t methods is POLQA (perceptual objective listening quality assessment) which was adopted as ITU-T P863 [6]. It is particularly developed for HD Voice and is a successor of P862 PESQ which is not suitable for higher bandwidth voice codecs such as G722.2, which is also called WB-AMR. The POLQA is formed by OPTICOM, SwissQual and TNO, the group of companies who won the competitive standardization process run by the ITU-T, and KPN, who owns patents on the TNO technology [7]. Measuring MOS can be subjective to implementation of VoLTE in UE. Differences, such as the size of Jitter Buffer in the UE can lead to different MOS figures with same jitter profile LTE Small Cell Optimization RTP delay difference (ms) Jitter (RFC3550) Sample no. Figure 14.25 Jitter variation as a function of measured delay differences between consecutive RTP packets in relatively constant conditions; used AMR codec bitrate = 23.85 kbps when measured using different UEs. This can make the comparison between networks difficult if not impossible as the results may not be comparable. The factors mostly impacting the quality of the voice are the selection of the codec, used bandwidth, packet loss ratio, jitter and excessive delay of an individual RTP packet as well as the implementation of VoLTE in the UE. As VoLTE voice traffic is usually delivered using UM on RLC layer, only the optimization of the scheduler, delay budget algorithms and ensuring reliable delivery of the packets can provide a difference to the quality of the voice from the LTE network point of view. Delay The measure often used to indicate delay variations profile is jitter and it is specified by IETF in RFC3550 [8] (0-1)+100-1.01-10-1) This works very well to indicate delay variations when they are maintained in relatively small range, example of which is shown in Figure 14.25. The picture shows the delay difference from the expected 20 ms inter-packet delay - ideal case here equals to 0 ms. The spikes at around 8 ms mark are caused by a single HARQ retransmission while ~16 ms case