Multipath
Understanding Multipath Propagation
Multipath is one of the most important phenomena in wireless communications. In any environment with reflective surfaces (buildings, ground, vehicles), the transmitted signal arrives at the receiver as the sum of multiple copies, each traveling a different path. The interaction of these copies creates a complex, time-varying channel.
Effects of Multipath
- Fading: Constructive and destructive interference causes signal strength to vary rapidly with position (Rayleigh fading) and time. Deep fades can reduce signal by 20-40 dB.
- Delay spread: Different path lengths cause arriving copies to be spread in time. This causes intersymbol interference for wideband signals.
- Frequency-selective fading: Different frequencies experience different fading patterns. Wideband signals see varying gain across their bandwidth.
Multipath Mitigation
- OFDM: Divides the signal into narrow subcarriers that each experience flat fading. Cyclic prefix absorbs delay spread.
- Diversity: Multiple antennas (spatial diversity), multiple frequencies (frequency diversity), or multiple time slots (time diversity) reduce the probability of deep fades.
- Equalization: Digital signal processing compensates for the channel frequency response.
Frequently Asked Questions
What is multipath propagation?
Multipath occurs when a signal reaches the receiver via multiple reflected, diffracted, and scattered paths. The copies arrive at different times with different phases, causing interference patterns that create fading (signal strength variations) and signal distortion.
Is multipath always harmful?
Multipath causes fading and intersymbol interference, which are challenges. However, MIMO technology exploits multipath by using the different paths as independent data channels, actually increasing capacity. Rich multipath environments enable higher MIMO throughput.
How does OFDM handle multipath?
OFDM divides the channel into many narrow subcarriers, each narrow enough to experience flat fading. The cyclic prefix absorbs multipath echoes (delay spread), preventing intersymbol interference. Each subcarrier can be equalized independently with a single complex multiplication.