StdDev Figure 14.30 RTP delay variations caused by UE aggregating RTP packets over 10-minute period. Step changes are caused by UE switching between 20 and 40 ms scheduling; used AMR codec bitrate = 23.85 kbps Also eNodeB may decide to aggregate RTP packets in downlink direction resulting in similar impact. While there is certainly a gain in reduced PDCCH usage, it adds to the delay variations and jitter. As there seem to be many variables resulting in variations to MOS figures, such as jitter buffer size in UE, use of sound equalizers to compensate the quality of the tiny speaker in UE and even volume setting in some cases, it is impossible to compare networks by using absolute MOS values. However, it is evident that controlling the delay is an important factor to maintaining good voice quality. As smaller peak delays (less than 60 ms) in this particular case, shown in Figure 14.30, seemed to have little or no impact on POLQA MOS, it is not surprising that HARQ retrans- missions correlate poorly with MOS. The impact of HARQ BLER variations was verified in live network with moderate non-GBR loading and relatively low GBR loading to have no correlation with average POLQA MOS as shown in Figures 14.31 and 14.32. Packet Loss During the testing it was also found that RTP Packet Loss Ratio seemed to have no or very small impact on POLQA MOS as is shown in Figure 14.33. MOS is impacted by both packet loss and delay variations, but it seems to be a bit unclear whether dropping a packet after certain delay is a better option than putting a lot of emphasis delivering it too late. Coverage Impact on Voice Quality The previously shown measurement results in this chapter concentrated on voice quality within reasonable coverage where UL BLER can be maintained at low level. When the pathloss increases there comes a point where BLER cannot be maintained any longer. Therefore, the increased number of retransmissions leads to increased delays and jitter and the probability of dropping RTP packets increase. LTE Small Cell Optimiza