What is multipath fading? In communication systems, the wide beam of the ground station's antenna can cause multiple signal paths due to factors like terrain, buildings, and sea conditions. As a result, the receiver may receive electromagnetic waves that arrive via different routes—such as reflection, refraction, and direct propagation. This phenomenon is known as the multipath effect. The signals arriving through these various paths are not consistent in phase or timing, leading to signal fading. Additionally, the different arrival times of these signals create inter-symbol interference (ISI), especially when the signal strength from multiple paths is high and the delay difference is significant. This kind of error cannot be reduced by increasing transmission power, and the resulting signal degradation is referred to as multipath fading. It is a major source of ISI and can severely impact digital communications, radar detection, and other related technologies.
Multipath Fading Classification
1. **Rayleigh Fading**
When the delays between different signal paths are small and the transmitted signal has a narrow bandwidth (such as low-rate digital signals), the channel affects all frequency components similarly. In this case, the received signal experiences only random amplitude variations with minimal distortion. This type of fading is called flat fading.
If a cosine wave is transmitted, the received signal becomes an amplitude- and phase-modulated wave with random characteristics. From a frequency perspective, the single-frequency signal spreads into a narrowband stochastic process. The envelope of this signal follows a Rayleigh distribution, hence it is commonly referred to as Rayleigh fading.
**Environmental Conditions:**
- Typically occurs in areas far from the base station.
- No direct path exists between the transmitter and receiver.
- A large number of reflected signals are present, with random directions.
- Each reflected signal has statistically independent amplitude and phase.
2. **Frequency Selective Fading**
When the delays between different paths are significant and the signal has a wide bandwidth (or high data rate), the channel affects different frequency components differently. This leads to both amplitude fluctuations and waveform distortion. The digital signal may spread in time, causing overlapping between adjacent symbols and resulting in inter-symbol interference. This type of fading is known as frequency selective fading.
Multipath Fading Characteristics
The main features used to describe multipath fading include depth of fading, fading rate, duration of fading, and level pass rate.
1. **Level Pass Rate**: The average number of times the signal envelope crosses a specific threshold with a positive slope per unit time.
2. **Average Fading Duration**: The ratio of the probability that the signal envelope is below a certain level to the corresponding level pass rate.
From a spatial perspective, the received signal strength varies as the mobile station moves. Fast fading occurs due to local reflectors, while slow fading results from changes in terrain and spatial loss. From a temporal perspective, the delay spread causes the pulse width to increase at the receiver, which is a key characteristic of multipath effects. Understanding these phenomena is crucial for improving communication reliability and performance.
Lenovo Chromebook Series
S-yuan Electronic Technology Limited , https://www.syuanelectronic.com