Prerequisites

Before You Begin

This chapter applies OFDM, MIMO, and multiple access theory to the IEEE 802.11 family of wireless LAN standards. It requires familiarity with multipath fading and channel models (Chapter 8), OFDM modulation and the time-frequency resource grid (Chapter 14), and multiple access techniques including FDMA, TDMA, and random access (Chapter 19).

Multipath fading, delay spread, and coherence bandwidth(Review ch08)
Self-check: Can you explain how the RMS delay spread $\tau_{\mathrm{rms}}$ determines the coherence bandwidth $B_c \approx 1/(5\tau_{\mathrm{rms}})$ and why OFDM subcarrier spacing must satisfy $\Delta f \gg 1/T_u$ to experience flat fading per subcarrier?
OFDM: FFT-based modulation, cyclic prefix, and guard interval(Review ch14)
Self-check: Can you derive the OFDM signal model $x(t) = \sum_{k} X_k e^{j2\pi k \Delta f t}$ and explain how the cyclic prefix eliminates inter-symbol interference for channels with delay spread $\tau_{\max} < T_{\mathrm{CP}}$ ?
Multiple access: FDMA, TDMA, CSMA, and random access protocols(Review ch19)
Self-check: Can you describe the ALOHA protocol, derive its maximum throughput of $1/(2e)$ , and explain how carrier sensing improves performance in CSMA?

Chapter 25 Notation

Key symbols introduced or heavily used in this chapter.

Symbol	Meaning	Introduced
$N_{\mathrm{FFT}}$	FFT size for OFDM (64, 128, 256, 512, 1024, or 4096 depending on standard)	s01
$N_{\mathrm{SD}}$	Number of data subcarriers per OFDM symbol	s01
$N_{\mathrm{SP}}$	Number of pilot subcarriers per OFDM symbol	s01
$T_{\mathrm{GI}}$	Guard interval (cyclic prefix) duration	s01
$DIFS$	DCF interframe space: time a station must sense idle before transmitting	s02
$SIFS$	Short interframe space: minimum gap between frame and acknowledgement	s02
$W$	Contention window size in CSMA/CA backoff	s02
$\tau$	Transmission probability per slot in Bianchi model	s02
$RU$	Resource unit in 802.11ax OFDMA (26, 52, 106, 242, 484, 996, or 2 $\times$ 996 tones)	s03
$TWT$	Target Wake Time: scheduled wake/sleep mechanism in 802.11ax	s03
$MLO$	Multi-Link Operation in 802.11be	s04

← Ch 24 802.11 Physical Layer