multipath

Multipath occurs when a transmitted radio signal bounces off buildings, terrain, metal structures, or other obstacles before reaching the receiver antenna. Instead of a clean direct path, the signal arrives via several different routes, each at slightly different times and with different amplitudes. This stacking of delayed copies creates constructive and destructive interference at the receiver, causing the signal to fluctuate in strength and quality.

The delay between the earliest and latest arriving signal copies is called delay spread. In urban environments, delay spread might range from microseconds to tens of microseconds, depending on the size of the area and density of reflectors. A signal that takes 1 microsecond longer to arrive on a reflected path will arrive 300 meters later in physical distance. When multiple delayed versions of the same signal interfere, they can either reinforce each other at certain frequencies or cancel each other out at others, creating deep fades in the received signal across the spectrum.

Impact on communications systems

Multipath fading is a primary source of signal degradation in mobile radio, WiFi, and cellular systems operating in non-line-of-sight conditions. A receiver tuned to a frequency where two multipath copies arrive nearly 180 degrees out of phase will experience a severe fade at that frequency, while adjacent frequencies may receive strong signals. This frequency-selective fading can corrupt data unless the system compensates through equalization, diversity reception, or coding techniques. Fast-fading occurs when the receiver or transmitter moves rapidly, causing the multipath configuration to change faster than the receiver can track it.

Engineers combat multipath through several practical methods. Diversity techniques use multiple antennas spaced to receive somewhat independent multipath patterns, since at least one antenna is likely to have a strong signal at any given instant. Direct-sequence spread spectrum and orthogonal frequency-division multiplexing (OFDM) systems handle multipath more gracefully than narrowband systems because their wider bandwidth resolves individual reflections rather than blending them together. In indoor wireless systems, antenna placement and polarization choices are tuned to avoid the worst multipath nulls in critical coverage areas.

Multipath is unavoidable in mobile radio; no receiver can distinguish whether a reflected signal is legitimate or unwanted. Modern systems assume multipath will exist and design their waveforms and receivers accordingly. Free-space propagation, where line-of-sight is unobstructed and reflections are negligible, is the exception in real deployments. Understanding the multipath environment of a specific site through propagation surveys or modeling is essential for reliable link budgets.