LTE: TTI Bundling

Why Bundling?

TTI Bundling optimizes the uplink (cell edge) coverage for services like VoLTE. For services such as VoIP, the packet size is small and the inter-arrival time of VoIP packets is constant (i.e., AMR codec provides one packet every 20ms during active period and one silence indicator (SID) at 160ms). In a cell edge scenario, the probability of decoding PUSCH as error is more so instead of waiting for HARQ feedback from eNB, the UE performs initial transmission and re-transmissions in 4 consecutive Transmit Time Intervals (TTIs)

Once TTI Bundling is enabled/activated the UE send the same packet but with different error detection and correction bits in 4 consecutive TTIs

TTI Bundling reduces a lot of signalling overhead as the eNB doesn’t need to send Ack/Nack for every (re-) transmission instead it sends Ack/Nack for the entire bundle

Latency is also reduced as no waiting time (for HARQ feedback) is required between the re-transmissions

The simulation results indicated that TTI bundling provides a cell edge gain of more than 4 dB in terms of the sustainable path loss

TTI Bundling Configuration

• Configuration of TTI Bundling is dedicated i.e., per UE basis. TTI Bundling might be configured by eNB in situation like UE is operating at its maximum output Tx Power
• TTI Bundling IE is under RadioResourceConfigDedicated => mac-MainConfig => ul-SCH-Config
• TRUE indicates that TTI bundling is enabled while FALSE indicates that TTI bundling is disabled.
• TTI bundling can be enabled for FDD and for TDD only for UL/DL configurations 0, 1 and 6
• For TDD, E-UTRAN does not simultaneously enable TTI bundling and Semi-Persistent Scheduling (at least until rel-10)
• E-UTRAN does not simultaneously configure TTI bundling and SCells with configured uplink (uplink Carrier Aggregation)
• Default configuration of TTI Bundling is ‘release’
• The UE indicates the support of TTI Bundling with FGI 28

 TTI Bundling Operation

Initial Bundle Transmission

For DCI0 detected in subframe n-4, the UE shall transmit the first PUSCH in subframe n and non-adaptive retransmissions in n+1, n+2, and n+3 with corresponding Redundancy Versions set as 0, 2, 3, and 1

UE shall expect Ack/Nack in subframe n+7

Bundle re-Transmission

For DCI0 and/or Nack detected in subframe n-9, the UE shall adjust the first PUSCH transmission in n and non-adaptive retransmissions in n+1, n+2, and n+3 with corresponding Redundancy Versions set as 0, 2, 3, and 1

UE shall expect Ack/Nack in subframe n+7

• Number of TTIs in a bundle is always equal to 4
• Each transmission/re-transmission of a bundle uses same HARQ Process
• Within a bundle HARQ re-transmissions are non-adaptive and triggered without waiting for feedback for previous transmissions
• The HARQ feedback of a bundle is only received for the last TTI of the bundle regardless of whether a transmission in that TTI takes place or not (e.g. when a measurement gap occurs). Say for example, in above figure, measurement gap starts in subframe#1 (the measurement GAP is until subframe#6) ⇨ The UE should still expect HARQ feedback in subframe#7 even though (re-) transmissions in subframes #1, #2, and #3 didn’t actually occur
• A retransmission of a TTI bundle is also a TTI bundle. As shown in the figure above, after receiving NACK at subframe#7, the UE performs retransmission as a bundle starting from subframe #16
• For FDD, when the TTI bundling is enabled, there shall be 4 uplink HARQ Processes. For TDD, there are 3 HARQ processes for UL/DL Configuration 0 and 6 whereas 2 HARQ processes used in case of UL/DL Configuration 1
• For transmission of PUSCH that corresponds to Msg3 TTI bundling does not apply