Variable TTI Bundling for FDD Latency Reduction
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Solution Overview
Problem
In wireless communication systems, particularly in LTE, the re-transmission of data due to errors caused by poor RF conditions leads to increased latency as the transmitter waits for HARQ responses, and conventional TTI bundling does not efficiently manage data transmission and acknowledgement processes.
Innovation Solution
The method involves variable bundle sizes and non-consecutive subframe transmissions with intervening subframes, allowing for early acknowledgement and cancellation of subsequent data transmissions, thereby reducing latency and resource usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If data is re-transmitted multiple times until successful reception, then reliability of data transmission is improved, but latency increases due to waiting periods for HARQ responses
Solution Approach 1:
The transmitter pre-determines a bundle size N and schedules N transmissions in advance with predetermined intervals, rather than waiting for HARQ responses between transmissions. This preliminary planning allows multiple transmission attempts to occur back-to-back without idle waiting periods, reducing latency while maintaining reliability through multiple redundancy opportunities
2Reliability
If TTI bundling transmits the same data four times in four consecutive TTIs, then robustness of data reception is improved, but latency is reduced only compared to individual transmissions with waiting periods
Solution Approach 1:
The bundle size N is made variable rather than fixed at four transmissions. The transmitter can adapt N dynamically based on channel conditions, QoS requirements, and traffic patterns. This dynamic adjustment allows optimization between reliability and latency trade-offs, enabling smaller bundle sizes when latency is critical and larger bundle sizes when robustness is prioritized
3Duration of action of stationary object
If transmissions occur in consecutive subframes, then continuity of transmission is improved, but resource efficiency deteriorates due to inability to cancel unnecessary transmissions
Solution Approach 1:
The system introduces feedback mechanisms where HARQ responses or scheduling grants received during the transmission sequence can trigger cancellation of remaining transmissions. This feedback-driven cancellation allows the system to maintain transmission continuity when needed while avoiding wasteful use of channel resources when data has already been successfully received or retransmission is no longer required
4Reliability
If bundle size N is increased for more transmission attempts, then reliability is improved, but channel resource consumption increases
Solution Approach 1:
The bundle size N is changed from a fixed value to a variable parameter that can be adjusted based on channel quality indicators, historical transmission success rates, QoS requirements, and current network load. This parameter adaptation allows the system to use larger bundle sizes only when necessary for reliability while conserving channel resources during good channel conditions or when latency is critical
Data Source
AI summary
A base station and a user equipment (UE) device may communicate over an air interface using a frequency division duplexing (FDD) configuration that provides a time sequence of consecutive subframes for both uplink and downlink communications. A transmitter (either the UE device or the base station) may select a sequence of N subframes for N transmissions of the same data to a receiver using transmission time interval (TTI) bundling, wherein successive subframes in the sequence are separated by one or more subframes. The transmitter may transmit the data M times (where M<N) and may receive an acknowledgement during the one or more subframes between subframe M and subframe M+1 in the sequence that the receiver has successfully received the data. In response to the acknowledgement, the transmitter may cancel transmission of the data in each subframe of the sequence occurring after subframe M.