Path loss

Industrial, scientific, and medical bands are reserved portions of the radio spectrum, defined by the ITU Radio Regulations [23], that are employed in body-centric wireless communication applications and, more in general, for other industrial, medical, and scientific applications. The majority of textile antennas developed to date are intended for operation in some of those ISM bands, especially in the 2.45 GHz, by far the most popular for wearable antennas, and 5.8 GHz bands. The first band represents a good trade-off between antenna dimensions (inversely proportional to fiequency) and path loss (increasing with frequency), whereas the second is more convenient when... 

Introduction Radio Wave Basics Free Space Path Loss... 

A typical problem in the design of a radio frequency communications system requires the calculation of the power available at the output terminals of the receive antenna. Although the gain or loss characteristics of the equipment at the receiver and transmitter sites can be ascertained from manufacturer s data, the effective loss between the two antennas must be stated in a way that allows for the characterization of the transmission path between the antennas. The ratio of the power radiated by the transmit antenna to the power available at the receive antenna is known as the path loss and is usually expressed in decibels. The minimum loss on any given path occurs between two antennas when there are no intervening obstructions and no ground losses. In such a case when the receive and transmit antennas are isotropic, the path loss is known as free space path loss. 

The term (A. /4tt D is the free space path loss. Expressed in decibels with appropriate constants included for consistency of units, the resulting equation for free space path loss, written in terms of frequency, becomes... 

Lfs = free space path loss, dB Gr = gain of receive antenna, dBi Lr = line loss of receiver downlead, dB... 

Assuming a 50-km path the radius of the first Fresnel zone at the path midpoint is approximately 61m at 1 GHz, 194 m at 100 MHz, and 612 m at 10 MHz. From these examples, it is apparent that free space path loss conditions do not often exist for lower frequency systems. If there is less than 0.6 Fresnel zone radius clearance on a given path, then it is presumed that diffraction effects must be considered. 

Free space path loss The amount of attenuation of RF energy on an unobstructed path between isotropic antennas. 

In radio propagation studies one measure of interest is the path loss I, defined as the ratio between the transmitted power Wt and the received power w,. It is frequently expressed in decibel as... 

In the next subsections we describe the three mentioned propagation models with the aim of determining their respective path losses. 

Assuming isotropic and lossless antennas (Gt = Gr = 1), the path loss is given by... 

Note that, from Eq. (16.10), in free-space conditions the path loss rate due to the distance is 20 dB/decade or 6 dB/octave. Moreover, although it has been said that the free-space model describes an ideal propagation condition, it can indeed be accurately used in satellite communication systems and short hne-of-sight radio links. 

Note that, from Eq. (16.14), the path loss rate due to the distance is 40 dB/decade or 12 dB/octave. In the same way, the loss rate due to the antenna heights is —20 dB/decade or —6 dB/octave. In other words, a gain is obtained with the increase of the antenna height. A flat terrain condition can be assumed for... 

Because the Okumura model is totally based on information provided by empirical curves, its automated version can become cumbersome. Hata (1980) has proposed an empirical formulation that fits the Okumura curves within a useful range of application. Within such a range the Hata model gives predictions almost indistinguishable from those provided by Okumura. The general path loss L of the Hata formulation, in decibels, for distances d, in kilometers, and base and mobile antenna heights h( and hr, in meters, within the range... 

The parameters considered here are the same as those defined previously in the subsection on the Hata model. The basic loss for rural areas are the those expressed in Eq. (16.26) and Eq. (16.27). The path loss is as follows ... 

The Lee model (Lee, 1986) is based on the well-accepted assumption that the ratio between the powers received at two different distances from the base station is proportional to the ratio of these distances to the power of the path loss coefficient. Accordingly, the model is completely determined if the received power at a given distance and the path loss coefficient for the required environment are known. These two... 

TABLE 16.17 One-Mile Intercept Path Losses and Path Loss Coefficients for Different Environments... 

These probabilities are plotted in Fig. 16.64 for different values of standard deviation and path loss... 

From the theory that has been developed it can be seen that the parameters affecting the probabilities /3 and /x for cell coverage are the path loss coefficient a, the standard deviation cr , the required threshold Wr, and a certain power level K measured or estimated at a given distance Xq from the base station. 

Path loss The ratio between the transmitted power and the received power. 

Figure 16.72 illustrates the link loss factors, maximum nadir angle 0, Earth central angle y, and earth-space time delay as a function of satellite altitude. The path losses are shown for several sateUite frequencies... 

Wind and seismic activity can cause microwave dishes to go out of alignment. Earthquake-resistant towers and mounts can help prevent alignment failure, even for wind-related problems. Redundant systems should be considered part of your solution. A duplicate microwave system might lead to a false sense of security. Consider a nonmicrowave backup, such as fiber, for a primary microwave link. Smoke, heavy rain, and snow storms can cause enough path loss to disable otherwise sound wireless systems. 

It is assumed that each node has a path loss model for each service channel Cm,rn = 1., Nsch-... 

To determine the distance between robots, we use the well-known formulas of path loss ... 

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