The Problem With One Antenna
Traditional wireless systems use one antenna to send and one antenna to receive. This is called SISO (Single Input, Single Output).
The problem?
- You can only send one stream of data at a time
- Want more throughput? You need more bandwidth
- But bandwidth is expensive and limited
What if you could send multiple streams at the same time, on the same frequency?
That’s exactly what MIMO does.
Multiple Antennas = Multiple Streams
MIMO stands for Multiple Input, Multiple Output.
- Multiple Input → multiple antennas at the transmitter
- Multiple Output → multiple antennas at the receiver
The magic: with multiple antennas on both ends, you can send separate data streams simultaneously over the same frequency.
This is called spatial multiplexing.
Each stream takes a different path through space. The receiver can separate them because they arrive with different characteristics.
How Does the Receiver Separate the Streams?
This is where multipath becomes your friend.
In a real environment:
- Signals bounce off walls, buildings, and objects
- Each stream travels a slightly different combination of paths
- The receiver uses math to untangle them
Ironically, the “messy” multipath environment that causes problems for simple systems is actually good for MIMO.
In a perfectly clear line-of-sight with no reflections, all streams would arrive the same way. The receiver couldn’t tell them apart.
MIMO Configurations
MIMO systems are described by their antenna count: Tx x Rx
| Configuration | Transmit Antennas | Receive Antennas | Max Streams |
|---|---|---|---|
| 2x2 | 2 | 2 | 2 |
| 4x4 | 4 | 4 | 4 |
| 8x8 | 8 | 8 | 8 |
Read it as: “2 transmit antennas, 2 receive antennas”
Impact on Throughput
How many streams can you send? It depends on the bottleneck.
Throughput multiplier = min(Tx antennas, Rx antennas)
The side with fewer antennas limits you.
| Setup | Calculation | Throughput Boost |
|---|---|---|
| 2 Tx, 2 Rx | min(2, 2) = 2 | 2x |
| 4 Tx, 4 Rx | min(4, 4) = 4 | 4x |
| 4 Tx, 2 Rx | min(4, 2) = 2 | 2x (receiver is bottleneck) |
| 8 Tx, 8 Rx | min(8, 8) = 8 | 8x |
More antennas = more simultaneous streams = higher throughput — without using more bandwidth.
Where is MIMO Used?
MIMO is everywhere in modern wireless:
WiFi:
- 802.11n introduced MIMO (up to 4x4)
- 802.11ac uses MIMO + wider channels
- 802.11ax (WiFi 6) uses 8x8 MIMO with MU-MIMO (multi-user)
Cellular Networks:
- 4G LTE uses 2x2 or 4x4 MIMO in most deployments
- 5G NR uses Massive MIMO with 32, 64, or even 128 antennas at the base station
Why it’s so important:
- Spectrum is expensive and limited
- MIMO lets you get more data through the same spectrum
- It’s one of the key technologies that made 4G and 5G possible
MIMO is one of the biggest capacity boosters in modern wireless.