Massive MIMO & Spatial Multiplexing
Scaling Spectral Efficiency
1. From MIMO to Massive MIMO
While traditional MIMO (used in Wi-Fi 5) typically uses 2, 4, or 8 antennas, Massive MIMO jumps significantly in scale. A standard 5G Massive MIMO array often features 64 transmit (T) and 64 receive (R) elements. This density allows for incredibly sharp beamforming and a technique known as Spatial Multiplexing.
Massive MIMO Pilot: 3D Beamforming
64T64R Spatial Multiplexing Laboratory
Standard Beamforming (Maximum Gain).
By shifting the phase of each antenna, the base station makes waves add up at the target's location, and cancel out everywhere else.
The antenna array calculates a "Null" to ensure zero energy hits the interfering user, allowing frequency reuse in the same cell.
Massive MIMO uses the spatial dimension to deliver multi-gigabit speeds without needing more spectrum.
2. Spatial Multiplexing: The Virtual Pipe
Spatial multiplexing allows a single base station to send different data streams to different users on the exact same frequency and at the exact same time. This is made possible by the base station having a "Spatial Fingerprint" for every user.
3. 64T64R: The Engineering Standard
A 64T64R array (64 transmit, 64 receive) is the workhorse of mid-band 5G (n77/n78). These arrays are not just antennas; they are computers. Each element has its own power amplifier and phase shifter.
- Vertical Sectoring: Unlike 4G, which mostly steered beams horizontally, Massive MIMO can steer beams vertically (3D Beamforming), allowing it to target specific floors in a high-rise building.
- Interference Rejection: By focusing energy tightly on the user, Massive MIMO reduces the "noise" felt by neighboring cells, increasing the overall network capacity.
4. The Cost of Complexity: Pilot Contamination
To calculate the channel matrix $H$, the base station needs "Pilots" (training signals) from the devices. However, because the number of orthogonal pilots is finite, neighboring cells often use the same pilots.
Conclusion
Massive MIMO is essentially a transition from "Broadcasting" to "Unicasting" at the physical layer. As we move toward 6G, we expect to see Extremely Large Aperture Arrays (ELAA) with thousands of elements, turning entire building facades into antennas that can track tens of thousands of users with centimeter precision.