What is the effect of multipath fading on RF propagation?

1 Multipath fading definition

Multipath fading is the variation or attenuation of the signal strength of a radio wave due to multiple paths that it can take from the transmitter to the receiver. These paths may include direct line of sight, reflection, refraction, diffraction, or scattering by obstacles or the environment. Depending on the relative phases, amplitudes, and delays of the signals arriving at the receiver, they may interfere constructively or destructively, resulting in fluctuations in the signal quality.

2 Multipath fading causes

Multipath fading can be caused by various factors, such as the distance between the transmitter and the receiver, the speed and direction of their movement, the frequency and bandwidth of the signal, the polarization and antenna characteristics, and the terrain and atmospheric conditions. For example, in urban areas, buildings and vehicles can reflect or block the signals, creating multiple paths with different lengths and angles. In rural areas, hills and trees can also affect the signal propagation. In addition, weather phenomena such as rain, fog, or snow can attenuate or scatter the signals.

3 Multipath fading types

Multipath fading can be classified into two main types: large-scale fading and small-scale fading. Large-scale fading, also known as path loss or shadowing, is the gradual decrease of the average signal power over a long distance or behind an obstacle. It depends on the propagation environment and the antenna heights. Small-scale fading, also known as fast fading or Rayleigh fading, is the rapid variation of the signal amplitude and phase over a short distance or time. It depends on the multipath delay spread and the Doppler spread of the signal.

4 Multipath fading effects

Multipath fading can have various effects on mobile communications, from signal distortion to signal outage and signal degradation. Signal distortion can be caused by inter-symbol interference (ISI), which is the overlap of adjacent symbols in a digital signal, resulting in errors or loss of information. To mitigate ISI, techniques such as equalization, diversity, or coding can be used. Signal outage can cause deep fades or nulls in the signal, resulting in dropped calls or connection failures. To overcome deep fades, techniques such as power control, handover, or retransmission can be used. Signal degradation can cause noise or interference in the signal, reducing the signal-to-noise ratio (SNR) or the signal-to-interference ratio (SIR), resulting in poor audio or video quality or low data rate. To improve SNR or SIR, techniques such as filtering, modulation, or channel estimation can be employed.

5 Multipath fading measurement

Multipath fading can be measured by various parameters, such as the received signal strength indicator (RSSI), the bit error rate (BER), the signal-to-noise ratio (SNR), the signal-to-interference ratio (SIR), the delay spread, or the Doppler spread. These parameters can be used to evaluate the performance or quality of service (QoS) of a mobile communication system. For example, RSSI can indicate the signal coverage or availability, BER can indicate the signal accuracy or reliability, SNR can indicate the signal clarity or fidelity, SIR can indicate the signal interference or congestion, delay spread can indicate the signal distortion or dispersion, and Doppler spread can indicate the signal variation or fluctuation.

6 Multipath fading modeling

Multipath fading can be modeled by various mathematical models, such as the free space model, the log-normal model, the Okumura-Hata model, the Rayleigh model, the Rician model, or the Nakagami model. These models can be used to simulate or predict the behavior or characteristics of a multipath fading channel. For example, the free space model can describe the large-scale fading in a clear line of sight environment, the log-normal model can describe the large-scale fading in a shadowed environment, the Okumura-Hata model can describe the large-scale fading in an urban environment, the Rayleigh model can describe the small-scale fading in a non-line of sight environment, the Rician model can describe the small-scale fading in a line of sight environment, and the Nakagami model can describe the small-scale fading in a general environment.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?