Multipathpropagation

Multipath propagation is the phenomenon in which a transmitted radio signal reaches a receiver via multiple paths. These paths arise from reflections, diffractions, and scattering of the signal by objects such as buildings, terrain, water surfaces, and interfaces between materials. The received signal is the sum of several versions of the transmitted signal, each with its own delay, amplitude, and phase. The superposition can be constructive or destructive, causing rapid fluctuations in signal strength known as small-scale fading, as well as longer-term variations due to changes in the environment.

In wireless communications, multipath produces delay spread and frequency-selective fading. Each path introduces a different propagation delay, so parts of the signal overlap in time, which can cause inter-symbol interference if the symbol duration is not long enough. The channel can be modeled as an impulse response h(t) that is a sum of scaled and time-delayed copies of the transmitted signal. Motion of transmitter, receiver, or objects causes Doppler shifts, broadening the received spectrum and making the channel time-variant.

Effects of multipath include degradation of signal quality, fading margins, and variable bit error rates. In some contexts, multipath is exploited to increase capacity, notably in multiple-input multiple-output (MIMO) and spread-spectrum systems, where multiple paths carry independent information.

Mitigation and exploitation techniques include diversity (spatial, frequency, or time), equalization, rake receivers, MIMO, and orthogonal frequency-division multiplexing (OFDM) with guard intervals and channel coding. Understanding multipath is essential for designing robust wireless links in urban, indoor, and vehicular environments.