# Quality of Experience in 6G Networks: Outlook and Challenges
## 2 Quality of Experience in the Cellular Evolution
### The Fifth Generation--Generation Software  
ITU's IMT-2020 specifications set the minimum downlink for 5G networks to be 20 Gbps. In response, various technologies have been introduced in Release 15 and beyond to make this requirement viable. For example, 5G New Radio (5G) allows access to the mmWave spectrum (24-100 GHz range) in addition to the original sub-6-Ghz spectrum that is commonly shared with 4G networks [37]. This dual access allows for both range (for the sub-6-Ghz) as well as speed and frequency reuse (for the mmWave). Evolutions of new radio (NR) are discussed in Releases 16 and 17 (respectively in refs. [2; 3]). The 20 Gbps requirement will be a significant step towards an enhanced mobile broadband service (eMBB). This is one of three significant services expected in 2020 and beyond, with the other two being mMTC and URLLC. The expectation is that the eMBB will be introduced with the initial rollouts of 5G (i.e., based on Release 15), with the other two introduced in the later releases (i.e., Releases 16 and 17). The emergence of mMTC came as a response to the increasing dependence on IoT applications and the IMT-2020 requirements on dense connectivity and low power consumption. Earlier instances (e.g., NB-IoT and LTE-M) have been defined in earlier releases (i.e., Releases 13 and 14), with NB-IoT addressing bursty IoT communications with low-to-no delay requirements and LTE-M focusing on delay-sensitive applications. With the introduction of 5G NR, substantial support will be made for further deployment models and service requirements. Coexistence measures (both between NB-IoT, LTE-M, and mMTC and between mMTC and eMBB) are made in Release 16. The emphasis on URLLC is that it supports both high mobility (i.e., up to 500 km/h) and low latency. A central aim of URLLC is support for vehicular-to-anything (V2X) communications and intelligent transportation systems, including railway communications. However, URLLC is also directed at services such as AR/VR, remote monitoring, tactile Internet, smart grids, Industry 4.0, and Industrial IoT (IIoT). Through a service view, the combined offering of eMBB, mMTC, and URLLC stresses the need for great flexibility on the part of the network, especially at the core. This need resulted in two critical actions. The first is a strong dependence on softwarization in 5G core; the second is introducing an analytics-based element in the network. In 5G networks, softwarization essentially allows for easily instantiating services across the network in a manner that harmonizes resource allocation while maintaining stability [38]. To support this, a dependence on a two-tier cloud computing hierarchy (i.e., cloud-edge) allows for slicing the physical network resource towards different services, e.g., eMBB, mMTC, and URLLC. Variants on these services can also be realized--even ones that are geographically localized. This results in \"campus solutions\" (i.e., a localized 5G instance in an office, factory, residential, stadium, etc.) that renders a different service offering depending on the localized requirements [39]. In addition to this, softwarization will make it easier to satisfy IMT-2020 latency requirements through its reliance on edge servers. The 5G core will include a network data analytics function (NWDAF) [19] to complement the increased dependence on resource slicing. This function will help manage the increasing network complexity, especially towards realizing autonomous policy-based (or intent-based) management. 5G networks will be rolled out in phases, with the initial phase exhibiting a change in the radio interface. This entails connecting 5G NR to the 4G network's EPC, and this initial stage is called the nonstandalone phase. In the standalone phase, 5G NR will be connected to the 5G core, and a dual-interface phone would be able to perform handover (or roaming) between the two networks. The rollout of the first phase (based on Release 15) is already taking place.