TTI Bundling

With all the hype created around IMS and LTE, operators have started questioning network vendors if they are supporting RAN specific features for VoLTE. TTI bundling is one of the features among many others that can help VoIP (VoLTE) calls in LTE.

TTI Bundling is LTE feature to improve coverage at cell edge or in poor radio conditions. UE has limited power in uplink (only 23dBm for LTE) which can result in many re transmissions at cell edge (poor radio). Re transmission means delay and control plan overhead which may not be acceptable for certain services like VoIP. To understand TTI bundling one need to have the basic idea of Hybrid Automatic Repeat Request (HARQ) and Transmission Time interval (TTI).

HARQ

HARQ is a process where data at mac layer is protected against noisy wireless channels through error correction mechanism. There are couple of different versions of HARQ but in LTE we have a type known as 'Incremental Redundancy Hybrid ARQ'. When receiver detects erroneous data, it doesn't discard it. On the other hand, sender will send the same data again but this time, with different set of coded bits. The reciever will combine the previously recieved erroneous data with newly attempted data by the sender. This way the chances of successfully decoding the bits improve every time. This will repeat as long as the receiver is not able to decode the data. The advantage of this method is  that with each re-transmission, the coding rate is lowered. Whereas in other types of HARQ, it might use the same coding rate in every re-transmission

TTI

TTI is LTE smallest unit of time in which eNB is capable of scheduling any user for uplink or downlink transmission. If a user is receiving downlink data, then  during each 1ms, eNB will assign resources and inform user where to look for its downlink data through PDCCH channel. Check the following figure to understand the concept of TTI

Now coming to TTI Bundling ...

HARQ is a process where receiver combines the new transmission every time with previous erroneous data. There is one drawback however, that it can result in delay and too much control overhead in case of poor radio conditions if the sender has to attempt many transmissions. For services like VoIP this means bad end user experience. Well, there is another way- Instead of re-transmitting the erroneous data with new set of coded bits, why not send few versions (redundancy versions) of the same set of bits in consecutive TTI and eNB sends back Ack when it successfully decodes the bits. I hope  the figure below will make it clear. This way we are avoiding delay and reducing control plane overhead at mac layer