5G and Beyond

This chapter introduces a family of rate-compatible polar (RC-Polar) codes for next-generation communication systems (e.g., 6G systems). First, a new class of RC-Polar code, named parallel concatenated polar (PCP) code, is described. This code is constructed by the parallel concatenation of multiple (punctured) polar codes and the so-called information-bit divider. The special structure of PCP enables incremental retransmissions to produce various transmission rates, and, hence, PCP code can be used for HARQ-IR systems. It is remarkable that in PCP code, a capacity-achieving (punctured) polar code is used at every (re)transmission. Thus, PCP is capacity-achieving for an arbitrary sequence of code rates. Focusing on finite lengths, furthermore, RC punctured polar (RCPP) codes are described. For the construction of RCPP codes, the main challenging is to jointly optimize RC puncturing patterns and a common information set which is good for all the codes in the family. To address such problem, this chapter presents an efficient method to construct a good RCPP code by introducing hierarchical puncturing and information-copy technique. Due to the special property of hierarchical puncturing and the use of information-copy, each punctured polar code in the family can use its own optimized information set. Thus, RCPP code can yield attractive performances for a wide range of code rates in HARQ-IR systems. Via numerical results, it is shown that RCPP code can attain a nontrivial performance gain over the other rate-compatible codes (e.g., RC-LDPC codes). Therefore, the special structure of RCPP code, introduced in this chapter, would play a key role in designing a good RCPP code for 6G systems.

Multiple access is an essential physical-layer technique in wireless communication networks that allows multiple mobile users to access the network simultaneously. Driven by the upsurge of devices expected in 6G and beyond, future wireless communication networks are foreseen to operate in dynamic regimes ranging from underloaded (where the number of scheduled devices is smaller than the number of transmit antennas on each access point) to overloaded (where the number of scheduled devices is larger than the number of transmit antennas on each access point). Besides, each transmitter is required to simultaneously serve devices with heterogeneous capabilities, deployments, as well as qualities of channel state information at the transmitter (CSIT) since the devices for 5G and beyond tend to be more diverse including low-end units such as Internet of Things (IoT) and machine-type communications (MTC)-type devices and high-end equipment such as smartphones with varied user deployments and applications. The resulting requirements for massive connectivity, high throughput, as well as quality of service (QoS) heterogeneity have recently sparked interests in redesigning multiple access techniques for the downlink of communication systems. This chapter first reviews the state-of-the-art multiple access techniques including their benefits and limitations, followed by introducing the promising multiple access candidate, rate-splitting multiple access (RSMA) for 6G and beyond, and a comprehensive comparison among all multiple access techniques. The challenges and future trends of using RSMA will be summarized in the end.

Massive multiple-input multiple-output (MIMO) technology has become one of the key technologies for fifth generation (5G) wireless systems and beyond due to its potential to offer high spectral efficiency simultaneously to many users, with simple signal processing. These remarkable gains are obtained through the use of many antennas at the base station to spatially multiplex many users on the same time-frequency resource. This chapter provides a comprehensive overview of massive MIMO. A completed massive MIMO transmission protocol under time-division duplex operation is first presented. Then, fundamental aspects including favorable propagation, channel hardening, pilot contamination, and use-and-then-forget capacity bounding technique are discussed in detail. Finally, a range of important topics for future research on massive MIMO is suggested.

Aggressive frequency reuse through denser deployment of base stations (BS), termed network densification, provides cell-splitting gains, which have traditionally translated to higher network throughput. In fact, one can show that the sum throughput increases linearly with the BS density under seemingly reasonable assumptions including standard power-law path loss model. However, these assumptions do not hold when we approach ultra-densification because of which the linear relationship between network throughput and network densification does not exist beyond a certain base station density. In fact, it can be easily shown that the inclusion of a realistic path-loss model, such as the dual-slop path-loss model, may cause the network throughput to fall and even crash when BS density is increased beyond a certain point. In this chapter, we discuss in detail all key factors, such as path-loss models, the height difference between base station and user equipment antennas, scaling of user equipment density, network access restrictions, and traffic characteristics, that may affect densification gains. We evaluate the impact of these factors on the performance metrics including signal-to-noise ratio coverage probability and throughput. Through our analysis, we identify factors that have higher impact on densification gains than the others. In order to understand the interplay between these factors and their joint impact on densification gains, we present useful case studies in which two or more of these factors are jointly considered.

Drones, also known as unmanned aerial vehicles (UAVs), are becoming a promising solution for a wide range of applications in current and upcoming cellular networks. As a new type of users, UAVs are increasingly being used in wireless networks. UAVs are also being used as a new type of base stations (BSs). The fact that they are able to move, makes UAVs a viable solution as flying BSs to cover e to hard-to-reach areas. More so, their flexibility, agility, adaptability, and cost-effectiveness make them an attractive alternative to conventional rooftop or pole-mounted BSs, at least in certain situations. In addition to aerial users or BSs, UAVs can be exploited as relays. Such new paradigms bring new opportunities but also stress the current cellular networks in various ways. This chapter discusses the integration of UAVs in cellular communication networks. After a brief history of UAV, different communication scenarios based on UAVs are illustrated. Then, standardization studies are explained and summarized. Next, key new features of UAVs including channel modeling, trajectory design, and interference-aware transmission strategies are elaborated on. Finally, several important open problems of UAV-based cellular networks are discussed.

The ongoing deployment of 5G cellular systems is continuously exposing the inherent limitations of this system, compared to its original premise as an enabler for Internet of Everything applications. These 5G drawbacks are spurring worldwide activities focused on defining the next-generation 6G wireless system that can truly integrate far-reaching applications ranging from autonomous systems to extended reality. To date, the fundamental architectural and performance components of 6G remain largely undefined and open to speculations. In this chapter, we present a holistic vision that identifies the main principles that can guide the design and development of a 6G system. In particular, we discuss, in detail, why 6G will not be a simple exploration of more spectrum at high-frequency bands such as terahertz frequencies, but it will rather be a convergence of a number of technological trends. 6G will also be largely driven by a new breed of exciting Internet of Everything services. To this end, we first outline the primary drivers of 6G systems, in terms of applications and accompanying technological trends. Then, we introduce a new set of service classes and expose their target 6G performance requirements. We then explore the enabling technologies for the introduced 6G services and define a comprehensive set of research problems that come hand in hand with the identified technologies. We conclude by providing our observations on the challenges and opportunities that will define the road toward 6G. Ultimately, this chapter can be used to stimulate comprehensive, out-of-the-box research around 6G technologies.

This chapter is an introduction to the NG-RAN architecture, which is the foundation of 5G as specified by the 3rd Generation Partnership Program (3GPP). It presents the standardized architecture and motivates the design choices (CU-DU split, CP-UP split, architecture options, and others) made by 3GPP. Some definitions and concepts specific to 3GPP architecture specifications are presented (including Dual Connectivity) in order to better understand the distinctive features of NG-RAN. Additional insights into the standardization process and timeline for 5G are also included to give additional insights into the deployment scenarios foreseen for 5G and their evolution. The NG-RAN architecture is the basis for all future 5G network functionality, and it is already shaping a range of features which are currently studied or in the process of being specified by 3GPP: positioning and multicast/broadcast are two such examples, and they are discussed in this chapter showing how they are anchored to NG-RAN architecture choices.

The physical layer of NR is designed to support a large set of use cases from day one and operate tightly with Long Term Evolution (LTE). In addition, NR is designed so that new use case and design can be added on in later releases. It is further designed to support performances in terms of throughputs, latency, energy efficiency, deployment flexibility and different spectrums. NR shares some basics with LTE. NR is, however, designed to support a larger spectrum range and wider carriers than LTE. NR supports spectrum ranges from around 500 MHz to up to 52.6 GHz in its first release, and work is being done to expand this beyond 52.6 GHz. When it comes to data rate and particularly latency, NR has taken a large step compared to LTE. All of this will enable and grow the current use cases used by cellular technology. By that, the design of NR will support the general technology development during the 2020 decade and 2030 decade, wherein connectivity is a key enabler for all sorts of applications and uses.

To satisfy the demanding requirements of 5G NR system, new error-correcting codes have been introduced in 5G NR for both data and control channels. Low-density parity-check (LDPC) codes are adopted to support data channels, replacing the turbo codes used in 4G LTE system. Polar codes are adopted to support control channels, replacing the tail-biting convolutional codes in 4G LTE. This chapter describes in detail the design features of the adopted LDPC codes and polar codes and related operations such as CRC attachment, segmentation, interleaving, and rate matching. Finally, simulation curves are provided to demonstrate the superior performance and implementation advantages of the sophisticated code designs.

Before a UE can properly communicate within a network, it must carry out cell search to find, synchronize, and identify a cell. Then, it can acquire basic system information and perform random access to access the cell. This chapter introduces the fundamental concepts of 5G NR cell search and random access. We explain in detail the design of synchronization signals (SSs) and physical broadcast channel (PBCH) block (i.e., SSB) for cell search, basic system information acquisition, different physical random-access channel (PRACH) formats and configurations for random-access preamble transmission, and the four-step random-access procedure. We share the insights on how to utilize these new concepts introduced in NR to enable an unified initial access framework for supporting various deployment scenarios and flexible operation in different frequency bands.

The fifth generation (5G) wireless access technology, known as New Radio (NR), features flexibility to support a variety of usage scenarios. One of the basic concepts in 5G NR is bandwidth part (BWP), which is, at a high level, a set of contiguous resource blocks configured inside a channel bandwidth. BWP spans across many 5G NR specifications developed by the 3rd Generation Partnership Project. Understanding how BWP operates is vital to understanding 5G NR. This chapter provides an overview of the essentials of BWP in the NR technical specifications. We describe fundamental BWP concepts, BWP configuration methods, and BWP switch mechanisms. We also discuss user equipment capabilities in terms of BWP support and share our thoughts on use cases of BWP for NR deployments.

Ultra-reliable and low-latency communications (URLLC) has been identified as one of the most important use cases for 5G technologies. In this chapter, we provide an overview of the URLLC-related features in NR, which is the 5G standards for radio access network developed in 3rd Generation Partnership Project (3GPP). The discussion focuses on the physical layer design that enables the low latency and high reliability over the air interface. We provide the background regarding the use case analysis and the corresponding performance requirements. The physical layer design considerations for URLLC in the first release (Rel-15) of NR and the URLLC-specific enhancements in NR Rel-16 are described in detail.

The 3rd Generation Partnership Project (3GPP) Rel-16 introduced New Radio (NR) operation in unlicensed spectrum. This is a promising feature that is expected to accommodate large bandwidth for NR worldwide. Generally speaking, operation in unlicensed spectrum uses the Rel-15 NR design as a baseline with some design enhancements to support operation in unlicensed spectrum. Those enhancements are necessary to meet spectrum regulations and optimize NR operation in shared spectrum.

This chapter provides an overview of unlicensed spectrum operation requirements and challenges and radio access design changes as compared to licensed operation and highlights the targeted deployments and spectrum.

Historically, the main driver for location-based services has been requirements from regulatory authorities (e.g. emergency caller location requirements). However, today, many public and private entities demand delivery of location information to enable commercially motivated location-based services, which often require higher location accuracy and precision. Consequently, 5G supports several enhanced location services capabilities and NR native positioning technologies to address the diverse location requirements resulting from new applications and industry verticals. In this chapter, we provide an overview of the location services and position location capabilities in the 5G system. We describe the location services architecture and summarize the basic location services procedures. After a brief overview of the position location fundamentals, we describe the NR native positioning methods specified in 3GPP Release 16. Finally, we summarize the positioning reference signals which were defined for NR to support the various positioning methods.

As an advanced NR feature, integrated access and backhaul (IAB) is specified in 3GPP Rel-16 to enable wireless backhauling using NR air interface and layer-2 relaying. IAB is considered as an important NR capability to resolve the backhauling issue and facilitate cell densification. In this chapter, we introduce the architecture and the key features of IAB as specified in 3GPP Rel-16 from physical layer, radio protocol, and radio access network perspectives.

Today, mobile operators are starting to deploy Fifth-Generation (5G) networks to expand the coverage ubiquity of broadband wireless service. In contrast, in-flight connectivity remains limited, and its quality of service does not always meet the expectations. Embracing 5G New Radio (NR) in air-to-ground (A2G) communication systems can help narrow the gap between airborne and ground connectivity. In this chapter, we focus on 5G NR-based direct A2G communications. We first provide an overview of the existing A2G systems which are based on earlier generations of mobile technologies. Then we confirm the feasibility of NR A2G systems with a performance study in a range of bands from below 7 GHz to millimeter wave frequencies. The results show that NR A2G systems can provide significantly improved data rates for in-flight connectivity. We also identify the major challenges associated with NR A2G communications, discuss enhancements to counteract the challenges, and point out fruitful avenues for future research.

The 3rd Generation Partnership Project (3GPP) completed the first global fifth-generation (5G) system standard in its Release 15, paving the way for making 5G a commercial reality. So, what is next in 5G system evolution to further expand the 3GPP ecosystem? Enabling 5G system to support satellite communications is one direction under exploration in 3GPP. There has been a resurgence of interest in providing connectivity from space, stimulated by technology advancement and demand for ubiquitous connectivity services. The ongoing evolution of 5G standards provides a unique opportunity to revisit satellite communications. In this chapter, we focus on the 5G radio access network known as the new radio (NR) and study how to adapt the NR air interface for satellite links. We first provide an overview of use cases and a primer on satellite communications. We then identify key technical challenges faced by NR evolution for satellite communications and propose solutions to overcome them.