# Quality of Experience in 6G Networks: Outlook and Challenges
## 2 Quality of Experience in the Cellular Evolution
### QoE in the First Four Generations
#### 2.1.2 QoE in 3G Networks  
The earlier instances of 3G deployments (around 2001) coincided with a boom in Internet usage. Personal Digital Assistants (PDAs) and phones with QWERTY interfaces were already introduced at the time. However, smartphone-like devices were introduced only around 2005, together with the earlier deployments of the first evolution to 3G, namely high-speed downlink packet access (HSDPA). Remarkably, all 3G evolutions were primarily software upgrades to the initial Universal Mobile Telecommunications System (UMTS) infrastructure [26].  
The last (pre-LTE) evolution in 3G networks saw downlink data rates up to 56 Mbps. Simultaneously, the network core underwent a change towards the IP multimedia subsystem (IMS), whereby a view of data-switched communications was introduced based on the session initiation protocol (SIP). This transformation facilitated a stronger coupling between cellular network cores and the Internet at large. It also allowed for portable mobility between different radio interfaces (e.g., cellular to WiFi).  
3G networks exhibited a shift in terms of involving user impression of the service in network management. Network management consideration relied on a quality of service (QoS)-based view. Estimates are made of the network performance in terms of delay, jitter, call drop ratio, etc. The network operation is amended or provisioned to enhance its performance. However, complementing voice with multimedia in 3G resulted in expanding consideration to a QoE-based view [27]. A vital motivator of this shift was a twofold emerging realization at the time: Enhancements in networks cannot be done in the absence of understanding the impact on user perception and of recognizing that factors other than network performance may affect user experience.  
Specifically, QoE is concerned with the user perception of the delivered service. The work in [28] takes a long view of QoE's development over the years and how it migrated from its inception in the context of TV communications to more generalized aspects. The authors converge on a definition that QoE is \"the degree of delight or annoyance of a person whose experiencing involves an application, service, or system. It results from the person's evaluation of the fulfillment of his or her expectations and needs with respect to the utility and/or enjoyment in the light of the person's context, personality, and current state\". The 2017 updated ITU-T definition is similar in stating that QoE is \"the degree of delight or annoyance of the user of an application of service\", with an additional note that while \"recognizing ongoing research on this topic, this is a working definition which is expected to evolve for some time\" [29].  
The natural subjectivity of these QoE measures is generally not overlooked, neither in terms of \"evaluation of the fulfillment\", \"expectations and needs\", nor a \"person's context, personality, and current state\". Various efforts have been made over the years to objectify (or standardize) the QoE measure to address this. These include the objective MOS techniques noted above. They also include efforts to identify how values on a particular QoE scale can translate into specific network performance (e.g., [30] or [31]). A more comprehensive review of the matter can be found in [32].