HARQ is a technique where receiver gets and combines the new transmitted data with previous erroneous data, but not all the time this method is convenient if the UE is in poor radio condition. Transmitter needs to re-transmit the data and wait for ACK/NACK and of course it will cause too much control overhead and delay. In VoLTE service the re-transmissions might be serious bad experience for user.
TTI Bundling is a LTE feature which improves cell edge uplink coverage, and it is used for services like VoLTE. In TTI Bundling mechanism UE sends same data with different redundancy versions (PUSCH) in multiple consecutive sub-frames and gets only one ACK/NACK for the entire bundle.
HARQ: Hybrid Automatic Repeat Request
TTI: Transmission Time Interval
PUSCH: Physical Uplink Shared Channel
ACK: Acknowledgement
NACK: No Acknowledgement