s are most likely caused by two consecutive HARQ retransmissions. Typical drive test for MOS analysis contains series of, for example, 6-second-long audio samples where each sample gets analysed by the POLQA MOS algorithm. Analysed results are stored together with other measured statistical and radio parameters for further analysis. During drive test data collection for MOS analysis, jitter is one of the commonly collected measurement values. As jitter averages the delay, it may not indicate long individual delays VoLTE Optimization RTP delay difference (ms) Jitter (RFC3550) Sample no. Figure 14.26 Jitter variation as a function of measured delay differences between RTP packets where one individual packet arrives 110 ms late; used AMR codec bitrate = 23.85 kbps well. In the example shown in Figure 14.26, measurement collection was done over 170 RTP packets, jitter provided average of 2.9 ms while highest jitter value over the period is about 9 ms even though this measurement period contains a single peak delay value of 80 ms. POLQA MOS value in this example case was relatively poor with a value of 2.5. No RTP packets were lost indicating that even just one RTP packet having relatively long delay may have high impact on the overall MOS figure. When the impact of peak delay within a MOS sample was analysed using very large number of 6-second-long MOS samples in varying radio conditions, a correlation between the peak delay and MOS value became evident as can be seen in Figure 14.27. The larger the RTP RTP peak delay (ms) Figure 14.27 MOS versus peak RTP delay during repeated 6 seconds MOS sample. Average MOS per peak delay is calculated and compared to the average MOS within 20 ms RTP peak delay. Jitter buffer size in UE is unknown; used AMR codec bitrate = 23.85 kbps LTE Small Cell Optimization 20 ms 20 ms 20 ms Figure 14.28 Twenty-millisecond RTP scheduling by UE peak delay within the MOS sample, the more the average MOS per peak RTP delay value decreases from the average MOS (calculated as average MOS with RTP p