Distributed large-scale MIMO is a promising option for coping with the projected explosion in mobile traffic. It involves multiple Access Points (APs) that are connected to a central server via wired backhaul and act as a distributed MIMO transmitter, serving multiple users via spatial precoding. As is well known, large downlink (DL) spectral efficiencies can be achieved with TDD operation, pilots sent in the uplink (UL), and DL-UL channel reciprocity. With APs made of inexpensive hardware and connected via, e.g., Ethernet, synchronization and reciprocity calibration are the main hurdle for implementing a truly distributed MU-MIMO system. This work studies mechanisms for RF calibration that can enable distributed high-performing large-scale MIMO operation. We propose methods for relative calibration of the APs in order to ensure TDD reciprocity while not relying on an explicitly self-calibrating RF design. As our analysis and simulations suggest, the proposed methods significantly outperform existing self calibration methods without requiring additional signaling overhead and can enable TDD reciprocity for calibration of non-colocated networks.
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