When a radio signal is transmitted, it does not travel in a single straight line to the receiver. Instead, it bounces off buildings, walls, vehicles, and other obstacles, creating multiple copies of the same signal that travel different paths. Each path has a different length, so these copies arrive at the receiver at different times. This causes signal distortion called Inter-Symbol Interference (ISI), where adjacent symbols overlap in time.
Multipath propagation is the phenomenon where a transmitted radio signal arrives at the receiver via multiple paths (direct, reflected, diffracted, scattered), each with different delays and amplitudes. The receiver sees a composite of all these copies.
- The transmitter sends a single signal pulse
- The signal takes multiple paths: direct line-of-sight, reflection off buildings, diffraction over obstacles, scattering from rough surfaces
- Each path has a different physical length, causing different travel times
- The receiver captures all copies simultaneously — they add up (superpose)
- If path delays are significant relative to the bit period, adjacent bits overlap — this is ISI
- The composite signal can be stronger (constructive) or weaker (destructive) depending on phase alignment
The delay spread (difference between earliest and latest arriving paths) determines the severity of ISI.
- Causes Inter-Symbol Interference (ISI) when delay spread exceeds bit duration
- Results in frequency-selective fading — some frequencies are boosted, others are attenuated
- More severe in urban environments with many reflecting surfaces
- Can be exploited using diversity techniques (multiple receive antennas)
- Doppler spread from motion causes time-varying multipath effects
- Built from: Channel Fading — the constructive/destructive combination of multipath signals causes fading
- Related: Modulation — modulation scheme choice affects how vulnerable the signal is to multipath-induced ISI
- Related: Diversity Antenna — using multiple antennas reduces multipath problems
- Related: DSSS — spreading code helps filter out multipath noise
- In dense urban areas, multipath can create 10+ distinct signal paths
- High-speed mobility (e.g., in a car) causes rapidly changing multipath patterns
- Simple amplitude-based received signal strength indicators (RSSI) cannot distinguish multipath components
- OFDM (used in Wi-Fi, 4G, 5G) converts one wide band into many narrow subcarriers, each less affected by frequency-selective multipath