Tommi Laitinen

Theory and Practice of the FFT/Matrix Inversion Technique for Probe-Corrected Spherical Near-Field Antenna Measurements With High-Order Probes

A complete antenna pattern characterization procedure for spherical near-field antenna measurements employing a high-order probe and a full probe correction is described. The procedure allows an (almost) arbitrary antenna to be used as a probe. Different measurement steps of the procedure and the associated data processing are described in detail, and comparison to the existing procedure employing a first-order probe is made. The procedure is validated through measurements.

Reproduction of Arbitrary Multipath Environments in Laboratory Conditions

We discuss the synthesis of arbitrary multipath environments in a spherical volume of space (test zone), with a limited number of feed antennas (probes). The probes can be of any type, e.g., simple dipoles. The required number of probes is proportional to the area of the sphere enclosing the test zone. The signal received by a two-port mobile terminal antenna model placed in the test zone is examined through simulations, using measured real-world propagation channel data. We study how the received signal and the channel capacity are affected by truncation errors and a certain noise level in the probe excitations. This kind of synthesis enables the testing of mobile terminals under realistic operating conditions in laboratory environments. The synthesis is not limited to far-field scenarios, but near-field effects can be generated as well.

Measurement-Based Analysis of Spatial Degrees of Freedom in Multipath Propagation Channels

A method of estimating spatial degrees of freedom (DoF) from measured multipath propagation channels in the multiple-input single-output regime is presented. The DoF of the multipath channels on the transmit (Tx) side is derived by means of the spherical-wave expansion of electromagnetic fields radiated from a Tx antenna array having a certain aperture size. The DoF estimates are independent of particular realization of antenna elements on the Tx aperture. For a given aperture size, the DoF provides the number of the Tx antenna elements for efficient improvement of the system performance by utilizing the spatial diversity. Having confirmed the soundness of our DoF estimation method by channel measurements in an anechoic chamber, the DoF of indoor multipath channels is analyzed. The DoF on the Tx side of the considered multipath channels reveals larger values when the antenna aperture size is increased at a fixed frequency, and when the frequency increases for a fixed antenna aperture size. For a fixed frequency, increasing the antenna aperture size is more effective in observing extra DoF in the obstructed and non-line-of-sight channels than in the line-of-sight channels. Furthermore, for a fixed antenna aperture size, the use of the higher frequency brings larger DoFs in many propagation scenarios. The results also show that electrically smaller antennas are more efficient in observing the DoF. Finally, the solid angle of multipath is derived as a Tx antenna-independent metric of the multipath richness. It is defined as an angular range of dominant multipath clusters subtended on a unit sphere. Our analysis method is extendable to the multiple-input multiple-output regime in a straightforward manner.

Modified Test Zone Field Compensation for Small-Antenna Measurements

A method is presented by which full 3-D antenna pattern measurements can be performed without an anechoic measurement environment. The method is a field compensation technique that allows compensating for the effect of arbitrary incident test zone fields. Simulations are performed to demonstrate that the method works reliably in various kinds of surroundings. The applicability range of the method is estimated in terms of the accuracy of the determined far-field antenna pattern with a given uncertainty in individual measurement values. Finally, measurement results are presented that are in line with the simulations and verify the functioning of the method.

On a MIMO-OTA testing based on multi-probe technology

Standardization work of the OTA test method for MIMO terminals is currently on-going. This paper discusses, mainly from the electromagnetics viewpoint, field synthesis with multi-probe technology with the emphasis on 2-D field synthesis using a circular array. Furthermore, opportunities and practical challenges of multi-probe technology for MIMO-OTA testing are discussed briefly.

Fast Fourier Transform preprocessing for accelerated plane wave based spherical near-field far-field transformation

Near-field measurement and transformation techniques are widely applied to characterize the radiation pattern of antennas. Spherical near-field measurements have been researched widely and various techniques with different probe compensation capabilities and complexities exist. Among those techniques applicable for (almost) arbitrary probes and based on spherical wave translations, the crucial computational relaxations have been gained through the use of a Fourier Transform based preprocessing of the measurement data. It is shown in this paper that the same Fourier Transform based preprocessing step can be applied in conjunction with the plane wave based probe-corrected near-field far-field transformations. The collection of probe signals is split into smaller sub sets by an Inverse Fast Fourier Transform. These sub problems can be solved with a reduced overall complexity and also a full probe correction is achieved.

Azimuthal Mode Decomposition Method for Cubical Surface Scanning With the Application to Spherical-Mode Field Synthesis

In this letter, a method of azimuthal mode decomposition is presented for cubical surface scanning. This method can be applied either in near-field-to-far-field (NF-FF) transformation in near-field antenna measurements or in synthesizing the fields using the virtual array principle. The method is presented for spherical-wave-mode field synthesis application in this letter. The method allows a reduction in the computational burden of the matrix inversion for synthesizing the fields. Requirements for the sampling locations on the cubical scanning surface and for the probe orientations and polarizations at each sampling location allowing the azimuthal-mode decomposition are provided.

Hybrid fast fourier transform-plane wave based near-field far-field transformation for “body of revolution” antenna measurement grids: The cylindrical case

Near-field antenna measurements are a common technique to characterize the radiation behavior of antennas. Due to the measurement in the radiating near field, a postprocessing near-field far-field transformation is required to finally compute the radiation pattern. For the near-field transformation a low numerical complexity as well as full probe correction capabilities are highly desirable. In this paper a hybrid near-field far-field transformation algorithm is presented which combines a Fast Fourier Transform preprocessing with the plane wave based fully probe corrected near-field transformation of low numerical complexity being suitable for “body of revolution” antenna measurement grids. The Fast Fourier Transform splits the probe signals into azimuthal Fourier modes which are processed individually by the plane wave based transformation algorithm. In this way the computation time can be reduced while the numerical complexity is unchanged. In this paper the hybrid technique is applied to cylindrical near-field measurements which are superior for the measurement of fan beam antennas.

Separation of radiation from two sources from their known radiated sum field

This paper presents a technique for complete and exact separation of the radiated fields of two sources (at the same frequency) from the knowledge of their radiated sum field. The two sources can be arbitrary but it must be possible to enclose the sources inside their own non-intersecting minimum spheres so that the closest distance between the surfaces of the two spheres is of the order of at least a few wavelengths.

Toward accurate antenna measurements using multi-probe systems

In this paper a few important aspects related to spherical multi-probe antenna measurements and their future research and development needs are discussed. In particular, the significance of the test zone field compensation technique as an enabler for accurate antenna measurements with multi-probe systems is addressed.