Mobile Radio Wave propagation channel- Path loss Models

Transcription

1 Mobile Radio Wave propagation channel- Path loss Models 3.1 Introduction The wireless Communication is one of the integral parts of society which has been a focal point for sharing information with different group of people scattered at various locations. After the advent of telephones, letters &telegrams have lost their meaning, in the same way, after mobile phone was invented it has totally revolutionized the way of communication by opening up new applications which are beyond comprehension and replaced the traditional land line phones. Today we cannot imagine communication in the world without mobiles phone. All mobile system technologies have made life easier. The history of the mobile telephony dates back to the 1920 s with the use of wireless radio by the Police Departments in the United States. In 1940 the first mobile telephony was invented and its capacity was along with it is limited and manoverability. In 1885 Guglielmo Marconi invented antenna and gave the first demonstration of wireless telegraphy words. It is a long radio wave transmission, and consumes high power (> 200Kw) and have a tall antennas which have line of sight communication. In 1920 Marconi discovered short radio waves. In the new communication system invented by Marconi, tall antennas are replaced by short antennas and line of sight concept is totally eradicated. This invention has made revolutionary changes in Mobile communications. From then onwards mobile communication is developing continuously and it became a large industry as of today [6]. A number of mechanisms are behind the electromagnetic wave propagation but they are attributed only to three basic mechanisms such as Reflection, Diffraction and Scattering. 18

2 3.1.1 Reflection when the electromagnetic waves falling on the object some of the signal power may be reflected back to its origin rather than signal is carried out by all the way is known as Reflection is as shown in figure 3.1 The transmitted radio wave nearly travels in one path to the receiving antenna, which means that there is no LOS between transmitting antenna to the receiving antenna. Thus, the signal received by the antenna is the total of all the signal components transmitted by the antenna meant for transmission. For example the surface of the Earth, buildings, and walls etc. causes the reflection of signals Diffraction When the Radio waves strikes on the surface and changes its direction. The Radio path between the transmitter and the receivers obstructed by the surface with sharp irregular edges As the wave bends around the obstacle, even though when LOS does not exist. In practice the height of the mobile antenna is lower than the base station antenna as there may be high rise buildings or hills in the region. Figure 3.1 Basic propagation mechanism of mobile communication (courtesy Google.com) 19

3 3.1.3 Scattering Scattering occurs when the medium through which the wave travels consists of objects with dimensions that are smaller compared to the wavelength, or the number of obstacles per unit volume is quite large. Ex Foliage, Street signs, Lamp posts Interference The signal at the point which is received by the antenna is generally weak because of interference from other signals. This is due to the same network or may be due to manmade barriers. Propagation models are mainly focused on predicting the average received signal strength at a given distance from the transmitter, as well as the predictability of the signal strength in close spatial proximity to a particular location [7] Fading Every wireless communication system consists of three major channel propagation impairments such as short-term fading, long term fading and the corruptive effect of cochannel interference. Fading is caused due to the climatic conditions, geographical topology and due to changes in environment. The motion of mobile station through different regions namely urban, suburban and rural areas also varies the receiver signal strength ultimately leading to fading. Fading of the signal between the transmitter and receiver causes due to two important factors. The propagation models are characterising the rapid fluctuations of the received signal strength over short distance at short period of time. In small scale fading the received signal power may vary three or four orders of magnitude (30 or 40 db). The local 20

4 average received signal will gradually decrease is predicted by large scale propagation models. The path loss depends up on antenna height, environment and distance. The predicted path loss will be constant for a given mobile distance. In practice the particular clutter (buildings, trees) the path loss models will be different by the large scatter evident in the measurement. If a mobile is driven around a base station (BS) at a constant speed, then the local mean signal level will typically appear. The process by which this distribution comes about is known as shadowing or slow fading and the variation occurs over distances compared to the widths of buildings and hills in region of the mobile usually tens or hundreds of meters. The standard deviation of the shadowing distribution is known as location variability is depends on Frequency, Antenna height and the environment. It is greatest at suburban areas and smallest for small areas 9.2 and 9.3 is 8dB. The application of a log-normal distribution for shadowing models can be justified as follows; if contribution to the signal attenuation along the propagation path is considered to act independently Multipath fading Multipath fading has a major bearing on cellular telecommunications. The important of multipath fading in cellular communications is expressed by two reasons. One is the mobile station or user is likely to be moving, and as a result the path lengths of all the signals being received are changing. The second is that many objects around may also be on the move. Automobiles and even people are likely to cause reflections that may have a significant effect on the received signal. The process by which this distribution comes about is known as shadowing or slow fading the variation occurs over distances comparable to the widths of buildings and hills in the region of mobile, usually tens or hundreds of meters. 21

5 3.2 Path loss The Radio wave propagation in free space depends on the frequency of the signal and obstacles in its path [8].These obstacles will change the strength of the signal. As the signal is obstructed the signal path is lost and changes its direction. This phenomenon is called path loss is technically stated as the ratio of transmitted power to the receive power. Suppose x(t) of power P t. is transmitted through a given channel and the received power Pr is the received signal power can be expressed as Pr Pt. Gt Gr / Lt L LLr (3.1) The Estimate of path loss is an important component in communication system design. In mobile communication this assumes greater significance in view of constantly changing environmental conditions [9]. It is the reduction in power density (attenuation) of electromagnetic waves as it propagates through space. In the view of wide geographical area, different types of terrain a single prediction technique may not be suitable (adequate) for all regions. To identify a suitable prediction technique field strength measurements needed to be conducted in different regions and a comparison of these with different prediction techniques has to be carried out Prediction methods To understand the various elements affecting radio signal, path loss is one able to predict the loss for a given path. To predict the coverage that may be achieved for a particular base stations, broadcast stations, etc. The field strength values are converted in to path loss relative to free space with respect to power with distance up to 30 Km for suburban. In general the path loss increases with distance and decreases with increase in 22

6 antenna height. The path losses are high when areas are followed by suburban, rural areas [10] [11]. The prediction (or) assessment can be fairly accurate for the free space scenarios. But in real life terrestrial applications it is not easy because there are many factors which are to be taken into consideration and it is not always possible to gain accurate assessments of the effects they will have Characteristics of path loss In this section the following propagation path loss characteristics of line-of-sight (LOS) and non-line-of-sight (NLOS) systems are described, the free space equations, path loss models, and the empirical path loss formula [12] is highlighted Free Space Propagation Loss Equation The free space transmission loss or propagation loss equation for Omni directional transmit and receive unity gain (G=1) antennas separated by r meters. This equation, also known as Friis equation (Parsons and Gardiner, 1989), is given by For two antennas separated by r meters, having a transmit antenna gain G T given by And a receive antenna gain G R given by The propagation loss equation in free space is 23

7 The propagation path loss (L F ), expressed in db is obtained from equation 3.5. It is described as 10logGT 10 logg _ R 20 log( c / f / 4 r For unity gain, isotropic (that is, ideal Omni directional) transmit and receiving antennas and unobstructed LOS transmission, the basic transmission loss L B is described (Or) From this basic LOS transmission loss equation, conclude that the received power (relative to the transmitted power) decreases by 6dB for every doubling of distance and also for doubling of the radio frequency. 3.3 Path Loss Models Path loss is the major component in the analysis and design of the link budget of telecommunication systems. Path loss is commonly used in both wireless and signal communications [13]. 24

8 There so many factors like free space loss, Refraction Diffraction, Reflection, Aperture will affect the path loss. Propagation models are broadly classified in to three types 1. Empirical models 2. Semi-deterministic models 3. Deterministic models Empirical model An Empirical models are those based on observation and measurements alone. These models are mainly used to predict the path loss. An Empirical model based on extensive field measurements is used to predict the average path loss along the radio path. These models can be used to develop relationships for forecasting and explaining trends. These relationships and trends are not relevant. Mechanically Empirical models are set of equations which are derived from wide field measurements. These are simple and efficient to use the measurements were made with these models absolutely perfect for environments with the similar characteristics features. This path is from the base station antenna to the mobile antenna. Experimental path loss curves are generated by measuring the received signal strength (RF carrier) and subtracting it from the transmitted signal power. For example, if they have unity gain Omni directional antennas, then the transmit power PT. = +30 dbm, and at a given location the received carrier power P T =-105 dbm, then the path loss L P is The input parameters for the empirical models are usually qualitative and not very specific like urban area and suburban area. It is referred that the empirical models like Hata model Cost -231, Okummura and Hata are among very two popular empirical propagation models. There are many empirical prediction models like, Cost 231 Hata 25

9 model, Okummura Hata model, Sakagami-Kobo model, Cost 231 Wolfish-Ikegami model. Here smaller means sub urban areas and the larger means urban areas. When h e is the CPE, (Customer Premises Equipment) antenna height above the ground. Observation of (7) to (9) reveals that the path loss exponent of the predictions made by the COST- 231,2.1GHz band, the model predictions are based on the measurements from three different environments namely urban, suburban and the rural. Table 3.1Antenna parameters Parameters Range Frequency MHz Distance 1-10m Base Station antenna height m Link distance 1-20Km Okummura Hata Model In 1968 Okummura carried out large measurements of base station to mobile signal attenuation and developed a set of curves giving median attenuation relative to free space path loss. This is fully an empirical prediction method which is entirely based upon an extensive series of measurements made in and around Tokyo city between 200MHz and 2GHz. So in 1980, Hata model developed a closed form of expressions for Okumura s data is known as Okummura-Hata model. It is an empirical model in the frequency range of 150MHz to 1920MHz & distances from 1 to 100 Km. It can be extended to 3GHz with distance of 1Km. It can be used for base station antenna heights ranging from 30m to 1000m. 26

10 .The method aims dividing the prediction area into a series of clutter and gradient categories, namely open, suburban and urban. These are summarised as follows. Open area It is the area having open space and no tall tree or buildings should be in path, and a plot of land should be cleared at least m e.g. farm land, rice fields, open fields. Suburban area It is the area having Village or highway scattered with trees and houses, there is a possibility of some obstacles near the mobile tower however it should not be very congested. Urban area It is the area to construct the city or large town with huge buildings and houses with two or more storeys, larger villages with close houses and tall, thickly grown trees. Okummura takes urban areas as a reference and applies correction factors for conversion in to the other classifications. This is a sensible choice as such areas avoid the larger variability present in suburban areas. The Okummura Hata model is one of the empirical model used to calculate the Path Loss [14] from the 150MHz to 1500MHz. It is used to identify the heights of the antenna at base station and the mobile. These are important factors to calculate the Path Loss. Masahuru Hata created empirical models that provide good fit to the measurement taken by Okummura for the transmitter receiver separation is distance(d) and it is more than 1Km and the expression is developed by Hata is called Okummura-Hata is shown by Equation (3.11). Hata presented the area propagation laws as a standard formula and supplied correction equations to other applications. Hata model is based on Okumura s field test results to 27

11 predicts various equations for path loss with different types of clutters. The limitations of the Hata Model test results include its frequency s(150mhz to 1.8GHz), the distance from BTS(50m to20km), and the height of base station antenna (30m to 200m) and the height of mobile antenna(1m to 10m). Mathematically, the path loss of Hata model is expressed for different environments such as the urban, suburban and open area. Hata model produces good results for the hilly rural terrains because the geographic details of this environment are not specified. Okumura s model assumes that the path loss between the TX and Rx in the terrestrial propagation environment can be expressed as The path loss for urban, suburban and rural clutters is expressed as Where a (hm) is a correction factor. For small to medium sized cities, this factor is given by a (hm)=(1.1log(f)-0.7)hm-(1.56log(f))-0.8 (3.13) The a (hm) value for the larger city, the frequency will be nearly or greater than 1.8GHz, a(hm)=12.78hm db (3.14) The a(hm) value for the frequency will be nearly or greater than 2.1GHz, 28

12 a (hm)=12.854hm-20.02db (3.15) Though Hata model does not have any one of the path specific corrections which is available in Okummura model and the predictions of the Hata model compare very closely with the original Okummura model. The above propagation models are plotted together with experimental results to see which one estimates the path loss more accurately. Where, PL=Path Loss, d=distance. MS antenna correction factor PLfs=Path loss for free space PLbm=Path loss for basic medium Gb=Gain at base station Gm=Gain at main station COST 231 MODELS The practical use of cellular plan was extended by Cost 231.The Cost -231 models is used widely for calculating the wireless mobile systems. This model is used only in particular band width from 500MHz to 2.1GHz (2000MHz) because this is the frequency of the mobile. The below given equation gives the Path Loss in db s [3.10] and the equation is 29

13 Where, Ncost= ( log10 (hb))/10 (3.18) The cm value in the above equation will be 0(zero) for the smaller cities and the cm value will be 3dB for larger cities [15]. When h e is the CPE (Customer Premises Equipment) antenna height above the ground. To evaluate the applicability of the model for the COST GHz band, the model predictions are based on the measurements from three environments namely urban, suburban and the rural. Chapter Summary This chapter clearly described preoperational characteristics, Shadowing, different types of fading s, and their mitigation techniques also represented. Different empirical models like Hata model, Cost 231 model and ECC 33 model have discussed. 30