Compact adaptive multi-antenna navigation receiver

Kurzfassung

The efficient detection and mitigation of jammers and interferers is an important feature of a robust receiver for satellite navigation. To this end, a multi-antenna approach with suitable digital-beamforming and array-processing procedures that allow the directed nulling of unwanted signals is by far superior to a single-element solution [1]. However, the number of interferers or multipath signals that can be suppressed simultaneously is determined by the number of antenna elements. In a conventional array architecture, where the distance between the elements is usually one-half free-space wavelength, the physical size of the antenna may limit possible applications. Particularly in an environment with numerous interferers from different directions, many antenna elements are required, and it will be difficult to integrate the resultant large terminal antenna into moving platform structures like aircraft and vehicles. To overcome this problem, both the single-element dimensions and the inter-element distances have to be reduced significantly. A major drawback of small and dense antenna arrays is mutual coupling since it affects the accuracy of null positioning and the degree of interference suppression. Therefore, novel techniques have been developed to cope with strong mutual coupling. In this paper, a concept of a compact navigation receiver employing a miniaturized antenna array is presented. The work has been performed within the project KOMPASSION (Compact Adaptive Terminal Antenna for Interference-free Satellite Navigation), which is funded by the Space Administration of the German Aerospace Center (DLR) on behalf of the Federal Ministry of Economics and Technology. In order to miniaturize the size of the single antenna elements, microstrip patches on a substrate with high permittivity have been developed. In this context, different structures and array configurations have been investigated. The problem of mutual coupling has been tackled by a combination of hardware and software techniques of which the basis was developed in [2]. At first, the array reception patterns of the highly coupled antenna elements are decomposed by means of a decoupling and matching network (DMN) to an orthogonal subspace of beams so that every output of the network represents a different orthogonal beam. After RF processing by means of CMOS-integrated RF front ends and A/D converters, the proposed digital-beamforming algorithms must operate in the beam space, rather than in the usual element space. The challenges are set upon the fact that the beams differ in their shape, gain (SNR) and radiation efficiency. To support the compactness of the receiver, the single front ends will be fabricated on an application-specific integrated circuit (ASIC) in 0.18-m CMOS technology combining several receiving paths. A baseband calibration signal using a reserved pseudo-random-noise (PRN) sequence is also fed into the front ends, to correct for variations in the analogue signal path. The digital system includes several analog-to-digital and digital-to-analog converters (ADCs and DACs) connected to a field programmable gate array (FPGA), where high-rate base-band processing modules are employed. The FPGA interacts with a personal computer (PC) that controls these modules and computes the directions of arrival and interference (DoA/DoI) as well as the final position, velocity, and time (PVT). The eigen-decomposition of the antenna-array radiation patterns and the overall system performance have been investigated by simulation in detail for different types of single elements and array configurations. It has been demonstrated, that DoA/DoI estimation is feasible with this approach, as well as the efficient generation of nulls by suitable beamforming algorithms to suppress interference and jamming. We will present the conceptual design for a compact adaptive multi-antenna receiver and simulated results of the system performance and its components. Currently, a demonstrator is being built, comprising a quadratic array of 2x2 patch antenna elements placed at an inter-element distance of a quarter of the free-space wavelength. The actual state of the demonstrator development will be reported and first results from intermediate testing will be given. [1] M. V. T. Heckler, M. Cuntz, A. Konovaltsev, L. A. Greda, A. Dreher, and M. Meurer, “Development of robust safety-of-life navigation receivers” IEEE Trans. Microw. Theory Tech., vol. 59, no. 4, pp. 998–1005, April 2011. [2] J. Weber, C. Volmer, K. Blau, R. Stephan, and M. A. Hein, “Miniaturized antenna arrays using decoupling networks with realistic elements,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 6, pp. 2733–2740, June 2006.