TTI(Transmission Time interval) Bundling FDD

The packet bundling that is done in LTE, which concatenates several packets into one transmission burst thereby saving a lot of air interface overhead.
The purpose of TTI Bundling is to improve cell edge coverage in poor radio conditions. 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 retransmissions is that it saves a lot of signaling overhead. Latency is also reduced as no waiting time is required between the retransmissions. In case the bundle is not received correctly, it is repeated in the same way as an ordinary transmission of a packet.

Or else

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 transmission. In this case, TTI bundling can be activated on UE and eNodeB by RRC.

or else

HARQ is a process where receiver combines the new transmission every time with previous erroneous data. There is one drawback however, that it can result in delay and too much control overhead in case of poor radio conditions if the sender has to attempt many transmissions. For services like VoIP this means bad end user experience. Well, there is another way- Instead of re-transmitting the erroneous data with new set of coded bits, why not send few versions (redundancy versions) of the same set of bits in consecutive TTI and eNB sends back Ack when it successfully decodes the bits.

The benefits of TTI bundling are:

1. Coverage achieved without delay in packet reception at eNB
2. Uplink resources over multiple TTIs can be granted via a single grant;
3. There is only one HARQ feedback message per bundle

TTI bundling take the MAC TB, make several redundancy versions corresponding to the entire MAC TB and then transmit    them   over 4 consecutive TTIs. Only when the last redundancy version of the transport block is received by eNodeB, the HARQ feedback is sent.

How TTI bundling would be enabled/disabled

•  The decision to enable/disable TTI bundling would be taken by RRM based on the pathloss and PHR reports received from eNodeB MAC.
• RRM then sends this information to RRC which further sends RRCConnectionReconfiguration message to UE with TTI bundling field enabled/disabled. When UE sends RRCConnectionReconfigComplete then RRC will send  MAC_RECONFIGURE_UE_ENTITY_REQ to eNB-MAC with TTI bundling enabled/disabled.
• L3 should also check if UE supports the TTI bundling (bit-3 for R8 UEs and bit-28 for R9 UEs in Feature Group Indicators).

 

How does things work when a UE is TTI bundling enabled

When a UE is TTI bundling enabled then for each new grant received it transmit a MAC TB for consecutive 4 ticks.

Example:

First transmission of a new data bundle is a new data transmission with RV=0 where rest 3 transmission are non-adaptive retransmissions without waiting for HARQ with RV=2, 3, 1. In this case RTT of UE would be 16 and number of HARQ process is 4. HARQ is received at UE for last TTI of a bundle. If NACK is received then the retransmission will happen at +9 TTI from the reception of NACK and if the retransmission of adaptive then DCI0 is sent at +5 TTI from the reception of NACK. Retransmission of a bundle is also bundle.

Following is a sequence diagram illustrating with a NACK use cases-

When TTI bundling is configured, the parameter TTI_BUNDLE_SIZE provides the number of TTIs of a TTI bundle. TTI bundling operation relies on the HARQ entity for invoking the same HARQ process for each transmission that is part of the same bundle. Within a bundle HARQ retransmissions are non-adaptive and triggered without waiting for feedback from previous transmissions according to TTI_BUNDLE_SIZE. The HARQ feedback of a bundle is only received for the last TTI of the bundle (i.e the TTI corresponding to TTI_BUNDLE_SIZE), regardless of whether a transmission in that TTI takes place or not (e.g. when a measurement gap occurs). A retransmission of a TTI bundle is also a TTI bundle. TTI bundling is not supported when the UE is configured with one or more SCells with configured uplink.

36.321 7.5 TTI_BUNDLE_SIZE value

The parameter TTI_BUNDLE_SIZE is 4.