Hossein Shokri-Ghadikolaei

Millimeter Wave Cellular Networks: A MAC Layer Perspective

The millimeter-wave (mmWave) frequency band is seen as a key enabler of multigigabit 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 leads 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. This paper provides an integrated view on MAC layer issues for cellular networks, identifies new challenges and tradeoffs, and provides novel insights and solution approaches.

Beam-searching and transmission scheduling in millimeter wave communications

Millimeter wave (mmWave) wireless networks rely on narrow beams to support multi-gigabit data rates. Nevertheless, the alignment of transmitter and receiver beams is a time-consuming operation, which introduces an alignment-throughput tradeoff. A wider beamwidth reduces the alignment overhead, but leads also to reduced directivity gains. Moreover, existing mmWave standards schedule a single transmission in each time slot, although directional communications facilitate multiple concurrent transmissions. In this paper, a joint consideration of the problems of beamwidth selection and scheduling is proposed to maximize effective network throughput. The resulting optimization problem requires exact knowledge of network topology, which may not be available in practice. Therefore, two standard-compliant approximation algorithms are developed, which rely on underestimation and overestimation of interference. The first one aims to maximize the reuse of available spectrum, whereas the second one is a more conservative approach that schedules together only links that cause no interference. 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.

The Transitional Behavior of Interference in Millimeter Wave Networks and Its Impact on Medium Access Control

Millimeter-wave (mmWave) communication systems use a 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 contention-based 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.

Spectrum Pooling in MmWave Networks: Opportunities, Challenges, and Enablers

Motivated by the specific 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 networks of different operators, study the impact of beamforming at both base stations and 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 to use the resources more efficiently than 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 article will help stimulate further research in this area.

Distributed Multiuser Sequential Channel Sensing Schemes in Multichannel Cognitive Radio Networks

Effective spectrum sensing strategies enable cognitive radios (CRs) to identify and opportunistically transmit on under-utilized spectral resources. In this paper, sequential channel sensing problems for single and multiple secondary users (SUs) networks are effectively modeled through finite state Markovian processes. More specifically, a model for single user case is introduced and its performance is validated through analytical analysis. Then, in order to address multiple SUs case, this model is extended to include the modified p-persistent access (MPPA) protocol. Since the scheme utilized experiences a high level of collision among the SUs, to mitigate the problem appropriately, p-persistent random access (PPRA) protocol is considered, which offers higher average throughput for SUs by statistically distributing their loads among all channels. The structure and performance of the proposed schemes are discussed in detail, and a set of illustrative numerical results is presented to validate and compare the performance of the proposed sense-access strategies.

Spectrum Sharing in mmWave Cellular Networks via Cell Association, Coordination, and Beamforming

Spectrum sharing is not typically used in current cellular networks, because it only provides a slight performance improvement while requiring a heavy coordination among different cellular operators. However, these problems can be potentially overcome in millimeter wave (mmWave) networks, thanks to beamforming both at base stations and at user equipments. This paper investigates up to which extent spectrum sharing in mmWave networks with multiple cellular operators is a viable alternative to the traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework to characterize the performance gain that the users can experience by spectrum sharing as a function of the underlying beamforming, operator coordination, bandwidth, and infrastructure sharing scenarios. The framework is based on a joint beamforming and base station association optimization with the objective of maximizing the long-term throughput of the users. Asymptotic and non-asymptotic performance analysis reveals that 1) spectrum sharing with light on-demand intra- and inter-operator coordination is feasible, especially at higher mmWave frequencies (e.g., 73 GHz); 2) directional communications at the user equipments substantially alleviate the disadvantages of spectrum sharing; 3) larger number 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, intra-operator coordination can still bring gains by balancing the network load; and 5) critical control signals among base stations, operators, and user equipments should be protected from the adverse effects of spectrum sharing, e.g., by exclusive resource allocation. The results of this paper can provide fundamental insights for future standardization and spectrum policy.This paper investigates the extent to which spectrum sharing in millimeter-wave (mmWave) networks with multiple cellular operators is a viable alternative to traditional dedicated spectrum allocation. Specifically, we develop a general mathematical framework 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 equipment 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, intra-operator 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.

Analytical and learning-based spectrum sensing time optimisation in cognitive radio systems

In this study, the average throughput maximisation of a secondary user (SU) by optimising its spectrum sensing time is formulated, assuming that a priori knowledge of the presence and absence probabilities of the primary users (PUs) is available. The energy consumed to find a transmission opportunity is evaluated, and a discussion on the impacts of the number of PUs on SU throughput and consumed energy are presented. To avoid the challenges associated with the analytical method, as a second solution, a systematic adaptive neural network-based sensing time optimisation approach is also proposed. The proposed scheme is able to find the optimum value of the channel sensing time without any prior knowledge or assumption about the wireless environment. The structure, performance and cooperation of the artificial neural networks used in the proposed method are explained in detail, and a set of illustrative simulation results is presented to validate the analytical results as well as the performance of the proposed learning-based optimisation scheme.

Intelligent Sensing Matrix Setting in Cognitive Radio Networks

Setting a powerful spectrum sensing and access policy increases the throughput of cognitive radio networks (CRNs). In this paper, the problem of maximizing the average throughput of a CRN through setting proper sensing sequences is investigated. In addition, a systematic neural network-based optimization approach is developed which avoids challenges associated with the conventional analytical solutions. The proposed intelligent learning and optimization cycle, based on a cooperation between two kinds of well-known artificial neural networks, finds the optimal sensing sequence for each secondary user without any prior knowledge or presumptions about the wireless environment. The structure of the proposed scheme is discussed in detail, and its efficiencies are verified through a set of illustrative numerical results.

Design aspects of short-range millimeter-wave networks: A MAC layer perspective

Increased density of wireless devices, ever growing demands for extremely high data rate, and spectrum scarcity at microwave bands make the millimeter-wave frequencies an important player in future wireless networks. However, millimeter-wave communication systems exhibit severe attenuation, blockage, and deafness, and may need microwave networks for coordination and fall-back support. To compensate for high attenuation, mmWave systems exploit highly directional operation, which in turn substantially reduces the interference footprint. The significant differences between millimeter-wave networks and legacy communication technologies challenge the classical design approaches, especially at the MAC layer, which has received comparatively less attention than PHY and propagation issues in the literature so far. In this article, the MAC layer design aspects of short-range millimeter- wave networks are discussed. In particular, we explain why current mmWave standards fail to fully exploit the potential advantages of short-range mmWave technology, and argue for the necessity of new collision-aware hybrid resource allocation frameworks with on-demand control messages, the advantages of a collision notification message, and the potential of multihop communication to provide reliable mmWave connections.

Green sensing and access: energy-throughput trade-offs in cognitive networking

Limited spectrum resources and the dramatic growth of high data rate applications have motivated opportunistic spectrum access exploiting the promising concept of cognitive networks. Although this concept has emerged primarily to enhance spectrum utilization and to allow the coexistence of heterogeneous network technologies, the importance of energy consumption imposes additional challenges, because energy consumption and communication performance can be at odds. In this article the approaches for energy efficient spectrum sensing and spectrum handoff, fundamental building blocks of cognitive networks, are investigated. The trade-offs between energy consumption and throughput, under local as well as under cooperative sensing, are characterized. We also discuss the additional factors that need to be investigated to achieve energy efficient cognitive operation under various application requirements.