Since around 1980, a new generation of wireless technology has arisen approximately every 10 years. First-generation (1G) and second-generation (2G) began with voice and eventually introduced more and more data in third-generation (3G) and became highly popular in the fourth-generation (4G). To increase the data rate along with low latency and mass connectivity the fifth-generation (5G) networks are being installed from 2020. However, the 5G technology will not be able to fulfill the data demand at the end of this decade. Therefore, it is expected that 6G communication networks will rise, providing better services through the implementation of new enabling technologies and allowing users to connect everywhere. 6G technology would not be confined to cellular communications networks, but would also comply with non-terrestrial communication system requirements, such as satellite communication. The ultimate objectives of this work are to address the major challenges of the evolution of cellular communication networks and to discourse the recent growth of the industry based on the key scopes of application and challenges. The main areas of research topics are summarized into (i) major 6G wireless network milestones; (ii) key performance indicators; (iii) future new applications; and (iv) potential fields of research, challenges, and open issues.
One of the most influential technological advances in recent history has been mobile communication. Five (AMPS, GSM, UMTS, LTE, and 5G) wireless networking networks have also been adapted to date. Approximately, in every decade since 1980, a new generation has appeared. 1G and 2G began with voice and eventually added more and more 3G data and highly popular 4G data [
The International Telecommunication Union (ITU) defines the vision and specifications; three standard scenarios, high data rate (Gb/s), minimum delay (ms)) and dense device connections (one million/sq-km), should be met by 5G [
6G will be further improved and extended on the foundation of 5G. 6G aims to increase data rate 10 to 100 times higher than 5G. Bare minimum delay for delay-sensitive applications, advanced system capacity and higher spectrum efficiency will some other features. Moreover, 6G will promote improvements in coverage, facilitate higher mobility with mass interconnection, and uphold the development of an overall intelligent mobile Society [
The goal of this work is to address the significant challenges of the evolution of mobile cellular communication networks and to keep the focus on the recent growth of the industry dealing with the key areas of application and challenges. The major factors of this work are summarized into (i) major milestones of 6G wireless networks; (ii) key performance indicators; (iii) potential emerging applications; and (iv) potential research areas, challenges, and open issues. To achieve an accurate, concrete, and succinct conclusion, these areas are analyzed grounded on their particular sub-domains. This article will greatly contribute to introduce new pathways for future research guidelines for researchers through presenting some novel sources that could help in the development of 6G networks.
The rest of the article is organized as follows. The evolution of wireless networks is provided in Section 2. The major milestones of 6G wireless networks are provided in Section 3. The open research issues are discussed in Section 4. Finally, Section 5 concludes the work.
The main achievements of the five generations (1G−5G) of wireless communication networks are summarized in
In the last part of the 1970s, the 1G of wireless cellular technology was uncovered. It, an analog telecommunication standard designed for audio transmissions, was mainly available in Scandinavia, UK, and Japan. The throughput was up to 2.4 kbps. In 1G, Frequency Modulation (FM) and Multiple Access Frequency Division (FDMA) transmission technology was used. The total dedicated bandwidth was 30 kHz. Nevertheless, it had a lot of drawbacks, for instance (i) insecure or no encryption because of analog modulation scheme, (ii) can handle inadequate subscribers because of the FDMA technique used, (iii) insecure radiation, and absence of a handoff and so on. Overall, it was a limited-service and not unique worldwide [
The first system of the 2nd generation, Global Systems for Mobile Communications (GSM), was revealed in the first part of the year 1990. 200 kHz bandwidth was dedicated for 2G and achieved a data rate up to 9.6 kbps. It used Gaussian Minimum Shift Keying (GMSK) modulation and Time Division Multiple Access (TDMA for creating a very basic phase of voice communication. It was the era when global mobile communication came under a unified standard. Secrecy due to digital encryption, prolonged battery life due to less power consumption, and enhanced device capacity are some mentionable achievement of 2G. 2G faced problems due to lower data rate, 2.5G or GPRS overcome that drawback [
A new feature, packet switching, started through GPRS. 2G uses this technology along with circuit switching that was adopted in GSM, which able to increase the data rate up to 50 kbps with keeping similar GMSK, TDMA, and 200 kHz BW used in GSM. The next evolutionary step is Enhanced Data GSM Environment (EDGE). It is well known as pre-3G radio technology and is the target to provide up to 200 kbps of data rate. Using the same transmission technology and BW, the EDGE standard is based on the current GSM standard but the difference is in the modulation technique. It uses Eight-Phase Shift Keying (8PSK) along with GMSK. 8PSK provides higher data rates but a lower coverage area compared to GMSK. To boost packet-switched services, the EDGE was introduced to allow a multimedia application that is an example of a high-speed data application [
3G, the result of research carried out by the International Telecommunication Union (ITU), is the 3rd generation of wireless cellular technology. The spectrum between 400 MHz to 3 GHz was assigned for 3G. It uses the Wideband Code Division Multiple Access (W-CDMA) and High-Speed Packet Access (HSPA) technologies to give improved internet access with audio and video streaming competencies. HSPA is a combination of two protocols, High-Speed Downlink Packet Access (HSDPA) and High-Speed Uplink Packet Access (HSUPA), which expands as well as enhances the performance of in effect W-CDMA protocols for 3G mobile telecommunication networks. Evolved HSPA (also known as HSPA+), an enhanced 3GPP (3rd Generation Partnership Project) standard, was launched in the last part of 2008, with consequent wide-reaching use starting in 2010. 3.9G Long Term Evolution (LTE) contains features that surpass those of 3G mobile communications [
LTE is a radio access technology based on orthogonal frequency-division multiplexing (OFDM). It supports up to 20 MHz BW and Multiple-Input Multiple-Output (MIMO), which is the key technology behind high data rates and spectrum efficiency. It guarantees enhanced connection quality. Adaptive beamforming ensures the adaptation of radiation patterns for signal gain and mitigates interference. The peak mobile data speeds were increased by LTE technology to 100 Megabits per second (Mbps). This technology roadmap is extended to LTE-Advanced (LTE-A) [
By providing a complete and reliable solution based on IP, a 4G system enhances the predominant communication networks. The wireless community has extensively investigated three main lines of research to ensure the features of 4G wireless networks:
5G wireless networks have attracted considerable interest from both academia and industry since the fourth quarter of 2014 [
These overwhelming 5G network specifications have already given rise to a whirlwind of innovative thinking with an intellect of perseverance to bring revolutionary novel technology to practice. Only five years ago, mm Wave mobile networks were thought of as imaginary; they are now considered to be unavoidable. Moreover, the use of massive MIMO techniques incorporated with mm-wave along with small cell geometries seems to be a symbiotic fusion to maximize throughput [
As of April 2019, South Korea was the leading country to implement a significant large-scale 5G deployment for around 85 cities. The total number of 5G BSs was around 86,000. Overall, it is estimated that nearly 65% of the global population will gain access to the coverage of superfast 5G internet within 2025 It is mentionable that 85% of the 5G BSs were positioned in six cities, including Seoul, Busan, and Daegu, where a distributed architecture spectrum of 3.5 GHz with data rate speed deployed tested speeds ranging from 193 to 430 Mbit/s [
The following subsections highlight the major milestones of 6G wireless networks such as potential technologies, key performance indicators, potential emerging applications, and potential research areas.
In general, 6G will go further than the internet Services and it will be provided from the core to the end devices of the network to support universal AI services. In the design and optimization techniques of 6G frameworks, protocols and operations, AI will perform a very important character [
Enabling technology | Features | Challenges | |
---|---|---|---|
Spectrum | ✓ Terahertz (THz). |
✓ High bandwidth, |
✓ Circuit design, high propagation loss. |
Intelligence | ✓ Learning for value of information assessment. |
✓ Intelligent and autonomous selection of the information to transmit. |
✓ Complexity, unsupervised learning. |
PHY | ✓ Full duplex Out-of-band channel estimation Sensing and localization | ✓ Continuous TX/RX and relaying. |
✓ Management of interference and scheduling. |
Network architectures | ✓ Multi-connectivity and cell-less architecture. |
✓ Seamless mobility and integration of different kinds of links. |
✓ Scheduling, need for new network design. |
The key KPIs of 6G wireless networks are addressed in this section, namely peak data rate, mobility requirements, connected devices/Km2, area traffic capability, latency, reliability, spectral network, and energy efficiency. In addition, a detailed study of KPIs between the 5G and 6G networks is provided in
Since the beginning of telecommunications, consumer data rate demands have been rising. The data rates of the 1G were a few kbps that increased to a few Gbps in 5G. To obtain wider bandwidth, terahertz, visible light, and so on, 6G is indeed to operate on a higher frequency. 6G will facilitate a data rate of up to 10 to 100 times relative to 5G, promoting the Tbps maximum bit rate. The expectation is that the data rate will increase to 1 Tbps to allow the future smart city to autonomously handle different activities. The data rate is projected to grow from 1Gbps in 5G to at least 10 Gbps per user for individual users, and up to 100 Gbps in certain circumstances in evolving 6G networks [
In order to boost wireless communications, 6G will use smart frameworks to provide an additional degree of freedom (DoF), providing an unparalleled power. Smart reflective surfaces will be installed on a wide scale in buildings [
Low latency means communication is quick and rapid. We want to transmit our packets very quickly and there shouldn't be much delay in processing. The allowable 6G latency limit is 10 μSec [
High reliability and low latency are also needed for online gaming. The eRLLCS can combine the protection features with mMTC and uRLLC in 5G with higher reliability criteria greater than 99.999999999% (Nine 9's) in 6G wireless communication systems [
A range of heterogeneous radios in the equipment can support 6G applications. This allows multiple connectivity techniques, with users connected to the network as a whole (i.e., through various complementary technologies) and not to a single cell, that can increase the existing borders of cells. The devices will be able to seamlessly transition among different heterogeneous links (e.g., sub6 GHz, mmWave, Terahertz, or VLC) without manual intervention or configuration, which will provide QoS guarantees that are in line with the most challenging mobility requirements envisioned for 6G, low latency even in ultra-high mobility scenarios (up to 1000 km/h) [
Another use case for next-generation wireless communication is mMTC. This is the domain at which IoTs come in without the presence of human beings. It is machine type communication where the calls, texts, and commands go from one machine to another machine. No human interactions are required and the machine can communicate with each other by following the instructions. It is expected that future mobile technology will accommodate 107 devices/Km2 [
Sensor networks and IoTs will be linked to one another in a cooperative manner and with many base stations to boost the performance of the system such as wearable devices, control and tracking devices, self-driving vehicles, smart grids, industrial automation and control devices, and medical and health-related devices. Communication between these devices can be through peer-to-peer or multi-hop cooperative relay methods. A different communication network architecture that can handle different content-driven applications/network needs various applications or devices. Therefore, with all of these criteria in mind, preparation and optimization of next-generation wireless networks would require a unique approach [
The world is entering into a new era of technology. Demand for better channel capacity and backhauling has also increased as the number of connected devices per meter square area has increased. The extremely dense sensor network generates more data regularly than Tara-Bytes (TB) [
Wireless protocols have been developed for many unique applications in previous wireless generations (1G-to-5G). Nevertheless, to build massive IoTs or mMTCs, designers need to develop some power-efficient and cost-effective devices. This massive IoT connection contributes to the growth of vehicular communication called V2X (vehicle-to-infrastructure), such as autonomous driving [
6G will meet and exceed many demands, including high energy efficiency distribution, most importantly from the viewpoint of widespread use of the Internet of Things (IoTs) and with a multi-minute sensor eco-system. Besides, the expansion of smartphones’ battery-recharge capability must be discussed in line with the suggestion that their capacity and ability to cope with advanced multimedia signal processing leap increases as their electricity consumption increases [
5G use cases have three main classes, such as URLLC, enhanced mobile broadband (eMBB), and massive machine type communication (mMTC). However, new communication scenarios in the future networks in the 2030s, which encompass holographic calls, haptics, autonomous connected vehicles, massive URLLC (mURLLC), human-centric services, Bio-Internet of things (B-IoT), nano-Internet of things (N-IoT), and mobile broadband reliable, low-latency communication, which require bandwidth not available even in the current millimeter wave spectrum, are considered be a critical driver towards the new era of the wireless networks (6G). In addition, consider XR (i.e., combining mixed reality, augmented reality, and virtual reality) and brain-computer interaction that requires 5G-eMBB high data rates, low-latency, and high reliability [
Although 5G offers numerous advantages, there is a need to propose services that are more human-centric. 6G is expected to widely support the human-centric communication concept, where the human can access and/or share physical features. The human-bond communication concept is proposed to allow access to the five human senses. It is foreseen that there will be an increasing number of new communication devices in the 2030s. These new communication devices can be wearable devices, integrated headsets, and implantable sensors [
This use case is based on a remote connection with an ultrahigh accuracy [
After using holographic communication to transfer a virtual vision of close-to-real sights of people, events, environments and so on, it is beneficial to remotely exchange the physical interaction through the tactile internet in real time [
With the This use case deals with fully autonomous connected vehicles that offer complete unmanned mobility, safe driving, smart infotainment, and enhanced traffic management [
Research areas | Challenges and open Issues |
---|---|
Radio frequency (RF) link integration | ✓ Design of RF hardware. |
AI | ✓ Reinforcement learning for SON. |
3D networking | ✓ 3D propagation modelling. |
Holographic radio | ✓ Design of holographic MIMO. |
Reliability-latency | ✓ Characterization of energy and spectrum needs for rate-reliability-latency targets. |
6G protocol designs | ✓ Design of scheduling, coordination, and signalling protocols that do not require pre-determined, rigid frame structures. |
This article discussed the substantial issues of the evolution of wireless communication networks. We first reviewed the major successes and challenges of 1G through 5G. We did this by discussing, for each generation, aspects of regulation, services, innovations, and other issues. The major milestones of 6G wireless networks including key performance indicators, potential emerging applications, and potential research areas have discussed. Looking at 6G, key proposed innovations we foresee are optical free-space indoor communications, wireless charging, and energy harvesting, and extensive use of machine learning to facilitate innovative services.
The authors would like to thanks the editors of CMC and anonymous reviewers for their time and reviewing this manuscript.