Abstract-The millimeter wave (mmWave) frequency band is seen as a key enabler of multi-gigabit wireless access in future cellular networks. In order to overcome the propagation challenges, mmWave systems use a large number of antenna elements both at the base station and at the user equipment, which lead to high directivity gains, fully-directional communications, and possible noise-limited operations. The fundamental differences between mmWave networks and traditional ones challenge the classical design constraints, objectives, and available degrees of freedom. This paper addresses the implications that highly directional communication has on the design of an efficient medium access control (MAC) layer. The paper discusses key MAC layer issues, such as synchronization, random access, handover, channelization, interference management, scheduling, and association. The paper provides an integrated view on MAC layer issues for cellular networks, identifies new challenges and tradeoffs, and provides novel insights and solution approaches.

Abstract-Millimeter wave (mmW) wireless networks are capable to support multi-gigabit data rates, by using directional communications with narrow beams. However, existing mmW communications standards are hindered by two problems: deafness and single link scheduling. The deafness problem, that is, a misalignment between transmitter and receiver beams, demands a time consuming beam-searching operation, which leads to an alignment-throughput tradeoff. Moreover, the existing mmW standards schedule a single link in each time slot and hence do not fully exploit the potential of mmW communications, where directional communications allow multiple concurrent transmissions. These two problems are addressed in this paper, where a joint beamwidth selection and power allocation problem is formulated by an optimization problem for short range mmW networks with the objective of maximizing effective network throughput. This optimization problem allows establishing the fundamental alignment-throughput tradeoff, however it is computationally complex and requires exact knowledge of network topology, which may not be available in practice. Therefore, two standard-compliant approximation solution algorithms are developed, which rely on underestimation and overestimation of interference. The first one exploits directionality to maximize the reuse of available spectrum and thereby increases the network throughput, while imposing almost no computational complexity. The second one is a more conservative approach that protects all active links from harmful interference, yet enhances the network throughput by 100% compared to the existing standards. Extensive performance analysis provides useful insights on the directionality level and the number of concurrent transmissions that should be pursued. Interestingly, extremely narrow beams are in general not optimal.

Motivated by the intrinsic characteristics of mmWave technologies, we discuss the possibility of an authorization regime that allows spectrum sharing between multiple operators, also referred to as spectrum pooling. In particular, considering user rate as the performance measure, we assess the benefit of coordination among the networks of different operators, study the impact of beamforming both at the base stations and at the user terminals, and analyze the pooling performance at different frequency carriers. We also discuss the enabling spectrum mechanisms, architectures, and protocols required to make spectrum pooling work in real networks. Our initial results show that, from a technical perspective, spectrum pooling at mmWave has the potential for a more efficient spectrum use than a traditional exclusive spectrum allocation to a single operator. However, further studies are needed in order to reach a thorough understanding of this matter, and we hope that this paper will help stimulate further research in this area.F. Boccardi's work was carried out in his personal capacity and the views expressed here are his own and do not reflect his employer's ones.

While the current generation of mobile and fixed communication networks has been standardized for mobile broadband services, the next generation is driven by the vision of the Internet of Things and mission critical communication services requiring latency in the order of milliseconds or submilliseconds. However, these new stringent requirements have a large technical impact on the design of all layers of the communication protocol stack. The cross layer interactions are complex due to the multiple design principles and technologies that contribute to the layers' design and fundamental performance limitations. We will be able to develop low-latency networks only if we address the problem of these complex interactions from the new point of view of sub-milliseconds latency. In this article, we propose a holistic analysis and classification of the main design principles and enabling technologies that will make it possible to deploy low-latency wireless communication networks. We argue that these design principles and enabling technologies must be carefully orchestrated to meet the stringent requirements and to manage the inherent trade-offs between low latency and traditional performance metrics. We also review currently ongoing standardization activities in prominent standards associations, and discuss open problems for future research.

Millimeter wave (mmWave) communication systems use large number of antenna elements that can potentially overcome severe channel attenuation by narrow beamforming. Narrow-beam operation in mmWave networks also reduces multiuser interference, introducing the concept of noise-limited wireless networks as opposed to interference-limited ones. The noise-limited or interference-limited regime heavily reflects on the medium access control (MAC) layer throughput and on proper resource allocation and interference management strategies. Yet, these regimes are ignored in current approaches to mmWave MAC layer design, with the potential disastrous consequences on the communication performance. In this paper, we investigate these regimes in terms of collision probability and throughput. We derive tractable closed-form expressions for the collision probability and MAC layer throughput of mmWave ad hoc networks, operating under slotted ALOHA. The new analysis reveals that mmWave networks may exhibit a non-negligible transitional behavior from a noise-limited regime to an interference-limited one, depending on the density of the transmitters, density and size of obstacles, transmission probability, operating beamwidth, and transmission power. Such transitional behavior necessitates a new framework of adaptive hybrid resource allocation procedure, containing both contention-based and contention-free phases with on-demand realization of the contention-free phase. Moreover, the conventional collision avoidance procedure in the contentionbased phase should be revisited, due to the transitional behavior of interference, to maximize throughput/delay performance of mmWave networks. We conclude that, unless proper hybrid schemes are investigated, the severity of the transitional behavior may significantly reduce throughput/delay performance of mmWave networks.

This paper investigates the extent to which spectrum sharing in mmWave networks with multiple cellular operators is a viable alternative to traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework by which to characterize the performance gain that can be obtained when spectrum sharing is used, as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on joint beamforming and cell association optimization, with the objective of maximizing the long-term throughput of the users. Our asymptotic and non-asymptotic performance analyses reveal five key points: (1) spectrum sharing with light on-demand intra-and inter-operator coordination is feasible, especially at higher mmWave frequencies (for example, 73 GHz); (2) directional communications at the user equipments substantially alleviate the potential disadvantages of spectrum sharing (such as higher multiuser interference); (3) large numbers of antenna elements can reduce the need for coordination and simplify the implementation of spectrum sharing; (4) while inter-operator coordination can be neglected in the large-antenna regime, intraoperator coordination can still bring gains by balancing the network load; and (5) critical control signals among base stations, operators, and user equipment should be protected from the adverse effects of spectrum sharing, for example by means of exclusive resource allocation. The results of this paper, and their extensions obtained by relaxing some ideal assumptions, can provide important insights for future standardization and spectrum policy.

M illimeter-wave (mmWave) wireless communications is one of the most promising candidates to support extremely high data rates in future wireless networks [1][2][3]. MmWave communications are attractive for many applications such as ultra short-range communications, augmented reality, data centers, vehicular networks, mobile offloading, mobile fronthauling, and in-band backhauling. Due to their great commercial potential, several international activities have emerged to standardize mmWave communications in wireless personal and local area networks (WPANs and WLANs). Examples include IEEE 802.15.3c, ECMA 387 [1], IEEE 802.11ad [3], WirelessHD, WiGig, and recently the IEEE 802.11ay study group on next generation 60 GHz. 1Special propagation features and hardware constraints of mmWave systems introduce many new challenges in the design of efficient physical, medium access control (MAC), and routing layers.The severe channel attenuation, vulnerability to obstacles, directionality of mmWave communications, reduced interference footprint, and potentially high signaling overhead demand a thorough reconsideration of traditional protocol design principles, especially at the MAC layer.In this article, we focus on short-range mmWave networks. Compared to [1-3], which survey either the existing standards or the research literature, we deliver original contributions based on the features specific to mmWave networks that have mostly been ignored in the design of the existing mmWave standards. To distinguish this article from [4] that discusses MAC layer design for mmWave cellular networks, we should notice the following important differences, which are more relevant to our studies, between short range and cellular networks:• Short-range networks may rely on carrier sensing among terminals. • They may use multihop communications, which may also affect traffic patterns. In this article, we show that, contrary to mainstream belief, a mmWave network may exhibit both noise-limited and interference-limited regimes. We highlight the significant mismatch between transmission rates of control and data messages that may degrade the performance and limit the range of use cases that can be supported in future mmWave networks. We also raise the prolonged backoff time problem and discuss the beam training overhead and its consequences such as the alignment-throughput trade-off. To address these new problems, we discuss the necessity of new collision-aware hybrid resource allocation protocols that facilitate concurrent transmissions with quality of service (QoS) guarantees, and also the need for a more efficient retransmission policy. We argue the benefits of a hybrid reactive/proactive control plane to minimize the signaling overhead and propose, for this purpose, a new MAC layer message, which is also able to alleviate the prolonged backoff time. Finally, we discuss the potential of multihop communication techniques to compensate for the error-prone mmWave physical layer, provide reliable mmWave connections, and extend mmWave communication...

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