  The traditional centralized baseband processing architecture is faced with
the bottlenecks of high computation complexity and excessive fronthaul
communication, especially when the number of antennas at the base station (BS)
is large. To cope with these two challenges, the decentralized baseband
processing (DPB) architecture has been proposed, where the BS antennas are
partitioned into multiple clusters, and each is connected to a local baseband
unit (BBU). In this paper, we are interested in the low-complexity distributed
channel estimation (CE) method under such DBP architecture, which is rarely
studied in the literature. The aim is to devise distributed CE algorithms that
can perform as well as the centralized scheme but with a small inter-BBU
communication cost. Specifically, based on the low-complexity diagonal minimum
mean square error channel estimator, we propose two distributed CE algorithms,
namely the aggregate-then-estimate algorithm and the estimate-then-aggregate
algorithm. In contrast to the existing distributed CE algorithm which requires
iterative information exchanges among the nodes, our algorithms only require
one roundtrip communication among BBUs. Extensive experiment results are
presented to demonstrate the advantages of the proposed distributed CE
algorithms in terms of estimation accuracy, inter-BBU communication cost, and
computation complexity.
