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Voice over LTE (VoLTE) by Antti Toskala, Juha Kallio, Jukka Hongisto, Harri Holma, Miikka Poikselkä

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Chapter 7

VoLTE Radio Performance

This section considers the radio coverage, radio capacity and end to end latency which are key factors for end user voice call quality and for operator network dimensioning.

7.1 Coverage

Long term evolution (LTE) radio has been designed for low latency and high data rates, but providing good coverage requires further optimisation steps. Adaptive multirate (AMR) voice packets arrive every 20 ms and can be transmitted in 1 ms transmission time intervals (TTIs) which allows the terminal power amplifier to transmit only for a short time making uplink coverage a problem. The optimal solution for coverage would allow the terminal to have continuous transmission. The solution in LTE is to use TTI bundling where the same data is repeated in four consecutive TTIs. The other solution is to use layer 1 retransmissions which also increases the effective transmission time. Combining these two solutions also makes LTE coverage competitive for the voice service. TTI bundling and retransmissions are illustrated in Figure 7.1.

Figure 7.1 TTI bundling and retransmissions for improving uplink coverage. Reproduced with permission from John Wiley & Sons, Ltd.

7.1

TTI bundling requires terminal support. If TTI bundling is not supported, another option is to use medium access control (MAC) segmentation which does not need any specific terminal support. This solution splits ...

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