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Physical

Summary

  • The PHY (Physical Layer) is the bottom layer of 802.11 — it turns bits into radio waves and back, and is where nearly every "Wi-Fi 5 vs Wi-Fi 6" speed comparison actually lives.
  • Modern Wi-Fi speed gains come from four independent levers: wider channels, more spatial streams (MIMO), denser modulation, and more efficient multiplexing (OFDM/OFDMA) — generations stack improvements across all four rather than replacing one technique with another.
  • PHY features are negotiated per-link: a Wi-Fi 6 router and a Wi-Fi 4 phone will still connect, just at the older, lower-capability PHY that both sides support.

Modulation & Multiplexing

How data is actually encoded onto the radio signal, and how multiple data streams share the channel:1

Technique Introduced What It Does
DSSS 802.11b Spreads a signal across the whole channel using a chipping code; simple, but low throughput
OFDM 802.11a/g Splits a channel into many narrow, overlapping subcarriers transmitted in parallel; much more resistant to multipath interference
OFDMA 802.11ax (Wi-Fi 6) Divides subcarriers into Resource Units (RUs) so the AP can serve several clients in a single transmission, instead of one client per transmission

Table 1. Wi-Fi Modulation & Multiplexing Techniques

OFDM vs. OFDMA is about who gets the channel, not how fast it is

OFDM still gives the entire channel to one client at a time, even if that client only has a tiny packet to send. OFDMA's real win is efficiency in busy networks — many small transmissions (IoT sensors, VoIP) can be bundled into one airtime slot instead of each fighting for its own turn.

Modulation Order (Bits per Symbol)

Within OFDM, the modulation order sets how many bits each symbol carries — higher order means more throughput, but requires a cleaner signal:2

Modulation Bits/Symbol Requires
BPSK 1 Very robust, used for the slowest/most reliable rates
QPSK 2 Good range, moderate speed
16-QAM 4 Stronger signal needed
64-QAM 6 Standard "good signal" rate through Wi-Fi 5
256-QAM 8 Introduced with Wi-Fi 5, needs a strong, clean signal
1024-QAM 10 Introduced with Wi-Fi 6, close range only
4096-QAM 12 Introduced with Wi-Fi 7, very close range only

Table 2. Modulation Orders Used in 802.11

MIMO & Spatial Streams

MIMO (Multiple-Input, Multiple-Output) uses multiple antennas to send more than one data stream at once:3

Term Meaning
Spatial Stream One independent data stream sent over one antenna path
SISO Single antenna, single stream — pre-Wi-Fi 4 baseline
MIMO Multiple antennas, multiple streams to one client at a time
MU-MIMO Multiple antennas serve several clients simultaneously, each on its own stream(s)
Beamforming The AP shapes its antenna signal to focus energy toward a specific client, improving range and reliability
Nss (Number of Spatial Streams) Shorthand used in device specs, e.g. "2x2" = 2 antennas, 2 spatial streams

Table 3. MIMO Terminology

MU-MIMO direction matters

802.11ac (Wi-Fi 5) only supports MU-MIMO downlink (AP → clients). Uplink MU-MIMO (clients → AP) wasn't added until 802.11ax (Wi-Fi 6).

Channel Width & Bonding

Wider channels carry more data per unit time, at the cost of fewer non-overlapping channels being available:4

Width Available Since Notes
20 MHz 802.11b/a Baseline width, only option on 2.4 GHz in most regions
40 MHz 802.11n Two adjacent 20 MHz channels bonded together
80 MHz 802.11ac Four bonded 20 MHz channels
160 MHz 802.11ac (rarely used), 802.11ax Eight bonded 20 MHz channels; needs very clean spectrum
320 MHz 802.11be (Wi-Fi 7) Only possible in the 6 GHz band, where there's enough clean spectrum to support it

Table 4. Channel Bonding Widths

Guard Interval (GI)

The guard interval is a short pause between OFDM symbols that prevents multipath reflections from smearing into the next symbol:2

GI Length Trade-off
Long GI (800 ns) Traditional default, more robust in reflective environments
Short GI (400 ns) ~11% higher throughput, introduced in 802.11n, safe in most modern indoor environments
Extra-short GI (800 ns "1x", down to shorter options in 11ax) 802.11ax added additional GI options tuned for OFDMA's longer symbol durations

Table 5. Guard Interval Options

Forward Error Correction (Coding)

Coding schemes add redundancy so the receiver can correct small errors without asking for a retransmission:2

Scheme Introduced Notes
BCC (Binary Convolutional Coding) 802.11a Mandatory baseline, all 802.11 devices support it
LDPC (Low-Density Parity-Check) 802.11n (optional), more common by Wi-Fi ⅚ More efficient than BCC, especially at higher modulation orders; optional but widely implemented

Table 6. Coding Schemes

PHY Capabilities by Wi-Fi Generation

Putting the levers above together, this is roughly what each generation added on the PHY side:1

Wi-Fi Generation Max Channel Width Max Modulation MIMO Multiplexing
Wi-Fi 4 (802.11n) 40 MHz 64-QAM MIMO (single-user) OFDM
Wi-Fi 5 (802.11ac) 160 MHz 256-QAM MU-MIMO (downlink only) OFDM
Wi-Fi 6/6E (802.11ax) 160 MHz 1024-QAM MU-MIMO (up + down) OFDMA
Wi-Fi 7 (802.11be) 320 MHz 4096-QAM MU-MIMO (up + down) OFDMA + Multi-Link Operation (MLO)

Table 7. PHY Feature Comparison Across Wi-Fi Generations

This table complements, not duplicates, the generation table on the Overview page

The Overview page covers the headline feature per generation; this table breaks that headline down into its underlying PHY components (width, modulation, MIMO, multiplexing).

Useful Resources

  1. IEEE. (n.d.). IEEE 802.11 Standard. https://standards.ieee.org/ieee/802.11/7028/ ↩↩

  2. IEEE. (n.d.). IEEE 802.11-2020 — Part 11: Wireless LAN MAC and PHY Specifications. https://standards.ieee.org/ieee/802.11/7028/ ↩↩↩

  3. Wi-Fi Alliance. (n.d.). Discover Wi-Fi — Generations. https://www.wi-fi.org/discover-wi-fi ↩

  4. IEEE. (n.d.). IEEE 802.11 Standard. https://standards.ieee.org/ieee/802.11/7028/ ↩