Massive MIMO is one of the key technologies enabling the next generation of wireless communications, offering high spectrum efficiency, saving transceivers energy and providing many other advantages. The OpenAirInterface massive MIMO testbed is the world's first open source LTE compliant base station equipped with large antenna array, which can directly provide services to commercial user equipments (UEs). The testbed performs TDD channel reciprocity calibration to acquire accurate channel state information at transmitter (CSIT). It shows the feasibility of using massive MIMO already in current generation LTE standard by using Transmission mode (TM) 7-10, indicating the possibility of smoothly evolving the wireless network from 4G to 5G. We show here an innovation platform for solving 5G massive MIMO challenges, by giving the possibility of advanced algorithm testing, concept validation, channel measurements, etc.