TTI Bundling for better HARQ and Latency
TTI bundling enables a data block to be transmitted in consecutive TTIs, which are packed together and treated as the same resource during the scheduling process. TTI bundling makes use of HARQ gains and reduces the number of re-transmissions and round trip time (RTT). When user’s radio channel quality is degraded or the transmit power is limited (indication that UEs is closer to the cell edge), the TTI bundling can be triggered to improve the uplink coverage at cell-edge, reduce the number of different transmission segments at the RLC layer and the re-transmission overhead.
For UEs at cell-edge RLC packet is segmented first and mapped to MAC Transport Block (TB), then TTI bundling transmit same packets four times in one scheduling period to extend the coverage by increasing uplink transmission reliability. The eNodeB decides when to activate or deactivate the TTI bundling for certain users based on the measured signal-to-interference noise ratio (SINR) and PRB usage on uplink. The activation and deactivation is done by sending RRC reconfiguration message from eNodeB to UE.
The data block in a bundle of TTIs, where the chunk of each bundle (up to 4 chunks), is modulated with different redundancy versions within the same HARQ identity. In the case of unsuccessful decoding of the HARQ identity, the eNodeB sends NACK the UE, which requires re-transmission. The resource allocation during this operation is restricted to a certain number of PRB and transport block size (TBS) in order to improve the probability of decoding at lower data rates.
TTI Bundling as Procedure
The mechanism to transmit same packets four times in one scheduling instance expands the coverage by increasing the uplink transmission reliability with a better success rate gain. In addition, it guarantees that VoLTE packets are transmitted at cell-edge when resources are limited to improve the latency in bad radio conditions. TTI bundling is estimated to provide 2 – 4 dB uplink coverage improvement for VoLTE services which extend the cell radius for VoLTE service. Figure below describes the TTI bundling mechanism and provide comparison with a scheduling mechanism that depends only on RLC segmentation procedures.
The main advantage of TTI bundling is enhancing VoLTE uplink coverage when the UE has limited uplink transmit power. Thus, it guarantees VoLTE QoS for cell-edge users.
In a conventional scheduling mechanism, if the UE is not able to accumulate sufficient power to transmit a small amount of data, like a VoIP packet, the data packets can be segmented into smaller size packets that fit within the UE transmit power. Each segment will be transmitted with a separate HARQ process. This segmentation mechanism increases the amount of control information that needs to be transmitted resulting in the control channel load increases with the amount of segments as every segment requires new transmission resources on these channels. Additionally, at cell-edge the probability for HARQ NACK increases with the number of segments causing higher uplink BLER . Therefore, the need to utilize better segmentation method like TTI Bundling is important.
TTI Bundling Configuration
- Its Configuration is dedicated i.e. individual UE basis
- TTI Bundling might be configured by eNodeB in situation like UE is operating at its maximum output Tx Power
- TTI Bundling IE is sent under RadioResourceConfigDedicated —> mac-MainConfig —> ul-SCH-Config
- TRUE indicates that it is enabled while FALSE means it is disabled.
- It can be enabled for FDD and for TDD can be enabled for DL/UL configurations 0, 1 and 6 only
- For TDD, eNodeB does not simultaneously enable TTI bundling and SPS (at least until rel-10)
- eNodeB does not simultaneously configure TTI bundling and SCells with configured uplink (uplink CA)
- Default configuration of TTI Bundling is ‘release’
- The FGI bit 28 indicates that UE support it
Below figure shows RRC reconfiguration for TTI bundling.
- VoLTE Cell Capacity- Calculating Packet Size, PRBs and No. of Users
- Voice Codec Options for VoLTE Services
- VoLTE SIP Methods, Response Codes and Details
- Robust Header Compression – ROHC