multipath: one construct, many systems

In radio, multipath is the arrival of a transmitted signal by more than one path, as reflections off the ground, buildings, or ionospheric layers reach the receiver with different delays and phases and add constructively or destructively; the result is fading, time spreading, and distortion of the recovered signal. Haykin's Communication systems treats multipath fading and the delay spread it produces. The construct is the superposition of many delayed copies of one signal, and the same phenomenon recurs wherever a wave can reach a receiver by several routes, from undersea sound to light in a fibre.[1]

Unit

The delay spread and resulting fading depth across the received copies

Recovery means

Not applicable

Measured by

Characterised by the channel's delay spread, coherence bandwidth, and fading statistics

In underwater acoustics, multipath is the reception of a sound signal over several paths at once, as rays bend with the sound-speed profile and bounce off the surface and bottom, arriving with different travel times and amplitudes and interfering at the hydrophone. Urick's Principles of underwater sound describes this multipath structure and its effect on signal fidelity and ranging. The construct is identical to radio multipath, a single sound reaching the receiver by many routes, realised in the ocean waveguide and measured again by arrival spread and fading.[2]

Unit

The spread of arrival times and amplitudes across the multiple acoustic paths

Recovery means

Not applicable

Measured by

Assessed from the measured arrival-time structure and the resulting signal distortion versus range

In fibre optics, multimode fibre carries light in several guided modes that follow effectively different path lengths and so arrive at slightly different times, spreading a pulse through modal (intermodal) dispersion, the optical counterpart of multipath that limits the bit rate of multimode links. Agrawal's Fiber-optic communication systems treats intermodal dispersion and the pulse broadening it causes. The construct is the same multiple-path superposition as in radio and underwater sound, here arising from the fibre's modes and measured by pulse spreading per unit length.[3]

Unit

The pulse broadening per unit fibre length from the spread in modal path lengths

Recovery means

Not applicable

Measured by

Quantified by intermodal dispersion and the resulting bandwidth-distance product of the fibre