Modeling wireless fading channels via stochastic differential equations: identification and estimation based on noisy measurements

This paper is concerned with modeling and identification of wireless channels using noisy measurements. The models employed are governed by stochastic differential equations (SDEs) in state space form, while the identification method is based on the expectation-maximization (EM) algorithm and Kalman filtering. The algorithm is tested against real channel measurements. The results presented include state space models for the channels, estimates of inphase and quadrature components, and estimates of the corresponding Doppler power spectral densities (DPSDs), from sample noisy measurements. Based on the available measurements, it is concluded that state space models of order two are sufficient for wireless flat fading channel characterization.     

Propagation characteristics on microcellular urban mobile radiochannels at 910 MHz

The results of measurements made to determine propagation characteristics on urban mobile radio channels with low base-station antennas and line of sight between the base and mobile units are reported. Results show that multipath propagation conditions would be significantly less severe if small-celled systems were implemented. Root-mean-square delay spread averages computed by considering all multipath signal components with powers greater than 25 dB below the peak were reduced by a factor of approximately four in comparison to those typical in conventional systems. In addition, microcellular-type channels have Rician, rather than Rayleigh, envelope fading characteristics and correspondingly different frequency correlation statistics. The area bounded by the correlation envelope is introduced as a measure of channel quality, and these areas are shown to be larger for Rician channels than for Rayleigh channels     

Cavity ring-down measurements in flames using a single-mode tunable laser system

High-resolution pulsed cavity ring-down spectroscopy (CRDS) of OH in a flame has been demonstrated using a novel single-mode tunable laser (STL). This system operates by pulse amplification of the output of a single-mode diode laser in a modeless dye laser. Ring-down curves obtained using the narrow-bandwidth STL, for both strong and weak transitions, are shown to be well fitted by single exponentials. These results are demonstrated to be in direct contrast with those obtained using a standard dye laser, for which the bandwidth is comparable to the transition linewidths and ring-down curves require multi-exponential fits.Accurate lineshape analysis is thus made possible using the STL, allowing the temperature to be derived from the measured Doppler width. The resulting measurement is in good agreement with the value derived from a Boltzmann plot of data obtained using a conventional laser in a similar flame. The advantages of using the STL system for quantitative CRDS measurements are discussed, together with a suggestion for quantitative measurements of the ASE content of narrowband lasers using CRDS. PACS 82.33.Vx; 42.60.-v; 42.62.Fi     

Propagation measurement-based probability of error predictions fordigital land-mobile radio

This paper demonstrates that closed-form equations can be used to calculate error rates for π/4-shift differential quadrature phase-shift keying (DQPSK) directly from time-series measurements of impulse-response data on frequency-selective land-mobile radio channels. Results are quantitative and free from uncertainties that can accompany such predictions when they are made using qualitative relationships based on assumed channel characteristics. Calculations using the demonstrated method show that significantly greater signal-to-noise ratios are required on macrocellular (MC) channels than those required on microcellular (μC) channels for the same performance, In addition, it is shown that MC channels require channel protection, even for IS54 performance levels, whereas higher data rates (up to 250 kbps at 10^-3) and lower error probabilities (down to 10^-4 at 48.6 kbps) are possible on line-of-sight (LOS) μC channels without such protection. This is considered to be crucial information in the design of future systems. Through the analysis of quantitative results, not available elsewhere, it is shown that correlations between rms delay spread and performance measures, such as unprotected channel-error-rate floors and maximum data rates, are poor. Recommendations are, therefore, made regarding other parameters (the Rician K ratio and another parameter Vol, defined in the present paper) that can be derived through simple propagation measurements and used to predict these performance characteristics with greater assurance     

UTILIZATION OF TOOLS FOR SUCCESSFUL. TROUBLESHOOTING OF STRUCTURAL PROBLEMS

A structural failure problem was solved using an integrated and iterative program of testing and analysis. The steps taken in solving the problem were: analytical calculations; operational testing; qualifications of analytical results; problem identification; design of corrective action; and confirmatory testing.     

Propagation considerations for the design of an indoor broad-bandcommunications system at EHF

This paper reports the measurement and analysis of wideband propagation data for indoor radio channels at 40 GHz. Propagation characteristics are reported for two open-concept office areas of different sizes in two different buildings. Also, the results of measurements in one building are compared for system configurations in which either an omnidirectional or a narrowbeam antenna is employed at a base station for communications to multiple work stations with omnidirectional antennas. It is reported that, on a statistical basis, at the 90th percentile, multipath dispersion is the same for the two base-station antenna radiation patterns. Dispersion was, however, found to be lower in the smaller of the two measurement areas, where transmit/receive ranges were shorter. The 90th percentile of static RMS delay spread for this area was 19 ns compared with 45 ns for the larger area. Multipath spreads at the -25 dB relative power level were about 370 ns, compared with 140 ns in the larger area. In the larger area, global propagation loss was found to be well modeled by the one-way propagation equation with different range exponents (1.5 and 4) before and after a breakpoint at a range of 25 m. In the smaller area, the range exponent was found to be greater, being equal to 3.5. This is considered to be a result of reduced multipath infill, which would accompany the reduced dispersion. Temporal fading on fixed links with omnidirectional antennas was found to have depths such that a 14-dB fade margin is required for 99% reliability. Finally, spatial variations in received power at a given range indicated the requirement for a power margin between 4-7 dB for 99% reliability. These results are used in a link budget example for a broad-band indoor extremely high frequency (EHF) digital communications system     

Use of cell-site diversity in millimeter-wave fixed cellularsystems to combat the effects of rain attenuation

An examination of potential advantages of cell-site diversity with selection combining in the context of MM-wave fixed cellular systems is reported. The study involves simulation of converging radio links over weather radar images of the radar reflectivity factor, from which the specific attenuation of rain at 30 GHz is derived. The average correlation of attenuation on two converging links as a function of their angular separation is shown to indicate the potential benefits in the use of cell-site diversity, especially in heavy rain. Results show that diversity gain exhibits a dependence on angular separation &thetas; in the general form of sin^k (&thetas;/2). For links of identical lengths the model reduces to a root-sinusoidal shape (k=0.5), whereas links of unequal lengths lead to the ITU-R recommended model (k=1). Based on the sin^k (&thetas;/2) model and observation of the length ratio of the links, a set of criteria for determining the benefit of cell-site diversity for a given subscriber location is proposed     

Vortex lattice transitions in YNi2B2C

We have performed extensive small-angle neutron scattering (SANS) diffraction studies of the vortex lattice in single crystal YNi₂B₂C for B‖c. High-resolution SANS, combined with a field-oscillation vortex lattice preparation technique, allows us to separate Bragg scattered intensities from two orthogonal domains and accurately determine the unit cell angle, β. The data suggest that upon increasing field there is a finite transition width where both low- and high-field distorted hexagonal vortex lattice phases, mutually rotated by 45°, coexist. The smooth variation of diffracted intensity from each phase through the transition corresponds to a redistribution of populations between the two types of domains.