Real-time Performance Evaluation of Relative Calibration on the OAI 5G testbed

In this paper we describe the OpenAirInterface (OAI) C-RAN testbed deployed at Eurecom and present the real-time implementation and performance evaluation of a channel calibration scheme. The distributed MIMO operation in dense Time Division Duplex (TDD) radio networks requires accurate time and frequency synchronization and calibration for precoding. We achieve this by using a common reference and over-the-air (OTA) synchronization between remote radio units (RRUs), which is a much cheaper alternative to distributed synchronization using PTPv2-like protocols and special clock regeneration circuitry. Furthermore we perform channel measurements between the RRUs without interrupting the real-time operation in order to perform distributed channel reciprocity calibration which is required to exploit uplink (UL) channel estimates to infer the precoder performed on the downlink (DL) channel.