Transmit-Receive Processing in Massive MIMO Systems

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.

In general, transmitter-receiver design in multi-cell multi-user massive MIMO systems can be potentially simplified, as compared to state-of-the-art iterative optimization based schemes. As the imbalance between the number of independently fading antennas M at the BS and the number of users K in the cell becomes large, i.e. N >> K, a simple linear detector/precoder (e.g. matched filter) is close to optimal. However, given practical concerns such as non-zero correlation between antennas and physical limitation of antenna array sizes, coordinated designs with some limited coordination between adjacent cells can be beneficial.

The project focus on but are not limited to the following aspects of massive MIMO systems: linear transceiver design/analysis with practical impairments, channel estimation under pilot contamination, and energy-efficient communications through power control.