The preamble is the initial portion of a Wi-Fi frame transmission. Its main purpose is to:
- Alert receiving stations that a transmission is starting.
- Allow receivers to synchronize their radios (timing and frequency) with the incoming signal before decoding the frame’s data payload.
Why is it important?
Wi-Fi radios need to:
- Lock onto the signal’s timing
- Adjust frequency offset
- Prepare to demodulate and decode
802.11 modulation refers to the methods used to encode digital data onto radio waves for wireless communication in Wi-Fi networks, following the IEEE 802.11 standards.
What is modulation in 802.11?
- Modulation is the process of varying a carrier signal (usually a sine wave) to represent digital data (bits).
- In 802.11 Wi-Fi, modulation schemes determine how bits (0s and 1s) are transmitted over the air as radio signals.
- Different 802.11 standards use different modulation types to optimize data rates, range, and reliability.
Common modulation types in 802.11 standards:
| 802.11 Standard | Modulation Schemes | Description |
|---|---|---|
| 802.11b | DSSS with CCK (Complementary Code Keying) | Uses Direct Sequence Spread Spectrum, slower speeds, robust. |
| 802.11a/g | OFDM (Orthogonal Frequency Division Multiplexing) with BPSK, QPSK, 16-QAM, 64-QAM | Faster speeds with multiple subcarriers, more complex modulation. |
| 802.11n | OFDM with up to 64-QAM | MIMO introduced, higher throughput. |
| 802.11ac | OFDM with up to 256-QAM | Even higher data rates, wider channels, MU-MIMO support. |
| 802.11ax | OFDMA with up to 1024-QAM | Improved spectral efficiency, supports more users. |
The preamble enables this synchronization and readiness.
Structure of the preamble
The preamble structure varies by PHY type:
1. 802.11b (DSSS)
- Short preamble (optional):
- Shorter synchronization field to reduce overhead.
- Long preamble (default if short not supported):
- Longer synchronization field for better reception reliability.
2. 802.11a/g/n/ac/ax (OFDM PHYs)
The preamble includes:
| Field | Purpose |
|---|---|
| Short Training Field (STF) | Synchronize timing and detect signal presence. |
| Long Training Field (LTF) | Channel estimation and fine frequency/timing synchronization. |
| Signal Field (SIG) | Contains rate and length information for the payload. |