LTE Coverage Improvement by TTI Bundling

Compared to WCDMA, the LTE radio access has a significantly shorter Transmission Time Interval (TTI) in order to reduce end-to-end delays. However, if a User Equipment (UE) at the cell edge is limited by its available transmission power, it may not be able to transmit an entire VoIP packet during one TTI, since the instantaneous source data rate is too high. Thus TTI bundling has been recently introduced as a feature of LTE Rel. 8 to improve the uplink coverage. In TTI bundling, a VoIP packet is transmitted as a single PDU during a bundle of subsequent TTIs without waiting for the HARQ feedback. HARQ feedback is only expected for the last transmission of the bundle. This paper studies TTI bundling and compares it to the conventional RLC segmentation. The simulation results indicate that TTI bundling provides a gain of more than 4 dB in terms of the sustainable path loss.