Licensed-Assisted Access (LAA) enabled LTE operators to access unlicensed spectrum while adhering to Listen-Before-Talk (LBT) requirements. LAA is based on enhancements over 4G LTE technology. Differently, 5G New Radio (NR) technology is being designed from the start to support operation in unlicensed bands through a technology referred to as NRbased access to unlicensed spectrum (NR-U). A large amount of unlicensed spectrum has been allocated in millimeter-wave (mmWave) bands, making it an attractive candidate for NR-U. However, the propagation characteristics in mmWave often require beam-based transmissions. Beam-based transmissions enhance spatial reuse, but also complicate interference management due to the dynamic nature of the directional antennas. Therefore, some major design principles need to be revisited in NR-U to address coexistence. This paper elaborates on the design challenges, opportunities, and solutions for NR-U by taking into account beam-based transmissions and the worldwide regulatory requirements. In particular, different problems and the potential solutions related to channel access procedures, frame structure, initial access procedures, HARQ procedures, and scheduling schemes are discussed.

For a fair coexistence of multiple Radio Access Technologies (RATs) in unlicensed millimeter-wave (mmWave) bands, where directional transmissions/receptions are needed, two techniques are being considered for channel access: omnidirectional Listen-Before-Talk (omniLBT) and directional LBT (dirLBT). Under directional transmissions and receptions, om-niLBT is overprotective and depresses spatial reuse, while dirLBT enables spatial reuse but may create hidden node problems. In this regard, this paper proposes a paired LBT (pairLBT) solution, which aims at taking the best of omniLBT and dirLBT. The key idea is to perform directional LBT in paired-directions: the direction of the receiver and the opposite direction(s), so that spatial reuse is permitted while hidden nodes are avoided, hence stimulating a fair multi-RAT coexistence. We derive analytic expressions to properly configure the beamwidth and the energy detection threshold of LBT in the opposite direction(s) in a way such that the impact of hidden node problems is minimized.

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Abstract-The use of heterogeneous networks with multiple radio access technologies (RATs) is a system concept that both academia and industry are studying. In such system, integrated use of available multiple RATs is essential to achieve beyond additive throughput and connectivity gains using multi-dimensional diversity. This paper considers an aggregation module called opportunistic multi-MAC aggregation (OMMA). It resides between the IP layer and the air interface protocol stacks, common to all RATs in the device. We present a theoretical framework for such system while considering a special case of multi-RAT systems, i.e., a multi-band wireless LAN (WLAN) system. An optimal packet distribution approach is derived which minimizes the average packet latency (the sum of queueing delay and serving delay) over multiple bands. It supports multiple user terminals with different QoS classes simultaneously. We further propose a packet scheduling algorithm, OMMA Leaky Bucket, which minimizes the packet end-to-end delay, i.e., the sum of average packet latency and average packet reordering delay. We also describe the system architecture of the proposed OMMA system, which is applicable for the general case of the multi-RAT devices. It includes functional description, discovery and association processes, and dynamic RAT update management. We finally present simulation results for a multi-band WLAN system. It shows the performance gains of the proposed OMMA Leaky Bucket scheme in comparison to other existing packet scheduling mechanisms.

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