Chapter 27: Open Problems

Learning Objectives

• Identify the gap between current 3GPP channel models and the empirically observed spatial non-stationarity of XL-MIMO arrays
• Explain the O(N_AP^3) complexity barrier in ultra-dense cell-free massive MIMO and survey the distributed alternatives that trade off accuracy for scalability
• State the self-interference cancellation budget (around 150 dB) required for commercial full-duplex massive MIMO and describe the digital/analog/RF cascade
• Recall the $(L/L_F)^4$ DoF scaling of holographic MIMO surfaces (Pizzo-Marzetta-Sanguinetti) and the manufacturing constraints separating theory from practice
• Recognize the convergence of RIS and cell-free architectures and articulate the joint estimation and optimization problems that remain open
• Formulate a PhD-scale research question in each of the five open problem areas and relate it to the broader 6G research agenda