  As a key enabler for massive machine-type communications (mMTC), spatial
multiplexing relies on massive multiple-input multiple-output (mMIMO)
technology to serve the massive number of user equipments (UEs). To exploit
spatial multiplexing, accurate channel estimation through pilot signals is
needed. In mMTC systems, it is impractical to allocate a unique orthogonal
pilot sequence to each UE as it would require too long pilot sequences,
degrading the spectral efficiency. This work addresses the design of channel
features from correlated fading channels to assist the pilot assignment in
multi-sector mMTC systems under pilot reuse of orthogonal sequences. In order
to reduce pilot collisions and to enable pilot reuse, we propose to extract
features from the channel covariance matrices that reflect the level of
orthogonality between the UEs channels. Two features are investigated:
covariance matrix distance (CMD) feature and CMD-aided channel charting (CC)
feature. In terms of symbol error rate and achievable rate, the CC-based
feature shows superior performance than the CMD-based feature and baseline
pilot assignment algorithms.
