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This article investigates the slicing concept in the 5G Radio Access Network (RAN) with the related challenges and research problems. The objective is to identify the plausible options for implementing the slicing concept at the RAN level by the Mobile Network Operator (MNO) to respond to the needs of verticals. We start by identifying the different slice granularity options, i.e., how to define slices by combining customer and service needs. We then present how the 5G New Radio (NR) features can be used for facilitating slice implementation and provide typical configurations for different slice types from technology and RAN architecture perspectives. The main challenges for RAN slicing are then discussed, with a special attention to the resource allocation problem between slices sharing the same spectrum band. We also investigate the multi-tenant slicing implementation in terms of the openness of the network to third parties which is regarded as a key issue that may encourage vertical players to use operators' networks rather than deploying their own infrastructure.

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Low latency targets for Ultra-Reliable Low Latency Communications (URLLC) may be conflicting with their stringent reliability requirements due to the need for re-transmissions. We explore in this paper the different resource allocation schemes for transmissions and re-transmissions depending on the requirements of the underlying service and on the traffic characteristics, focusing on Industrial Internet of Things (IIoT). We namely consider schemes with individual reservation versus a pool of contention-based reserved resources. We provide novel resource allocation schemes for initial transmissions and re-transmissions and derive corresponding analytical models for loss rates. We then show how to set the system parameters that allow meeting the URLLC requirements with low resource consumption.