L-DACS1 Laboratory Demonstrator Development and Compatibility Measurement Set-up

The Future Communications Infrastructure (FCI) comprises a set of data link technologies for aeronautical communications. For the airport, a data link technology dubbed AeroMACS is currently developed within NextGen and SESAR which is strongly based on the WiMAX standard. ESA initiated the development of a future satellite-based communications system for aviation within their ESA Iris program, supplemented by work performed within SESAR. For air/ground communications, currently two candidate systems are under consideration – L-DACS1 and L-DACS2. Whereas L-DACS1 is a broadband system employing Orthogonal Frequency-Division Multiplexing (OFDM) as modulation scheme and frequency-division duplex (FDD), L-DACS2 is a narrowband single-carrier system utilizing time-division duplex (TDD). The final decision on L-DACS will be based on a set of evaluation criteria including laboratory prototype testing of L-DACS with respect to L-band compatibility. Current work on L-DACS is performed under the framework of SESAR. The corresponding SESAR project P15.2.4 „Future Mobile Data Link System Definition“ has started activities on L-DACS within an Early Task mid of February this year. Main goals of this Early Task are the refinement of L-DACS specifications, the development of evaluation criteria for L-band compatibility testing, and the set-up for the L-DACS evaluation. Besides the SESAR activities, DLR has already started to implement an L-DACS1 physical layer laboratory demonstrator in FPGA technology based on the current L-DACS1 specification. The demonstrator enables investigations of both the influence of the L-DASC1 waveform on the legacy L-band systems and the interference of the legacy L-band systems on the L-DACS1 receiver. These investigations are especially of interest for the so-called “inlay” deployment scenario, where L-DACS1 and DME share the L-band as common spectrum resource by implementing L-DACS1 channels of approximately 500 kHz bandwidth between two adjacent DME channels. Since the proof of L-band compatibility is the main scope of the L-DACS1 demonstrator, the focus is on the physical layer implementation in FPGA technology. The demonstrator set-up comprises a complete implementation of the physical layer of the L-DACS1 transmitter, including adaptive coding and modulation as well as the complete framing structure for forward and reverse link. The L-DACS1 receiver is implemented only partly in hardware. Mainly sampling and digital down-conversion followed by fast data storage are realized, i.e. a data grabber function. The subsequent receiver tasks, like synchronization, interference mitigation, channel estimation/equalization, decoding, and demodulation are realized in software. This concept allows rapid demonstrator set-up and high flexibility for receiver optimization. Recent results on L-DACS1 receiver optimization have been presented at last year’s DASC. The L-DACS1 physical layer laboratory demonstrator is finalized up to the intermediate frequency (IF) stage and the Radio Frequency (RF) frontend shall be available by May this year. First L-band compatibility tests at the labs of the German ATC authority DFS are scheduled for June/July this year. These tests will consider the outcomes from the SESAR project P15.2.4 with respect to the evaluation criteria. In addition, the L-DACS1 physical layer laboratory demonstrator will be adjusted to the L-DACS1 specification refinements as proposed by SESAR before testing. In the final paper, a short overview of the L-DACS1 system is presented followed by a description of the demonstrator concept. Moreover, the measurement set-up is presented in detail describing the different L-band compatibility tests performed at the DFS labs. Finally, measurement results are presented and a first evaluation of the L-band compatibility of L-DACS1 is given

Joint channel estimation and equalisation of fast time-varying frequency-selective channels

This paper addresses joint channel and data estimation for transmission over frequency-selective and time-varying channels based on a Doppler-variant channel impulse response (CIR). A discrete spreading function (DSF), which corresponds to the Doppler Fourier series of the time-varying channel taps, is introduced. The spectral leakage associated with the Fourier series representation is reduced by oversampling in Doppler-direction. This approach, however, requires joint channel estimation and equalisation (JCE). Therefore, a recursive least squares (RLS) algorithm for the DSF estimation is combined with the data detection in a reduced state diagram. Furthermore, an enhanced metric, previously introduced in De Broeck et al. (Proceedings of Second International Workshop on Multi-Carrier Spread-Spectrum, [1999]; published by Kluwer Academic Publishers: Dordrecht, 2000) for time-invariant channels, is applied here for path selection. The performance of this metric is investigated here for time-varying and frequency-selective (TV-FS) channels. The performance of the proposed approach is compared with the JCE algorithm which uses RLS estimation of a CIR by simulations

Airport Surface Propagation Channel in the C-Band: Measurements and Modeling

In this paper, we assess the path loss and the multipath fading characteristics of the airport surface channel based on the measurement data collected at Munich airport at 5.2 GHz. Our goal is to present channel models for the ground, tower and ramp area of the airport surface, which are easy to implement in a channel emulator and suitable for performance evaluation of the future aeronautical airport data link AeroMACS. The channel is in general nonstationary, but within one data frame and correspondingly one simulation run, the WSSUS assumption can be adopted for all scenarios related to an aircraft. However, the assumption loses its validity in the case of service vehicles moving in the NLOS ramp area. The proposed stochastic model for the small scale fading channel uses a sum-of-sinusoids based tapped delay line with a fixed channel parameters which are randomly chosen from the suitable intervals for each new simulation run. Incorporating scatterer movements into the model enables modeling of a nonstationary channel impulse response

Scatterer Based Airport Surface Channel Model

Based on measurements at Munich airport and the observation of the spreading function, this paper analyzes different types of scatterers which occur in the spreading function and their temporal behavior. The scatterer analysis reveals the non-stationary character of the airport surface channel and is useful for stochastic modeling. The proposed geometry-based, stochastic channel modeling approach may be realized by either randomly placing reflecting objects based on the scatterer analysis or utilizing an appointed airport environment. The proposed channel model yields a spreading function characteristic for the apron area