Downlink Pilots are Essential for Cell-Free Massive MIMO with Multi-Antenna Users
arxiv(2024)
摘要
We consider a cell-free massive MIMO system with multiple antennas on the
users and access points. In previous works, the downlink spectral efficiency
(SE) has been evaluated using the hardening bound that requires no downlink
pilots. This approach works well when having single-antenna users. In this
paper, we show that much higher SEs can be achieved if downlink pilots are sent
since the effective channel matrix does not harden when having multi-antenna
users. We propose a pilot-based downlink estimation scheme and derive a new SE
expression that utilizes zero-forcing combining. We show numerically how the
number of users and user antennas affects the SE.
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