  A massive multiple-input multiple-output (MIMO) system, operating in
Frequency Division Duplexing (FDD) mode of operation, suffers from
prohibitively high overhead associated with downlink channel state information
(CSI) acquisition and downlink precoding, due to the lack of uplink/downlink
channel reciprocity. In this paper, a heuristic edge-weighted vertex-coloring
based pattern division (EWVC-PD) scheme is proposed to alleviate the overhead
of a two-layer precoding approach, in a practical scenario where the user
clusters undergo serious angular-spreading-range (ASR) overlapping.
Specifically, under a constraint of limited number of subchannels, an
undirected edge-weighted graph (EWG) is firstly constructed, to depict the
potential ASR overlapping relationship among clusters. Then, inspired by
classical graph coloring algorithms, we develop the EWVC-PD scheme which
mitigate the ASR by subchannel orthogonalization between clusters possessing
serious ASR overlapping, and multiplexing the ones having slight ASR
overlapping. Simulation results reveal that our scheme efficiently outperforms
the existing pattern division schemes.
