Specified in TS 36.321
TTI bundling is a feature of LTE like other enhancements happened. TTI bundling triggered by UE informing the eNB about its power limitations at the present state.
When the base station detects that the mobile can't increase it's transmission power and reception is getting worse it can instruct the device to activate TTI bundling and send the same packet but with different error detection and correction bits in 2, 3 or even 4 consecutive transmit time intervals.
The advantage over sending the packet in a single TTI and then detecting that it wasn't received correctly which in turn would lead to one or more re-transmissions is that it saves a lot of signaling overhead. Latency is also reduced as no waiting time is required between the re-transmissions. In case the bundle is not received correctly, it is repeated in the same way as an ordinary transmission of a packet.
UL resources over multiple TTI’s can be assigned with a single grant (same format as for ordinary HARQ),
- decreases the signaling overhead
There is only one HARQ feedback message per bundle. requires less re-transmissions
- Low control signaling and the vulnerability to NACK to-ACK errors that can lead to data loss.
Due to no segmentation, RLC & MAC overhead headers as well as CRC is decreased
- more efficient use of radio resources.
Improves quality delay sensitive services, such as VoIP
Why do we need this?
The purpose of TTI Bundling is to improve cell edge coverage and in-house reception for voice.In LTE the UE has limited uplink power only 23dBm 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. This feature has more relevance for TDD over FDD as coverage issues are likely to be more challenging in TD-LTE. Simulation results reported in publications indicate a 4 dB gain due to TTI bundling on the UL.
How it is implemented ?
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'.
Typical case of TTI Bundling can be illustrated as shown below.
How to enable ttiBundling for an specific UE?
You just have to Enable a
ttiBundling IE as shown below:
- The granularity to match the number of transmissions exactly to required amount decreases.
- With segmentation, when a single segment is decoded, retransmissions of that segment can be stopped. For TTI bundling always a bundle of retransmissions is done.
- For that reason TTI bundling should be used only for the UEs that are at the cell edge.