  Sixth-generation (6G) mobile communication networks are expected to have
dense infrastructures, large antenna size, wide bandwidth, cost-effective
hardware, diversified positioning methods, and enhanced intelligence. Such
trends bring both new challenges and opportunities for the practical design of
6G. On one hand, acquiring channel state information (CSI) in real time for all
wireless links becomes quite challenging in 6G. On the other hand, there would
be numerous data sources in 6G containing high-quality location-tagged channel
data, e.g., the estimated channels or beams between base station (BS) and user
equipment (UE), making it possible to better learn the local wireless
environment. By exploiting this new opportunity and for tackling the CSI
acquisition challenge, there is a promising paradigm shift from the
conventional environment-unaware communications to the new environment-aware
communications based on the novel approach of channel knowledge map (CKM). This
article aims to provide a comprehensive overview on environment-aware
communications enabled by CKM to fully harness its benefits for 6G. First, the
basic concept of CKM is presented, followed by the comparison of CKM with
various existing channel inference techniques. Next, the main techniques for
CKM construction are discussed, including both environment model-free and
environment model-assisted approaches. Furthermore, a general framework is
presented for the utilization of CKM to achieve environment-aware
communications, followed by some typical CKM-aided communication scenarios.
Finally, important open problems in CKM research are highlighted and potential
solutions are discussed to inspire future work.
