Peter D. Holm

A new heuristic UTD diffraction coefficient for nonperfectly conducting wedges

A new heuristic UTD diffraction coefficient for non-perfectly conducting wedges is proposed. The coefficient is an extension of the heuristic one given by Luebbers (1984) and as simple as that to compute. In the case of forward-scattering and neglecting the surface wave effects, the new coefficient gives a result close to Maliuzhinetss (1958) solution, also deep in the shadow region where the previous one fails. Moreover, it makes the special care used by Luebbers to deal with grazing incidence unnecessary.

UTD-diffraction coefficients for higher order wedge diffracted fields

An asymptotic expansion of a multiple integral for knife-edge diffraction is derived. The expansion expresses the field in terms of single-edge diffraction. By considering this expression, a similar expansion for higher order UTD diffracted fields is proposed. By means of a set of transition functions, the result removes some of the shortcomings of the original set out of the UTD when the incident field is not a ray-optical field.

Wide-Angle Shift-Map PE for a Piecewise Linear Terrain—A Finite-Difference Approach

A wide-angle parabolic wave equation solution is presented using shift-map and finite-difference techniques. The corresponding split-step Fourier solution is well known. The solution using finite-difference technique, where the standard parabolic wave equation is modified into the so-called Claerbout equation allowing propagation angles up to 45deg from the paraxial direction, is also well known. Here, we present an extension to that solution in which the shift-map technique is incorporated into the finite-difference scheme allowing a varying terrain to be considered. The result is a solution that corresponds to the well known split-step solution, which is believed to perform well for terrain slopes up to 10deg-15deg and discontinuous slope changes on the order of 15deg-20deg. This solution is a first-order one with respect to the terrain slope. However, when using the finite-difference technique, it is also possible to find a second-order solution with respect to the terrain slope. This new solution performs well for slopes up to about 15deg and discontinuous slope changes up to about 30deg, which is an improvement.

Urban peer-to-peer MIMO channel measurements and analysis at 300 MHz

Multiple-input multiple-output (MIMO) systems operating at frequencies in the upper VHF and lower UHF region is attractive for peer-to-peer communication applications where robustness is of high importance, e.g., in tactical networks and emergency response systems. When designing and evaluating such systems, knowledge of realistic propagation conditions is required. This paper presents results from an urban MIMO measurement campaign at 300 MHz. Measurements are performed along 25 receiver routes and for three fixed transmitter locations, using antenna arrays mounted on two cars. Channel characteristics and ergodic capacity for the 7 times 7 MIMO channels are extracted from the measured data. A path-loss model is derived for the measured scenario, and the distributions of the large-scale fading, the Ricean K-factor, the delay spread, and the ergodic capacity are studied in detail. The correlation distance for the different channel parameters is also examined. Furthermore, the analysis reveals that several of the channel parameters are correlated, and also have a strong correlation with the capacity.

Calculation of higher order diffracted fields for multiple-edge transition zone diffraction

A well-known problem in conventional uniform theory of diffraction (UTD) is multiple-edge transition zone diffraction. Here, higher order diffracted fields have to be used in order to improve the result, and these fields are added by means of a series. Unfortunately, this series will converge slowly for many edges, which means that a good enough result might require a large number of terms. However, by summing up the series properly, the computation time can be kept reasonable, which will be shown for knife edges. The summation technique is though valid for wedges as well.

Wave propagation over a forest edge - parabolic equation modelling vs. measurements

This paper presents a propagation model, based on the parabolic wave equation technique (PE), for calculating the attenuation of propagation in a forest environment. A finite difference scheme is chosen to simplify implementation of boundary conditions. The standard parabolic equation is also modified into the so-called Claerbout equation to accept angles up to 45/spl deg/ from the paraxial direction. Furthermore, the implementation of the phase shift map solution for piecewise linear terrain would perform well for terrain slopes up to 10-15/spl deg/. The model has been tested against measurements in a fir forest environment.

A log-likelihood ratio for improved receiver performance for VLF/LF communication in atmospheric noise

Receiver-improving techniques are important for submarine communication at the LF/VLF bands, due to very demanding reception conditions. At the surface, the interference environment is dominated by the atmospheric noise, which is highly impulsive in character and may impede the reception of the radio signals. To handle the demanding channel and the impulsive interference environment, error correction and an adapted receiver are necessary. In this paper, we propose a log-likelihood ratio (LLR) as soft output from the demodulator suitable for atmospheric noise. The radio system is assumed to use minimum shift keying (MSK) and a low parity density check (LDPC) code. It is shown that the proposed interference-adapted LLR improves the performance substantially in atmospheric noise compared to when the LLR is designed for AWGN. The performance is also compared to a solution, where the soft output from the demodulator is simplified to a limiter, and to a solution when a larger system bandwidth is used in combination with a limiter. It is concluded that the proposed interference-adapted LLR achieves the best performance in the comparison, although the performance could probably be further improved, when compared to the obtained Shannon capacity of this particular interference.

Wave propagation over a forest - edge parabolic equation modelling vs. GTD modelling

This paper presents a comparison between a wave propagation model based on the parabolic wave equation (PE) technique and a model based on the geometric theory of diffraction (GTD) technique, for calculating the propagation attenuation in a forest environment. To compare the PE model and the GTD model, we have made the comparisons against measurements in a fir forest environment.

Link and Network Capacity Gains in Ad Hoc Networks Utilizing MIMO-Techniques

Multiple-Input Multiple-Output (MIMO) antenna systems is a promising technique for achieving substantially increased capacities and robustness in future tactical wireless networks. The purpose of this work has been to investigate the theoretical link and network capacity gains that can be achieved by employing MIMO-techniques in wireless ad hoc networks. We study these gains from a theoretical viewpoint and derive a closed-form expression of the network capacity for a reservation based MAC protocol that utilizes traffic adaptation. The link and network capacities are thereafter examined in urban environments, using two different MIMO channel models. Furthermore, the effect of utilizing MIMO-systems on the mean route lengths and the number of used links are investigated.

Improved robustness and capacity in modulated retro-reflective (MRR) optical communication links

Recent field trials have shown that modulated retro-reflective (MRR) optical communications is a potentially feasible technique for applications demanding high data rates. Data rates over 10 Mbit/s has been demonstrated with Multiple Quantum Well (MQW) modulators in experimental MRR systems. An MQW-based MRR has a variable reflectance and information can thus be transmitted to the receiver by modulating the intensity of the reflected signal. However, current experimental systems normally use binary modulation (e.g. on-off keying) and the data rate is then strictly limited by the modulation speed of the retro-reflector. Instead, by employing a multiple-level modulation scheme the data rate can be increased substantially. Herein, we discuss if the use of different signal processing techniques, commonly used in radio communication systems, may improve the robustness and capacity of MRR free-space optical communication links. Techniques of interest are mainly error-correcting codes, link adaptation and high-level modulation schemes. Furthermore, we apply some of these techniques on measured channel data that has been collected in recent field trials. Through simulations we demonstrate the potential gains that can be achieved through the use of link adaptation and multiple-level modulation. Finally, a brief comparison with competing radio techniques is given.