TTI Bundling in LTE

TTI Bundling:- Today we are going to discuss an LTE feature to improve mainly VoLTE performances at cell edges or in poor radio conditions.

Need to TTI Bundling?- TTI bundling as its name suggest is a bundling(together as a package) of TTI(smallest unit of time in which eNodeB is scheduling the UE).

       In normal scenario(without TTI bundling) at cell edges or in poor RF condition, the UE informs eNodeB about its power getting maximum value (UE has limited power 23dBm in LTE) therefore the chances of re-transmission of Voice Packets is increasing(Multiple NACK). Which further leads in delay of VoIP packets arriving(It is not acceptable in case of Voice call).

Note: Normally 1 HARQ retransmission is taking a delay of 8ms.

How it Works?- In TTI bundling we are sending a transport block multiple times in a consecutive subframe(Normally 4) without waiting for HARQ ACK/NACK messages. A combined ACK/NACK can be sent after processing all the transmissions of a transport block.

                     Actually, in TTI Bundling, we are sending multiple redundancy(4) versions of the same set of bits in consecutive(Bundled) TTI and eNodeB sends back ACK when it successfully decodes the data. This way we are avoiding delay (Which is mainly due to RTT of HARQ).

Note: TTI Bundling uses QPSK modulation.