Applications of ultrawideband in communications and ranging

Impulse Radio Ultrawideband (UWB) systems are based on the transmission of short time sub-nanosecond impulses. High data transfer rates as well as precise accuracy for distance and position estimation (ranging) are potentially achievable with these types of signals, while promising devices with low complexity and low power consumption. Despite of many advantages, several problems are needed to be addressed. The transmit power of the UWB systems are generally limited by spectral masks, and therefore the UWB received power is expected to be low. Analog to digital conversion cannot be performed with reasonable power consumption due to the large. For communications, optimal performance can be achieved by detection methods based on analog correlation but applying these methods to UWB systems is very challenging. Strict time synchronization and channel estimation are required. Detection method with less synchronization constraint such as energy detection is more preferable for UWB systems. We propose a novel UWB receiver structures based on a comb filter. The comb filter is a feedback loop with an analog delay element, and it is used for performing a coherent combination to improve the signal-to-noise ratio of the receiving signal and also to suppress interference from other systems. The comb based receiver can be used for both communications and ranging applications because, with proper setting, the output signal of the comb filter is the effective channel impulse response. The coherent combination process in the comb filter gives energy detection the benefits that cannot be achieved by conventional approaches. For ranging, conventional methods based on analog impulse correlation perform very well in high SNR scenarios but large error bias is expected when the SNR is low. The SNR improvement from the comb filter can improve the ranging performance of UWB radar system greatly

Ranging with LDACS : Results from Measurement Campaign

The current air traffic management (ATM) system in civil aviation is globally standardized, well-established and highly reliable. However, it still relies on techniques which have been developed several decades ago. With the continuously and rapidly growing demand for air transportation, it is predicted that the ATM system will reach its capacity limit in the near future. A major ATM modernization process including communications, navigation and surveillance (CNS) technologies is currently ongoing under SESAR in Europe and NextGen in the US. For the future aeronautical communications, analog voice shall be replaced by digital data to support more complicated information exchange as well as higher capacity. L-band Digital Aeronautical Communication System (LDACS) is the proposal for the future ATM data link. In this system, multicarrier OFDM signals with 500 kHz effective bandwidth per channel are used for transmission. With respect to aeronautical navigation, the performance and capacity of the legacy systems, e.g. distance measurement equipment (DME), are limited and will not be able to catch up with the growing demand for higher precision and more efficient flight-route management. The future navigation system will heavily rely on global navigation satellite systems (GNSS). Despite the far superior performance, the main concerns for the satellite-based approach are availability, continuity and integrity. The fundamental problem for satellite-based systems is the weak receiving power because of the large distance between the satellites and the aircraft. As a result, the satellite-based navigation system can be easily interfered, either intentionally or unintentionally. There is a need for a parallel backup navigational infrastructure to ensure availability, continuity and integrity. This backup is commonly referred to as alternative positioning, navigation and timing (APNT). The solutions for APNT are mostly ground-based systems as the sources of error would be different from that of satellite-based systems. Intensifying and enhancing the already existing DME system is the most commonly discussed and most-ready solution. The major drawback of this solution is that DME is using the precious aeronautical spectrum very inefficiently. Intensifying DME usage would even increase this inefficiency and would finally block the L-band spectrum for any other future CNS system – a consequence which shall be considered thoroughly. Aiming at efficient spectrum usage, the German Aerospace Center (DLR) has proposed to consider LDACS as APNT solution. Although the LDACS system is primarily designed for communications, the navigational function can be added without any major modifications. The network of LDACS ground stations can be used as pseudolites for estimating the aircraft positions. Additionally, LDACS has capability to be operated in parallel with DME, and therefore it is possible to combine the range calculation from both systems. Two LDACS flight trials to assess the usability of the system as APNT solution were conducted by DLR in November 2012 and November 2013. The goal of the first campaign was to implement a core structure of the LDACS system for navigation and test its performance in a realistic scenario with an airborne receiver. The results confirmed that the LDACS signal is a good ranging source for navigation. Multipath propagation and co-site DME interference were identified as significant factor on the performance. More understanding of their impact in different scenarios and developing techniques for mitigating these effects can significantly improve the ranging performance. As a result, the second measurement campaign was performed in November 2013. This contribution will focus on the measurement setup and the LDACS ranging results from this campaign. In this campaign, only one ground station (LDACS transmitter) was considered, therefore the results are assessed by ranging performance. The main goal was to verify the LDACS ranging performance in diverse flight scenarios. Several flight scenarios such as flying in different altitudes from FL100 to FL380, circling around the ground station in low altitude and flying in a strong in-band DME interference environment were performed with the measurements aircraft (LDACS receiver). The LDACS ranging performance is analyzed based on the measurement data and several mitigation techniques are investigated. We propose particle filtering and the Doppler smoothing filter to improve the ranging performance as well as mitigating the interference from the multipath propagation and DME. The results show that the proposed algorithms can greatly improve the ranging performance with reasonable computational complexity

Impact of the DME interference on the LDACS1 ranging performance

The communications, navigation and surveillance (CNS) technologies in air traffic management for civil aviation are currently undergoing a major modernization process. The legacy systems, which were established over several decades ago, such as analog voice communication systems, distance measuring equipment (DME) and VHF omnidirectional range (VOR) may not meet the continuously growing demands in the near future. In communications, L-band digital aeronautical communication system – type 1 (LDACS1) is one of the most promising candidates for the future air traffic management data link. Using digital communication systems, more complex information can be exchanged between pilots and controllers as well as high-capacity data transfer can be achieved. In navigation, the Global Navigation Satellite System (GNSS) is foreseen to be the primary means for high precision navigation and complex flight trajectories. However, there are still concerns about possible GNSS outages due to intentional or unintentional interference. As a result, a parallel backup navigational infrastructure commonly-referred to as alternative positioning navigation and timing (APNT) is needed. The proposed APNT solutions mainly focus on utilizing ground-based navigational systems because their source of errors would be different from that of GNSS. Several works have been done on the extension of LDACS1 functionality toward navigation and surveillance, and the LDACS1 system has been proposed as one of the APNT solutions. The LDACS1 system was not primarily designed for ranging application but only a few minor modifications are needed to exploit its navigation functionality. LDACS1 is a ground-based cellular system. The networks of LDACS1 ground stations can be used for an aircraft to estimate its position using trilateration. At the moment, enhancing and intensifying the use of the DME system is the most favoured solution for APNT. The major disadvantages for this option are the cost of installing additional DME infrastructure and its inefficient use of L-band spectrum. One of the advantages of using LDACS1 as APNT is that the deployment of LDACS1 ground stations is planned for the future aeronautical communication systems, and navigational functionality can be added without installing any additional infrastructure. Another key advantage of LDACS1 is its spectral deployment, the LDACS1 signal has 500 kHz effective bandwidth and is planned to be allocated in the same frequency band as DME, i.e. the lower part of L-band (960 - 1164 MHz), using inlay approach. The 500 kHz LDACS1 channels will be placed in the 1 MHz spectral gaps between two adjacent DME channels. The LDACS1 system is planned to be able to work in parallel with DME, and therefore both systems can work together as APNT solutions by combining the range calculation in the situations where either of the systems does not have a sufficient number of ground stations available. It is important to ensure the good performance for both systems and several studies on this topic are ongoing. The German Aerospace Center (DLR) has conducted two measurement campaigns in 2012 and 2013 to assess the usability of the LDACS1 system as APNT solution. Several flight scenarios were performed to evaluate the LDACS1 ranging performance and the analysis of the measurement data gave the conclusion that LDACS1 signals offer an excellent ranging source [1], [2]. One of the measurement scenarios recorded during the 2013 measurement campaign focused on the impact of DME interference on the LDACS signals. During this scenario, the measurements aircraft (LDACS1 receiver) flew at FL250 directly over a testing DME station. To test the feasibility of LDACS1 inlay deployment, similar flight route was flown twice and the DME channel is tuned to 500 kHz and then 1.5 MHz away from the tested LDACS1 carrier frequency. Different signal-to-interference ratio (SIR) can be observed along the flight paths. Additionally, the co-site DME interference, i.e. the interrogating DME signal transmitted from the aircraft, were visible at the receiver in most of the flight scenarios. This event occurred with a very low duty cycle but the damage to LDACS1 received signal can be very severe as the on-board DME transmitted power is much higher than the LDACS1 received power from the ground. The worst case scenario occurs when the LDACS1 received signal is heavily faded due to banking as the view of the LDACS1 receiving antenna was blocked. The LDACS1 ranging performance with the presence of DME interference is analyzed based on the measurement data and mitigation techniques are investigated. The in-band DME interference can be suppressed using methods based on pulse blanking. These methods have been studied and proved to be effective for the interference mitigation in the case of LDACS1 communication systems [3]. In this work similar approaches are adopted for the LDACS1 ranging applications. Since the LDACS1 ranging errors caused by DME interference occur in bursts with low duty cycle, the ranging algorithms that accommodate prior information in the range estimation are suitable methods to suppress this type of error bias. In this work, we propose the use of particle filtering and Doppler smoothing filter for the DME interference mitigation. Particle filtering is a variant of sequential Bayesian filter where the dynamic of the estimated parameters is taken into account in the estimation. Doppler smoothing filter is one interpretation of a well-known Hatch filter. This method uses the Doppler information to smooth the raw LDACS1 range estimates from maximum likelihood algorithm or particle filtering and, as a result, sudden error bursts can be suppressed. [1] D. Shutin et al., “LDACS1 Ranging Performance - An Analysis of Flight Measurement Results,” Digital Avionics Systems Conference (DASC 2013), USA, 2013. [2] T. Thiasiriphet et al., ““Application of Bayesian Filtering for Multipath Mitigation in LDACS1-based APNT Applications,” in ION GNSS+ , USA, 2014.” [3] U. Epple, M. Schnell, “Overview of Interference Situation and Mitigation Techniques for LDACS1”, 2011 Digital Avionics Systems Conference (DASC 2011), USA, 2011

Application of Bayesian Filtering for Multipath Mitigation in LDACS1-based APNT Applications

Civil avionic systems have been undergoing a major transformation during the past few years. To keep up with the growing demand for higher capacity and efficiency, the analog aeronautical communication systems have to be exchanged with a more efficient digital transmission. Moreover, the civil air traffic navigation systems need to be improved to provide more precise and reliable navigation information for the aircraft to improve the operation efficiency. The L-band digital aeronautical communication system – type 1 (LDACS1) is one of the most promising candidates for the future air traffic management data link. The LDACS1 signal has 500 kHz effective bandwidth and shall be allocated in the lower part of L-band (960 - 1164 MHz). One of the key advantages of LDACS1 is its spectral deployment: it is foreseen to allocate the LDACS1 channels in spectral gaps between two adjacent channels distance measuring equipment (DME) – a currently used radio navigation system. The civil aeronautics relies heavily on DME and VHF omnidirectional radio range (VOR). These legacy systems were established over several decades ago. They have limited performance that does not meet the future demands. Hence, the future navigation system is planned to rely on global navigation satellite systems (GNSS). The fields of GNSS navigation have been improved greatly during the last decades, and validations of flight procedures using GPS have already been done. However, integrity, continuity and availability of the navigation information are also important factors. These topics are major concerns for the use of GNSS navigation because of the large distance between the satellites and the aircrafts. The received signal can be interrupted by intentional or unintentional interference. Consequently, the GNSS failures might temporary occur. Therefore, there is a need for a parallel backup navigational infrastructure. This backup is commonly referred to as alternative positioning, navigation and timing (APNT). At the moment, several APNT solutions are being considered. One possible solution is to intensify the use of the DME system by increasing the density of the DME stations. However, this solution has some major disadvantages. First, the additional infrastructure might involve costly installation. Moreover, large portion of the L-band spectrum will be assigned to an old and spectrally inefficient technology. This gives difficulty for the frequency allocation of the mid- and long-term future communications in L-band. To support efficient spectrum usage, we propose to consider the future communication system LDACS1 as APNT solution. LDACS1 is a ground-based cellular communications system. Using ground stations (GS) as pseudolites, an airplane can exploit trilateration to estimate its position. The LDACS1 system is not optimized for ranging, yet it can be well exploited for navigation with only a few minor modifications. The deployment of LDACS1 GSs is planned for the future aeronautical communication systems and no additional infrastructure is needed for enabling navigational functionality. The LDACS1 system is also planned to be able to work in parallel with DME, and therefore it is possible to combine the range calculation from both systems in the situations where not a sufficient number of LDACS1 stations is visible. To assess the usability of the LDACS1 system as APNT solution, two measurement campaigns were conducted by DLR in November 2012 and 2013. The analysis of the measurement data gave the conclusion that LDACS1 signals offer an excellent ranging source. However, the results also indicate that multipath propagation can have a significant impact to the ranging performance, especially at low altitudes [1]. With the signal bandwidth of 500 kHz, the multipath in the range of few hundreds of meters are not easily resolvable. It is well-known that in such situation the ranging performance of the conventional correlation-based technique such as delayed lock loop (DLL) are largely biased. Unresolvable multipath environments cannot be easily avoided and would be a major problem for any low bandwidth ground-based APNT solutions (DME, LDACS1). Multipath mitigation is a very challenging task as the propagation channel is dynamic with the number of paths varies with time. A new algorithm that can perform robust range estimation under such dynamic multipath channels is needed to ensure good accuracy and integrity of the LDACS1-based APNT system. One of the considered algorithms for multipath mitigation in LDACS1-based APNT is particle filtering, which is a variant of sequential Bayesian filter implemented by Monte Carlo methods. For this algorithm, the underlying process model can be non-linear/non-Gaussian and is especially designed for dynamic channel conditions. The parameters estimation is based on a posterior density, and it uses a movement model to incorporate the temporal correlation of the change of the estimated parameters. It was demonstrated in [2] that this method can be efficiently used for the pseudo-range estimation of LOS component in dense and dynamic multipath environments in the GNSS receiver platform. The algorithm can correct the range error bias introduced by unresolvable multipath and give a much better ranging performance compared to the conventional DLL receiver. The fundamental drawback of the particle filtering is the complexity but it has been shown in [2] that the computation effort can be reduced greatly through the use of two-fold Bayesian filtering methods involving Kalman filter, Grid-based method and the particle filter. Here we propose to exploit this technique for LDACS1 ranging applications and demonstrate the algorithm performance on measured signals. The final paper starts with a brief description of the LDACS1 signal model, measurement campaign setup and the channel conditions. The detailed descriptions of the applied algorithm based on sequential Bayesian filtering and the application in the LDACS1 platform are then discussed. The analysis of the measurement data is focused on the dynamic multipath scenarios. Finally the obtained results and the future investigation directions are discussed

Modelling Distance Measurement Equipment (DME) signals interfering an airborne GNSS receiver

This publication describes an end-to-end model to generate Distance Measurement Equipment (DME) signals as an interference source to airborne Global Satellite Navigation Systems (GNSS). Both satellite navigation systems, the Global Positioning System (GPS) and GALILEO, use the lower L-band 1 for wideband navigation services and are sharing the same frequency band with DME. Any GNSS Receiver operating in the mentioned bands will receive DME signals and will have to deal with them as interference. This publication describes a model to rebuild the measured DME signals at the receiver input to allow simulations of the interference effect. Prior to this work we only found models based on propagation estimation. No model existed which is based on real world measurements of DME signals. Thus, the German Aerospace Center (DLR) has car- ried out a Fight measurement campaign at the European DME hotspot near Frankfurt (Main), Germany. From the data of the measurement campaign we have developed the new model. This measurement based model is much more accurate than the existing models since it accounts for the propagation and the DME transmission and the GNSS receiver antenna effects. We provide this model to the community to allow a more realistic forecast of the DME-GNSS interference situatio

LDACS1 Ranging Results with Doppler Smoothing from New Flight Experiments

In recent years the German Aerospace Center (DLR) has been actively working on a proposal to deploy an L-band Digital Aeronautical Communication System type 1 (LDACS1) for future Alternative Positioning, Navigation, and Timing (APNT) services. In 2012 a flight measurement campaign has been performed to validate LDACS1-based navigational functionality. The results indicated a strong influence of multipath propagation on ranging performance. To better characterize multipath environment for future ground-based APNT services a second measurement has been performed in November 2013. This paper outlines the November 2013 measurement campaign and provides corresponding range estimation results that make use Bayesian filtering methods and Doppler smoothing to mitigate multipath and improve range estimation

An Aeronautical Air-Ground Channel Investigation for APNT Applications

Over the course of the last years, civil aeronautics have been undergoing a perpetual innovation: the way we communicate, navigate, and survey the airspace is currently being redefined. In the past, pilots mainly relied on DME (distance measuring equipment) and VOR (VHF omnidirectional radio range) for navigation. Compared to state-of-the-art navigation aids these systems offer only a limited performance while being spectrally inefficient. Therefore, it is planned in the future to rely on global navigation satellite systems (GNSS) for navigation. GNSS offers a highly superior navigation performance as compared to that obtained with legacy DME/VOR infrastructure. To guarantee the required degree of integrity, GNSS systems will be accompanied with ground or satellite based augmentation systems (G/SBAS). However, an increased use of GNSS for aviation brings new challenges. Especially integrity, continuity, and availability of navigational information are of exceptional importance in a safety-of-life environment. Due to the low power levels received from in-orbit satellites, GNSS signals are susceptible interferences, both intentional and unintentional. Hence a ground based navigational backup system, referred to as alternative positioning, navigation and timing (APNT), needs to be employed. These systems should be used when GNSS services become temporarily unavailable. Different proposals for APNT are currently being discussed. One approach is based on the legacy system DME. The major advantage of DME is that it allows reusing the existing infrastructure; in fact, DMEs can be currently used as an APNT system. However, a large drawback of this solution is the continued use of old, spectrally-inefficient technology. But, what is more important, is that once the transition to GNSS-based air traffic management has been realized, a DME-based APNT solution will not be able to support the same level of navigational information in terms of accuracy or precision. Thus, alternative APNT approaches that provide accurate positioning, while at the same time allowing for a “gentle” phasing out of DMEs, are needed. One such example is the future communication system LDACS1, which signals can be also used for ranging and positioning. Hereby, communication infrastructure can also be employed for navigation purposes. Independently of the underlying technology, the majority of the APNT proposals are ground-based systems with planned allocation in the L-band. If, however, the future APNT systems are to deliver highly accurate positioning information, it is crucial to understand the characteristics of the ground-to-air (G2A) L-band wireless propagation channel in more detail. Yet this remains a challenging task as channel models for L-band G2A useable for testing and validation of range estimators are sparse. The existing models are extensions of classical statistical channel models developed for wireless communication applications. Thus, dependable simulations of the navigation performance of the proposed systems are hard to realize. To address this challenge, DLR has conducted in 2013 an extensive channel measurement campaign in the L-band. To this end, the setup consisted of a ground-based transmitter located on top of a building in an airport, and a receiver in a research aircraft Dassault Falcon 20E . For both the transmitter and receiver commercial L-band antennas were employed. The measurement bandwidth was set to 10MHz with a center frequency at the lower end of the L-band at 965 MHz; the transmit power was set to 39 dBm. Different flight patterns were tested in order to allow an extensive investigation of the channel characteristics. These include flights at different altitudes and transmitter-receiver separations, approach on the airport, take-off and landing. First preliminary analysis of the recorded data identifies an interesting characteristic of the L-band G2A channel relevant for navigation applications: for all altitudes multiple propagation paths can be observed. This effect is especially profound at low altitudes. The multipath is prone to occur in close vicinity to the transmitter, but excess delays sometimes reaching up to a few kilometers. The additional propagation paths are also strongly correlated in Doppler domain. As the result, for ranging and positioning applications such multipaths cause a strong interference to the direct path, significantly biasing the output of the correlator tuned to the line of sight (LOS) path. Furthermore, the spatial proximity of the different propagation paths complicates their separation and detection. This makes the compensation of the multipath effects a very challenging task. Especially for systems with low bandwidths, such as DME or LDACS1 that are considered for APNT services, multiple propagation paths represent a significant error source. Data collected in previous flight trials show that errors in the region of hundreds of meters are possible when ranging with a low bandwidth system in strong multipath environment. In the final paper we will discuss the above mentioned problems in more details. We will begin by outlining the goals of the channel measurement campaigns and explain the measurement setup used in our measurements. The latter includes a short description of both the hardware and software employed during the campaign as well as the applied algorithms. Then, we will concentrate on the analysis of the measurement data. Hereby a special focus is placed on the multipath characterization of the G2A channel using a novel superresolution multipath estimation algorithm. The paper is concluded with a discussion of the obtained results and outline of the future investigation directions