Prerequisites

Before You Begin

This forward-looking chapter draws on OFDM waveform design, massive MIMO, RF hardware impairment modelling, and millimeter-wave propagation to survey the key candidate technologies for 6G wireless systems. A working knowledge of these foundations will allow the reader to appreciate the engineering trade-offs that shape the 6G research landscape.

OFDM waveform structure and 5G NR numerology(Review ch14)
Self-check: Can you describe the OFDM signal model, explain the role of the cyclic prefix, and relate subcarrier spacing to the time-frequency resource grid in 5G NR?
Massive MIMO fundamentals and channel hardening(Review ch18)
Self-check: Can you explain how an $M$ -antenna base station serves $K$ single-antenna users with linear precoding, describe channel hardening and favourable propagation, and state the achievable sum-rate scaling with $M$ ?
RF hardware impairments: phase noise, PA nonlinearity, ADC quantisation(Review ch23)
Self-check: Can you model phase noise, quantify the impact of PA nonlinearity on wideband signals, and describe the distortion introduced by low-resolution ADCs?
mmWave and sub-THz propagation, hybrid beamforming, near-field basics(Review ch27)
Self-check: Can you apply the close-in (CI) path loss model at mmWave frequencies, design a hybrid analog-digital beamforming architecture, and compute the Fraunhofer distance $d_F = 2D^2/\lambda$ for a given array aperture?

Notation for This Chapter

Symbols introduced in this chapter. See also the NGlobal Notation Table master table.

Symbol	Meaning	Introduced
$d_R$	Rayleigh distance (boundary between near and far field): $d_R = 2D^2/\lambda$	s03
$D$	Physical aperture of the antenna array	s03
$N$	Number of antenna elements (XL-MIMO array)	s03
$P_{\mathrm{SI}}$	Self-interference power at the full-duplex receiver	s04
$\beta_{\mathrm{SI}}$	Total self-interference cancellation factor (linear)	s04
$\eta_k$	Transmit scaling coefficient for device $k$ in AirComp	s05
$g(\cdot)$	Target aggregation function in over-the-air computation	s05
$\mathcal{V}_n$	Visibility region of scatterer for array element $n$ (XL-MIMO)	s03
$R_{\mathrm{FD}}$	Achievable rate in full-duplex mode	s04
$R_{\mathrm{HD}}$	Achievable rate in half-duplex mode	s04

← Ch 32 Spectrum: From FR3 to Sub-THz