Chapter Summary

Chapter Summary

Key Points

• 1.

  The spectral efficiency of cell-free massive MIMO under imperfect CSI is governed by the coherent combining gain $(\sum_l \gamma_{lk})^2$ in the SINR numerator. This quadratic scaling in the number of contributing APs is the fundamental advantage over co-located and small-cell architectures, where each user benefits from at most one site.

• 2.

  The use-and-then-forget (UatF) bound provides a clean, computable SE lower bound by treating the channel estimate as perfect for beamforming and accounting for estimation error as worst-case Gaussian noise. The hardening-based bound is tighter when channel hardening holds but requires statistical CSI at the receiver.

• 3.

  Pilot contamination creates coherent interference that does not vanish as the number of APs grows. In cell-free systems, this is mitigated by geographic pilot assignment (co-pilot users are far apart) and MMSE combining that exploits spatial signature differences.

• 4.

  Under max-min fair power control, cell-free massive MIMO achieves 5--10 times higher 95%-likely per-user rate than small cells with the same total antenna count. The improvement is concentrated at the cell edge, where distributed APs convert interference into signal.

• 5.

  The 95%-likely per-user rate is the primary fairness metric. Max-min power control is solved via bisection over SOCPs. Proportional fairness provides a balanced alternative with higher sum-rate at the expense of slightly lower min-rate.

• 6.

  Energy efficiency is quasi-concave in AP density: there exists an optimal $\lambda_{\text{AP}}^*$ that balances SE gains from macro-diversity against hardware power costs ($P_{\text{AP}} + P_{\text{fh}}$ per AP). Reducing per-AP hardware cost — through PoE, passive optics, and sleep modes — is the key enabler for dense cell-free deployment.

• 7.

  Ultra-dense cell-free massive MIMO (500--2000 APs/km$^2$) is a leading candidate for the 6G radio architecture. The CommIT contribution by Ngo, Caire, Ashikhmin, and Larsson provides a deployment roadmap including two-tier fronthaul, O-RAN integration, and traffic-adaptive AP activation.