802.11be (Wi-Fi 7)

802.11be extends the modulation to 4096-QAM ($M = 4096$, $\log_2 M = 12$ bits per symbol):

The throughput gain over 1024-QAM (MCS 11) is: $$\Delta R = \frac{12 \times 5/6}{10 \times 5/6} = 1.2 \quad (20\%)$$

However, the required SNR increases significantly. For a target BER of $10^{-2}$ (before FEC):

$$\text{SNR}_{4096\text{-QAM}} \approx \text{SNR}_{1024\text{-QAM}} + 6 \text{ dB}$$

This limits 4096-QAM to short-range, high-SNR scenarios (typically $< 5$ m from the AP with line of sight).

802.11be supports 320 MHz channel bandwidth, double the maximum of 802.11ax. This is enabled by the 6 GHz band (5.925--7.125 GHz in the US), which provides 1200 MHz of contiguous spectrum — enough for three non-overlapping 320 MHz channels.

OFDM parameters for 320 MHz:

• FFT size: 4096
• Data subcarriers: $4 \times 996 = 3984$ (four 996-tone segments, minus DC and guard tones: effectively $\sim$3920 data tones)
• Total subcarriers: 4096
• Symbol duration: 12.8 $\mu$s (same as 802.11ax)
• Guard interval: 0.8 / 1.6 / 3.2 $\mu$s

Peak PHY rate (320 MHz, 16 SS, MCS 13, GI = 0.8 $\mu$s): $$R_{\mathrm{peak}} = 16 \times \frac{3920 \times 12 \times 5/6}{13.6 \times 10^{-6}} \approx 46.1 \text{ Gbps}$$

Preamble puncturing allows transmission on non-contiguous portions of the 320 MHz channel, with any 20 MHz sub-channel independently puncturable.

Multi-Link Operation (MLO) allows a device to establish a single logical connection that spans multiple frequency bands (e.g., 2.4 GHz + 5 GHz + 6 GHz) simultaneously:

Architecture:

• A Multi-Link Device (MLD) has multiple affiliated STAs, one per link (band).
• A single MAC address and association are maintained across links.
• Traffic can be distributed across links at the packet level.

Operating modes:

• Simultaneous Transmit and Receive (STR): All links active simultaneously. Requires sufficient antenna isolation between bands.
• Non-STR: Only one link active at a time per STA, but the AP can receive on multiple links simultaneously.
• Enhanced MLO (eMLO): Dynamic link switching based on channel conditions, traffic load, and latency requirements.

Benefits:

1. Throughput aggregation: Total throughput $\approx \sum_i R_i$.
2. Latency reduction: A packet can be sent on whichever link is idle first, avoiding CSMA/CA contention delays on any single link.
3. Reliability: If one link experiences interference, traffic shifts to other links.

Beyond the three pillar features, 802.11be introduces:

1. 16 spatial streams: Doubling from 802.11ax's 8 streams, though practical implementations will likely use 4--8 streams.

2. Multi-RU allocation: A single STA can be assigned multiple non-contiguous RUs within a single OFDM symbol, improving scheduling flexibility.

3. Enhanced OFDMA: Larger RU sizes (up to $4 \times 996$), more flexible RU combinations, and improved trigger-based UL.

4. Restricted TWT: Strengthened TWT enforcement to guarantee low-latency service periods for time-sensitive applications.

5. Coordinated spatial reuse (C-SR): Multiple APs coordinate their transmissions (via a coordinator AP) to enable simultaneous transmissions that would otherwise be prevented by CSMA/CA. This is a step toward coordinated multi-AP operation.