J. Bach Andersen

Antenna arrays in mobile communications: gain, diversity, and channel capacity

Are antenna arrays in mobile communications different from arrays in other applications? The answer is yes, sometimes, and it is the purpose of this paper to explain, in a tutorial fashion, when this is the case, and what this means for path loss in link calculations. One aspect is the classical gain of an antenna, which we have to understand in a new way. Another aspect is the possibility for two arrays, in a scattering environment, to create parallel channels, and thus, in effect, act as many independent antennas at the same time, carrying much more traffic over the same bandwidth.

Integrated antennas for hand-held telephones with low absorption

Antennas for hand-held telephones must have high efficiency due to the limited power available, and recent demands call for integrated antennas in hand-held telephones. A full short circuit planar inverted-F antenna (FS-PIFA) is investigated, which concentrates the current near the antenna, and has a relatively higher gain directed away from the users head thereby reducing the losses in the human body, which is important from the point of view of efficiency and possible health hazards. For design and analysis of the antenna including parts of the human body the finite difference time domain (FDTD) method is employed, and measurements of average received power and spatial peak specific absorption rate, SAR, values were performed, to verify simulation results. Measurements of the antenna gain in realistic environments with a person showed a gain about 2 dB less than a half wavelength whip on the same telephone, while the local SAR was typically 0.1-0.2 mW/cm/sup 3/, a factor of ten smaller than present standards and handportables. >

Theoretical limitations on radiation into muscle tissue

The radiation into muscle tissue from a planar array of magnetic dipoles is discussed. A figure of merit (gain) of an applicator involving power density at a point is defined. For constant illumination the gain is almost independent of frequency for a fixed aperture. Illuminations maximizing gain are tapered near the edge of the array and focused.

A Multiport Patch Antenna for Mobile Communications

Application of the circular patch antenna at the mobile is discussed. Diversity branches are available from the same patch by feeding at different backplane ports. The conditions for diversity action and appropriate analysis of the antenna are reviewed.

A general formulation of applied potential tomography.

Reconstruction of the conductivity of a medium from a set of known values of the boundary potential and boundary normal current is described. The geometry is general, two or three dimensional. The method is applied to two cases, a circular medium and an infinite medium under a plane. In both cases the conductivity is found as an expansion on a system of orthogonal functions. In a linear approximation the conductivity may explicitly be reconstructed. The problem is an ill posed one, the condition numbers are found. Some numerical examples are included.

Propagation aspects of datacommunication over the radio channel-a tutorial

The various error mechanisms in the time-varying, frequency-selective mobile channel are discussed from a propagation point of view. Starting with a two-ray model, both the frequency and time domain are discussed for the narrowband case, where the delay spread is less than the symbol period, and for the more general broadband case. the major importance of the product of bandwidth and delay spread is stressed.<<ETX>>

Quasi-static Profile Reconstruction of a Circular Cylinder

Reconstruction of a radially varying conductivity in a circular cylinder is considered by exciting the cylinder with a quasi-static current and measuring the potential distribution on the surface. A nonlinear integral equation is derived from which the conductivity distribution may be obtained from Fourier harmonics of the surface potential. A linear version is used for studies of uniqueness, resolution and accuracy. Using the orthogonality of shifted Legendre polynomials an explicit solution for the conductivity is given, avoiding numerical inversion problems. The solution is only unique if the conductivity on the axis is known. The resolution is determined by the maximum number N of Fourier harmonics, and the sensitivity of the solution increases rapidly with N. The nonlinear version, although non-unique in principle, is solved numerically by iteration with a considerable improvement of the reconstruction.

Polarization Properties of the Helix Antenna

The mode structure and surface fields of a helix with a dielectric core have been found. Conditions for constructing an antenna with perfect circular polarization in all directions have been identified and by defining a circular polarization index (CPI) optimum antennas may be found. The CPI essentially measures the degree of satisfaction of the Rumsey-conditions, ¿ = ±jZoH¿, for the surface fields. With a price of a smaller bandwith the presenc~ of a suitable dielectric is able to improve the wide angle polarization properties of the helix antenna.

Overview of antenna problems and solutions for multi-Gb/s links

Fundamental limitations in link budgets for wireless indoor networks are considered for millimeter and THz frequencies. Single small antennas at those frequencies are a bottle neck due to the small absorption areas and the ensuing path loss. Although different for LOS links and links in diffuse surroundings, there are no theoretical limitations, if complexity is ignored. Complexity and power consumption may be reduced by using analog solutions where possible, and in the paper we focus on phase conjugation methods, specifically in retrodirective arrays. Considering power solutions only, mimicking optical networks, we call attention to an array of rectennas.

A stochastic model in mobile communications

We propose a stochastic model for the variations in the transfer function for the mobile communication environment based on the complex Gaussian distribution. The model is a slight generalization of the classical and allows the characteristics of the scenario to be frequency dependent. Under the assumptions of this model we show that the group delay and instantaneous Doppler shift both follow a Students t distribution with two degrees of freedom.