TTI Bundling

TTI bundling is a feature mainly used for extending VoIP (or other real-time traffic) uplink cell edge coverage. If a UE is at cell edge, due to its limited available transmit power, it can't transmit on many RBs, otherwise the power per RB would be too low to achieve a successful tranmission. In this case, TTI bundling can be activated (via RRC signalling). The benefits of TTI bundling are:

- uplink resources over multiple TTIs can be granted via a single grant;

- There is only one HARQ feedback message per bundle

- Less RLC/MAC overhead because RLC SDUs are not segmented

TTI boundling take the RLC SDU, make several redundancy versions corresponding to the entire RLC SDU and then transmit them over several (2,3 or 4) consecutive TTIs. Only when the last redundancy version of the transport block is received by eNB, the HARQ feedback is sent. This is similar to IR in HARQ, but transmit before receiving the HARQ NACK.

Unlike TTI boundling, HARQ re-transmission could also improve the probability of successful reception. However, re-tranmission means overhead (HARQ ACK/NACK) and more importantly, a significant delay (8ms on per re-transmission). Therefore, TTI boundling is more desirable especially for real-time applications.

RLC layer could also segment the RLC SDU into smaller RLC PDUs, which can be transmitted via lower MCS, therefore also help to achieve successful transmission on the uplink. However, RLC segmentation has a few drawbacks, including more RLC/MAC header overhead, more control signalling (resource grant and HARQ), and possibly a VoIP packet loss due to HARQ feedback error (note RLC is UM here).

For details please refer to:

"LTE coverage improvement by TTI bundling" Riikka Susitaival, Michael Meyer,

Ericsson Research