A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> To meet the needs of next-generation high-data-rate applications, 60 GHz wireless networks must deliver Gb/s data rates and reliability at a low cost. In this article, we surveyed several ongoing challenges, including the design of cost-efficient and low-loss on-chip and in-package antennas and antenna arrays, the characterization of CMOS processes at millimeter-wave frequencies, the discovery of efficient modulation techniques that are suitable for the unique hardware impairments and frequency selective channel characteristics at millimeter-wave frequencies, and the creation of MAC protocols that more effectively coordinate 60 GHz networks with directional antennas. Solving these problems not only provides for wireless video streaming and interconnect replacement, but also moves printed and magnetic media such as books and hard drives to a lower cost, higher reliability semiconductor form factor with wireless connectivity between and within devices. <s> BIB001 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> This tutorial presents an overview of the technological advances in millimeter-wave (mm-wave) circuit components, antennas, and propagation that will soon allow 60-GHz transceivers to provide multigigabit per second (multi-Gb/s) wireless communication data transfers in the consumer marketplace. Our goal is to help engineers understand the convergence of communications, circuits, and antennas, as the emerging world of subterahertz and terahertz wireless communications will require understanding at the intersections of these areas. This paper covers trends and recent accomplishments in a wide range of circuits and systems topics that must be understood to create massively broadband wireless communication systems of the future. In this paper, we present some evolving applications of massively broadband wireless communications, and use tables and graphs to show research progress from the literature on various radio system components, including on-chip and in-package antennas, radio-frequency (RF) power amplifiers (PAs), low-noise amplifiers (LNAs), voltage-controlled oscillators (VCOs), mixers, and analog-to-digital converters (ADCs). We focus primarily on silicon-based technologies, as these provide the best means of implementing very low-cost, highly integrated 60-GHz mm-wave circuits. In addition, the paper illuminates characterization techniques that are required to competently design and fabricate mm-wave devices in silicon, and illustrates effects of the 60-GHz RF propagation channel for both in-building and outdoor use. The paper concludes with an overview of the standardization and commercialization efforts for 60-GHz multi-Gb/s devices, and presents a novel way to compare the data rate versus power efficiency for future broadband devices. <s> BIB002 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> Wireless systems require increasingly large system bandwidths that are only available at millimeter-wave frequencies. Such spectrum bands offer the potential for multi-gigabit-per-second data rates to low-cost massively broadband® devices. To enable mobile outdoor millimeter-wave cellular-type applications, it is necessary to determine the coverage potential of base stations in real-world environments. This paper presents the results of a measurement campaign of 38 GHz outdoor urban cellular channels using directional antennas at both the mobile and the base station, and assesses outage probabilities at two separate transmitter locations on the campus of The University of Texas at Austin. Our measurements demonstrate the viability of directional antennas and site-specific planning for future mm-wave cellular, and show that cell radii of ∼ 200 M will provide a very high probability of coverage in an urban environment. As production costs for millimeter-wave technologies continue to fall [1], we envision millimeter-wave cellular systems with dense base station deployments as a cost effective means of delivering multi-Gbps data rates to mobile cell phone and internet users. <s> BIB003 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> In this paper, we investigate the loss caused by multiple humans blocking millimeter wave frequencies. We model human blockers as absorbing screens of infinite height with two knife-edges. We take a physical optics approach to computing the diffraction around the absorbing screens. This approach differs to the geometric optics approach described in much of the literature. The blocking model is validated by measuring the gain from multiple-human blocking configurations on an indoor link. The blocking gains predicted using Piazzi’s numerical integration method (a physical optics method) agree well with measurements taken from approximately 2.7 dB to -50 dB. Thereofre, this model is suitable for real human blockers. The mean prediction error for the method is approximately -1.2 dB, and the standard deviation is approximately 5 dB. <s> BIB004 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> New research directions will lead to fundamental changes in the design of future 5th generation (5G) cellular networks. This paper describes five technologies that could lead to both architectural and component disruptive design changes: device-centric architectures, millimeter Wave, Massive-MIMO, smarter devices, and native support to machine-2-machine. The key ideas for each technology are described, along with their potential impact on 5G and the research challenges that remain. <s> BIB005 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> The global bandwidth shortage facing wireless carriers has motivated the exploration of the underutilized millimeter wave (mm-wave) frequency spectrum for future broadband cellular communication networks. There is, however, little knowledge about cellular mm-wave propagation in densely populated indoor and outdoor environments. Obtaining this information is vital for the design and operation of future fifth generation cellular networks that use the mm-wave spectrum. In this paper, we present the motivation for new mm-wave cellular systems, methodology, and hardware for measurements and offer a variety of measurement results that show 28 and 38 GHz frequencies can be used when employing steerable directional antennas at base stations and mobile devices. <s> BIB006 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> Wireless data traffic is projected to skyrocket 10 000 fold within the next 20 years. To tackle this incredible increase in wireless data traffic, a first approach is to further improve spectrally efficient systems such as 4G LTE in bands below 6 GHz by using more advanced spectral efficiency techniques. However, the required substantial increase in system complexity along with fundamental limits on hardware implementation and channel conditions may limit the viability of this approach. Furthermore, the end result would be an extremely spectrally efficient system with little room for future improvement to meet the ever-growing wireless data usage. The second approach is to move up in frequency, into an unused nontraditional spectrum where enormous bandwidths are available, such as at millimeter wave (mmWave). The mmWave option enables the use of simple air interfaces since large bandwidths can be exploited (e.g., 2 GHz) to achieve high data rates rather than relying on highly complex techniques originally aimed at achieving a high spectral efficiency with smaller bandwidths. In addition, mmWave systems will easily evolve to even higher system capacities, because there will be plenty of margin to improve the spectral efficiency as data demands further increase. In this paper, a case is made for using mmWave for a fifth generation (5G) wireless system for ultradense networks by presenting an overview of enhanced local area (eLA) technology at mmWave with emphasis on 5G requirements, spectrum considerations, propagation and channel modeling, air-interface and multiantenna design, and network architecture solutions. <s> BIB007 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> With the severe spectrum shortage in conventional cellular bands, millimeter wave (mmW) frequencies between 30 and 300 GHz have been attracting growing attention as a possible candidate for next-generation micro- and picocellular wireless networks. The mmW bands offer orders of magnitude greater spectrum than current cellular allocations and enable very high-dimensional antenna arrays for further gains via beamforming and spatial multiplexing. This paper uses recent real-world measurements at 28 and 73 GHz in New York, NY, USA, to derive detailed spatial statistical models of the channels and uses these models to provide a realistic assessment of mmW micro- and picocellular networks in a dense urban deployment. Statistical models are derived for key channel parameters, including the path loss, number of spatial clusters, angular dispersion, and outage. It is found that, even in highly non-line-of-sight environments, strong signals can be detected 100-200 m from potential cell sites, potentially with multiple clusters to support spatial multiplexing. Moreover, a system simulation based on the models predicts that mmW systems can offer an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks with no increase in cell density from current urban deployments. <s> BIB008 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> With the explosive growth of mobile data demand, the fifth generation (5G) mobile network would exploit the enormous amount of spectrum in the millimeter wave (mmWave) bands to greatly increase communication capacity. There are fundamental differences between mmWave communications and existing other communication systems, in terms of high propagation loss, directivity, and sensitivity to blockage. These characteristics of mmWave communications pose several challenges to fully exploit the potential of mmWave communications, including integrated circuits and system design, interference management, spatial reuse, anti-blockage, and dynamics control. To address these challenges, we carry out a survey of existing solutions and standards, and propose design guidelines in architectures and protocols for mmWave communications. We also discuss the potential applications of mmWave communications in the 5G network, including the small cell access, the cellular access, and the wireless backhaul. Finally, we discuss relevant open research issues including the new physical layer technology, software-defined network architecture, measurements of network state information, efficient control mechanisms, and heterogeneous networking, which should be further investigated to facilitate the deployment of mmWave communication systems in the future 5G networks. <s> BIB009 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> We model and analyze a User-Equipment (UE) based wireless network selection method where individuals act on their stochastic knowledge of the expected behavior off their available networks. In particular, we focus on networks with millimeter-wave (mmWave) radio. Modeling mmWave radio access technologies (RATs) as a stochastic 3-state process based on their physical layer characteristics in Line-of-Sight (LOS), Non-Line-of-Sight (NLOS), and Outage states, we make the realistic assumption that users have no knowledge of the statistics of the RATs and must learn these while maximizing the throughput obtained. We develop an online learning-based approach to access network selection: a user-centric Multi-Armed Bandit Problem that incorporates the cost of switching access networks. We develop an online learning policy that groups network access to minimize costs for RAT selection, analyze the regret (loss due to uncertainty) of our algorithm. We also show that our algorithm obtains optimal regret and in numerical examples achieves 24% increase in total throughput compared to existing techniques for high throughput mmWave RATs that vary over a fast timescale. <s> BIB010 </s> A Survey of Client-Controlled HetNets for 5G <s> B. FAST TIME VARYING RATs (mmWAVE) <s> The new frontier in cellular networks is harnessing the enormous spectrum available at millimeter wave (mmWave) frequencies above 28 GHz. The challenging radio propagation characteristics at these frequencies, and the use of highly directional beamforming, lead to intermittent links between the base station (BS) and the user equipment (UE). In this paper, we revisit the problem of cell selection to maintain an acceptable level of service, despite the underlying intermittent link connectivity typical of mmWave links. We propose a Markov Decision Process (MDP) framework to study the properties and performance of our proposed cell selection strategy, which jointly considers several factors such as dynamic channel load and link quality. We use the Value Iteration Algorithm (VIA) to solve the MDP, and obtain the optimal set of associations. We address the multi user problem through a distributed iterative approach, in which each UE characterizes the evolution of the system based on stationary channel distribution and cell selection statistics of other UEs. Through simulation results, we show that our proposed technique makes judicious handoff choices, thereby providing a significant improvement in the overall network capacity. Further, our technique reduces the total number of handoffs, thus lowering the signaling overhead, while providing a higher quality of service to the UEs. <s> BIB011
Several white papers by several industry groups predit [4],  , 5G wireless systems will provide, at the very least, higher data rates (between 10 Gbps in an indoor office environment to 25 Mbps in a very dense crowd, with an average of 50 Mbps for general use), end-to-end latency on the order of 1 ms, and handle up to 150, 000/km 2 connections in a crowd-like environment. The key elements of 5G identified by many telecom companies are  -  : • Peak Data Rate: 10 Gbps per client, 4x that of 4G • End-to-End Latency: 1 ms, 1/50 that of 4G • Scale of Connections: 1 million/km 2 , 100x that of 4G In order to meet these goals, 5G will not only require access to diverse (licensed, shared licensed, and unlicensed) spectrum  ,  , but leverage new spectrum bands from 400 MHz to 100 GHz  ,  and to create capability to handle dense HetNets  . Millimeter-wave (mmWave) radio technologies is increasingly looked at as one of the primary new RATs for 5G given the scarcity of spectrum at microwave frequencies BIB005 - BIB009 . A combination of cost-effective hardware, high-gain and steerable antennas, larger carrier bandwidth allocations all translate to higher data rates for mmWave communications for small-cell and indoor applications on the order of 200m BIB006 ,  . However, mmWave is also characterized by higher path-loss exponents and an inability to penetrate obstructions such as the human body. There exist many studies on mmWave wireless communication in the 60 GHz band BIB002 - BIB001 . These studies characterize the free space propagation loss and the higher loss due to attenuation from non-Line-of-Sight (NLOS) channels. However, these bands may not be suitable for cellular due to the unlicensed nature of this band and the coexistence of 802.11ad, and the primary candidates are 28, 38, 71-76 and 81-86 GHz for indoor applications BIB007 . Outage studies for 38 GHz were done in Texas in 2012 BIB003 and the first statistical channel models were measured for 28 and 73 GHz in dense urban neighborhoods (New York City) in 2013 BIB008 , which analyzed a range of parameters: path loss, shadowing, mmWave line-of-sight(LOS)/NLOS/Outage probabilities, angular orientation of the BSs and clients, and number of clusters. Furthermore, the authors showed strong evidence for the existence of a third state, Outage, characterized by a complete loss of signal, and proposed modeling mmWave with a three-state channel model defined by the degrees of signal obstruction. mmWave technology is particularly sensitive to obstruction, with drastic changes in the path-loss exponent for VOLUME 5, 2017 different mmWave frequency bands BIB009 . These increase from values of 1.8 and 2 for 28 GHz and 73 GHz with a direct line-of-sight component to values upwards of 4.5 and 2.69 once the direct path is blocked. Furthermore, human-body blockage can have a significant effect on signal quality, with attenuation between 20-35dB BIB004 ,  with the loss of the line-of-sight. However, with exceedingly high throughput upwards of 10+ Gbps for 73 GHz demonstrated only in the past year  , a large path-loss exponent only helps the case that these high throughput mmWave BSs can be used as dense small cells in HetNets to handle extremely high throughput and high volume traffic. In BIB010 , the authors apply online learning using multiarmed bandits to RAT Selection for HetNets with fastchanging mmWave channels to maximize throughput while minimizing parameterized switching costs, with optimal total regret. Limited feedback from the BS in the form of a parameter describing the channel state between client and BS is sent to each client, and the client then uses this knowledge of the channel to discover and exploit the ''best" BS in terms of average throughput using a upper-confidence-bound-type algorithm in which the client is only allowed to switch BS associations at specific times. An alternative approach that also leverages the Markovian nature of channel state changes is considered in BIB011 , which directly considers dynamic channel load and link quality but not switching costs. However, the use of mmWave in HetNets is a relatively new area of study, and there remain lots of work to be done to better understand how to interchangeably use mmWave alongside other existing networks.