Dear Colleagues,

The congestion of the bandwidth scarce sub-6 GHz spectrum has rendered the migration to the millimeter-wave spectrum (30–300 GHz) for the forthcoming 5G and beyond cellular networks an essential feature. Currently, millimeter-wave bands are primarily used for wireless networks of diverse topologies, such as backhaul/fronthaul, fixed, and adhoc-wireless access corresponding ones. Such networks are subject to strong line-of-sight conditions and leverage highly-directional antennas that compensate for the high channel attenuation.

As far as millimeter-wave cellular networks are concerned, they need to ensure uninterrupted, reliable, and high-data-rate multi-user access. As a consequence, they pose significant challenges. In particular, the need for multi-user connectivity requires the investigation of new beamforming schemes that in turn create the need for novel channel estimation, signal combining, and detection approaches. Furthermore, power-constraints in combination with the quality-of-experience and range requirements necessitate new transceiver designs. Moreover, the mobile nature of the corresponding networks and the higher susceptibility of these bands to blockages could render the communication intermittent, which is going to substantially compromise the high reliability required in future networks if no solutions are brought forward. In addition, the inherent directionality of millimeter-wave links and the user equipment mobility necessitate advancements in areas such as dynamic-cell formation, cell-free architectures, low-complexity beam steering and tracking algorithmic design, cooperation through relaying and device-to-device (D2D) communications, fast handover algorithms in dense networks, highly accurate channel estimation and modeling, efficient network management, etc.

In this context, this Special Issue will report on new advancements in millimeter-wave cellular networks that include, but are not limited to, the following topics:

• Novel application scenarios and key performance indicators (KPIs)
• Transceiver and antenna design
• Massive multiple-input multiple-output (MIMO) and beamforming schemes
• Mobility management approaches with emphasis on beam steering and tracking
• Medium access control (MAC) and radio resource management (RRM) protocol design
• Cooperative communications, e.g. relaying, D2D
• Dynamic cells and cell-free architecture
• Ultra-dense (UDN) networks
• Channel and transceiver hardware impairment models
• Reconfigurable intelligent surfaces
• Artificial intelligence (AI)-based approaches for system and network optimization
• Performance analysis, optimization, and information-theoretic limits
• Demonstrators and testbeds
• Optimization methods
• Internet of Things (IoT)
• Network planning
• Green network design
• Radio frequency (RF) energy harvesting approaches

Dr. Konstantinos Ntontin 
Dr. Alexandros-Apostolos A. Boulogeorgos
Guest Editors

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• 5G
• 6G
• artificial intelligence
• beamforming
• beyond 5G
• cell-free
• channel estimation
• channel modeling
• dynamic cells
• machine learning
• medium access control
• millimeter wave
• mobility management
• theoretical framework
• ultra-dense networks