Juha Laurila

Statistical characterization of urban spatial radio channels

We present a statistical analysis of wideband three-dimensional channel measurements at base station locations in an urban environment. Plots of the received energy over azimuth, elevation, and delay planes suggest that the incident waves group to clusters in most measured transmitter positions. A super-resolution algorithm (Unitary ESPRIT) allows one to resolve individual multipath components in such clusters and hence enables a detailed statistical analysis of the propagation properties. The origins of clusters-sometimes even individual multipath components-such as street apertures, large buildings, roof edges, or building corners can be localized on the city map. Street guided propagation dominates most of the scenarios (78%-97% of the total received power), while quasi-line-of-sight over-the-rooftop components are weak(3%-13% of the total received power). For this measurement campaign, in 90% of the cases, 75% of the total received power is concentrated in the two strongest clusters, but only 55% in the strongest one. Our analysis yields an exponential decay of power with 8.9 dB//spl mu/s, and a standard deviation of the log-normally distributed deviations from the exponential of 9.0 dB. The power of cross-polarized components is 8 dB below copolarized ones on average (vertical transmission).

Wideband 3D characterization of mobile radio channels in urban environment

This paper describes three-dimensional (3D) radio channel measurements at the base station site in an urban environment. We introduce a measurement concept which combines an RF switched receiver array and a synthetic aperture technique and allows full 3D characterization of the channel. Additionally, dual-polarized patch antennas as array elements enable full determination of the polarization properties of the impinging signals. We describe measurements at over 70 different transmitter positions and three receiver array sites with different sectors and antenna heights. Our results show that the received energy is concentrated within identifiable clusters in the azimuth-elevation-delay domain. We demonstrate that the observed propagation mechanisms are mainly determined by the environment close to the base station. Street canyon propagation dominates also when the receiver array is at or even above rooftop level with the studied measurement distances. Thus, the azimuth spectrum at the BS site is fairly independent of the location of the mobile. Signal components propagating over the rooftop are often related to reflections from high-rise buildings in the surroundings.

3-D double-directional radio channel characterization for urban macrocellular applications

We measured the spatial properties of the three-dimensional (3D) double-directional radio channel in urban macrocell environments separately at both ends of the link. In this paper, we study propagation conditions pertaining to reception and transmission at the mobile terminal, measured using a wideband channel sounder and a dual-polarized spherical antenna array. We were able to refine the results of the measurements conducted at the base station, and extend the study to full double-directional 3D channels. Individual propagation paths could be identified precisely, in some cases even considerable scattering from lampposts was observed. Our results show that over-rooftop-dominated propagation often occurs via building roofs with LOS to the base station antenna, acting as strong secondary signal sources. Based on measurements along continuous routes we demonstrate that the dominant propagation mechanisms can vary considerably when the mobile moves in the environment. We also present typical directional properties of the 3D radio channel at the mobile terminal in urban macrocell environments characterized by street canyons, showing how the angular distribution of energy is correlated with the excess delay.

Geometry-based directional model for mobile radio channels -principles and implementation

Adaptive antennas are used in mobile radio systems for improving coverage and increasing capacity. For realistic system design and simulation, channel models are required in order to assess the performance of such systems. We propose the so-called ‘geometry-based stochastic channel model’ (GSCM), which is easy to implement and has a low complexity even when the directional dimension of the channel is taken into account. In this model, we prescribe the probability density function (PDF) of the location of the scatterers. From this, the angularly resolved impulse response can be computed by a simple ray tracing. We then present various extensions of the model in urban environments, namely for diffraction losses and wave guiding by street canyons. We also discuss methods for efficiently implementing GSCM. Copyright

Downlink beamforming for frequency division duplex systems

We propose a downlink beamforming method for an adaptive antenna system utilizing our new spatial covariance matrix transformation (SCMT). Therefore we first estimate the azimuthal power spectrum (APS) of the received uplink data for all users within the same physical channel. Afterwards we modify the APS of the user and the interference to prevent beampointing errors for the desired user and improve the nulling capabilities. Finally we construct the spatial covariance matrices at the downlink frequency and use this spatial information for antenna weight calculation. The approach is applicable for any adaptive antenna system with FDD. It outperforms direction of arrival based methods by at least 10 dB in SNIR. Applying the SCMT we approach the upper downlink limit within 4 dB in urban environments.

Influence of the scatterer distribution on power delay profiles and azimuthal power spectra of mobile radio channels

We consider the distribution of scatterers that are responsible for the temporal and angular dispersion of mobile radio channel models. In a first step, we concentrate on the local scatterers around the mobile station. Various approximations for this distribution are analyzed, and their effect on the PDP (power delay profile) and APS (azimuthal power spectrum) are derived. We find that a one-sided Gaussian distribution gives the best agreement with measurements. Measured PDPs are usually exponential and APSs are usually Laplacian. We analyze the difference between single and multiple scattering. Finally, we consider the influence of scatterers that, are situated far away from the mobile station.

Signal Power Distribution in the Azimuth, Elevation and Time Delay Domains in Urban Environments for Various Elevations of Base Station Antenna

We investigate signal power distribution in the azimuth-of-arrival, elevation-of-arrival and time-of-arrival domains for various positions of the base station antenna located below the rooftop as well as at rooftop level. This article is based on a multiparametric stochastic model we introduced in 2004, as combination of a statistical part, describing an array of buildings randomly distributed at the terrain, and a waveguide model, describing a grid of straight streets with buildings along them. Joint signal power distributions in azimuth-time delay and elevation-azimuth planes are obtained and compared to high-resolution 3-D measurements carried out in downtown Helsinki. A good agreement between theoretical predictions and the measurements is obtained basically, and also regarding the wave-guiding effect and antenna height dependencies. A satisfactory physical explanation, which accounts for the character of the specific building topography, the height and tilt of the antennas, is found. Finally, we present a numerical experiment of changing the base station antenna height, its directivity, and tilt. By this we show that the proposed stochastic approach allows to predict and control a-priori main parameters of smart antenna based only on knowledge of specific features of built-up terrain

Downlink performance of adaptive antennas with null broadening

We compare different null broadening schemes and their ability to decrease the radiated interference for the co-channel users in a space division multiple access system with adaptive antennas. By link level simulations of the GSM 1800 system in urban and suburban environments we show that a gain of up to 8 dB in SNIR is feasible with null broadening. Therefore adequate null broadening seems to be a must in the downlink beamforming process.

Spatial characterization of urban mobile radio channels

We present statistical evaluations of wideband spatial dual-polarized radio channel measurements. Some 80 different street-level transmitter positions in downtown Helsinki, Finland, were measured. We chose three different receiver array sites below, at, and above the rooftop level of the surrounding urban environment. A super-resolution algorithm allows the determination of the directions of arrival of incident radio waves at a 16/spl times/62 elements base station antenna array. The main conclusion of our investigations is that the incident waves tend to cluster in azimuth, elevation and/or delay, which we justify by detailed statistical evaluations. Clustering is mainly defined by the surroundings of the base station. The cumulative distribution function of the powers of these clusters shows that in 90% of all cases, 75% of the total received power is concentrated in the two strongest clusters. Street guided propagation dominates at all three receiver sites, even if the base station is above the rooftop level.

Semi-blind separation and detection of co-channel signals

We propose a semi-blind algorithm for smart antennas that, in addition to some structural signal properties, utilises user identifiers that are available in existing cellular systems. Our algorithm estimates, first, the row span of the adequately composed data matrix and then projects the obtained basis vectors iteratively to the finite alphabet (FA) constellation. We call the projection part of our algorithm DILSF (decoupled iterative least-squares with subspace fitting). Assuming two co-channel users in a GSM-like TDMA system and a Rayleigh fading channel with finite angular spread, we obtained a BER<10/sup -3/ with only 3 antennas, spaced 10/spl lambda/ apart. We also demonstrate that co-channel signals are resolvable even if they are not separable in angle. We gain a factor of up to 70 in computational complexity by employing subspace tracking algorithms and demonstrate the robustness of the method against array imperfections.