4G radio has been built for low latency and high data rates, but delivering proper coverage needs further optimization procedures.
Adaptive multirate (AMR) voice packets arrive every 20 ms and can be transmitted in 1 ms TTIs. This allows the terminal power amplifier to transmit only for a short time which makes uplink coverage problematic. 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 is a method in which UE transmit a PUSCH in multiple subframes in a row (4 subframes according to current specification).
TTI bundling and retransmissions for improving uplink coverage
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.