What are the key design challenges in Massive MIMO systems?

Massive MIMO (64-256+ antennas) introduces significant design challenges. Channel estimation overhead scales with users, not antennas, using TDD reciprocity; FDD requires excessive feedback. Pilot contamination from pilot reuse in adjacent cells creates interference floor even with infinite antennas; solutions include pilot coordination and blind estimation. Hardware complexity requires low-cost, low-power components with calibration for reciprocity; hybrid beamforming (analog + digital) reduces RF chains. Computational complexity of precoding/detection (ZF, MMSE) requires O(K³) operations; iterative detectors and machine learning approaches help. Channel hardening and favorable propagation assumptions may not hold in real deployments. Thermal management for dense antenna arrays is critical.