Towards a flexible harmonised 5G air interface with multi-service, multi-connectivity support

Current 4G air interfaces (AIs), by which we mean existing cellular standards and their evolution, including internetworking with other technologies such as WLAN, lack the flexibility required for support of multiple and diverse services envisaged for 5G, such as Massive Machine-Type Communications – mMTC, Ultra-reliable Machine-Type Communications – uMTC, and Extreme Mobile Broadband – xMBB, with their diverging and sometimes conflicting requirements. Evolutions of 4G AIs may meet some of the KPIs of individual 5G services, but not the key 5G requirement to natively integrate multi-service support. The evolutionary alternative would hence result in the deployment of multiple 4G AI evolutions, leading to a very heterogeneous system – a mix of different RATs (Radio Access Technologies). With ambitious 5G goals for support of services with different (and often diverging) requirements, a highly flexible 5G air interface design will be required to answer this demand. One way to solve these issues is to harmonize the air interface variants with the goal to find an optimal compromise between potentially highly specialized solutions for specific services, and the broader goal to only have one air interface supporting multiple services. This paper will describe the approach adopted by EU H2020 / 5G-PPP project “METIS-II” for a harmonized 5G AI and its impact on protocol stack level. In particular, we will:  Show how AIs in currently deployed networks do not meet many of the 5G performance requirements, such as multi-connectivity, support for a wide range of frequencies, and multi-service support;  Elaborate the concept of harmonisation of different AIs towards a common user plane design and capture our key design principles;  Present the most promising set of AI variants with respect to identified KPIs and identify potential AI variants to be integrated in the 5G system; and  Perform an initial analysis of harmonisation possibilities. Keywords—air interface design, multi-connectivity, multiservice support, cellular protocol stack, 5G waveforms, user plane design