# Data-Driven Construction of User Utility Functions from Radio Connection Traces in LTE
## 4 Estimation of QoS Thresholds on a Per-Service Basis
### Step 2: Estimation of Minimum Qos Thresholds
Figure 1: Expected impact of user behavior for broad service classes. In full-buffer data services, all data are available at the beginning of the connection, meaning that the associated traffic pattern consists of a few, very long bursts in which data are transmitted at full speed. Thus, the user terminal demands as many resources as possible until all the data are transmitted. It is assumed here that the user tends to give up the session when the download time exceeds a certain threshold. Such an action should be reflected both in connection duration and traffic volumes per connection, as shown in Figure 1a. The x-axis represents the mean DL PDCP connection throughput, measured only considering active (and non-last) TTIs, which are selected as the QoS indicator with the largest impact on QoE for these services. The primary x-axis represents the connection duration, while the secondary y-axis represents the total DL data volume per the connection. The solid curve represents the median of the distribution of connection duration, whereas the dashed line represents the median of the distribution of the total DL data volume. For clarity, the shaded area labeled as the degraded state comprises connections whose link conditions are unacceptable for the user, which are more likely to be interrupted. As observed in the figure, it is expected that users will try to maintain a connection until a maximum duration is reached. On the right of the figure, as the link performance improves, the connection duration is reduced, since data are transmitted faster. In contrast, the data volume per connection remains constant, since it is not conditioned by link performance beyond a certain point (i.e., the user ends the connection before downloading the complete data). Thus, the minimum QoS threshold, \(TH_{min}\), in full-buffer data services is estimated as the average DL PDCP throughput below which connection duration drops. Streaming services are also affected by user throughput, meaning that the selected QoS indicator is again DL PDCP throughput. However, a different behavior is expected for connection duration and data volume. Streaming sessions consist of long connections with large data volume distributed in many bursts. Unlike full-buffer data services, streaming services are elastic, meaning that a good link performance does not necessarily lead to a reduction of session duration. Thus, connection duration may not be a good QoS indicator to reflect user behavior. Instead, DL session throughput, calculated by dividing the total DL data volume by the connection duration (including silent periods), may reflect the quality of the downloaded material. Figure 1b shows the expected impact of user behavior for streaming services, representing the relationship between DL PDCP throughput and DL session throughput. The solid line represents the median session throughput and the shaded area defines the degraded state. As shown, in the degraded state, the session throughput decreases as the DL PDCP throughput decreases. Once the DL PDCP throughput is good enough, the session throughput remains constant, showing that the latter is not conditioned by the former. Thus, the minimum QoS threshold, \(TH_{min}\), for streaming services is the value of the DL PDCP throughput below which the median session throughput starts to decrease. In a VoLTE service, the connection duration is the most representative indicator for the characterization of user behavior. However, unlike full-buffer data, the QoS indicator with the strongest impact on QoE is packet delay. Figure 1c shows the expected impact of user behavior in VoLTE by representing the variation of connection duration caused by changes in DL packet delay. As in previous sub-figures, the solid line represents the median of connection duration and the shaded area is the degraded state. It is observed that the median connection duration should drop when the DL packet delay increases above a certain limit.