  A massive multiple-input multiple-output (MIMO) system, which utilizes a
large number of base station (BS) antennas to serve a set of users, suffers
from pilot contamination due to the inter-cell interference (ICI). In this
letter, a graph coloring based pilot allocation (GC-PA) scheme is proposed to
mitigate pilot contamination for multi-cell massive MIMO systems. Specifically,
by exploiting the large-scale characteristics of fading channels, an
interference graph is firstly constructed to describe the potential ICI
relationship of all users. Then, with the limited pilot resource, the proposed
GC-PA scheme aims to mitigate the potential ICI by efficiently allocating
pilots among users in the interference graph. The performance gain of the
proposed scheme is verified by simulations.
