From massive MIMO to C-RAN: The OpenAirInterface 5G testbed

5G will be much more than just a new radio interface - 5G will fundamentally change the way networks are operated. Traditional telecoms equipment will be replaced successively with general purpose computing platforms. This change will affect both the core network and the radio network, where it is also called cloud-RAN (C-RAN). The C-RAN architecture allows for flexible splits between different processing elements in the radio network, for an optimal trade-off between processing in the cloud or in the remote radio unit (RRU). OpenAirInterface (OAI) is an open source initiative that today provides Rel-8/Rel-10 3GPP compliant reference implementation of eNodeB, UE, RRH and EPC that runs on general purpose computing platforms. Already today OAI offers several functional splits for its 4G radio stack, for example between the Radio Cloud Center (RCC) and a Remote Radio Unit (RRU). Moreover, Eurecom is currently deploying a C-RAN network at its premises in Sophia-Antipolis, France, using a low-cost solution for RRU based on off-the-shelf equipment. In this paper we are going to describe the OAI C-RAN architecture with a special emphasis on the possibilities to do multi-cell distributed MIMO processing. In particular we will show how to we can apply our recently developed TDD reciprocity calibration scheme to this distributed setting and integrate it seamlessly into the normal LTE operation.