Claes Beckman

Shared networks: making wireless communication affordable

In the wake of the substantial financial commitments incurred by European and other UMTS network operators in the form of licence fees, licensees have turned to network sharing as a means of reducing the capital requirements needed to bring 3G services to market. The reception from European regulators has been mixed, due to concerns that this inhibits competition, slow buildout, or otherwise result in reduced consumer benefits. The authors believe that the generic product life cycle model provides insights that indicate that network sharing, within an appropriately constructed regulatory framework, is not a threat to vigorous competition in the 3G industry, and in fact is one of the keys to stimulating the development of advanced, ubiquitous, affordable services.

Glare and contrast sensitivity before and after Nd:YAG laser capsulotomy

Abstract. The benefits of Nd:YAG laser capsulotomy on visual peformance were evaluated in 13 patients with after‐cataract following extracapsular cataract extraction. Besides clinical evaluation we measured: visual acuity, letter contrast sensitivity with and without the presence of a circular glare source (glare angle: 8°). Finally, the patients were asked to evaluate their subjective glare problems. In all patients, visual acuity had decreased markedly compared to measurements 2 months after cataract surgery. Before capsulotomy we observed impaired letter contrast sensitivity and increased intraocular light scatter compared to normals. Following capsulotomy, VA and contrast sensitivity both improved by a factor of 2. The subjective glare problems diminished, although most patients showed minor decreases in intraocular light scatter. The limited glare induced loss of letter contrast sensitivity, present before capsulotomy, remained after treatment. Our study indicates that after‐cataract impairs resolution and contrast sensitivity without necessarily causing a marked increase in light scatter as measured at a large angle.

Dual-polarized array for signal-processing applications in wireless communications

A novel dual-polarized antenna array designed for a spatial division multiple access (SDMA) system working in the 1850-1990-MHz band is designed and built. The antenna is designed to have similar element patterns, and measurements of S-parameters and radiation patterns are presented. The array signal processing performance of the array using all elements and with no compensation for mutual coupling or differences in element patterns is studied through direction-of-arrival (DOA) estimation using total least squares estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT). The results show that the accuracy of the DOA estimates is quite acceptable for wireless communication applications.

Disputing Viking navigation by polarized skylight

The widely held notion that the Vikings utilized polarization of skylight on overcast days for navigational purposes is demonstrated to have no scientific basis. The use of polarized skylight for navigation under partly cloudfree skies should be treated with caution and skepticism.

The Evolution of Base Station Antennas for Mobile Communications

This paper gives a general overview of the design of base station antennas for mobile communications. It explains underlying theoretical and practical implementation aspects in mobile communication networks of today and the future. In the first part the fundamental parameters of a base station antenna are discussed in the context of radio network design. In particular we discuss parameters such as gain, radiation patterns, frequency bands and power handling and put them in the context of cell planning, propagation and capacity. In the final parts of the paper we give an overview of the underlying theory of diversity and MIMO systems.

Measurements of the spatio-temporal polarization characteristics of a radio channel at 1800 MHz

We present a combined measurement and analysis of the spatial, temporal, and polarisation characteristics of a 1800 MHz radio channel. The measurements were performed using a correlative channel sounder. A complex chirp signal with a bandwidth of 150 MHz was transmitted by a vertically polarized omnidirectional antenna and received by a 8+8 dual polarized planar antenna array. Measurements were taken at two different base station locations, one being suburban and the other urban. Using this data we characterize the radio channel in terms of azimuth and delay spread of the vertical and horizontal polarization.

Optical properties of diffractive, bifocal, intraocular lenses

The resolution of diffractive, bifocal, intraocular lenses was studied with regard to pupil displacement and diameter size through computer simulations, bench measurements, and patient vision-acuity measurements. Good agreement was obtained between these three methods of investigation. In particular, we find that pupil displacements of the order of 1 mm reduce the resolution considerably for these lenses.

Confocal fundus imaging with a scanning laser ophthalmoscope in eyes with cataract.

AIMS/BACKGROUND--The study aimed to determine the influence of increased intraocular light scatter on the contrast in scanning laser ophthalmoscope (SLO) images and to examine to what extent SLO images can visualise the fundus through media opacities due to cataract. METHODS--Intraocular light scatter was estimated from measurements of letter contrast sensitivity before and after cataract surgery in five eyes. SLO images were obtained before and after surgery using confocal apertures of 1, 2, 4, and 10 mm, at laser wavelengths of 633 and 780 nm. Visibility of the fundus was determined by measurements of retinal contrast. SLO images were compared with standard fundus photographs. RESULTS--SLO images obtained before surgery revealed details of the retina that were unresolvable in the fundus photographs because of light scattering. By using one of the three smallest apertures, image contrast was further improved. However, no simple relations between aperture size, estimated light scatter, and image contrast could be found. CONCLUSION--SLO imaging was found to be superior to fundus photography for viewing the retina in eyes with cataract. Owing to the inhomogeneous nature of cataracts, the optimal choice of confocal aperture and laser wavelength is not simple and must be individualised.

Comparison of three methods of evaluating glare

Abstract A Humphrey Automatic Refractor Model 570 was used to measure the impairment of visual acuity for low contrast optotypes as a result of glare for normal subjects and for subjects with cataracts. This was compared with a direct measure of intraocular light scatter as measured by a compensating technique and with a subjective assessment of glare determined by a questionnaire. Only a weak correlation (r = 0.28) was found between the decrease in low contrast letter acuity and intraocular light scatter. The subjective assessment of glare sensitivity was not correlated with the decrease in low contrast letter acuity, but correlated to some extent (r = 0.55) with the measurement of intraocular light scatter.

Evaluation of antenna diversity performance for mobile handsets using 3-D measurement data

An evaluation of two-branch antenna diversity on mobile telephones is performed in the presence of head and hand phantoms to model the influence of the user. Two mobile telephone prototypes for the 1800-MHz band are considered. The evaluation is done by measuring the three-dimensional (3-D) complex radiation patterns from each antenna with the prototypes positioned near the phantom head and hand (i.e., in talk position) and calculating the diversity performance for three different scenarios of the incoming field. The two-branch antenna setups on both prototypes produce a calculated diversity gain of about 9 dB for selection diversity at the 99% reliability level.