What Are the Modulation Schemes Used in WiFi?
WiFi, based on the IEEE 802.11 standards, relies on various modulation schemes to transmit data over the air. A modulation scheme determines how data is encoded onto a carrier signal for wireless transmission. The choice of modulation significantly impacts speed, range, and reliability. As WiFi technology has evolved over time—from 802.11b to 802.11ax (WiFi 6) and beyond—different modulation techniques have been adopted, each offering improvements in efficiency, data rates, and robustness against interference.
This document explains the key modulation schemes used in WiFi networks, their characteristics, and how they affect overall performance.
What Is Modulation in Wireless Communication?
Modulation is the process of varying a carrier signal to transmit information. In the context of WiFi, digital modulation schemes are used to encode binary data (1s and 0s) onto electromagnetic signals. The modulation technique defines how these bits are represented in terms of signal amplitude, frequency, or phase.
WiFi uses several digital modulation techniques, primarily:
- BPSK – Binary Phase Shift Keying
- QPSK – Quadrature Phase Shift Keying
- 16-QAM – 16-level Quadrature Amplitude Modulation
- 64-QAM – 64-level Quadrature Amplitude Modulation
- 256-QAM – 256-level Quadrature Amplitude Modulation
- 1024-QAM – 1024-level Quadrature Amplitude Modulation (introduced in WiFi 6)
These schemes differ in complexity and data capacity. More complex schemes transmit more bits per symbol but require better signal quality (higher SNR).
Modulation Schemes by WiFi Standard
| 802.11 Standard | Maximum Modulation | Approximate Max Data Rate | Frequency Band |
|---|---|---|---|
| 802.11b | CCK (uses QPSK/BPSK) | 11 Mbps | 2.4 GHz |
| 802.11a/g | 64-QAM | 54 Mbps | 2.4 GHz / 5 GHz |
| 802.11n (WiFi 4) | 64-QAM | 600 Mbps | 2.4 GHz / 5 GHz |
| 802.11ac (WiFi 5) | 256-QAM | 6.9 Gbps | 5 GHz |
| 802.11ax (WiFi 6) | 1024-QAM | 9.6 Gbps | 2.4 GHz / 5 GHz / 6 GHz (WiFi 6E) |
Overview of Major Modulation Schemes
- BPSK (Binary Phase Shift Keying) –
This is the simplest modulation scheme, where each bit is represented by one of two phases. It is highly robust and used in lower-speed transmissions or poor signal conditions. - QPSK (Quadrature Phase Shift Keying) –
A more advanced method than BPSK, using four phase shifts to represent two bits per symbol. Offers double the throughput of BPSK under similar conditions. - QAM (Quadrature Amplitude Modulation) –
This family of schemes combines amplitude and phase variation to represent multiple bits per symbol:- 16-QAM transmits 4 bits per symbol
- 64-QAM transmits 6 bits per symbol
- 256-QAM transmits 8 bits per symbol
- 1024-QAM transmits 10 bits per symbol
Higher QAM levels require a cleaner signal environment but offer significantly higher data rates.
OFDM – The Backbone of Modern WiFi
Most WiFi standards from 802.11a onward use OFDM (Orthogonal Frequency-Division Multiplexing) as the primary transmission method. OFDM divides the channel into multiple smaller sub-carriers, each of which is modulated separately using one of the schemes mentioned above. This approach improves robustness to interference and enables parallel data transmission.
In WiFi 6 (802.11ax), the modulation continues to use OFDM but introduces OFDMA (Orthogonal Frequency-Division Multiple Access), allowing multiple users to transmit simultaneously on different subcarriers. OFDMA enhances network efficiency, especially in dense environments.
Modulation and Signal Quality (SNR)
Higher-order modulation schemes allow more data per symbol but require a higher Signal-to-Noise Ratio (SNR). Below is a general guide:
| Modulation Scheme | Bits per Symbol | Required SNR (Approx) |
|---|---|---|
| BPSK | 1 | ~6 dB |
| QPSK | 2 | ~9 dB |
| 16-QAM | 4 | ~16 dB |
| 64-QAM | 6 | ~22 dB |
| 256-QAM | 8 | ~30 dB |
| 1024-QAM | 10 | ~35 dB |
This illustrates that using 1024-QAM provides the highest data rates, but only if the connection is strong and interference is low.
Impact on WiFi Performance
Modulation schemes have a direct influence on key performance aspects of WiFi:
- Speed: Higher modulation schemes enable faster data transmission.
- Range: Lower modulation schemes maintain connectivity at greater distances.
- Stability: Adaptive modulation allows devices to downgrade when signal degrades, maintaining connection quality.
Adaptive Modulation in WiFi
WiFi devices use adaptive modulation, meaning they adjust the modulation scheme dynamically based on signal conditions. If the signal is strong and the interference is low, the device may use 1024-QAM. If the signal weakens, it may downgrade to 256-QAM or lower, ensuring a stable connection even at reduced speeds.
Conclusion
The modulation schemes used in WiFi have evolved significantly to meet growing demands for speed, reliability, and efficiency. From basic BPSK in early 802.11 standards to high-capacity 1024-QAM in WiFi 6, each modulation technique plays a role in determining how efficiently data can be transmitted over the air. Understanding these schemes provides insights into how WiFi networks operate and why performance varies under different conditions. As wireless technology continues to advance, even more sophisticated modulation methods are likely to be introduced to support future applications and bandwidth needs.