The capabilities of fifth-generation (5G) wireless access must extend far beyond those of previous generations’ — offering extremely high data rates, exceptionally low latency, ultra-high reliability, energy efficiency and extreme device densities. Long-Term Evolution (LTE) high-speed wireless communications and new radio-access technologies will facilitate extension to higher frequency bands, access/backhaul integration, device-to-device communication, flexible duplex, flexible spectrum use, multi-antenna transmission, ultra-lean design and user/control separation.
What Is 5G?
5G radio access technology will be a key component of the Networked Society. It will address high traffic growth and increasing demand for high-bandwidth connectivity. It will also support massive numbers of connected devices and meet the real-time, high-reliability communication needs of mission-critical applications.
5G will provide wireless connectivity for a range of new applications and use cases, including wearables, smart homes, traffic safety/control, critical infrastructure, industry processes and very-high-speed media delivery. As a result, it will also accelerate the development of the internet of things (IoT).
The overall aim of 5G is to provide ubiquitous connectivity for any kind of device and any kind of application that may benefit from being connected.
5G networks will not be based on one specific radio-access technology. Rather, 5G is a portfolio of access and connectivity solutions addressing the demands and requirements of mobile communication beyond 2020.
The specification of 5G will include the development of a new flexible air interface, NX, which will be directed to extreme mobile broadband deployments. NX will also target high-bandwidth and high-traffic-usage scenarios, as well as new scenarios that involve mission-critical and real- time communications with extreme requirements in terms of latency and reliability.
In parallel, the development of narrowband IoT (NB-IoT) in 3rd Generation Partnership Project (3GPP) standards is expected to support massive machine connectivity in wide-area applications. NB-IoT will most likely be deployed in bands below 2 GHz and will provide high capacity and deep coverage for enormous numbers of connected devices (see Figure 1).
Ensuring interoperability with past generations of mobile communications has been a key principle of the information and communications technology (ICT) industry since the development of the Global Standard for Mobile (GSM) communication and later wireless technologies within the 3GPP family of standards. In a similar manner, LTE will evolve in a way that recognizes its role in providing excellent coverage for mobile users, and 5G networks will incorporate LTE access (based on orthogonal frequency-division multiplexing (OFDM)) along with new air interfaces in a transparent manner toward both the service layer and users.
Around 2020, much of the available wireless coverage will continue to be provided by LTE, and it is important that operators with deployed 4G networks have the opportunity to transition some or all of their spectrum to newer wireless access technologies. For operators with limited spectrum resources, the possibility of introducing 5G capabilities in an interoperable way, thereby allowing legacy devices to continue to be served on a compatible carrier, is highly beneficial and, in some cases, even vital.
At the same time, it is essential that LTE evolve to the point in which it is a full member of the 5G family of air interfaces — especially given the fact that the initial deployment of new air interfaces may not operate in the same bands. The 5G network will enable dual-connectivity between LTE operating within bands below 6 GHz and the NX air interface in bands within the range 6 GHz to100 GHz. NX should also allow for user-plane aggregation (i.e., joint delivery of data via LTE and NX component carriers).
In order to enable connectivity for a wide range of applications with new characteristics and requirements, the capabilities of 5G wireless access must extend far beyond those of previous mobile communication generations. New capabilities will include massive system capacity, high data rates everywhere, low latency, ultra-high reliability and availability, low device cost and energy consumption, and energy-efficient networks.
Massive System Capacity
Traffic demands for mobile communication systems are predicted to increase dramatically. Predictions are included in ICT-317669 of the Mobile and Wireless Communications Enablers for the Twenty-twenty Information Society (METIS) Project and in the November 2015 Ericsson Mobility Report. To support this traffic in an affordable way, 5G networks must deliver data with much lower cost per bit compared with the networks of today. Furthermore, the increase in data consumption will result in an increased energy footprint from networks. 5G must therefore consume significantly lower energy per delivered bit than current cellular networks.
The exponential increase in connected devices — such as the deployment of billions of wirelessly connected sensors, actuators and similar devices for massive machine connectivity —will place demands on the network to support new paradigms in device and connectivity management that do not compromise security. Each device will generate or consume small amounts of data to the extent that it will individually, or even jointly, have a limited effect on the overall traffic volume. However, the sheer number of connected devices seriously challenges the ability of the network to provision signaling and manage connections.
High Data Rates Everywhere
Every generation of mobile communication has been associated with higher data rates compared with the previous generation. In the past, much of the focus has been on the peak data rate that can be supported by a wireless access technology under ideal conditions. However, a more important capability is the data rate that can actually be provided under real-life conditions in different scenarios.
· 5G should support data rates exceeding 10 Gbps in specific scenarios, such as indoor and dense outdoor environments.
· Data rates of several hundred megabits per second should generally be achievable in urban and suburban environments.
· Data rates of at least 10 Mbps should be accessible almost everywhere, including sparsely populated rural areas in both developed and developing countries.
Low latency will be driven by the need to support new applications. Some envisioned 5G uses, such as traffic safety and control of critical infrastructure and industry processes, may require much lower latency compared with what is possible with the mobile communication systems of today.
To support such latency-critical applications, 5G should allow for an application end-to-end latency of 1 millisecond or less, although application-level framing requirements and codec limitations for media may lead to higher latencies in practice. Many services will distribute computational capacity and storage close to the air interface. This will create new capabilities for real-time communication and will allow ultra-high service reliability in a variety of scenarios, ranging from entertainment to industrial process control.
Reliability and Availability
In addition to low latency, 5G should also enable connectivity with ultra-high reliability and ultra-high availability. For critical services, such as control of critical infrastructure and traffic safety, connectivity with certain characteristics, such as a specific maximum latency, should not merely be typically available. Rather, loss of connectivity and deviation from quality of service requirements must be extremely rare. For example, some industrial applications might need to guarantee successful packet delivery within 1 millisecond with a probability higher than 99. 9999 percent.
Cost and Energy Consumption
Low-cost, low-energy mobile devices have been a key market requirement since the early days of mobile communication. However, to enable the vision of billions of wirelessly connected sensors, actuators and similar devices, a further step has to be taken in terms of device cost and energy consumption. It should be possible to make 5G devices available at low cost and with a battery life of several years without recharging.
Although device energy consumption has always been prioritized, energy efficiency on the network side has recently emerged as an additional key performance indicator. There are three main reasons for this:
· Energy efficiency is an important component in reducing operational cost, and it is also a driver for better dimensioned nodes, leading to lower total cost of ownership.
· Energy efficiency enables off-grid network deployments that rely on medium-sized solar panels as power supplies, thereby enabling wireless connectivity to reach even the most remote areas.
· Energy efficiency is essential to realizing operators’ ambition of providing wireless access in a sustainable and more resource-efficient way.
The importance of these factors will increase further in the 5G era, and energy efficiency will therefore be an important requirement in the design of 5G wireless access.
Fundamentally, applications such as mobile telephony, mobile broadband and media delivery are about information for humans. In contrast, many of the new applications and uses behind the requirements and capabilities of 5G are about end-to-end communication between machines. To distinguish them from the more human-centric wireless communication uses, these applications are often called machine-type communication (MTC).
Although spanning a wide range of applications, MTC applications can be divided into two main categories — massive MTC and critical MTC — depending on their characteristics and requirements.
Massive MTC refers to services that typically span large numbers of devices, usually sensors and actuators. Sensors are extremely low cost and consume low amounts of energy in order to sustain long battery life. Clearly, each sensor normally generates a small amount of data, and low latency is not a critical requirement. Though actuators are similarly limited in cost, their energy footprints are likely to consumer low to moderate amounts of energy.
Sometimes, the mobile network may be used to bridge connectivity to the device by means of capillary networks. Here, local connectivity is provided by means of a short-range radio access technology — Wi-Fi, Bluetooth or 802. 15. 4/6LoWPAN, for example. Wireless connectivity beyond the local area is then provided by the mobile network via a gateway node.
Critical MTC refers to applications such as traffic safety and control, control of critical infrastructure and wireless connectivity for industrial processes. Such applications require high reliability and availability in terms of wireless connectivity, as well as low latency. On the other hand, low device cost and energy consumption are less critical for massive MTC applications. Although the average volume of data transported to and from devices may be small, wide instantaneous bandwidths are useful in meeting capacity and latency requirements (see Figure 2).
There is much to gain from a network’s ability to handle as many different applications as possible — including mobile broadband, media delivery and a wide range of MTC applications — by means of the same basic wireless access technology and within the same spectrum. This avoids spectrum fragmentation. And it also allows operators to offer support for new MTC services for which the business potential is inherently uncertain without having to deploy a separate network and reassign spectrum specifically for these applications.
To support increased traffic capacity and enable the transmission bandwidths needed to support high data rates, 5G will extend the range of frequencies used for mobile communication. This includes new spectrum below 6 GHz, as well as spectrum in higher frequency bands.
The ITU-R and individual regulatory bodies have yet to identify specific candidate spectrum for mobile communication in higher frequency bands. The mobile industry remains agnostic about particular choices, and the entire frequency range up to approximately 100 GHz is under consideration at this stage — though there is significant interest in large contiguous allocations that can provide dedicated and licensed spectrum for use by multiple competing network providers.
The lower part of this frequency range, below 30 GHz, is preferred from the point of view of propagation properties. At the same time, large amounts of spectrum and the possibility of wide transmission frequency bands of the order of 1 GHz or more are more likely above 30 GHz.
Spectrum relevant for 5G wireless access therefore ranges from below 1 GHz up to approximately 100 GHz, as Figure 3 shows.
It is important to understand that high frequencies, especially those above 10 GHz, can only serve as a complement to lower frequency bands. And they will mainly provide additional system capacity and wide transmission bandwidths for extreme data rates in dense deployments. Spectrum allocations at lower bands will remain the backbone for mobile communication networks in the 5G era, providing ubiquitous wide-area connectivity.
The World Radio Conference (WRC)-15 discussions have resulted in an agreement to include an agenda item for IMT-2020, the designated ITU-R qualifier for 5G, in WRC-19. The conference also reached agreement on a set of bands that will be studied for 5G, with direct applicability to NX. Many of the proposed bands are in the millimeter-wave region and include:
· 24.25 GHz to 27. 5 GHz, 37 GHz to 40.5 GHz, 42.5 GHz to 43.5 GHz, 45.5 GHz to 47 GHz, 47.2 GHz to 50.2 GHz, 50.4 GHz to 52.6 GHz, 66 GHz to 76 GHz and 81 GHz to 86 GHz, which have allocations to the mobile service on a primary basis
· 31.8 GHz to 33.4 GHz, 40.5 GHz to 42.5 GHz and 47 GHz to 47.2 GHz, which may require additional allocations to the mobile service on a primary basis
The mobile industry will strive to gain access to spectrum in the 6 GHz to 20 GHz range, but regulators seem to be following policy directions focused on frequency bands above 30 GHz. In the United States, the FCC has issued two Notices of Proposed Rule Making (NPRM) on bands above 24 GHz. Ofcom has likewise indicated a preference for bands above 30 GHz within the mobile industry.
Licensed spectrum will continue to serve the capacity needs of the mobile industry, though novel sharing arrangements for spectrum will become progressively more important as restricted opportunities for new spectrum start to affect incumbent services such as satellite communication and radiolocation. Two examples of sharing arrangements include licensed shared access (LSA) planned in Europe for the 2. 3-GHz band and the Citizens Band Radio Service for 3.5 GHz in the United States.
Beyond extending operation to higher frequencies, there are several other key technology components relevant for the evolution to 5G wireless access. These components include access and backhaul integration, device-to-device communication, flexible duplex, flexible spectrum usage, multi-antenna transmission, ultra-lean design and user/control separation.
Access and Backhaul Integration
Wireless technology is frequently used as part of the backhaul solution. Such wireless backhaul solutions typically operate under line-of-sight conditions using proprietary radio technology in higher frequency bands, including the millimeter-wave band.
In the future, the access (base-station-to-device) link will also extend to higher frequencies. Furthermore, to support dense, low-power deployments, wireless backhaul will have to extend to cover non-line-of-sight conditions, similar to access links.
In the 5G era, the wireless access link and wireless backhaul should not be seen as two separate entities with separate technical solutions. Rather, backhaul and access should be seen as an integrated wireless access solution able to use the same basic technology and operate using a common spectrum pool. This will lead to more efficient overall spectrum use as well as reduced operation and management effort.
The possibility of limited direct device-to-device (D2D) communication has recently been introduced as an extension to the LTE specifications. In the 5G era, support for D2D as part of the overall wireless access solution should be considered from the start. This includes peer-to-peer user-data communication directly between devices, but also, for example, the use of mobile devices as relays to extend network coverage.
D2D communication in the context of 5G should be an integral part of the overall wireless access solution, rather than a stand-alone solution. Direct D2D communication can be used to offload traffic, extend capabilities and enhance the overall efficiency of the wireless access network. Furthermore, direct D2D communication should be under network control to avoid uncontrolled interference to other links. This is especially important in the case of D2D communication in licensed spectrum.
Frequency-division duplex (FDD) has been the dominating duplex arrangement since the beginning of the mobile communication era. In the 5G era, FDD will remain the main duplex scheme for lower frequency bands. However, time-division duplex (TDD) will play a more important role for higher frequency bands — particularly those above 10 GHz — targeting extremely dense deployments.
In dense deployments with low-power nodes, the TDD-specific interference scenarios (direct base-station-to-base-station and device-to-device interference) will be similar to the normal base-station-to-device and device-to-base-station interference that also occurs for FDD.
Furthermore, for the dynamic traffic variations expected in dense deployments, the ability to dynamically assign transmission resources (time slots) to different transmission directions may allow for more efficient use of the available spectrum.
To reach its full potential, 5G should therefore allow for flexible and dynamic assignment of TDD transmission resources. This stands in contrast to current TDD-based mobile technologies, including TD-LTE, for which there are restrictions on the downlink and uplink configurations — and for which there typically exist assumptions about the same configuration for neighbor cells and neighbor operators.
Flexible Spectrum Use
Since its inception, mobile communication has relied on spectrum licensed on a per-operator basis within a geographical area. This will remain the foundation for mobile communication in the 5G era, allowing operators to provide high-quality connectivity in a controlled-interference environment.
However, per-operator spectrum licensing will be complemented with the possibility of spectrum sharing. Such sharing may be between a limited set of operators, or it may occur in license-exempt scenarios. The U.S. Citizens Broadband Radio Service in the 3.5-GHz band and the 5-GHz unlicensed spectrum are examples of managed and unlicensed sharing regimes, respectively.
New air interfaces such as NX are most likely to be well served by more conventional licensed allocations of spectrum. This is mainly because there is a need to establish a basic foundation for the technology to operate in an independent manner while interoperability is established with technologies such as LTE. At some point, further allocations of spectrum for 5G may benefit from the mobile industry’s experience regarding sharing approaches in lower cellular bands.
Multi-antenna transmission already plays an important role in current mobile communication generations, and it will be even more central in the 5G era because of the physical limitations of small antennas. Path loss between a transmitter and receiver does not change as a function of frequency, so long as the effective aperture of the transmitting and receiving antennas does not change. The antenna aperture does reduce in proportion to the square of the frequency, and the use of higher antenna directivity can compensate for that reduction. The 5G radio will employ hundreds of antenna elements to increase antenna aperture beyond what may be possible with current cellular technology.
In addition, the transmitter and receiver will use beamforming to track one another and improve energy transfer over an instantaneously configured link. Beamforming will also improve the radio environment by limiting interference to small fractions of the entire space around a transmitter, likewise limiting the effect of interference on a receiver to infrequent stochastic events. Beamforming will also prove to be an important technology for lower frequencies; for example, to extend coverage and to provide higher data rates in sparse deployments.
Ultra-lean radio-access design is important to achieve high efficiency in 5G networks. The basic principle of ultra-lean design can be expressed as follows: Minimize any transmissions not directly related to the delivery of user data. Such transmissions include signals for synchronization, network acquisition and channel estimation, as well as the broadcast of different types of system and control information.
Ultra-lean design is especially important for dense deployments with a large number of network nodes and highly variable traffic conditions. However, lean transmission is beneficial for all kinds of deployments, including macro deployments.
Its ability to enable network nodes to enter low-energy states rapidly when there is no user data transmission makes ultra-lean design an important component in delivering high network energy performance. Ultra-lean design will also enable higher achievable data rates by reducing interference from non-user-data-related transmissions.
User and Control Separation
Another important design principle for 5G is the decoupling of user data and system control functionality. The latter includes the provisioning of system information — that is, the information and procedures needed for a device to access the system.
Such a decoupling will allow separate scaling of user-plane capacity and basic system control functionality. For example, user data may be delivered by a dense layer of access nodes, while system information is only provided via an overlaid macro layer on which a device also initially accesses the system.
It should be possible to extend the separation of user data delivery and system control functionality over multiple frequency bands and radio access technologies (RATs). For example, the system control functionality for a dense layer based on new high-frequency radio access could be provided by means of an overlaid LTE layer.
User and control separation is also an important component for future radio access deployments relying heavily on beamforming for user data delivery. Combining ultra-lean design with a logical separation of user-plane data delivery and basic system connectivity functionality will enable a much higher degree of device-centric network optimization of the active radio links in the network. Because only the ultra-lean signals related to the system control plane need to be static, it is possible to design a system in which almost everything can be dynamically optimized in real time.
An ultra-lean design combined with a system control plane logically separated from the user data delivery function also provides higher flexibility in terms of evolution of the RAT as, with such separation, the user plane can evolve while retaining system control functionality.
5G is the next step in the evolution of mobile communication and will be a key component of the Networked Society. In particular, 5G will accelerate the development of the internet of things. To enable connectivity for a wide range of applications and use cases, the capabilities of 5G wireless access must extend far beyond those of previous generations. These capabilities include very high achievable data rates, very low latency and ultra-high reliability. Furthermore, 5G wireless access needs to support a massive increase in traffic in an affordable and sustainable way, implying a need for a dramatic reduction in the cost and energy consumption per delivered bit.
5G wireless access will be realized by the evolution of LTE for existing spectrum in combination with new radio access technologies that primarily target new spectrum. The key technology components of 5G wireless access include access and backhaul integration, device-to-device communication, flexible duplex, flexible spectrum usage, multi-antenna transmission, ultra-lean design, and user and control separation.
Afif Osseiran is director of radio communications for industry area telecom at the Ericsson CTO Office in Stockholm.