–Bundling–
(noun) the selling of different items, typically of hardware or software, together as a package.
“the bundling of the browser and the operating system”

TTI bundling in LTE does not escape from the definition above. LTE uses HARQ (Hybrid Automatic Repeat Request) to deal with air interface transmission errors. HARQ is a combination of Automatic Repeat Request (ARQ) with Forward Error Correction (FEC). In other words, the data wrongly received is retransmitted while new data is added. In order to know if the transmission has errors or not, ACK/NACK flags are utilized. This system works well with normal data, but is not enough for delay sensitive services such as VoLTE. With TTI bundling the UE can use in a single grant for PUSCH, 4 consecutive TTI for transmitting while is in a VoLTE session. The data the UE will transmit in these four consecutive TTI will be exactly the same but will differ one of the other by the redundancy version (RV).

TTI bundling process

TTI Bundling Process

  1. The scheduler (at the eNB) will determine if the UE that is using VoLTE service can benefit from TTI bundling or not. This decision is taken based on the channel quality of the UE. Normally SINR is used to establish the threshold of when to use and when not to use TTI Bundling. While TTI bundling is good when the UE is in poor radio conditions, it is not if the UE is in a good radio environment.
  2. If the UE is using VoLTE and in bad radio condition then the RRC signalling is configured so that TTI Bundling can be used.
  3. The UE then will transmit data into four consecutive TTI using a single grant PUSCH.
  4. After the eNB receives the last TTI it will submit and HARQ feedback message for the entire bundle.
  5. Data is retransmitted within 16 ms

Pros and Cons of TTI Bundling

While the UE has a VoLTE service, this feature is very beneficial as the overhead of RLC and MAC using TTI bundling is much less than the one generated in normal RLC segmentation. This reduces delay, which is critical for VoLTE.
The downside is that the UL throughput is reduced. TTI bundling transmissions are limited to QPSK modulation and the resource assignment is just 3 PRBs, which impacts negatively UL throughput.

Cheers!

Diego Goncalves Kovadloff

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