Multipath effect and its characteristics

In wireless communication systems, the multipath effect occurs when a moving object, such as a car, travels between buildings and obstacles. As the signal propagates, it is reflected off various surfaces, creating multiple paths that reach the receiver. The received signal is a combination of the direct wave and these reflected waves, leading to interference. This interference can cause signal fading, where the strength of the received signal fluctuates over time. The electrical length of each path changes dynamically, causing the phase relationship between the arriving signals to vary. These random fluctuations in phase and amplitude result in the overall signal experiencing rapid variations, known as fading. Since the phase differences depend on frequency, the interference effects also vary with frequency, a phenomenon called frequency selectivity. In broadband systems, this can lead to intermodulation distortion, where different frequency components interact unpredictably. Moreover, due to varying delays along different paths, signals sent at the same time arrive at different times at the receiver. This causes overlapping of narrow pulses, which can distort the original signal. In addition to direct and ground-reflected waves, scattered waves from objects along the propagation path contribute to the multipath effect. Multipath propagation has several characteristics. From a waveform perspective, it transforms a carrier signal into a narrowband signal with modulated envelope and phase—commonly referred to as a fading signal. Spectrally, it causes frequency dispersion, spreading the signal across a broader range of frequencies. The signal envelope often follows a Rayleigh distribution, hence Rayleigh fading. There are three types of selective fading: time-selective, frequency-selective, and spatial-selective. Time-selective fading results from Doppler shifts caused by relative motion between the transmitter and receiver. Spatial-selective fading arises from the spread of arrival angles at the receiver antenna array. Frequency-selective fading happens when the channel introduces time delay spread, causing intersymbol interference and signal distortion. Coherence bandwidth is used to measure the degree of frequency selectivity. A smaller ratio of coherence bandwidth to signal bandwidth indicates stronger frequency selectivity, while a larger ratio suggests weaker selectivity. In urban environments, where line-of-sight propagation is often blocked, multipath effects are more pronounced. Even when a direct path exists, reflections from buildings and the ground create additional signal paths. These multipath components combine at the receiver, leading to unpredictable signal variations. Objects in motion within the environment can further enhance fading, even if the receiver itself is stationary. Each path has its own gain, phase shift, and delay, contributing to the complexity of the received signal. Small-scale fading, caused by multipath, leads to rapid fluctuations in signal strength, random frequency modulation due to Doppler shifts, and echo-like extensions from multiple delays. Factors influencing small-scale fading include the number of multipath components, the speed of the mobile station, the movement of surrounding objects, and the signal's transmission bandwidth. If the signal bandwidth exceeds the channel’s coherence bandwidth, the signal becomes distorted, but fading may not be as severe. Doppler shift is another key factor. When a mobile station moves, it causes a frequency shift in the received signal. The magnitude of the shift depends on the angle between the direction of motion and the incoming signal. A positive shift occurs when the mobile station moves toward the source, while a negative shift happens when it moves away. This Doppler effect broadens the signal spectrum, increasing the effective bandwidth. For example, consider a mobile station traveling a distance d between two points, X and Y. The difference in signal path lengths causes a phase change, which translates into a frequency shift. This shift, known as the Doppler frequency (fd), depends on the mobile station’s velocity and the angle of incidence of the signal. As the mobile station moves, the Doppler spread increases, affecting the quality of the received signal. Understanding multipath effects is essential for designing robust wireless communication systems, especially in complex urban environments where signal degradation is common. By accounting for these factors, engineers can develop techniques to mitigate fading and improve signal reliability.

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