Multipath propagation

Multipath propagation. In wireless communication and radio astronomy, it refers to the phenomenon where a transmitted signal reaches a receiving antenna via two or more distinct paths. This occurs due to reflection, diffraction, and scattering caused by obstacles in the environment, such as buildings, mountains, or the ionosphere. The multiple copies of the signal, each with potentially different phases, amplitudes, and delays, combine at the receiver, leading to complex interference effects that can significantly impact system performance.

Overview

Multipath propagation is a fundamental characteristic of most modern radio wave transmission environments, from urban cellular networks to deep-space communication with NASA probes. The phenomenon was first systematically studied in the early 20th century by pioneers like Guglielmo Marconi during transatlantic experiments, where signals reflected off the ionosphere created multiple paths. It is a central consideration in the design of systems like GPS, digital television (e.g., ATSC standards), and Wi-Fi networks governed by the IEEE 802.11 family of standards. Understanding multipath is also crucial in fields like radar used by the Royal Air Force and sonar employed by the United States Navy.

Causes and mechanisms

The primary physical mechanisms causing multipath are wave propagation interactions with the environment. Reflection occurs when a signal bounces off smooth, large surfaces like the walls of the Empire State Building, bodies of water, or the ground. Diffraction allows waves to bend around obstacles such as hills or building edges, as described by the Huygens–Fresnel principle. Scattering happens when a signal encounters objects small compared to its wavelength, like foliage or street signs, radiating energy in many directions. In satellite communications, the troposphere and ionosphere can refract signals, creating additional paths. The specific environment, from the canyons of Manhattan to the Mariana Trench, dictates the dominant mechanisms and path delay spread.

Effects on signal quality

The interference of multiple signal copies at the receiver leads to several detrimental effects. Constructive and destructive interference cause rapid spatial variations in signal strength known as fading, with Rayleigh fading and Rician fading being common statistical models. This fading can cause severe bit error rate increases in digital systems like those using quadrature amplitude modulation. Time dispersion, where delayed copies arrive, leads to intersymbol interference, particularly problematic for high-speed data in standards like 4G/LTE and 5G NR. In GPS receivers, multipath can introduce significant errors in pseudorange measurements, affecting positional accuracy for users from the United States Coast Guard to civilian smartphone applications.

Mitigation techniques

Engineers employ numerous techniques to combat multipath degradation. In digital television broadcasting, the ATSC standards use a long guard interval in the 8VSB modulation to tolerate delay spread. Spread spectrum techniques, such as those used in GPS and code-division multiple access systems, leverage correlation properties to isolate the direct path. Orthogonal frequency-division multiplexing, the basis for Wi-Fi (IEEE 802.11a/g/n/ac) and 4G/5G, divides the signal into many narrow subcarriers to minimize intersymbol interference. Diversity schemes, including space diversity using multiple antennas as seen in MIMO systems, frequency diversity, and time diversity through forward error correction, are also critical. Advanced receivers may use equalizers or rake receivers, the latter famously used in CDMA2000 networks.

Applications and examples

While often a challenge to mitigate, multipath effects are also exploited in certain applications. Radar systems, such as those on the Lockheed Martin F-35 Lightning II, can use multipath reflections off the sea or ground for over-the-horizon radar. In wireless sensor networks, techniques like fingerprinting for indoor positioning systems rely on the unique multipath signature of a location. Radio astronomy observatories like the Arecibo Observatory had to account for multipath within their massive reflector dishes. Furthermore, the study of multipath propagation in the ionosphere has been essential for long-range shortwave radio communication, historically used by entities like the British Broadcasting Corporation for international broadcasts.