OFDM Subcarriers - PHY Architecture
OFDM (Orthogonal Frequency Division Multiplexing) is the physical layer technology in every 802.11 standard from 802.11a (1999) onward. Understanding how subcarrier count, FFT size, guard interval, and spatial streams interact determines what you can achieve at any given distance and environment.
Why OFDM Works - Multipath Immunity
DSSS (used in 802.11b) spreads one wide signal - multipath echoes corrupt the entire channel. OFDM divides the channel into dozens of narrow subcarriers, each with a very long symbol period. A multipath echo arriving 800 ns late affects only a tiny fraction of the symbol's duration - the Cyclic Prefix absorbs it before the FFT window begins. This is why OFDM delivers dramatically better performance in reflective indoor environments than spread spectrum.
Narrower subcarriers = longer symbol duration = more multipath tolerance. 802.11ax quadrupled symbol duration (3.2µs → 12.8µs) by using 4× narrower subcarriers.
Copy of the LAST portion of the symbol prepended as a PREFIX. Receiver waits for CP to clear, then starts FFT. CP absorbs multipath echoes that arrive within the CP window. CP duration = GI duration.
OFDM subcarriers are orthogonal - peak of each subcarrier falls on zero-crossings of all others. Enables dense packing without inter-subcarrier interference. Requires precise frequency synchronisation.
Known reference symbols on specific subcarriers. Receiver uses pilots to estimate phase offset and frequency drift over time. Pilots remain constant while data subcarriers change every symbol.
Subcarrier Count by Generation
| Standard | BW | FFT | Total Subs | Data | Pilots | Null | Spacing | Symbol | GI Options |
|---|---|---|---|---|---|---|---|---|---|
| 802.11a/g | 20 MHz | 64 | 52 | 48 | 4 | 12 | 312.5 kHz | 4.0 µs (3.2+0.8) | 800 ns |
| 802.11n (HT) | 20 MHz | 64 | 56 | 52 | 4 | 8 | 312.5 kHz | 4.0 µs / 3.6 µs SGI | 800/400 ns |
| 802.11n (HT) | 40 MHz | 128 | 114 | 108 | 6 | 14 | 312.5 kHz | 4.0 µs / 3.6 µs SGI | 800/400 ns |
| 802.11ac (VHT) | 80 MHz | 256 | 242 | 234 | 8 | 14 | 312.5 kHz | 4.0 µs / 3.6 µs SGI | 800/400 ns |
| 802.11ax (HE) | 20 MHz | 256 | 242 | 234 | 8 | 14 | 78.125 kHz | 16.0 µs (12.8+GI) | 0.8/1.6/3.2 µs |
| 802.11ax (HE) | 80 MHz | 1024 | 996 | 980 | 16 | 28 | 78.125 kHz | 16.0 µs | 0.8/1.6/3.2 µs |
| 802.11be (EHT) | 320 MHz | 4096 | 3920 | 3888 | 32 | 176 | 78.125 kHz | 16.0 µs | 0.8/1.6/3.2 µs |
802.11ax moved from 312.5 kHz subcarrier spacing to 78.125 kHz - exactly 4× narrower. This means 4× longer symbol duration (12.8 µs vs 3.2 µs) for the same 20 MHz bandwidth. Longer symbols mean the same-size cyclic prefix provides 4× more multipath tolerance, and OFDMA resource units can be assigned precisely to individual users.
Guard Interval - Cyclic Prefix Reference
Rule: GI must exceed RMS delay spread of the environment. Typical indoor office: 30–100 ns delay spread → 400 ns GI safe. Industrial/warehouse with metal racking: 200–600 ns → 800 ns GI required. Outdoor with large structures: 600-2000 ns → 800 ns or 1600 ns. Short GI (SGI) in 802.11n/ac is negotiated per-frame in HT-SIG/VHT-SIG-A. Never force SGI in high-delay environments - PHY errors increase.
MIMO - Spatial Streams vs STBC
Send DIFFERENT data on each antenna simultaneously. Each stream takes a different multipath path to the receiver. Requires SNR ≥ ~20 dB per stream. Multiplies throughput by the number of streams. Client must have ≥N receive chains for N-stream SU-MIMO.
Send the SAME data redundantly across multiple antennas using Alamouti orthogonal codes. TX1: [S1, −S2*]. TX2: [S2, S1*]. Single-chain receiver recovers both symbols. Trading throughput for reliability and range extension. Cannot combine with spatial multiplexing on the same stream.
| Config | Streams | Chains | Peak (80 MHz, Mbps) | Peak (160 MHz, Mbps) | Use |
|---|---|---|---|---|---|
| 1×1 SISO | 1 | 1T/1R | 292.5 | N/A | IoT sensors, entry-level devices, max range mode |
| 2×2 2SS | 2 | 2T/2R | 867 | N/A | Standard client devices (phones, laptops), typical home AP |
| 3×3 3SS | 3 | 3T/3R | 1300 | 2600 | Mid-range enterprise APs, prosumer routers |
| 4×4 4SS | 4 | 4T/4R | 1733 | 3466 | High-density enterprise APs, WPA3-Enterprise 192-bit |
| 8×8 8SS (ac) | 8 | 8T/8R | 6933 | N/A | 802.11ac Wave 2 DL MU-MIMO, high-capacity campus APs |
Nsts (Space-Time Streams) ≠ Nss (Spatial Streams). When STBC is used: Nsts = 2 × Nss (each stream transmits on 2 space-time slots). Without STBC: Nsts = Nss. HT-SIG field encodes Nsts; the MCS index encodes Nss. STBC limit per 802.11n/ac: Nsts ≤ 2 × Nss. Maximum STBC for 1 spatial stream = 2 space-time streams.