The increase in demands of wireless applications has raised questions on capacity, spectral/energy efficiency attained by conventional MIMO system. The massive or large scaled multiple input multiple output (MIMO) systems have achieved vast importance due to better attainable data rate and minimal interference/fading effect. In this paper, massive MIMO systems in which large number of antennas are operational at the base station is studied. Substantial increase in capacity is obtained with the implementation of this system. Also, all noise and fading are removed due to its large degree of freedom. But its performance is limited by only one factor which is called as pilot contamination. Pilot contamination puts constraints on the maximum achievable capacity. In this paper, pilot contamination problem in massive MIMO systems is addressed and ways of mitigation are suggested. Reduction in cell size, i.e., SCN (small cell networks) and pre-coding techniques are two methods which are discussed in this paper to eliminate effect of pilot contamination.

massive MIMO systems, pilot contamination, pre-coding, SCN (small cell networks), dirty paper coding (DPC), ZF (zero-forcing), MMSE (minimum mean square error, MF (matched filter) pre-coding.