Multipath propagation

Lore & Background

Multipath interference is a wave phenomenon whereby a wave from a source travels to a detector via two or more paths, and the components interfere constructively or destructively. The condition necessary is that the components of the wave remain coherent throughout their travel. The interference arises because the components have generally traveled different lengths, as measured by optical path length, and thus arrive out of phase. The signal due to indirect paths interferes with the required signal in amplitude and phase, a condition called multipath fading.

Where the magnitudes of the signals arriving by various paths have a Rayleigh distribution, it is known as Rayleigh fading. Where one component dominates, a Rician distribution provides a more accurate model, known as Rician fading. Where two components dominate, the behavior is best modeled with the two-wave with diffuse power (TWDP) distribution. These descriptions are commonly used and accepted but are generic and abstract, hiding or approximating the underlying physics.

In analog television transmission, multipath causes ghosting, seen as a faded duplicate image to the right of the main image. In radar processing, multipath causes ghost targets that move and behave like normal targets, deceiving the receiver. In digital radio communications such as GSM, multipath can cause errors due to intersymbol interference, which equalizers, orthogonal frequency division modulation, and rake receivers may address. In GPS receivers, multipath effects can cause a stationary receiver's output to indicate random jumping or creeping.

Reader's Guide

Multipath propagation has significant implications across various radio systems. In analog facsimile and television, it causes jitter and ghosting, degrading image quality. In radar, it creates ghost targets that are particularly bothersome because they mimic real targets, requiring ground maps and altitude filtering to minimize deception. In digital radio communications like GSM, multipath introduces intersymbol interference, which can be corrected using equalizers, orthogonal frequency division modulation, or rake receivers. For GPS receivers, multipath degrades accuracy, causing stationary units to appear to jump or creep, and while moving the effect may be hidden, it still reduces location and speed precision.

In wired media such as power lines and telephone local loops, impedance mismatch causes signal reflection, leading to multipath propagation. High-speed power line communication systems often use multi-carrier modulations like OFDM to avoid intersymbol interference. The ITU-T G.hn standard uses OFDM with a cyclic prefix, adapting parameters for different wiring types. DSL modems also use OFDM to communicate despite multipath from mixed wire gauges or bridge taps, which may be removed if training is unsatisfactory.

The mathematical model of multipath uses an impulse response approach, where a transmitted Dirac pulse results in multiple received pulses at different times due to different path lengths. The delay spread, defined as the time between the first and last received impulses, is a key parameter. The channel transfer function, obtained via Fourier transform, shows a sequence of peaks and valleys, with the coherence bandwidth roughly inversely proportional to the multipath time.

The Physics Governing Every Radio Signal

Radio waves, as electromagnetic radiation, are subject to a suite of physical phenomena as they journey from transmitter to receiver. Reflection, refraction, diffraction, absorption, polarization, and scattering all play roles in shaping how a signal arrives. In the idealized case of free space, the behavior is elegantly simple: power density follows the inverse-square law, meaning that doubling the distance from a point-source antenna reduces the received power to one-quarter. The electric and magnetic field strengths each halve over that same doubling of distance. In the real atmosphere, however, the picture is more complex. Although the atmosphere is thin enough that radio waves travel at nearly the speed of light, variations in density and temperature introduce slight bending of the wave path over long distances. These subtle refraction effects must be factored into any practical link budget, and at certain frequencies, precipitation and water vapor add further attenuation. Understanding these interactions is the foundation for every application, from amateur radio frequency selection to the design of reliable mobile telephone networks, radio navigation aids, and radar systems.

Line-of-Sight Propagation and Its Practical Limits

The most intuitive mode of radio transmission is the direct, straight-line path between two antennas, commonly called line-of-sight or direct-wave propagation. This is the dominant mechanism at VHF frequencies and above, and the only viable mode at microwave frequencies. On Earth's surface, the practical range is bounded by the visual horizon, roughly forty miles or sixty-four kilometers, a limit set by the curvature of the planet and the heights of the transmitting and receiving antennas. This mode underpins an enormous range of technologies: cellular and cordless telephones, walkie-talkies, wireless local area networks, FM and television broadcasting, point-to-point microwave relay links, and radar. Satellite communication extends the line-of-sight concept to extraordinary distances, with home dish antennas receiving signals from geostationary satellites roughly twenty-two thousand miles overhead, and ground stations tracking spacecraft billions of miles away. A critical nuance is the ground-plane reflection effect: the interference between the direct beam and its ground-reflected counterpart often produces an effective inverse-fourth-power distance law rather than the simpler inverse-square law, significantly affecting link design at VHF.

Ground Wave Propagation and Subsurface Penetration

At frequencies between thirty and three thousand kilohertz, vertically polarized radio waves can propagate as surface waves that cling to and follow the curvature of the Earth. This ground-wave mode works by the wave interacting with the conductive surface of the ground, allowing the signal to bend around hills and travel beyond the visual horizon. AM broadcast stations and many amateur radio operators rely on this mechanism to cover their local listening areas. As the frequency drops further into the very low and extremely low frequency ranges, attenuation with distance diminishes dramatically, enabling ground waves to carry information across the globe. These lower frequencies possess an additional remarkable property: they can penetrate significant distances through water and solid earth. This capability makes VLF and ELF waves indispensable for military communication with submerged submarines and for maintaining contact in mining operations where conventional radio links are impossible. The lower the frequency, the more effectively the wave threads through conductive media, turning the planet itself into a communication medium.

Skywave Propagation and Less Common Mechanisms

In the medium-wave and shortwave bands, radio waves can be refracted by the ionosphere, a layer of charged particles suspended high in the atmosphere. When a signal is transmitted at an angle into the sky, the ionized layer bends it back toward Earth, allowing communication across transcontinental distances far beyond any line-of-sight horizon. This skywave mechanism is the backbone of international shortwave broadcasting and enables amateur radio operators to make contacts with stations in distant countries. Beyond this well-known mode, several less common propagation mechanisms serve specialized needs. Tropospheric scattering, or troposcatter, and tropospheric ducting at VHF frequencies exploit irregularities in the lower atmosphere to extend range. Near vertical incidence skywave, or NVIS, is employed when HF-band communication is needed within a few hundred miles, a range where conventional skywave angles are impractical. Together, these diverse propagation modes ensure that radio communication can be tailored to virtually any distance, environment, or operational requirement.