Large Scale Fading

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Large Scale Fading and
Channel modeling
Using Matlab

March 2012

Large scale fading and channel modeling

Using Matlab

Introduction
The fading phenomenon can be broadly classified into two different types: large-scale fading and small-scale fading. Large-scale fading occurs as the mobile moves through a large distance, for example, a distance of the order of cell size . It is caused by path loss of signal as a function of distance and shadowing by large objects such as buildings, intervening terrains, and vegetation. Shadowing is a slow fading process characterized by variation of median path loss between the transmitter and receiver in fixed locations. In other words, large-scale fading is characterized by average path loss and shadowing. On the other hand, small-scale fading refers to rapid variation of signal levels due to the constructive and destructive interference of multiple signal paths (multi-paths) when the mobile station moves short distances. Depending on the relative extent of a multipath, frequency selectivity of a channel is characterized (e.g., by frequency-selective or frequency flat) for small-scaling fading. Meanwhile, depending on the time variation in a channel due to mobile speed (characterized by the Doppler spread), short-term fading can be classified as either fast fading or slow fading. Figure 1.1 classifies the types of fading channels.

| | | | Fading channel| | | | |
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| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Large-scale fading| | | | | | Small-scale fading| | | | | | | | | | | | | | | | | | |

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| | | | | | | | | | | | | | | | | | | | | | | | | | | | | Path loss| | | Shadowing| | Multi-path fading| | Time variance| | | | | | | | | | | | | | | | |

| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Frequency selective fading| | | Flat fading| | | | Fast fading| | | Slow fading| | | | | | | | | | | | | | | | |

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Figure 1.2 Large-scale fading vs. small-scale fading.

The relationship between large-scale fading and small-scale fading is illustrated in Figure 1.2. Large-scale fading is manifested by the mean path loss that decreases with distance and shadowing that varies along the mean path loss. The received signal strength may be different even at the same distance from a transmitter, due to the shadowing caused by obstacles on the path. Furthermore, the scattering components incur small-scale fading, which finally yields a short-term variation of the signal that has already experienced shadowing.

Link budget is an important tool in the design of radio communication systems. Accounting for all the gains and losses through the wireless channel to the receiver, it allows for predicting the received signal strength along with the required power margin. Path loss and fading are the two most important factors to consider in link budget. Figure 1.3 illustrates a link budget that is affected by these factors. The mean path loss is a deterministic factor that can be predicted with the distance between the transmitter and receiver. On the contrary, shadowing and small-scale

Figure 1.3 Link budget for the fading

fading are random phenomena, which means that their effects can only be predicted by their probabilistic distribution. For example, shadowing is typically modeled by a log-normal distribution.

Due to the random nature of fading, some power margin must be added to...
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