Antenna Diversity
Understanding Antenna Diversity
Antenna diversity is the most effective technique for combating multipath fading in mobile communications. By providing multiple independent copies of the signal, diversity dramatically reduces the probability of a deep fade.
Diversity Techniques
- Spatial diversity: Antennas separated by > lambda/2. Decorrelated fading at each antenna. Most common technique.
- Polarization diversity: V and H polarized antennas on the same mast. Fading is largely independent between polarizations. Saves space vs spatial.
- Pattern diversity: Antennas with different beam patterns cover different angular regions. Different multipath contributions.
Diversity Combining
- Selection combining: Use the antenna with the strongest signal. Simplest. ~5 dB gain with 2 antennas.
- MRC (Maximum Ratio Combining): Coherently combine all antennas weighted by SNR. Best performance. ~10 dB gain with 2 antennas.
- Equal gain combining: Co-phase and add all antenna signals equally. Near-MRC performance. Simpler implementation.
Frequently Asked Questions
What is antenna diversity?
Diversity uses multiple antennas to combat fading: when one fades, another provides a good signal. Types: spatial (separation), polarization (V/H), pattern (direction). Diversity gain: 5-15 dB improvement in fade margin.
How much spacing is needed for spatial diversity?
Minimum lambda/2 for decorrelated fading at mobile terminals. 10-20 lambda at base stations (because scattering is local to the mobile, not the base). At 1.9 GHz (lambda=16cm): mobile 8cm, base station 1.6-3.2m.
Is MIMO the same as diversity?
Diversity uses multiple antennas to improve reliability (SNR gain). MIMO uses multiple antennas to increase capacity (spatial multiplexing). MIMO exploits the same multipath that diversity combats. MIMO can include diversity as a fallback mode.