L-band Digital Aeronautical Communication System Research Articles

Advanced Interference Mitigation Method Based on Joint Direction of Arrival Estimation and Adaptive Beamforming for L-Band Digital Aeronautical Communication System

The L-band digital aeronautical communication system (LDACS) is one of the candidate technologies for future broadband digital aeronautical communications, utilizing the unused L-band spectrum between distance measuring equipment (DME) channels. However, the higher signal power of DME complicates LDACS implementation. This paper proposes an advanced DME mitigation approach for the LDACS, integrating joint direction of arrival (DOA) estimation with adaptive beamforming techniques. The proposed method begins by exploiting the cyclostationary characteristics of signals, accurately obtaining the preliminary direction of the LDACS signal using the Cyclic-MUSIC method. Subsequent precise steering vectors (SVs) are selected through Capon spectrum search, followed by the reconstruction of the interference plus noise covariance matrix (INCM). Using the obtained SV and INCM, the weight vector is calculated and beamforming is performed. Simulation results validate that the proposed method not only accurately estimates the direction of LDACS signal but also efficiently mitigates DME interference, demonstrating a superior performance and reduced algorithmic complexity, even in scenarios with lower signal-to-noise ratios (SNRs) and the presence of DOA estimation errors. Additionally, the proposed method achieves a low bit error rate (BER), further validating its ability to ensure communication quality and enhance the reliability of LDACS.

Survey of IP-based air-to-ground data link communication technologies

The main purpose of an air traffic management system is to provide air traffic services to an aircraft moving within the controlled airspace. Very high frequency (VHF) radio in continental regions, as well as high frequency (HF) radio and satellite communications in remote areas, are used today as the primary way of delivering air traffic services. The technical limitations and constraints associated with the current technology, such as line-of-sight requirement, vulnerability to interference, and limited coverage, cause degraded voice quality and discontinuity in service. At the same time, voice-based communication may affect flight safety due to poor language skills, call sign confusion, and failure to use standard phraseology. For this reason, text-based communication over a VHF data link (VDL) has been proposed as an alternative. However, it is predicted that VDL will be insufficient to support increasing air traffic and intensive data exchanges due to its lack of mobility support and limited resources to ensure service continuity. This paper surveys next-generation data link technologies based on the current state of the "industry standard" for aeronautical communication. These include Aeronautical Mobile Airport Communication System (AeroMACS), L-band Digital Aeronautical Communications System (LDACS), and Airborne New Advanced Satellite Techniques & Technologies in a System Integrated Approach (ANASTASIA). The paper also surveys IP-based text communication solutions over these next-generation data links. We analyze the efficiency of the proposed solutions with regard to service continuity and aeronautical application requirements. We conclude the survey by identifying open problems and future trends.

Physical Layer Security for L-Band Digital Aeronautical Communication System with Interference Mitigation

As one of the main candidates for future civil aviation communications systems, the L-band digital aeronautical communication system (L-DACS) is expected to achieve secure and reliable transmission. Due to the broadcasting nature of air–ground wireless links, the L-DACS has the risk of being intercepted by malicious eavesdroppers, which negatively affects aviation communication security. In addition, because the spectrum of the L-DACS overlaps with the aviation distance measuring equipment (DME), the pulse interference caused by the DME signal may lead to the wireless link being more fragile and susceptible to wiretapping. In this paper, with a focus on enhancing wireless transmission security, we propose a comprehensive physical layer security (PLS) method for the L-DACS. The key to the proposed PLS method is restraining the transmission of the eavesdropper by injecting artificial noise into the transmitted signal while improving the transmission of the legitimate receiver through the adoption of pulse interference mitigation. First, to characterize the L-DACS in the secure scene, we derive the signal-to-interference-plus-noise ratio (SINR) of the legitimate receiver and any potential eavesdropper by constructing equivalent noise. Next, from the perspective of the information theory, we derive the closed form of the secrecy capacity of the L-DACS by employing the proposed PLS methods with three kinds of nonlinear interference mitigation: including ideal pulse blanking, peak threshold-based pulse blanking, and peak threshold-based pulse clipping. Finally, we compare and analyze different ways to enhance the secrecy capacity of the proposed PLS method using various interference mitigation methods.

Advancing the Security of LDACS

The “Single European Sky” air traffic management master plan foresees the introduction of several modern digital data links for aeronautical communications. The candidate for long-range continental communications is the L-band Digital Aeronautical Communications System (LDACS). LDACS is a cellular, ground-based digital communications system for flight guidance and communications related to safety and regularity of flight. Hence, the aeronautical standards, imposed by the International Civil Aviation Organization (ICAO), for cybersecurity of the link and network layer, apply. In previous works, threat-and risk analyses of LDACS were conducted, a draft for an LDACS cybersecurity architecture introduced, algorithms proposed, and the security of a Mutual Authentication and Key Establishment (MAKE) procedure of LDACS formally verified. However, options for cipher-suites and certificate management for LDACS were missing. Also, previous works hardly discussed the topic of post-quantum security for LDACS. This paper proposes a cell-attachment procedure, which establishes a secure LDACS communication channel between an aircraft and corresponding ground-station upon cell-entry of the aircraft. Via the design of a hybrid LDACS Public Key Infrastructure (PKI), the choice of a pre-or post-quantum Security Level (SL) is up to the communications participants. With that, this work introduces a full LDACS cell-attachment protocol based on a PKI, certificates, certificate revocation and cipher-suites including pre-and post-quantum options. Evaluations in the symbolic model show the procedure to fulfill LDACS security requirements and a communications performance evaluation demonstrates feasibility, matching requirements imposed by regulatory documents.

Adaptive Threshold Energy Detection Spectrum Sensing Method for L-Band Digital Aeronautical Communication System

Air travel is growing at an alarming rate. However, according to the latest research report by Eurocontrol on European aviation, the growth of air traffic will be limited by the available spectrum resources. As one of the means to provide air/ground (A/G) broadband communication, the L-band digital aeronautical communication system (LDACS) is becoming the preferred model for final deployment and has received continuous attention. Based on various spectrum measurement studies, the international civil aviation organization (ICAO) has identified multiple 1 MHz vacant bands between adjacent distance measuring equipment (DME) signals for LDACS. In order to improve spectrum efficiency, the concept of dynamic spectrum access (DSA) can be applied to the LDACS, which requires the use of spectrum sensing methods to detect the spectrum holes of DME users. In this paper, an adaptive threshold energy detection spectrum sensing method is proposed based on the characteristics of DME pulse signals. Firstly, the energy of the received signal is estimated to construct the detection statistics, and the equations of detection probability and false alarm probability are established. Secondly, the adaptive threshold is calculated using the maximum likelihood decision criterion under the assumption of a constant probability of false alarm. Thirdly, the detection statistic is compared to the adaptive threshold to determine the spectrum occupancy state in the decision-making stage. Finally, the result is transmitted back to the transmitter for the best spectrum resource allocation. According to the simulation analysis, the adaptive threshold energy detection-based sensing method outperforms the energy-difference detection method under low signal-to-noise ratio (SNR) conditions. Meanwhile, it has superior adaptability since the adaptive threshold can be adaptively changed according to the channel.

An optimised hardware architecture of the angular-domain cyclostationary detector for cognitive radio communications

This paper proposes a novel method to improve the performance and reduce the implementation design complexity of the angular domain cyclostationary feature detection (CFD). This method implements Orthogonal Frequency Division Multiplexing (OFDM) signal detection for cognitive radio applications. The originality of this work is based on using a selective sampling process for the OFDM signal detection by using only samples related to the cyclic prefix (CP). Using only the first NCP (CP length) angles of the angular domain cyclic autocorrelation function and eliminating the remaining angles has significantly improved the detector sensitivity. Results from Matlab simulations revealed that the proposed method reaches a detection probability of 100% at an SNR (signal-to-noise ratio) equal to −16 dB in the case of an additive white Gaussian noise (AWGN) channel. Moreover, the proposed method outperforms the other CFD methods with a performance range up to 5 dB. The proposed algorithm is coded using the VHDL language and implemented with the minimum of counters and comparators on Field-programmable gate array (FPGA) platform without using multipliers and DSP blocks. The proposed design requires a lesser power, logic and memory bits by 5.95, 60.96 and 87.5%, respectively, compared to the conventional angular domain CFD design. The proposed hardware architecture can be more efficient for spectrum sensing in many communication systems, such as LTE-Advanced and the L-band Digital Aeronautical Communication System (LDACS).

Moment-based modulation classification for Rician time-varying aeronautical channels

Modulation classification is a classical topic in the field of signal classification, and is of great significance in various applications including aeronautical communications. However, changes in the features of the signals transmitted through the Rician time-varying channels deteriorate the performance of modulation classification. In this study, a robust moment-based algorithm is proposed to counter the influence of the channels. The statistical correlation of the received samples are utilized for the estimation on the Rician factor. According to the derivation of the relationship between the moments of the transmitted samples and the received samples, the features utilized for classification are compensated mathematically. Finally, decision tree and random forest classifier are presented for classification. The estimation and classification performance of the proposed algorithm are analyzed for aeronautical en-route (EN) and arrival/takeoff (AT) channels through simulations. Meanwhile, real world signals of L-band digital aeronautical communication system (L-DACS1) are utilized for verification. The results show that our algorithm has high classification accuracy and low computational complexity.

LDACS Flight Trials: Demonstration and Performance Analysis of the Future Aeronautical Communications System

The L-band digital aeronautical communications system (LDACS) is a key enabler of the new air traffic services and operational concepts necessary for the modernization of the air traffic management (ATM). After its initial design, compatibility tests with legacy L-band systems, and functional demonstrations in the laboratory, the system is currently undergoing the standardization process of the International Civil Aviation Organization (ICAO). However, LDACS has not been demonstrated in flight yet. In this article, we present the first in-flight demonstration of LDACS, which took place in March and April 2019 in southern Germany and included four LDACS ground stations and one LDACS airborne station. We detail the experimental setup of the implemented LDACS ground and airborne stations together with the flight routes, the conducted experiments, and the frequency planning to ensure compatibility with legacy systems. In addition, we describe the demonstrated ATM applications and the security measures used to protect them. Based on the obtained measurement results, we evaluate the LDACS in-flight communication performance for the first time, including the achieved communication range, the measured end-to-end message latency, and the LDACS capability to provide quality of service by effectively prioritizing safety-relevant data traffic. Furthermore, we use the in-flight received signal power to assess the applicability of a theoretical path loss model. These flight trials contribute to the final steps in the development of LDACS by providing its in-flight communication performance and by demonstrating: first, its correct functionality in a realistic environment; second, its capability of supporting ATM applications and the advanced security measures that can be used to protect them; and third, its spectrum compatibility with legacy systems. We conclude that LDACS is ready to support ATM operations and that LDACS frequency planning can safeguard legacy systems successfully.

On the Design of Variable Filtered-OFDM Based LDACS for Future Generation Air-to-Ground Communication System

L-band digital aeronautical communication system (LDACS) has been newly presented for air-to-ground communication (A2GC) environment which is essentially based on orthogonal frequency division multiplexing (OFDM) scheme. This brief presents an innovative variable filtered-OFDM (VF-OFDM) based LDACS with the aid of variable digital filter utilizing the theory of Farrow structure and innovative interpolated band-pass method. Experimental observation has demonstrated that the resultant response of the filter satisfies the standard specification of LDACS spectral mask for different interpolation factors. It has also been shown that proposed VF-OFDM based LDACS system exhibits superior performance in terms of bit error rate (BER) and power spectral density (PSD) when compared to OFDM and F-OFDM based LDACS. Finally, superiority of the suggested architecture has been established by implementing it on Cyclone IV FPGA board.

High Performance Implementation of Next Generation Aeronautical Communication Systems

Current air traffic communication systems are mainly based on voice communication, with a newer digital communication technology called L-Band Digital Aeronautical Communication System (LDACS) being investigated in the Single European Sky Air Traffic Management Research (SESAR) project. An essential feature of this communication infrastructure is the encoding of data that guarantees reliable transport. While the encoding in the transmission path is straightforward, the decoding is computationally expensive due to the peculiarities of the convolutional codes used. As the target platform for the communication equipment is an embedded system, a proper system design is essential for the receiving path to ensure real-time processing. This paper therefore focuses on the hardware/software co-design of the functional system parts needed for decoding in the LDACS radio receiver. We describe the fundamental design considerations, followed by the actual implementation in form of software and FPGA-based hardware modules. Subsequently, the decoding solution was verified to prove a standard-compliant system. In an experimental validation, the actual system was fed with test data from a reference system. This allows conclusions to be drawn about system characteristics like data throughput, latency, and error correction. The resulting system demonstrates high-performance decoding that can exceed the desired requirements for quality and speed for use in the LDACS communication system.

Nested code division multiple access for distance measuring equipment interference mitigation in L‐band digital aeronautical communication system

SummaryAir‐to‐ground (A/G) communications have been transferred from the congested very high frequency (VHF) band to occupy the guard bands between adjacent distance measurement equipment (DME) navigation channels within the L‐Band. However, the inlayed channels, called L‐band digital aeronautical communication system (LDACS) channels, experience severe interference with the coexisting DME channels. In this paper, a DME interference detection and mitigation technique is proposed to improve the performance of the LDACS in the presence of DME interference. The DME energy detector based on Neyman–Pearson test is proposed, which has more advantages than the other techniques due to its noncoherent nature and simplicity. Furthermore, the proposed DME detection technique is performed by using an adaptive threshold to achieve the best trade‐off between the DME signal detection and false alarm. The proposed DME interference mitigation technique relies on the proposed concept of nested RF upconversion, where the in‐phase and the quadrature components of an LDACS signal are forwarded to a code division multiple access (CDMA) block in which they are transmitted only over the quadrature branch once DME interfering signal is detected. In this work, the closed form expression for the probability of DME detection, false alarm, and the detection error rate are derived. Simulation is performed, and the results show that the proposed technique provides improvement in the probability of DME detection and the detection error rate compared to the previous work. Also, simulation results shows that the proposed technique provides lower bit error rate and less complexity compared to the previous work.

Design and Implementation of Low Complexity Reconfigurable Filtered-OFDM-Based LDACS

L-band Digital Aeronautical Communication System (LDACS) aims to exploit vacant spectrum in L-band via spectrum sharing, and orthogonal frequency division multiplexing (OFDM) is the currently accepted LDACS waveform. Recently, various works dealing with improving the spectrum utilization of LDACS via filtering/windowing are being explored. In this direction, we propose an improved and low complexity reconfigurable filtered OFDM (LRef-OFDM) based LDACS using novel interpolation and masking based multi-stage digital filter. The proposed filter is designed to meet the stringent non-uniform spectral attenuation requirements of LDACS standard. It offers significantly lower complexity as well as higher transmission bandwidth than state-of-the-art approaches. We also integrate the proposed filter in our end-to-end LDACS testbed realized using Zynq System on Chip and analyze the performance in the presence of L-band legacy user interference as well as LDACS specific wireless channels. Via extensive experimental results, we demonstrate the superiority of the proposed LRef-OFDM over OFDM and Filtered-OFDM based LDACS in terms of power spectral density, bit error rate, implementation complexity, and group delay parameters.

Design and Simulation of Interference Mitigation Algorithm for an L-Band Digital Aeronautical Communication System

The LDACS 1 is an alternative to the current Very High Frequency (VHF) technology which meets needs of upcoming demands. LDACS 1, an Orthogonal Frequency Division Multiplexing (OFDM) operates in the L-Band between 960-1167MHz frequencies installed in the middle of two DME channels with 500 kHz spectral gap. In this paper, a LDACS 1 OFDM and LDACS 1 2x2 MIMO are designed and simulated without any interference. The Distance Measuring Equipment (DME) interference is introduced only to the LDACS1 OFDM, and significant degradation in the performance is observed. The pulse blanking technique helps in lowering the interference in the LDACS1 OFDM system, resulting in a performance close to the performance in the interference-free case. The BER performances for the conditions without interference, with interference and pulse blanking are compared. An improvement in the BER performance is seen with respect to design of LDACS 1 2x2 MIMO system.

Flight Trial Demonstration of Secure GBAS via the L-band Digital Aeronautical Communications System (LDACS)

The ground-based augmentation system (GBAS) is the cornerstone for enabling automated landings without the instrument landing system (ILS). Currently, GBAS is evolving to GBAS approach service type-D (GAST-D) for category III landings. This development toward GBAS GAST-D extends GBAS via the use of multiple frequencies (L1/L2 and L5) and the use of multiple global navigation satellite system constellations. GBAS requires correction data to be broadcast to aircraft, which is currently handled via the VHF data broadcast (VDB) datalink. However, VDB has several known shortcomings: 1) low throughput, 2) small area of operation, and 3) no cyber-security measures. In this article, we propose the use of the L-band digital aeronautical communications system (LDACS) for broadcasting GBAS correction data to address these shortcomings. In flight experiments conducted in 2019, we set up an experimental GBAS installation using LDACS. Broadcast data are secured using the timed efficient stream loss-tolerant authentication (TESLA) broadcast authentication protocol. Our results indicate that cryptographically secured GBAS data via LDACS can provide GAST-C and GAST-D services with high availability if cryptographic parameters are chosen appropriately.

Long Coherent Integration in Passive Radar Systems Using Super-Resolution Sparse Bayesian Learning

Maximizing the coherent processing interval (CPI) is crucial when performing passive radar detection on weak signal reflections. In practice, however, the CPI is limited by the target movement. In this work, the extent of the range and Doppler migration effects occurring when using a long CPI to integrate the returns from an L-band digital aeronautical communication system (LDACS) based passive radar is studied. In particular, our simulations underline the extensive Doppler migration effect that arises even for non-accelerating targets. To this end, the Keystone transform and fractional Fourier transform techniques are combined with the standard passive radar processing to enable the compensation of both range and Doppler migration effects. This non-model-based approach is, however, shown to have limitations, in particular for low signal-to-noise ratios and/or multitarget scenarios. To address these shortcomings, a novel model-based framework that allows to perform joint target detection and parameter estimation is developed. For this, a super-resolution sparse Bayesian learning approach is employed. This technique uses a multitarget observation model, which accurately accounts for the underlying range and Doppler migration effects and provides super-resolution estimation capabilities. This is particularly advantageous in the LDACS case since the narrow bandwidth generally limits the separation of closely spaced targets. The simulation experiments demonstrate the effectiveness of the algorithm and the advantages it provides when compared to the standard migration compensation approach.

A Power-Efficient Spectrum-Sensing Scheme Using 1-Bit Quantizer and Modified Filter Banks

Spectrum sensing is an efficient way to determine the spectrum availabilities over the frequency range of interest, aiding in improving the spectrum utilization in the cognitive radio (CR) systems. Conventional Nyquist multiband sensing entails higher computational capability for sampling, quantization, and subsequent processing, lending the approach infeasible for applications with limited power budgets. In this brief, a power-efficient spectrum-sensing technique is proposed, which explores an accuracy–complexity tradeoff. The presented spectrum-sensing architecture is based on 1-bit quantization at the CR receiver and implements it in hardware by a resource- and power-efficient approach, using a finite-impulse-response (FIR) filter-bank channelizer. The proposed scheme allows the complex operators like multipliers and quantizers to be replaced by the inverter logic and high-speed comparators, reducing the hardware complexity and power consumption. We validate the proposed scheme on a field-programmable gate-array (FPGA) emulator for an aeronautical $L$ -band digital aeronautical communication system (LDACS) application, and our results show that the proposed scheme achieves substantial resource reduction with at most 5% degradation in the detection accuracy in this case.

Spectral Coexistence of LDACS and DME: Analysis via Hardware Software Co-Design in Presence of Real Channels and RF Impairments

To meet the exponentially increasing air traffic, L-band (960--1164 MHz) digital aeronautical communication system (LDACS) has been introduced. The LDACS aims to exploit the vacant spectrum between incumbent Distance Measuring Equipment (DME) signals and envisioned to follow an orthogonal frequency division multiplexing (OFDM) approach to support high-speed delay-sensitive multimedia services. This paper deals with the design and implementation of end-to-end LDACS the transceiver on the Zynq System on Chip platform, consisting of FPGA as programmable logic (PL) and ARM as processing system (PS). We consider OFDM based LDACS and improve it further using windowing and/or filtering. We propose a hardware-software co-design approach and analyze various transceiver configurations by dividing it into PL and PS. We demonstrate the flexibility offered by such a co-design approach to choose the configuration as well as word-length for a given area, delay and power constraints. The transceiver is also integrated with the programmable analog front-end to validate its functionality in the presence of various RF impairments and wireless channels and interference specific to the LDACS environment. Via in-depth performance analysis concerning parameters such as out-of-band attenuation, DME interference, bit-error-rate, word-length, and complexity, we demonstrate wide bandwidth filtered OFDM as an attractive solution for the next generation LDACS.

Cognitive Radio Techniques for Utilizing the Primary L-Band Distance Measuring Equipment for Aeronautical Communications

The aeronautical communication has been switched from the congested very high frequency (VHF) band to occupy the L-band, namely, L-band digital aeronautical communication system type I (LDACSI). Critical navigation system in this band is the distance measurement equipment (DME). However, the DME channels provide a severe interference to onboard LDACSI receiver. In this paper, a cognitive radio (CR) network is proposed to allow efficient spectrum reuse with coexisting primary DME channels. In LDACSI-CR network, the proposed spectrum sensing technique is performed by using an adaptive threshold DME energy detector to achieve the best trade-off between the DME signal detection and false alarm. Moreover, dynamic spectrum access, namely, hybrid spectrum access (HSA) is newly introduced to enhance the LDACSI-CR throughput. Joint effect of HSA with different frame structures on the LDACSI throughput is proposed. Besides, optimal power allocation using genetic algorithm that maximizes the LDACSI-CR ergodic throughput for HSA with different frame structures is proposed. In this work, the closed form expression for the probability of DME detection, false alarm, detection error, the LDACSI throughput and data loss are derived. Simulation is performed and the results show the LDACSI throughput and the data loss for the proposed techniques. Besides, ergodic throughput maximization for HSA under constraints of the average LDACSI transmitted power and tolerable interference power on the DME signal is performed. Moreover, simulation results show that the proposed LDACSI-CR system performance provides lower bit error rate and less complexity compared to the previous work.

An Adaptive Energy Detection Scheme with Real-Time Noise Variance Estimation

Energy detection-based spectrum sensing techniques are ideally suited for power-constrained cognitive radio applications because of their lower computational complexity compared to feature detection techniques. However, their detection performance is dependent on multiple factors like accuracy of noise variance estimation and signal-to-noise ratio (SNR). Many variations of energy detection techniques have been proposed to address these challenges; however, they achieve the desired detection accuracy at the cost of increased computational complexity. This restricts the use of enhanced energy detection schemes in power-constrained applications such as aeronautical communication. In this paper, an adaptive low-complexity energy detection scheme is proposed for spectrum sensing in an L-band Digital Aeronautical Communication System (LDACS) at lower SNR levels. Our scheme uses a real-time noise variance estimation technique using autocorrelation which is induced by the cyclic prefix property in LDACS signals. The proposed technique does not incur dedicated hardware blocks for noise variance estimation, leading to an efficient hardware implementation of the scheme without significant resource overheads. The simulation studies of the proposed scheme show that the desired accuracy (90% detection accuracy with only 10% of false alarms) can be achieved even at $$-16.5$$ dB SNR, significantly lowering the SNR wall over existing energy detection schemes.

Impact Assessment of the L-Band Digital Aeronautical Communications System on the Joint Tactical Information Distribution System

The L-band digital aeronautical communications system (LDACS) is the future air–ground communications technology currently undergoing the International Civil Aviation Organization standardization process. As LDACS is intended to operate in the frequency band 960–1164 MHz, compatibility tests between LDACS and the legacy systems operating in this frequency band are necessary to ensure that no system is harmfully interfered. One of these systems is the joint tactical information distribution system (JTIDS), a technology employed by the tactical data link Link 16. In this paper, we present the results of an impact assessment of LDACS on JTIDS conducted through simulations. The extent of the impact has been quantified by simulating a wide variety of interference configurations, which are expected to cover most realistic interference conditions between LDACS and JTIDS. Baseband simulation models of both systems have been implemented and an interference scenario between LDACS and JTIDS has been defined. To evaluate the impact, the degradation of the signal-to-noise ratio (SNR) of JTIDS by the presence of LDACS has been determined. Default JTIDS transmissions, where the information is repeated at distant frequencies, do not show a significant degradation by the presence of strong LDACS interference, with an SNR loss lower than 1 dB in any considered interference scenario. Comparatively, a certain dependency on the specific LDACS deployment is noticeable for less protected JTIDS transmissions. Based on the observed interdependencies, recommendations for the deployment of LDACS are given in this paper, such that the impact of LDACS on JTIDS is minimized.