02-10-2012, 12:11 PM
Ray -Tracing Based Technique
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In wireless communication systems, wireless signals from a transmitter are transmitted to a receiver via multiple propagation paths. As shown in Fig. 1, the ground and buildings cause reflection, diffraction, or penetration in these paths. As a result, radio waves taking different propagation paths arrive at the receiver at different times and each propagation path has a different phase condition since it has a different path length. Therefore, at the receiver, different received power levels are observed for each frequency. This frequency selective fading, as it is called, has a tremendous impact on the communication quality of wideband wireless communication systems. Therefore, propagation characteristics including frequency selective fading must be considered when service areas or systems in wideband wireless communication systems are being planned.
There are various methods for estimating propagation characteristics. The ray-tracing method is one of the most well known and most effective methods for estimating the propagation characteristics in multiple-input multiple-output (MIMO) systems. However, MIMO propagation channel estimation by ray tracing requires an enormous amount of calculation resources. In this article, we introduce a ray-tracing technique for overcoming the computational complexity, which is a major problem in MIMO propagation channel estimation.
Ray-tracing method
Algorithms for the ray-tracing method are classified into two general types: the imaging method and ray-launching method. The imaging method derives a propagation path by using geometric optics from a combination of the transmission position, receiving position, and reflecting surfaces. On the other hand, the ray-launching method derives the propagation path using rays discretely launched at given regular intervals and searching for the rays that arrive at the received position. Estimating the propagation characteristics using the ray-tracing method requires three parameters: the propagation distance, incident angle to the reflecting surface, and complex permittivity of the reflecting surface. The propagation distance and the incident angle to the reflecting surface are derived from ray-tracing estimation results. The complex permittivity of the reflecting surface is a predetermined static parameter.
Issues facing ray tracing
As shown in Fig. 2, the ray-tracing method derives the propagation path including reflection, diffraction, and penetration from the transmitter to receiver using geometric optics. The estimation accuracy can be improved by increasing the number of reflections, diffractions, or penetrations in the case of the imaging method or by increasing the number of rays launched from the transmitter in the case of the ray-launching method. However, the receiving area must be configured for propagation channel calculation by the ray launching method since there is a very low probability that rays launched from the transmitter will arrive at the receiving point. Moreover, it is quite complicated to decide the receiving area. Therefore, in this article, we focus on the imaging method in order to introduce our new technique.
In the imaging method, it is necessary to search for the propagation path for the combination of all allocated building walls. Thus, in the case of single-input single-output systems, one must judge whether a reflection point exists on the wall a total of AB times when the number of building walls is set to A and the number of reflections is set to B. For instance, as shown in Fig. 2, a single reflection requires two judgments and a double reflection requires four. The computational complexity increases with the square of the number of walls, reflections, diffractions, and penetrations. Therefore, many computational complexity reduction techniques have been studied.
Special problem in applying ray tracing to MIMO systems
MIMO technology, which uses multiple antennas at the transmitter and at the receiver to improve the transmission rate, has recently been widely studied. IEEE 802.11n (IEEE: Institute of Electrical and Electronics Engineers) and WiMAX (worldwide interoperability for microwave access), which are recently standardized wireless schemes, both use MIMO technology. MIMO propagation channel evaluations that use the ray-tracing method have been reported. However, whether a reflection point exists on a wall must be judged a total of m × n × AB times when an m × n MIMO system is treated. Thus, there is the additional problem that the computational complexity increases in proportion to the number of antenna combinations when the ray-tracing method is applied to MIMO systems. The computational complexity, it is generally thought that a MIMO propagation path can be simulated by using the propagation path between a particular combination of antennas instead of those for all the transmitter and receiver antenna combinations.
Improvement of Accuracy of Ray-Tracing Algorithms
The ray-tracing method can provide site-specific predictions. Due to the fact that the environmental database may not be accurate and the materials of the objects in the regionof interest may not be known, the ray-tracing method can only provide approximate results for realistic propagation environments.
Another factor affecting the accuracy of the ray-tracing procedure is the incomplete account for all kinds of rays. This is because the more rays taken into account, the more computation time will be needed, leading to unacceptable efficiency. Examples of techniques used to improve the accuracy of ray-tracing algorithms are described in the following sections.
Additional Ray Mechanisms — Effect of Diffractions:
Diffractions from vertical and horizontal edges of buildings are important contributions to the received power. The over-roof top propagation is mainly due to diffractions from the horizontal edges. Methods for calculation of diffraction coefficients for metal or materials with finite conductivity were developed. A comparison among the perfectly absorbing wedge(PAW) method, UTD, and UTD heuristic methods can be found in. It is found that errors given by these three methods are comparable.
Rizk etal proposed a method to include the slope diffraction from wedges to improve the accuracy of calculation of the diffracted field in transition regions using classical UTD. Several decibels (approximately6dB) of improvement can be achieved. The diffraction from building corners (wedges) is taken into account in. New diffraction coefficients for objects with finite conductivity are developed. The artificial dip in the usual diffraction calculation is removed. Comparison with FDTD shows that the results of the new method are of good accuracy.