Newsflash: METIS-II views and considerations on 5G RAN Architecture and Functional Design

METIS-II white paper
Published on March 8th, 2016

METIS-II released a white paper on March 8th, 2016. It summarizes the initial views and considérations of METIS-II on 5G RAN Architecture and Functional Design. The paper starts by listing the main service types that are considered for 5G, namely extreme mobile broadband (xMBB), massive machine-type communications (mMTC) and ultra-reliable machine-type communications (uMTC), as well as the five specific use cases towards which METIS-II is performing the 5G RAN design, and which typical represent a mixture of services. It further describes the key requirements on the 5G RAN architecture that have been identified and derived from the diverse service and use case needs, and explicitly elaborates on the requirements posed by the notion of Network Slicing in 5G.

METIS-II is a EU-funded project within the framework of the 5th Generation Public Private Partnership (5G PPP).

Wide range of spectrum bands
METIS-II envisions the overall 5G RAN to operate in a wide range of spectrum bands to address the diverse services, for instance considering frequencies below 6 GHz as likely most suitable to support mMTC services with high coverage requirements, and spectrum above 6 GHz as essential to provide the massive capacity demanded by xMBB applications. Studies have shown that large contiguous spectrum bands are preferable for various reasons, in particular related to device complexity. In general, the 5G system will build upon a set of spectrum usage forms such as the use of dedicated licensed spectrum, horizontal sharing of bands with differentiation according to limited spectrum pools, mutual renting and unlicensed use, as well as vertical sharing of bands.

The overall 5G air interface to comprise multiple so-called air interface variants (AIVs)
Due to this wide range of bands and the stated service diversity, METIS-II envisions the overall 5G air interface (AI) to comprise multiple so-called air interface variants (AIVs), including the evolution of existing radio such as Long Term Evolution Advanced (LTE-A) and novel AIVs introduced in 5G, which may be tailored towards specific bands, cell types or services. For example, two AIVs designed for bands below 3 GHz and above 60 GHz, respectively, may be distinct in terms of frame structure, the importance of beamforming and related handling of control signals etc. The precise waveform(s) and physical layer (PHY) numerologies to be used for novel AIVs, which may be derived from various waveform families such as orthogonal frequency division multiplex (OFDM) or filterbank multi-carrier (FBMC) based solutions, are still under investigation. Multiple hypotheses are currently being pursued related to the overall AIV landscape: for example, multiple waveform families may jointly cover the space of bands and services, or a single waveform family may be tailored to cover all bands and services.

To which extent different AIVs can be harmonised towards a single AI protocol stack specification
A key question w.r.t. the overall RAN design is to which extent different AIVs can be harmonized towards a single AI protocol stack specification in order to reduce implementation and standards complexity and improve cost-efficiency for devices having to implement multiple AIVs. Regarding novel AIVs introduced in 5G, METIS-II is currently investigating the following three different kinds of harmonization (listed here in no particular order): 1) PHY harmonization of novel 5G AIVs that are potentially based on different waveform families (e.g. OFDM and FBMC based solutions), 2) Medium access control (MAC) layer (or higher) harmonisation towards a single specification that supports the usage of different waveform families on PHY layer, and 3) scaling of a single waveform (or waveform family) across all 5G frequency bands to support all 5G use cases. Among LTE-A evolution and novel 5G AIVs, on the other hand, the benefits of harmonization have to be weighed against the potential legacy constraints imposed towards novel air interface technology.

UP aggregation on layer 2, novel AIV on PDCP level
Beyond harmonization, METIS-II investigates to which extent user plane (UP) instances related to different bands can be logically aggregated on certain layers, and beyond which layer there would be a single control plane (CP) instance. In this respect, the preliminary assumption is that for the integration between multiple novel 5G AIVs, UP aggregation could take place on layer 2 i.e. MAC, radio link control (RLC) or packet data convergence protocol (PDCP) level, likely dependent on the physical network architecture, and likely with one common radio resource control (RRC) instance w.r.t. the CP. It is further assumed that the 5G RAN should allow to integrate LTE-A evolution and novel 5G radio technology on RAN level, even though this may not be done in all scenarios. Among various options which are being investigated in this context, a “UP aggregation among LTE-A evolution and novel 5G AIVs on PDCP level so far appears to be the most viable option.

Split between core network and RAN, shared common interface between core network and RAN
Regarding the overall 5G architecture, METIS-II envisions a logical split between core network (CN) and RAN, taking initial orientation in the 3GPP Evolved Packet System (EPS), though it is considered to move some functionalities from CN to RAN, for instance related to paging. It is further assumed that LTE-A evolution and novel 5G radio share common CN functions, and hence also share a common interface between CN and RAN.

Paradigm changes in resource management in 5G
In the context of a wide range of services and bands, and novel communication scenarios such as flexible time division duplex (TDD), device-to-device (D2D) communications, and moving cells, METIS-II is considering various paradigm changes related to resource management in 5G, as for instance the extension of the notion of a resource beyond conventional radio resources towards different types of access nodes along with their extensions and soft capabilities of network entities. Furthermore, context-aware interference management schemes are envisioned that minimize the dependency on inter-node interfaces, as well as more sophisticated Quality-of-Service (QoS) management schemes than in legacy systems, for instance with a mechanism in the RAN that translates air-interface-agnostic to air-interface-specific QoS metrics.

Changes in designing system access and mobility management procedures
Finally, METIS-II is considering various changes in 5G compared to legacy systems that will impact the design of system access and mobility management procedures. For instance, the introduction of a novel RRC “Connected Inactive is being discussed, allowing for faster state lean system design is being evaluated, through a minimization of “always-on” signals such as reference signals and system information and potentially self-contained transmission. Various further concepts are being investigated, related to paging, random access channel (RACH) for differentiated access, and a beam-centric system design, as elaborated in this paper.

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