[1] Array processing usually operates on identical sensors. In this paper an investigation is made to apply the direction finding algorithm MUSIC to an antenna array with nonidentical elements (heterogeneous array). This original solution is polarizationsensitive though it resorts to only one sensor at each point of the spatial sampling. As a major consequence of its structure, the computation of the MUSIC algorithm is based on an expression of the steering vector that integrates the spatial response of each sensor. This concept is then validated in the special context of HF radio direction finding, for which a deterministic model of the polarization at the exit of the ionospheric channel is derived. A multichannel radio receiving system has been developed and experimented using transmitters located more than 1000 km from the receiving site.
EEKTEL Louis, MARIE Franqois 1. S.R./I..K. UPKES-A CNRS 6075 Universite de Rennes I 35042 KENNES-Ccdex FRANCE
I INTRODUCTIONThis paper presents an original method of direction finding operating on an array of collocated antennas. At first. we derive the classical MUSIC algorithm for an heterogeneous array, i.e. made up of different sensors : their complex directional responses, which are supposed to be well known, arc integrated in the normalized steering-vectors. We underline the advantages induced by this technique in the field of radio direction finding. specially as two types of polarization are received (right or left circular polarization in the V.H.F range, ordinary or cxuaordinary polarization in the H.F. range). Then. we show that the diversity of the sensors c m replace to some extent the space diversity usually provided by classical arrays. Actually. the array manifold does not only depend on the geometry but also on the directivity functions of the sensors. In those conditions, it becomes feasible to find the directions of arrival with a small array aperture. and possibly with collwated sensors. At last. we present experimental validations in the H.F.(3-30MHz) range.
II MUSIC A1,GORITHM OPERATING ON AN ARRAY OF IDENTICAL
SENSORSThe parametric inclhods aim lo estimate parameters which are bound to the incident sources such as angles of arrival, power ... by the computation of P directional function from the steering vector of the may. For a sourcc which direction of arrival is denoted 6 (angle or couple of angles in a 3-D problem), this vector is written as F(6) is the common directional response of the NC sensors (supposed to be identical) and %(e) is the geometrical phase of sensor k (sensor I being chosen as a reference).The MUSIC algorithm is based on the orthogonality of the incident steering-vectors and the noise subspace of the datii covarimce inabix : the computation of the pseudo-spectrum uses the norm of the projection of the normalized slecring-vector b(6) = in that subspace:
la(g)Iwhere the set of vectors Vk delinc the noise suhspace.As shown below, the pseudo-spectrum PSSP(9) can be computed without the knowledge of the sensor responses F(6) :where biso (0) is the normalized stcering-vector for an array of iaotropic sensors and -Further on, 0-7803-4478-2/98/$10.00 0 1998 IEEE 207
Different applications of radio systems are based on the implementation of antenna arrays. Classically, radio direction finding operates with a multi channel receiving system connected to an array of receiving antennas. More recently, MIMO architectures have been proposed to increase the capacity of radio links by the use of antenna arrays at both the transmitter and receiver. The first part of this paper describes some novel experimental work carried out to examine the feasibility of applying MIMO techniques for communications within the HF radio band. A detailed correlation analysis of a variety of different antenna array configurations is presented. The second section of the paper also deals with HF MIMO communications, focusing on the problem from a modelling point of view. The third part presents a sensitivity analysis of different antenna array structures for HF direction finding applications. The results demonstrate that when modelling errors, heterogeneous antenna arrays are more robust in comparison to homogeneous structures.
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