The abundance of spectrum in the millimeter-wave (mm-wave) bands makes it an attractive alternative for future wireless communication systems. Such systems are expected to provide data transmission rates in the order of multi-gigabits per second in order to satisfy the ever-increasing demand for high rate data communication. Unfortunately, mm-wave radio is subject to severe path loss, which limits its usability for long-range outdoor communication. In this paper, we propose a multi-hop mm-wave wireless network for outdoor communication, where multiple full-duplex buffered relays are used to extend the communication range, while providing end-to-end performance guarantees to the traffic traversing the network. We provide a cumulative service process characterization for the mm-wave propagation channel with self-interference in terms of the moment generating function of its channel capacity. Then, we then use this characterization to compute probabilistic upper bounds on the overall network performance, i.e., total backlog and end-to-end delay. Furthermore, we study the effect of self-interference on the network performance and propose an optimal power allocation scheme to mitigate its impact in order to enhance network performance. Finally, we investigate the relation between relay density and network performance under a sum power constraint. We show that increasing relay density may have adverse effects on network performance, unless the selfinterference can be kept sufficiently small.

We analyze the delay performance of traffic dispersion in millimeter-wave (mm-wave) communications, where Nakagami-m fading channel is considered. To apply (mm, +)algebra network calculus in wireless communications, we develop a closed-form expression based on moment generating function (MGF), which characterizes the stochastic service process by staying in the bit domain, rather than transferring to the SNR domain. Subsequently, for traffic dispersion with mm-wave, we derive probabilistic delay bounds and effective capacity based on the obtained MGF of the cumulative service process. Besides, the impacts of several factors, e.g., the number of independent path, system gain (including antenna gain and adopted radio frequency) or Nakagami parameter, on the delay performance are studied. We not only comprehensively study the delay performance of traffic dispersion with mm-wave, but also demonstrate the feasibility and tractability of performance analysis.

Heterogeneous ultradense networks (H-UDNs) are one key enabler for fifth-generation (5G) wireless networks and beyond to satisfy the explosive growth of mobile data traffic, which exploits spatial reuse of scarce spectrum by deploying massive base stations (BSS) to boost network capacity and enhance network coverage. In this article, we present the system architecture for 5G H-UDNs, consisting of virtualized integrated ground-Air-space radio access networks (RANs) and core networks and study network coordination for virtualized H-UDN to efficiently manage computing resources and intercell interference. We look at a cloud-fog-computing coordination framework for efficient computing resource management by achieving reasonable computing task distribution and transfer; computing load balance for computing tasks among virtual computing resources to improve network performance and computing resource efficiency; and a macro-small cell coordination framework for virtualized H-UDN with nonorthogonal multiple access (NOMA) to efficiently manage intercell interference and improve network performance. The joint macro-small enhanced intercell interference coordination (eICIC) and small-small coordinated multipoint joint transmission (CoMP-JT) scheme can efficiently eliminate macro-small intercell interference and utilize small-small intercell interference.

The heterogeneous network (HetNet) is a key enabler to largely boost network coverage and capacity in the forthcoming 5G and beyond. To support the explosively growing mobile data volumes, wireless communications with millimeter- wave (mmWave) radios have attracted massive attention, and is widely considered as a promising candidate in 5G HetNets. In this article, we give an overview on the end-to-end latency of HetNets with mmWave communications. In general, it is rather challenging to formulate and optimize the delay problem with buffers in mmWave communications, since conventional graph-based network optimization techniques are not applicable when queues are considered. Toward this end, we develop an adaptive low-latency strategy, which uses cooperative networking to reduce the end-to-end latency. Then we evaluate the performance of the introduced strategy. Results reveal the importance of proper cooperative networking in reducing the end-toend latency. In addition, we have identified several challenges in future research for low-latency mmWave HetNets.

Low latency is critical for many applications in wireless communications, e.g., vehicle-to-vehicle (V2V), multimedia, and industrial control networks. Meanwhile, for the capability of providing multi-gigabits per second (Gbps) rates, millimeter-wave (mm-wave) communication has attracted substantial research interest recently. This paper investigates two strategies to reduce the communication delay in future wireless networks: traffic dispersion and network densification. A hybrid scheme that combines these two strategies is also considered. The probabilistic delay and effective capacity are used to evaluate performance. For probabilistic delay, the violation probability of delay, i.e., the probability that the delay exceeds a given tolerance level, is characterized in terms of upper bounds, which are derived by applying stochastic network calculus theory. In addition, to characterize the maximum affordable arrival traffic for mmwave systems, the effective capacity, i.e., the service capability with a given quality-of-service (QoS) requirement, is studied. The derived bounds on the probabilistic delay and effective capacity are validated through simulations. These numerical results show that, for a given sum power budget, traffic dispersion, network densification, and the hybrid scheme exhibit different potentials to reduce the end-to-end communication delay. For instance, traffic dispersion outperforms network densification when high sum power budget and arrival rate are given, while it could be the worst option, otherwise. Furthermore, it is revealed that, increasing the number of independent paths and/or relay density is always beneficial, while the performance gain is related to the arrival rate and sum power, jointly. Therefore, a proper transmission scheme should be selected to optimize the delay performance, according to the given conditions on arrival traffic and system service capability.

We study the maximum achievable rate of a two-hop amplified-and-forward (AF) relaying millimeter-wave (mm-wave) system, where two AF relaying schemes, i.e., half-duplex (HD) and full-duplex (FD) are discussed. By considering the two-ray mm-wave channel and the Gaussian-type directional antenna, jointly, the impacts of the beamwidth and the self-interference coefficient on maximum achievable rates are investigated. Results show that, under a sum-power constraint, the rate of FD-AF mm-wave relaying outperforms its HD counterpart only when antennas with narrower beamwidth and smaller self-interference coefficient are applied. However, when the sum-power budget is sufficiently high or the beamwidth of directional antenna is sufficiently small, direct transmission becomes the best strategy, rather than the AF relaying schemes. For both relaying schemes, we show that the rates of both AF relaying schemes scale as O(min{theta(-1)(m),theta(-2)(m)})with respect to beamwidth theta(m), and the rate of FD-AF relaying scales as O(mu(-(1/2))) with respect to self-interference coefficient mu. Also, we show that ground reflections may significantly affect the performance of mm-wave communications, constructively or destructively. Thus, the impact of ground reflections deserves careful considerations for analyzing or designing future mm-wave wireless networks.

Nowadays, the ever-increasing demands on higher data rates and better serviceperformance have posed extremely huge challenges to the existing wireless communicationswithin sub-6 GHz bands, mainly due to the spectrum scarcity in lowerfrequency bands. In recent years, the millimeter-wave (mm-wave) technology, as apromising candidate to meet the aforementioned demands, have attracted extensiveresearch attention, and has been regarded as one of the key enablers for theforthcoming the 5th generation (5G) mobile communications. The main featuresof mm-wave communications include: abundant spectral resources, high penetrationloss, severe path loss, weak multi-path effects, and narrow antenna beams, andthese particular features make the potential challenges and solutions with mm-wavediffer a lot from those in the conventional 6-GHz systems.

It is known that the high throughput and the low latency are two critical qualityof-service (QoS) aspects in future mobile networks, while the related research withmm-wave are fairly recent and insufficient in the past few years. Motived by theurgent needs for further development and the blanks remained in previous works,in this doctoral thesis, we investigate the throughput and the latency in mm-wavenetworks through conducting performance analyses and identifying design principles,with the objective of seeking clues for improving the QoS of mm-wave wirelesscommunications in practice.

Our main research regarding throughput and latency in mm-wave networksthat are included in this doctoral thesis can be categorized from the following threeaspects:

(i) Throughput of mm-wave relay networks: For indoor scenarios, we study thehalf-duplex (HD) relaying with mm-wave in the presence of random linkblockages, where a distance-based routing algorithm is proposed to maximizethe throughput. For outdoor scenarios, focusing on a two-hop amplifyand-forward (AF) relay network in the HD or the full-duplex (FD) mode, weinvestigate the impacts of beamwidth, ground reflections, and self-interferencecoefficient on the throughput, where Gaussian-type directional antenna modeland two-ray channel model are jointly adopted.

(ii) Latency analysis via stochastic network calculus: With the aid of stochasticnetwork calculus, we derive upper bounds for the probabilistic delay tokeep track of the latency performance of buffer-aided mm-wave networks. We mainly study mm-wave systems designed in tandem or parallel manners,and also consider a hybrid design that combines the tandem and parallelschemes in a flexible manner. Moreover, the capability of achieving low-latencymm-wave communications is characterized and investigated in terms of effectivecapacity, and the comparison among different transmission schemes isconducted to identify the respective strengths and proper conditions for theirapplications.

(iii) Traffic allocation for low-latency mm-wave systems: Traffic allocation schemesfor low latency in buffer-aided mm-wave networks are investigated. Due tothe use of buffers, the delay optimization problem hereby differs from thosewithout buffers, where the conventional graph-based network optimizationtechniques become intractable. We demonstrate the impacts of different trafficallocation schemes on the latency. For multi-hop networks with multipleparallel channels in each hop, we consider both local and global traffic allocationschemes, quantify their resulting end-to-end (E2E) latencies, and analyzethe respective strengths and weaknesses.

For buffer-aided tandem networks consisting of relay nodes and multiple channels per hop, we consider two traffic allocation schemes, namely local allocation and global allocation, and investigate the end-to-end latency of a file transfer. We formulate the problem for generic multi-hop queuing systems and subsequently derive closed-form expressions of the end-to-end latency. We quantify the advantages of the global allocation scheme relative to its local allocation counterpart, and we conduct an asymptotic analysis on the performance gain when the number of channels in each hops increases to infinity. The traffic allocations and the analytical delay performance are validated through simulations. Furthermore, taking a specific two-hop network with millimeter-wave (mm-wave) as an example, we derive lower bounds on the average end-to-end latency, where Nakagami-m fading is considered. Numerical results demonstrate that, compared with the local allocation scheme, the advantage of global allocation grows as the number of relay nodes increases, at the expense of higher complexity that linearly increases with the number of relay nodes. It is also demonstrated that a proper deployment of relay nodes in a linear mm-wave network plays an important role in reducing the average end-to-end latency, and the average latency decays as the mm-wave channels become more deterministic. These findings provide insights for designing multi-hop mm-wave networks with low end-to-end latency.