Abstract
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.
Highlights
Massive machine-type communications is a class of services planned for 5G and 6G systems to provide access for enormous numbers of connected devices, possibly on the order of tens of billions [1]–[3]
As a key enabler for massive machine-type communications, spatial multiplexing relies on massive multiple-input multiple-output technology to serve the massive number of user equipments (UEs)
Massive multiple-input multiple-output technology has been proposed as a way to improve the spectral and energy efficiency of such systems by exploiting the spatial multiplexing provided by the large numbers of antennas [4], [5]
Summary
Massive machine-type communications (mMTC) is a class of services planned for 5G and 6G systems to provide access for enormous numbers of connected devices, possibly on the order of tens of billions [1]–[3]. In [6], a pilot assignment scheme was devised to deal with pilot interference in multi-cell mMIMO based on the UEs’ large-scale coefficients, which are assumed to be known in all cells Their objective is to assign orthogonal pilot sequences to UEs with poor channel quality. You et al [10] proposed a pilot reuse strategy based on the channel covariance matrices for single-cell mMIMO systems with correlated channels They proposed a pilot allocation algorithm that assigns orthogonal pilot sequences to UEs with similar channel covariance matrices, measured by the covariance matrix distance (CMD) metric. Utilizing the proposed features along with the pilot allocation algorithm devised in [11], the numerical results show that the angular information can be exploited for multi-sector systems with spatially correlated channels, resulting in enhanced achievable rate and SER compared to baselines
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