TTI Bundling in FDD and TD-LTE - Sekhar - Expert Opinion - LTE University

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TTI Bundling in FDD and TD-LTE1Sekhar 13 Aug 2012 9:02 PM

Abstract

TTI Bundling has been introduced in FDD and TD-LTE to improve Uplink coverage. In this blog, we shall present thesalient features of TTI Bundling with respect to FDD and TD-LTE. We will first explore the motivation for TTI Bundlingand then compare it with other techniques. Finally, we shall address the implementation of TTI bundling in FDD andTD-LTE .

Introduction

TTI bundling is a technique used to send a transport block multiple times in consecutive subframes without waiting forHARQ ACK/NACK messages. Normally, a transport block is converted to multiple redundancy versions after codingand the first redundancy version is sent in a subframe. Subsequent transmissions of the transport block aredependent on the HARQ ACK/NACK which is sent 4 subframe durations later or more after the first transmission. InTTI bundling, the different redundancy versions can all be sent in consecutive subframes without waiting for theHARQ ACK/NACK feedback and a combined ACK/NACK can be sent after processing all the transmissions of atransport block.

The motivation for TTI bundling which is illustrated in Figure 1 is the low transmission power of some handsets, shortTTI length, and the long RTT of the HARQ transmissions. TTI bundling is expected to improve the UL coverage ofapplications like VOIP over LTE wherein low power handsets are likely to be involved. This feature has morerelevance for TDD over FDD as coverage issues are likely to be more challenging in TD-LTE. Simulation resultsreported in publications indicate a 4 dB gain due to TTI bundling on the UL.

TTI Bundling in FDD and TD-LTE - Sekhar - Expert Opinion - LTE U... http://lteuniversity.com/get_trained/expert_opinion1/b/sekhar/archive/...

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Alternative Approach

The alternative to TTI bundling is RLC segmentation wherein a VOIP payload is split into smaller size RLC PDUs asshown in Figure 2. The smaller RLC PDUs will result in smaller transport blocks which can be decoded with betteraccuracy. One drawback of this method is the potential overhead increase due to RLC segmentation due to multipleRLC headers needed. For a typical VOIP payload, it has been shown that as we increase the segmentation factorfrom 1 to 8, the overhead increases from 14% to 55%. Each RLC PDU which is mapped into a transport block willneed a separate PDCCH assignment message which will contribute to control signal overhead for such a scheme. There might be retransmissions of each of those transport blocks which will also potentially increase the controlsignaling overhead. In addition, since we are transmitting many small transport blocks, the chances of interpreting aNACK as a ACK also increases proportionately with the increase in the RLC segmentation size. Hence, RLCsegmentation has many disadvantages when we consider the transmission of a VOIP like payload from a powerlimited terminal.


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Overview of TTI Bundling

TTI bundling is used to achieve successful transmissions from power limited terminals. The process as shown inFigure 3 is typically triggered by UE informing the eNB about its power limitations at the present state. This could forexample happen at the edge of a cell when the terminal has to send high power but is limited by the power capabilityof the terminal. This triggers the eNB to transmit the various redundancy versions of the same transport block inconsecutive subframes or TTIs giving rise to the name TTI bundling. A single PDCCH allocation is sufficient for themultiple transmissions thus saving control overhead as compared to the RLC segmentation approach. A singleHARQ ACK/NACK for the combined transmissions is generated after processing the TTI bundle which can reduce theerror rate of the transport block as compared with processing a single redundancy version. This approach can alsoreduce the delay in the HARQ process compared to transmissions of the redundancy versions separated in timeusing the normal approach.

TTI Bundling Operation

As shown in Figure 4, TTI bundling enables up to 4 redundancy versions of the same transport block to be sent in 4consecutive subframes. In TD-LTE systems, the TDD configurations standardized allow only a maximum of 3consecutive UL subframes. A single RLC PDU is transmitted as multiple redundancy versions in consecutivesubframes using a single common allocation. The channel coding used in LTE enables easy generation of the


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multiple redundancy versions from which the transmissions in the TTI bundle are generated. A common RLC headeris shared across the TTI bundle and the same HARQ process identity is used for multiple transmissions in the TTIbundle. Combined processing of the redundant transmissions over multiple subframes leads to a better probability ofdetection of the transport block. Thus, with limited power, the UE has a better chance of a successful transmissionwith lesser latency using the TTI bundling method.

Summary: TTI bundling is a useful technique for improving coverage of VOIP handsets in LTE systems. It isapplicable to both FDD and TD-LTE deployments and can improve the link budget by up to 4 dB. Differences inimplementation exist between FDD and TD-LTE systems. TTI bundling helps achieve good latency performance forVOIP even at the edges of cells.


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TTI Bundling in FDD and TD-LTE - Sekhar - Expert Opinion - LTE University