MIMO in 802.11n Wireless Networks Part-1
MIMO (Multiple-Input Multiple-Output) is a key technology in modern wireless communication, particularly in 802.11n Wi-Fi, which significantly improves data throughput, reliability, and range compared to older standards like 802.11a/b/g.
Before MIMO, wireless networks used SISO (Single-Input Single-Output), where a single antenna was used for transmission and reception. Even if devices had multiple antennas, only one was active at a time. MIMO changes this by allowing multiple antennas to transmit and receive simultaneously, leading to higher data rates and better signal robustness.
MIMO vs. SISO
SISO (Single-Input Single-Output)
- Uses one transmit antenna and one receive antenna.
- Only one data stream is transmitted at a time.
- Performance is limited by multipath interference (where signals reflect off surfaces, causing delays and signal degradation).
MIMO (Multiple-Input Multiple-Output)
- Uses multiple transmit (Tx) and receive (Rx) antennas simultaneously.
- Each antenna can send or receive independent data streams, increasing throughput.
- Exploits multipath propagation (instead of suffering from it) to improve signal quality.
Key Differences
| Feature | SISO | MIMO |
| Antennas | 1 Tx, 1 Rx | Multiple Tx & Rx |
| Data Streams | Single | Multiple (Spatial Streams) |
| Multipath Handling | Suffers from interference | Benefits from multipath |
| Throughput | Limited | Significantly higher |
MIMO Components & Concepts:
(A) Spatial Streams
- MIMO transmits multiple independent data streams over the same frequency channel.
- Each stream is called a spatial stream because it takes a different path through space.
- Example: A 2×2:2 MIMO system (2 Tx, 2 Rx, 2 streams) doubles throughput compared to SISO.
(B) Radio Chains
- Each spatial stream requires a radio chain (a set of components for signal processing).
- A radio chain includes:
- Transmitter: Inverse Fourier Transform (IFT), amplifier, antenna.
- Receiver: Amplifier, Fourier Transform (FT), signal decoder.
- 802.11n requires multiple radio chains to handle multiple streams.
(C) MIMO Notation (T × R : S)
- T = Number of transmit antennas
- R = Number of receive antennas
- S = Number of spatial streams (must be ≤ T and R)
- Example: A 3×3:2 device has 3 antennas but only 2 streams (max speed of a 2-stream system).
Advanced MIMO Techniques
(A) Space-Time Block Coding (STBC)
- Improves reliability (not speed) by transmitting redundant copies of data across multiple antennas.
- Requires at least two radio chains but transmits only one spatial stream.
- Used in environments with high interference or weak signals.
(B) Maximal Ratio Combining (MRC)
- A receive-side technique that combines signals from multiple antennas to improve quality.
- Uses signal strength and phase information to reconstruct the best possible signal.
- Helps in low-SNR (weak signal) conditions.
(C) Beamforming
- Instead of transmitting signals equally in all directions (omnidirectional), beamforming focuses energy toward the receiver.
- Improves signal-to-noise ratio (SNR) and range.
- Achieved by adjusting phase shifts across multiple antennas.
Modulation & Coding in 802.11n
(A) Modulation and Coding Scheme (MCS)
- Defines the data rate based on:
- Modulation type (BPSK, QPSK, 16-QAM, 64-QAM)
- Coding rate (error correction strength, e.g., 1/2, 3/4, 5/6)
- Number of spatial streams
- 802.11n supports 77 MCS combinations, but most devices use only the first 32 (equal modulation).
(B) Forward Error Correction (FEC)
- Adds redundant bits to detect and correct errors.
- Two types in 802.11n:
- Convolutional Codes (legacy, used in 802.11a/g)
- Low-Density Parity Check (LDPC) (more efficient but optional)
- Code Rate (R): Ratio of payload bits to total transmitted bits.
- Example: R=1/2 means 1 data bit + 1 error-correction bit.
Data Rate Comparison (802.11a/g vs. 802.11n)
| Modulation | Coding Rate | 802.11a/g Speed (Mbps) | 802.11n (1×1, 20 MHz) Speed (Mbps) |
| BPSK | R=1/2 | 6 | 6.5 |
| QPSK | R=1/2 | 12 | 13.0 |
| QPSK | R=3/4 | 18 | 19.5 |
| 16-QAM | R=1/2 | 24 | 26.0 |
| 16-QAM | R=3/4 | 36 | 39.0 |
| 64-QAM | R=1/2 | 48 | 52.0 |
| 64-QAM | R=3/4 | 54 | 58.5 |
| 64-QAM | R=5/6 | Not used in 802.11a/g | 65.0 |
Conclusion:
MIMO in 802.11n revolutionized Wi-Fi by:
✅ Increasing throughput (multiple spatial streams).
✅ Improving reliability (STBC, MRC).
✅ Enhancing range (beamforming).
✅ Better handling of multipath interference.
Future Wi-Fi standards (802.11ac, 802.11ax and 802.11be) build on these MIMO principles, further improving speed and efficiency.
Next -> Part-2