This work is within the framework of the FP7 project ICT-317669 METIS, which is partly funded by the European Union.
The concept of massive MIMO has been introduced for cellular networks where a base station (BS) equipped with a very large number of antennas serves simultaneously multiple users. The massive MIMO technique exploits the additional spatial degrees of freedom to multiplex messages for several users on the same time-frequency resource, to maximize the beamforming gain by focusing the radiated energy towards the intended receivers and to minimize the intra-cell and inter-cell interference. Moreover, a large number of BS antennas can drastically reduce the transmit power which leads to a significant energy efficiency.
One of the practical challenges of massive MIMO systems is the need of accurate CSI at the BS. In principle, the CSI may be obtained through training and feedback in frequency division duplexing (FDD) mode. This approach has the drawback that the training overhead in terms of required CSI grows linearly with the number of transmit antennas. The other popular option for obtaining the CSI is to utilize channel reciprocity, particularly in time division duplexing (TDD) mode. However, due to the limited channel coherence time, the use of non-orthogonal pilots among different cells is inevitable, which causes the so-called pilot contamination problem and harms the channel estimate at the BS. In addition, If the channel is time varying, the delay between pilot and data transmission also represents an effect that should be further addressed.