Summary

Chapter 25 Summary: Wi-Fi (IEEE 802.11)

Key Points

• 1.

  802.11 OFDM PHY has evolved from 64-point FFT with 312.5 kHz SCS (802.11a, 1999) to 4096-point FFT with 78.125 kHz SCS (802.11ax/be). The $4\times$ narrower SCS in 802.11ax was the most significant PHY change, enabling OFDMA multi-user access and improving outdoor delay spread resilience. Channel bonding scales bandwidth from 20 MHz to 320 MHz (802.11be), with throughput scaling approximately linearly.

• 2.

  CSMA/CA with binary exponential backoff is the fundamental MAC mechanism. The Bianchi model shows that saturation throughput decreases with the number of contending stations because the collision probability $p = 1-(1-\tau)^{n-1}$ grows faster than the backoff adaptation. Typical MAC efficiency is 40--60% of the PHY rate due to contention overhead, DIFS, backoff, and preambles. EDCA provides QoS differentiation through per-AC contention parameters.

• 3.

  802.11ax (Wi-Fi 6) introduced OFDMA with resource units (26 to 2$\times$996 tones), trigger-based uplink, BSS coloring for spatial reuse, and TWT for IoT power saving. These features target a $4\times$ per-user throughput improvement in dense environments, shifting Wi-Fi from contention-only to scheduled multi-user access. The peak PHY rate reaches 9.6 Gbps (160 MHz, 8 SS, 1024-QAM).

• 4.

  802.11be (Wi-Fi 7) pushes peak throughput to 46 Gbps via 320 MHz channels (6 GHz band), 4096-QAM (MCS 12/13), and 16 spatial streams. Multi-Link Operation (MLO) is the most impactful feature for user experience: by sending packets on whichever link is idle first across multiple bands, MLO reduces tail latency from $\sim$20 ms to $< 1$ ms, enabling AR/VR and cloud gaming.

• 5.

  The hidden node problem remains a fundamental challenge in unlicensed spectrum. RTS/CTS mitigates but does not eliminate it. BSS coloring (802.11ax) and coordinated spatial reuse (802.11be) represent incremental solutions. Full coordinated multi-AP operation (802.11bn / Wi-Fi 8) will bring cellular-style coordination to Wi-Fi.

• 6.

  Wi-Fi vs. cellular design philosophies differ fundamentally: Wi-Fi uses distributed contention (CSMA/CA) in unlicensed bands with best-effort QoS, while cellular uses centralised scheduling in licensed spectrum with guaranteed QoS. 802.11ax/be increasingly borrow cellular concepts (OFDMA, scheduled access, power control), blurring the boundary. The choice between Wi-Fi and cellular for a given application depends on spectrum availability, deployment density, mobility, and latency/reliability requirements.