Fading
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Chapter 4
Fading and Shadowing
Fading is a significant part of any wireless communication design; it is important to predict what type of fading may occur, and if possible use a wireless standard capable of providing remedies for the fades. When no remedies are possible, it may be important to predict the likelihood of outage. We will review here several types of fading and provide models to predict their impact.
4.1 Large-Scale Shadowing
We start with fading that occurs on a large scale, and which correspond to conditions that may vary as one turns a corner, moves behind a large building, or enters a building. This large-scale fading is often referred to as shadowing.
4.1.1 Log-Normal Shadowing
Large-scale variations caused by shadowing of obstacles are shown to follow a log-normal distribution [11][12][13], which means that when measured in dB they follow a Gaussian distribution. Consequently shadowing effects they are usually incorporated into path loss estimates by the addition of a zero-mean Gaussian random variable, with standard deviation σ: N(0,σ), were σ is often estimated by empirical measurements. Commonly accepted values for σ are between 6 dB and 12 dB. Measured values of σ itself seem to display Gaussian distribution as well, in their variations from one area to another, and depend on the radio frequency, the type of environment (rural, suburban, or urban), base station and subscriber station height. Many measurement campaigns have been conducted and reported in the literature, as summarized in table 4.1.
[pic]
|Table 4.1: |
|Path loss exponent (n) and log-normal shadowing standard deviation (σ, in dB) — summary of values for various frequencies |
|reported for suburban or residential areas.
Chapter 4
Fading and Shadowing
Fading is a significant part of any wireless communication design; it is important to predict what type of fading may occur, and if possible use a wireless standard capable of providing remedies for the fades. When no remedies are possible, it may be important to predict the likelihood of outage. We will review here several types of fading and provide models to predict their impact.
4.1 Large-Scale Shadowing
We start with fading that occurs on a large scale, and which correspond to conditions that may vary as one turns a corner, moves behind a large building, or enters a building. This large-scale fading is often referred to as shadowing.
4.1.1 Log-Normal Shadowing
Large-scale variations caused by shadowing of obstacles are shown to follow a log-normal distribution [11][12][13], which means that when measured in dB they follow a Gaussian distribution. Consequently shadowing effects they are usually incorporated into path loss estimates by the addition of a zero-mean Gaussian random variable, with standard deviation σ: N(0,σ), were σ is often estimated by empirical measurements. Commonly accepted values for σ are between 6 dB and 12 dB. Measured values of σ itself seem to display Gaussian distribution as well, in their variations from one area to another, and depend on the radio frequency, the type of environment (rural, suburban, or urban), base station and subscriber station height. Many measurement campaigns have been conducted and reported in the literature, as summarized in table 4.1.
[pic]
|Table 4.1: |
|Path loss exponent (n) and log-normal shadowing standard deviation (σ, in dB) — summary of values for various frequencies |
|reported for suburban or residential areas.