There are several key 5G technology components relevant for the evolution to 5G wireless access.
Multi-Antenna Transmission
Multi-antenna transmission already plays an important role for current generations of mobile communication and will play an even more important role in the 5G era.
Especially for operation at higher frequencies, the use of multiple antennas for beam-forming at the transmitter and/or receiver site is a critical component to counter the worse propagation conditions at higher frequencies.
However, beam-forming will also be an important component for lower frequencies; for example, to further extend coverage and to provide higher data rates in sparse deployments.
Ultra-Lean Design
Ultra-lean radio-access design is important to achieve high efficiency in future wireless-access networks. The basic principle of ultra-lean design can be expressed as: 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.
By enabling network nodes to rapidly enter low-energy states when there is no user-data transmission, ultra-lean design is an important component for high network energy performance. Ultra-lean design will also enable higher achievable data rates by reducing interference from non-user-data-related transmissions.
User/Control Separation
Another 5G technology components for future wireless access is to decouple user data and system control functionality. The latter, for example, includes the provision of system information; that is, the information as well as the 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, a layer on which a device also initially accesses the system.
The separation of user data delivery and system control functionality should be possible to extend over multiple frequency bands and RATs. As an 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/control separation is also an important component for future radio-access deployments relying heavily on beam-forming 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.
Since only the ultra-lean signals related to the system control plane need to be static, it is possible to design a system where 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 the system control functionality.
Flexible Spectrum Usage
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 and 5G technology components, allowing operators to provide high-quality connectivity in a controlled-interference environment.
However, per-operator licensing of spectrum will be complemented with the possibility to operate under other spectrum regimes. This may include sharing of spectrum between a limited set of operators, as well as operation in unlicensed spectrum. Deviating from conventional per-operator spectrum licensing will mainly be relevant in frequency bands above 10GHz.
In high-frequency bands, the focus will be on very wide transmission bandwidths. It may be difficult to find sufficiently large spectrum blocks to allow for per-operator-dedicated spectrum supporting such bandwidths for multiple operators.
Furthermore, high-frequency bands will typically be used for very dense deployments for which one can expect much more dynamic traffic variations. Statically dividing the spectrum between different operators may, in such situations, not necessarily lead to the most efficient spectrum usage.
Rather, making it possible for operators to jointly access at least part of the spectrum in a dynamic way could, potentially, lead to more efficient overall spectrum utilization.
Flexible Duplex
FDD is also part of the essential 5G technology components. FDD has been the dominating duplex arrangement since the beginning of the mobile communication era. For lower frequency bands, FDD will remain the main duplex scheme in the 5G era.
However, for higher frequency bands, especially above 10GHz, targeting very dense deployments, TDD will play a more important role.
In very dense deployments with low-power nodes, the TDD-specific interference scenarios (direct base-station-to-base-station and device-to-device interference) will be more 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 very dense deployments, the ability to dynamically assign transmission resources (time slots) to different transmission directions may allow more efficient utilization of the available spectrum.
Therefore, to reach its full potential, 5G should allow for very flexible and dynamic assignment of the TDD transmission resources.
This is in contrast to current TDD-based mobile technologies, including TD-LTE, for which there are restrictions on the downlink/uplink configurations, and for which there typically exist assumptions about the same configuration for neighbor cells and also between neighbor operators.
Direct Device to Device Communication
The possibility for limited direct device-to-device (D2D) communication has recently been introduced as an extension to the LTE specifications. As part of 5G technology components, support for D2D 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. The possibility for direct D2D communication should extend the capabilities and enhance the overall efficiency of the wireless-access network.
Furthermore, in order to avoid uncontrolled interference to other links, direct D2D communication should be under network control. This is especially important for the case of D2D communication in licensed spectrum.
Access/Backhaul Integration
Wireless technology is already frequently used as part of the backhaul solution and is the last remaining of 5G technology components. Such wireless-backhaul solutions then typically operate under line-of-sight conditions using proprietary radio technology in higher frequency bands, including the millimeter wave (mmW) 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 therefore 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 utilization as well as reduced operation and management effort.
