Chapter Summary

Key Points

1.
ISAC uses a single signal for simultaneous communication and sensing, with three paradigms: communication-centric (OFDM + echoes), sensing-centric (FMCW + data), and joint design. The fundamental tradeoff is governed by the power split and beamforming design, quantified by the capacity-distortion region.
2.
The Liu/Caire capacity-distortion framework establishes that the optimal ISAC signal decomposes into a deterministic (sensing-optimal) and random (information-bearing) component. Gaussian signalling achieves the Pareto boundary. The tradeoff severity depends on the alignment between the communication channel $\mathbf{H}$ and the sensing matrix $\mathbf{A}$ --- connecting information theory to the imaging forward model of this book.
3.
ISAC waveform design via transmit covariance optimisation is a convex SDP. OFDM-ISAC reuses 5G NR infrastructure; FMCW-ISAC prioritises sensing; OTFS-ISAC (Yuan/Schober/Caire) exploits the delay-Doppler domain where both channels and targets are sparse, naturally producing the sensing matrix $\mathbf{A}$ .
4.
ISAC beamforming jointly optimises communication SINR and sensing CRB. SDR provides near-optimal solutions; null-space projection offers a simpler alternative for massive MIMO. RIS-assisted ISAC extends sensing to NLOS regions by adding virtual viewing angles.
5.
Bistatic and multi-static ISAC (Liu/Wan/Caire) turns the cellular network into a distributed radar. Blind interference management handles unknown data at the sensing receiver. With $N_b$ base stations, $N_b(N_b-1)/2$ bistatic baselines provide spatial diversity equivalent to multi-view imaging (Chapter 11).

Looking Ahead

Chapter 30 extends the ISAC framework to the full imaging pipeline: channel estimation AS imaging, where sparse channel estimators (LASSO, OAMP from Parts IV--V) directly reconstruct the environment. The sensing matrix $\mathbf{A}$ constructed from ISAC waveforms feeds into the reconstruction algorithms developed throughout this book, closing the loop from waveform design to scene reconstruction.

Bistatic and Multi-Static ISAC Exercises