# QoE Optimization in a Live Cellular Network through RLC Parameter Tuning
## 6 Proof of Concept
### Simulated Scenario
#### 6.1.2 Results and Discussion
The difference between the optimal buffer sizes for the different values of bandwidth available in the system is motivated by the fact that the more bandwidth is available in the system, the more data can be transmitted by the video server in each TTI. This means that in each TTI, more data of the RLC UM transmission buffer can be sent, and therefore, the increase of the delay produced by the accumulation of packets in the buffer will occur for a larger buffer size. For a constant server delivery rate, the lower the bandwidth available in the system, the more losses will occur since the RLC UM transmission buffer will fill more quickly. As a result, the users will experience a worse QoE when the bandwidth available in the system is lower. This results in smaller MOS values when the available bandwidth in the system is 5 MHz and larger values when the available bandwidth in the system is 10 MHz. In Figure 5, it is shown that the MOS value obtained in the experiments performed during the optimization process converges to a maximum value. With 5 MHz system bandwidth, the maximum value reached by the MOS is 3.32, while with 10 MHz system bandwidth, it is 4.28. It is also possible to observe in Figure 5 that less MOS improvement is found for the 10 MHz use case with respect to the 5 MHz case. This is because, in the 10 MHz use case, the starting conditions are good (the network has more resources available). In the file transfer service case, to configure the TM-based controller, the t-StatusProhibit and the t-PollRetransmit were selected as factors or parameters to optimize and the E2E throughput as the response metric to maximize. The initial range of values to be evaluated for t-StatusProhibit spanned from 0 ms to 600 ms, whereas the initial range of values to be evaluated for t-PollRetransmit spanned from 5 ms to 600 ms. The number of levels configured for each of these parameters was 3. The optimization process was terminated when the step size of all input parameters was less than or equal to 0.2. This value was calculated using heuristic methods, reaching a balance between model accuracy and pro  
Figure 4: Simulated scenario—RLC UM transmission buffer adjustment. cessing time. As the file transfer service presented more relaxed quality requirements than the real-time video streaming service, in this case, the bandwidth values selected to show the benefits of adjusting the RLC AM configuration parameters over the QoE presented by the file transfer service clients were 1.4 MHz and 3 MHz. QoE degradation in a file transfer service may be associated with a misconfiguration of RLC AM parameters. In RLC AM, control packets (STATUS PDUs) have priority in transmission over data packets (AMD PDUs). Likewise, retransmissions of AMD PDUs have priority over sending new AMS PDUs. The sending of an excessive number of STATUS PDUs and retransmissions can cause an increase in the delay of sending new information and, therefore, a deterioration in the QoE perceived by the user. As mentioned above, the t-PollRetransmit and t-StatusProhibit timers are used to control the frequency with which STATUS PDUs are sent. On the one hand, if these timers have very small values, the sending of STATUS PDUs will be very frequent, producing an increase in the delay of sending AMD PDUs. On the other hand, if the values of these timers are very large, STATUS PDUs will be sent infrequently, which can cause the transmission window to collapse and, therefore, lead to an increase in the packet loss rate. Figures 6-8 show the results of the experiments performed during the optimization process for bandwidths of 1.4 MHz and 3 MHz. From these figures, it can be inferred that the TM-based controller works correctly, providing the values of the RLC AM configuration parameters (t-StatusProhibit and t-PollRetransmit) that maximize the E2E throughput. These values are, in the 1.4 MHz case, 18 ms for t-PollRetransmit and 34.05 ms for t-StatusProhibit, and in the 3 MHz case, 56.12 ms and 267.52 ms.