MIMO
Understanding MIMO
MIMO exploits the multipath environment that was previously considered a nuisance. In a rich multipath channel, each transmit-receive antenna pair creates a unique propagation path. If these paths are sufficiently independent, they can carry independent data streams.
MIMO Types
- Spatial multiplexing: Send independent data on each stream. Multiplies throughput. Requires good channel conditions.
- Beamforming: Focus energy toward the user. Improves SNR and range. Works even with poor multipath.
- Diversity: Send the same data on multiple paths. Improves reliability. Used for coverage extension.
Massive MIMO (5G)
- Elements: 64-256 antenna elements at the base station.
- MU-MIMO: Simultaneously serve 8-16 users with spatial beams.
- Capacity gain: 5-10x compared to 4x4 MIMO.
C = min(M,N) x log2(1 + SNR/min(M,N)) bps/Hz
where M = TX antennas, N = RX antennas
4x4 MIMO at 20 dB SNR:
C = 4 x log2(1 + 100/4) = 4 x 4.7 = 18.8 bps/Hz
vs SISO: 6.7 bps/Hz (2.8x improvement)
Frequently Asked Questions
What is MIMO?
MIMO uses multiple antennas at both transmitter and receiver to create parallel spatial data channels. It multiplies throughput without additional bandwidth. 4G supports 2x2 to 8x8 MIMO. 5G uses Massive MIMO with 64-256 elements.
How does MIMO increase throughput?
In a rich multipath environment, each transmit-receive antenna pair creates a unique channel. Mathematical decomposition (SVD) reveals independent spatial streams. Each stream carries independent data, multiplying overall throughput.
What is Massive MIMO?
Massive MIMO uses 64-256 antenna elements at the base station to form very narrow beams directed at individual users (MU-MIMO). It provides 5-10x capacity gain by serving many users simultaneously with spatial separation.