# QoE Optimization in a Live Cellular Network through RLC Parameter Tuning
## 6 Proof of Concept
### Simulated Scenario
#### 6.1.2 Results and Discussion  
In the real-time video streaming service case, to configure the TM-based controller, the transmission buffer size of the RLC UM was selected as a factor or parameter to optimize and the MOS as the response metric to maximize. The initial range of values to be evaluated for the transmission buffer size ranged from 1 kB to 100 kB. For this configuration parameter, three levels were defined. The TM stop criterion was set to 0.5 as, with this value, a tradeoff between the optimization accuracy and the processing time was achieved. In the simulated scenario for very low system bandwidth values, the network would not have enough resources to meet the quality requirements of the real-time video streaming service in an acceptable way, and very low MOS values would be obtained at all times. Otherwise, when the bandwidth available in the system was very large in the simulated scenario, the network would be able to comfortably meet the quality requirements of the real-time video streaming service, minimizing the impact of the RLC layer's configuration on QoE. For this reason, two intermediate bandwidth values were selected in order to observe the benefits of a proper adjustment of the transmission buffer size of the RLC UM on the QoE of the real-time video streaming service: 5 MHz and 10 MHz. In an environment in which the packet generation rate is higher than the packet delivery rate, the QoE deterioration in a real-time video streaming service may be associated with the size of the RLC UM transmission buffer. On the one hand, if the RLC UM \begin{table}
\begin{tabular}{c c c} \hline  & **Real-Time Video** & **File Transfer** \\  & **Streaming Service** & **Service** \\ \hline Scenario & & 2 LTE cells \\  & & 10 users in each cell \\  & & Distance between eNBs 100 m \\ \hline System bandwidth & 5 and 10 MHz & 1.4 and 3 MHz \\ \hline RLC mode & UM & AM \\ \hline RLC parameters & Transmission buffer & t-StatusProhibit: \\  & size: 1–100 kB & 0–600 ms \\  & & t-PollRetransmit: \\  & & 5–600 ms \\ \hline Mobility & Static users & \\ \hline Transmission direction & Download & \\ \hline Services & Studied user & Real-time video & File transfer \\  & & streaming service & service (file 5 MB) \\  & & (MPEG-4, 3.6 MB, 30 fps) & \\ \hline Other users & Constant bitrate & Constant bitrate \\ \hline e-NodeB & Omnidirectional antennas & \\  & & SISO, EIRP\({}_{max}\)= 30 dBm \\ \hline Scheduler & Proportional Fair & \\ \hline Transmission Time & & 1 ms \\ Interval (TTI) & & \\ \hline Maximum delay & 150 ms & - \\ allowed in real-time & & \\ video service & & \\ \hline \end{tabular}
\end{table} 
Table 1: Configuration of ns-3 scenario. transmission buffer size were too small, a collapse of the buffer will be produced, causing the discarding of packets due to the lack of available space in the buffer. This would lead to an increment in the packet loss rate and, therefore, to a degradation in the QoE for the user. On the other hand, if the RLC UM transmission buffer size were too large, there would be an increase in the delay of sending packets due to the accumulation of packets in the buffer. In real-time services, packets that accumulate more than a certain delay are discarded. In this way, an excessive size of the RLC UM transmission buffer would also cause an increase in packet losses and, therefore, the deterioration of the QoE. In Figure 4, the buffer size used in each of the experiments performed during the optimization process is depicted. It can be observed that the optimization algorithm progressively reduces the search range for the transmitter buffer size values until it finds the value with which the user experiences the maximum QoE. With a 5 MHz system bandwidth, the optimal value for the transmitter buffer size is 4 kB, while with a 10 MHz system bandwidth, this value increases to 47 kB.