate in-orbit conditions in an anechoic chamber. This paper explains the estimation of interference, test methodology &amp; results and recommends the optimal configuration of the ground receiver for demodulation of the CDMA signal in the presence of interference from multiple narrow-band phase modulated signals. The proposed configuration has been proved successful in a recent GEO spacecraft mission.

Refining the search space of Alltop monomials

Alltop functions have applications to code-division multiple access (CDMA) systems and mutually unbiased bases (MUBs). Alltop functions construct MUBs and CDMA signal sets, and this motivates the continued search for further Alltop functions. The discovery of further Alltop functions is hindered by the computational complexity of verification. This paper narrows the search space through the introduction of a characteristic cubic polynomial and provides a verification process for Alltop monomials with a reduced computational complexity. Computational results lead to the conjecture that there are no further Alltop monomials.

Millimeter-Precision Ultrasonic DSSS Positioning Technique With Geometric Triangle Constraint

In this paper, we present a 3-D ultrasonic measurement technique that uses direct sequence spread spectrum (DSSS) with simultaneous multiple code-division multiple access (CDMA) signals. This technique can be applied in precise indoor locating systems. By placing three microphones close to one another separated by distances on the order of the ultrasound wavelength and imposing the constraint that one transducer and two from the three microphones form three triangles in the space (i.e., the triangle constraint) for 3-D ultrasonic DSSS trilateration measurement, millimeter 3-D positioning precision is achieved. The experimental results for a positioning system comprising a set of three microphones and four ultrasound transducers, each of them transmitting its respective 256-bit DSSS code, demonstrate that the proposed system can measure the spatial 3-D location of the receiver with a standard deviation of less than 1.21 mm. The proposed technique is suitable for potential future indoor positioning system (IPS) applications that require sub-centimeter 3-D positioning.

PPP\u2013RTK functional models formulated with undifferenced and uncombined GNSS observations

Technique PPP–RTK combines the advantages of both the Precise Point Positioning (PPP) and the Real-Time Kinematic (RTK) positioning. With the emergence of multi-frequency Global Navigation Satellite System (GNSS) observations, it is preferable to formulate PPP–RTK functional models based on original (undifferenced and uncombined) observations. While there exist many variants of the undifferenced and uncombined PPP–RTK models, a unified theoretical framework needs developing to link these variants. In this contribution, we formulate a class of undifferenced and uncombined PPP–RTK functional models in a systematic way and cast them in a unified framework. This framework classifies the models into a code-plus-phase category and a phase-only category. Each category covers a variety of measurement scenarios on the network side, ranging from small-, medium- to large-scale networks. For each scenario, special care has been taken of the distinct ionospheric constraints and the difference between Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA) signals. The key to systematically formulating these models lies in how to deal with the rank deficiency problems encountered. We opt for the Singularity-basis (S-basis) theory, giving rise to the full-rank observation equations in which the estimable parameters turn out to be the functions of original parameters and those selected as the S-basis. In the sequel, it becomes straightforward to derive for each scenario the user model as it, more or less, amounts to the single-receiver network model. Benefiting from the presented theoretical framework, the relationships and differences between various undifferenced and uncombined PPP–RTK models become clear, which can lead to the better use of these models in a specific situation.

An Optimized Vector Tracking Architecture for Pseudo-Random Pulsing CDMA Signals.

The vector tracking loop (VTL) has high tracking accuracy and a superior ability to track weak signals in GNSS. However, traditional VTL architecture is established on continuous Code Division Multiple Access (CDMA) signal and is incompatible with pseudolite positioning systems (PLPS) because PLPS generally adopts a pseudo-random pulsing CDMA signal structure to mitigate the near-far effect. Therefore, this paper proposes an optimized VTL architecture for pseudo-random pulsing CDMA signals. To avoid estimation biases in PLPS, the proposed VTL adopts irregular update periods (IUP) pre-filters which adjust the update cycles according to the active timeslot intervals. Meanwhile, as the active timeslots of different pseudolites do not overlap, the sampling time of the navigation filter inputs is inconsistent and time-varying, causing jitter degradation. Thus, the proposed VTL predicts the measurements so that they can be sampled at the same time, which improves tracking accuracy. Simulation is carried out to evaluate the performance of the proposed VTL. The results suggest that the proposed VTL outperforms the traditional pre-filter-based VTL and IUP pre-filter-based VTL.

A Novel Modified Artificial Bee Colony for DOA Estimation

Aims: We consider the Direction of Arrival (DOA) estimation for code division multiple access (CDMA) signals. Background: Solving this problem requires non-linear optimization and thus the speed of convergence becomes crucial. Objective: A novel Modified Artificial Bee Colony (MABC) has been proposed. We use secondorder Taylor series expansion of the cost function to ameliorate the searchability of artificial bee colony (ABC) for finding the globally optimal solution. Methods: The main idea is to harness the exploration and exploitation features. The optimum point of second-order Taylor expansion of cost function is used as a velocity factor of the ABC algorithm. Results: The proposed technique is used for solving the DOA estimation problem of a CDMA system. Simulation results confirm the performance improvement of our proposed algorithm. Conclusion: The cost function of the DOA estimation usually leads to a non-linear optimization problem. Using evolutionary algorithms can improve convergence rate of such problems.

An Application of Spectral-Amplitude-Coding Labels in Optical Signal Buffering Over Optical Packet-Switching Networks

Optical buffering is one of the essential functions in optical packet switching (OPS) network. This letter employs optical code-division multiple-access (OCDMA) signals of spectral-amplitude-coding (SAC) in a packet-buffering architecture. In the proposed scheme, the original OCDMA label of the incoming packet is identified and converted to a new label by a code-switching system. As multiple packets overlap in the time domain in a single device, SAC is adopted for packet encoding to avoid multiple access interference (MAI). The concepts of queuing system and Gaussian approximation are employed to model the buffer architecture. Simulations were conducted to demonstrate the tradeoff between packet dropping probability and the signal variations in the label identification process. The buffering performance in terms of bit-error rate (BER) is examined. The results show SAC labels perform well on packet buffering.

Power-Amplifier Stabilization Through Out-of-Band Feedback

This letter presents a power-amplifier (PA)-stabilization technique based on the low-frequency out-of-band feedback. The proposed structure replaces a conventional stability network to ensure unconditional stability at the frequencies up to the radio-frequency (RF) operating band, with the in- and above-band frequencies stabilized through the matching network design. The approach offers an alternative to inserting the lossy components in the RF signal path, which can significantly degrade the realizable in-band gain of the amplifier. The 900-MHz proof-of-concept PA based on a 10-W gallium-nitride (GaN) device is characterized under the continuous-wave (CW) and Wideband Code Division Multiple Access signal (W-CDMA) excitations and realizes 25.6-dB small-signal gain and 73% power added efficiency (PAE) at P3dB.

Pseudo-Noise Code Shifting Signal for AI Arranged UAV Networking

Communication problems standout as the key issue needing to be addressed in Unmanned Aerial Vehicle (UAV) applications. Nowadays, the application of Artificial Intelligence (AI) technology provides a more flexible and effective organizing strategy for the UAV formation, but it also brings the challenges of transmission speed and access speed for the UAV networking technology.This highlights the importance of multi-access technology in UAV communication. Code Division Multiple Access (CDMA) is a suitable multi-access technology for UAV networking, but low spectrum utilization limits its application. In this study, we use a Pseudo-Noise (PN) code shifting modulation method to improve the spectrum utilization of CDMA signals, which makes it more suitable for UAV communication applications. Theoretical analysis and simulations show that the PN code shifting modulation strategy can effectively improve the information transmission rate of the system without affecting the processing gain and its multi-access capability.

Load‐modulation technique without using quarter‐wavelength transmission line

A proposed method for achieving active load-modulation technique without using a quarter-wavelength transmission line is discussed and evaluated. The theoretical analysis shows that the active load-modulation can be achieved without using a quarter-wavelength line, where the main amplifier sees a low impedance when the input signal level is low, and this impedance increases in proportion to the amount of current contributed from the peaking amplifier. The peaking amplifier sees an impedance decreasing from infinity to the normalized impedance. To validate the method, a circuit was designed, simulated and fabricated using two symmetrical gallium nitride (GaN) transistors (6 W) to achieve a peak power of 12 W and 6 dB output back-off efficiency. The design operates with 400 MHz bandwidth at 3.6 GHz and showed an average efficiency of 50% at 6 dB back-off and an efficiency of 75% at peak power. The designed circuit was tested with CW and modulated signals, the amplifier showed an Adjacent Channel Power Ratio (ACPR) of 31–35.5 dB when tested with a wideband code division multiple access signal of 6 dB peak-average-power ratio (PAPR) at 35.5 dBm average power. Additional 20 dB of linearity improvement was achieved after adding a lineariser.

Positioning bias of different frequency observations in double-differenced relative positioning

In this study, the double-differenced positioning bias of different frequency observations (PBDFO) in a code division multiple access (CDMA) signal system is analyzed based on real-time kinematic (RTK) positioning using raw observations at different frequencies. To test and verify the existence of the DD PBDFO, a set of ultra-short baseline (25 m) GNSS observation data were collected. The stations NT03 and NT04 were located in Xi’an City, Shaanxi Province. The observation time ranged from UTC 11:00 to 17:00 for 10 consecutive days from DOY 012 to DOY 021 in 2017. The results showed that there are mm–cm differences in the RTK solution results for the raw observations at different frequencies, the mean bias and the standard deviation (STD) of the positioning results of GPS L1 and L2 in the East, North, and Up components are (−0.595 mm, 1.35 mm, −0.133 mm) and (3.11 mm, 3.32 mm, 6.44 mm), respectively, but for BDS B1 and B2, the corresponding values are (2.16 mm, −4.24 mm, 9.48 mm) and (3.35 mm, 4.85 mm, 7.41 mm), respectively. The PBDFO between GPS L1 and L2 frequencies is lower in magnitude and more stable than the PBDFO between BDS B1 and B2 frequencies, the mean and the STD of the PBDFO between GPS L1 and L2 obtained from the parameter estimation method in the East, North, and Up components are (−0.606 mm, 1.03 mm, −0.217 mm) and (0.162 mm, 0.169 mm, 0.276 mm), respectively; but for BDS B1 and B2, the corresponding values are (3.00 mm, −3.83 mm, 8.59 mm) and (0.375 mm, 1.14 mm, 2.47 mm), respectively. Both PBDFO correction model and parameter estimation approach can achieve consistency in terms of the RTK positioning results at different frequencies. The double-differenced (DD) PBDFO obtained from the direct difference method and parameter estimation approach are consistent and relatively stable for the same period of continuous days.

A 20.5 dBm Outphasing Class-F PA with Chireix Architecture at 3.5 GHz for RF Transmitter Front-end

An outphasing class-F power amplifier (PA) with Chireix architecture is presented in Global Foundry (GF) 0.13µm Complementary Metal Oxide Semiconductor (CMOS) technology. The proposed circuit is composed of two branch class-F PA and Chireix power combiner with a floating load. The designed outphasing amplifier can provide 20.5 dBm output power from 1.8 V power supply at 3.5 GHz with 52.9 % power added efficiency (PAE). Wideband Code Division Multiple Access (W-CDMA) signal is implemented as input signal to simulate ACPR and the adjacent channel leakage ratios (ACLRs) at ±5 MHz are -20.5 dBc/-20.7dBc. After DPD (Digital Pre-Distortion) ACLRs can achieve -44.5 dBc/-44.7dBc at ±5 MHz. Furthermore, the chip area including testing pads is 1.98*1.62 mm2.

Multiple-Access Interference and Multipath Influence Mitigation for Multicarrier Code-Division Multiple-Access Signals

Sidelobe raising, caused by multipath and multiple-access interference (MAI), hinders the signal detection and time-delay estimation of multicarrier code-division multiple-access (MC-CDMA) signals, which are crucial for localization and tracking applications as well as the sampling-instant configuration of demodulators. Therefore, we propose a method based on reiterative minimum mean-square error (RMMSE) filters to mitigate the MAI and multipath influence. The RMMSE filter is modified to adapt to the multicarrier modulation and symbol involvement characteristics of MC-CDMA signals. The superiority of the proposed method in terms of sidelobes suppression, compared with existing matched filter, least square estimator, least mean square, and single-carrier RMMSE methods, is verified through simulations in the presence of MAIs and short-time-delay multipath. The results show the advantages of the proposed method in terms of high temporal resolution and robustness to MAI influence. This study could contribute toward multipath detection and aid multipath mitigation techniques in strong MAI scenarios.

Spatial assessment of array antenna based joint SDMA-TDMA architecture for DS-CDMA signals

Fifth Generation (5G) technology is looking forward to coagulation of Multiple Access (MA) techniques for integrating individual benefits. Equidistant Direct-Sequence Code Division Multiple Access (DS-CDMA) signals with no relative separation by Propagation Delay suffer from severity of Multiple Access Interference (MAI). Joint implementation of Space Division Multiple Access (SDMA) and Time Division Multiple Access (TDMA) is proposed here in overcoming the limitations for equidistant sources. Spatial assessment toward effectiveness of DS-CDMA signals with appropriate beamforming in SDMA-TDMA architecture is explored here. Quadriphase Shift Keying (QPSK) modulated equidistant DS-CDMA sources having non-overlapping time deployment are transmitted through Additive White Gaussian Noise (AWGN) channel in occurrence of Near-Far Effect. Direction of Arrival (DOA) estimate and spatial behaviour of DS-CDMA sources are exemplified utilizing MUSIC (Multiple Signal Classification), Root-MUSIC and gold-MUSIC algorithms for Uniform Linear Array (ULA) antenna.

Feasibility analysis of non‐orthogonal waveforms for multiple input multiple output passive radar application

New non-orthogonal multiple access technology is proposed with the development of 5G, which is expected to support higher spectrum efficiency and massive terminal access, it enables the communication system to achieve better bit error rate performance and alleviate the spectrum congestion problem effectively under limited resources. With the design of spectrum sharing for radar and communication getting more attention, 5G non-orthogonal signal has become potential passive radar illumination source. In this article, an orthogonal multiple input multiple output (MIMO) radar waveform design metric by minimising the integration sidelobe level, is introduced firstly. Then based on sparse code multiple access (SCMA) technology, the non-orthogonal waveform is designed as the passive radar illumination sources. The distance and Doppler resolution of non-orthogonal waveform are evaluated by a more generalised MIMO ambiguity function. Simulation results demonstrate that non-orthogonal SCMA signals have competitive performance with orthogonal code division multiple access signals.