Difference between Harq and TTI bundling?

Hi Expert

Can you please explain what are the major difference between harq and tti bundling?

Harq results in a single Retx after each NACK or DTX
Tti bundling a combination of TX and 3 retx in 4 consecutive TTIs


TTI bundling:


TTI Bundling
In normal case, when network send a grant (DCI 0), UE transmit PUSCH at only one specific subframe (4 ms after the DCI 0 reception). TTI Bundling is a method in which UE transmit a PUSCH in multiple subframes in a row (4 subframes according to current specification). In other words, UE transmit a PUSCH in a ‘BUNDLED TTI’.

HARQ bundling:
ETSI TS 136 213 V10.6.0 (2012-07) page 69:
if two transport blocks are received on a serving cell, the HARQ-ACK bit for the serving cell is generated by spatially bundling the HARQ-ACK bits corresponding to the transport blocks;

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TTI bundling enables the data to be transmitted in consecutive TTIs as a single resource during the scheduling process. It makes use of HARQ gains. The HARQ is a technique when the receiver gets a new data with some errors then it tries to make correction if the error is minor, but if the error is not minor then it sends re-transmission request to the sender. … The ARQ mechanism takes care of residual errors which passed from HARQ

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