Vojnotehnički Glasnik (Jan 2011)
Effect of environment on the propagation of electromagnetic waves in GRC 408E digital radiorelay devices
Abstract
Quality transmission of digital signals from a transmitting radio-relay device to a receiving one depends on the impact of environmental effects on the propagation of electromagnetic waves. In this paper some of the most important effects are explained and modeled, especially those characteristic for the frequency range within which the GRC 408E operates. The modeling resulted in the conclusions about the quality of transmission of digital signals in the GRC 408E radio-relay equipment. Propagation of electromagnetic waves A radio-relay link is achieved by direct electromagnetic waves, provided there is a line of sight between the transmitting and receiving antenna of a radio-relay device. Electromagnetic waves on the road are exposed to various environmental influences causing phenomena such as bending, reflection, refraction, absorption and multiple propagation. Due to these environmental effects, the quality of information transmission is not satisfactory and a radio-relay link is not reliable. The approach to the analysis of the quality of links in digital radiorelay devices is different from the one in analog radio-relay devices. Therefore, the quality is seen through errors in the received bit ( BER ), the propagation conditions are taken into account, a reservation for the fading is determined by other means, etc.. Phenomena which accompany the propagation of electromagnetic waves in digital radio-relay links The propagation of direct EM waves is followed by the following phenomena: - attenuation due to propagation, - diffraction (changing table), - refraction (refraction), - reflection (refusing), - absorption (absorption) and - multiple wave propagation. Each of these has a negative effect on the quality of the received signal at the receiving antenna of the radio-relay device. Attenuation due to propagation of electromagnetic waves The main parameter for evaluating the quality of radio-relay links is the level of the field at the reception, i.e. the strength of a signal received at the entrance of the receiver. The error in the received bit (BER) is a function of the receiving field. By reducing the level of the field the BER increases and vice versa. The level of the receiving field in the absence of margin is called the nominal level of the receiving field. The difference between the nominal level and the receiving threshold represents a margin or a budget for the fading for the given BER. Diffraction is a phenomenon that follows the propagation of electromagnetic waves and indicates their ability to bend round the relief, uneven surfaces and other obstacles, during propagation through the environment. Diffraction is considered when the obstacles on the path of propagation of electromagnetic waves enter the first Fresnel zone, because then an error in the information transmission occurs. Refraction is the refraction of electromagnetic waves in the lower layers of the atmosphere and is caused by its unhomogeneity. The upper part of the EM wave front progresses faster and the wave bends towards the Earth. The phenomenon of EM wave bending towards the Earth is called refraction. Reflection When electromagnetic waves propagate near the Earth surface, a part of the wave front, reflected from the surface of the Earth, may arrive in the receiving antenna of radio relay equipment together with direct electromagnetic waves. EM waves (direct and reflected) are summed up vectorially in the receiver giving the resulting EM wave. This can cause a substantial reduction in the resulting field when compared to the field in ideal conditions, which leads to the error increase. Absorption or EM wave absorption occurs in all frequency bands and signifies a higher or lower level of attenuation of electromagnetic waves. It is taken into consideration in digital radio-relay devices which operate in the frequency range over 7 GHz. Multiple propagation of electromagnetic waves EM waves from the transmitter can reach the receiver in different ways, and such EMW propagation is known as multiple EM wave propagation in the literature. The result of multiple propagation can be the reduction of the EM field intensity or its complete disappearance. This phenomenon is called fading. Fading Fading is caused by short-term weakening of electromagnetic waves at the reception. During EMW propagation, the interaction between EM waves and objects occurs resulting in multiple copies of useful signals of different amplitude and delay values at the reception point, thus making the resulting EM field unstable. A large number of copies of the useful signal at the reception are caused by the effects of environmental impacts on the propagation of electromagnetic waves along the route, such as reflection, refraction, diffraction, and their combination. The total loss is equal to the sum of propagation weakening and fading weakening. Fading is divided into propagation fading and interference fading. Propagation fading is generally slow and does not depend on frequency. Interference fading occurs due to the appearance of multiple EM wave propagation and it can be flat or selective. Flat fading is the same in the entire frequency range. In selective fading there is degradation of basic signals, i.e. Intersymbol interference which is present at RR devices operating at higher frequencies with the data flow around 34 MBit/s and over. Model of the radio-relay system and the results of modeling The paper deals with a model as the one given in Fig. 2 The GRC 408E RR devices are supposed to be built into mobile call centers. The following phenomena are modeled: attenuation due to propagation, diffraction, reflection and fading. Each phenomenon is modeled for typical cases. The input signal in the GRC RR 408E device is a random binary sequence, modulated by a modulation device provided by the RR. Such a signal propagates through the particular medium towards the RR device receiving antenna. At the reception point, the transmitted and the received signal are compared in order to find an error due to the influence of some of the modeled phenomena. The modeling results are presented graphically for different effects of environmental impacts on the propagation of EM waves in RR digital devices. The graphs indicate the errors occurred during the propagation of EM waves. Conclusion Modeling the impact of environment on the propagation of EM waves in RR digital devices can show the influence of environment on the propagation of EM waves, as well as on the quality of transmission signals. The conclusion is that the choice of the terrain for setting a mobile communication center is essential for high-quality signal transmission. For high-quality transmission of digital signals in RR devices, another requirement is also important and that is the requirement for the line-of-sight transmission and free first Fresnel zone. While in analog RR devices the conditions of EMW propagation are not a key factor in calculating the quality of communication, the results of modeling show that the conditions of propagation in digital RR devices are an important factor in the calculation of the quality of communication. The obtained results are useful for modeling RR devices in mobile communication centers and for engineers working on the main design of communication systems in the Serbian Army. The next aim is to model the impact of environment on the propagation of EM waves in GRC RR 408E/34 digital devices which operate within a higher frequency range and at higher data flow speeds.
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