Uplink Reference Signal Density Reduction for Low SNR MTC Coverage
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Solution Overview
Problem
Current LTE systems face challenges in supporting Machine-Type Communications (MTC) due to low Signal-to-Noise Ratio (SNR), as the existing DMRS configuration is designed for medium or high SNR, leading to increased overhead when trying to improve channel estimation accuracy for MTC UEs.
Innovation Solution
A method and apparatus for uplink data transmission that reduces the density of reference signals in the frequency domain, allowing for coverage enhancement by using unused resource elements, and optionally increases density in the time domain, with the option of power boosting, to improve channel estimation accuracy for MTC UEs in low SNR conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the density of reference signals is increased to improve channel estimation accuracy in low SNR, then the accuracy of channel estimation is improved, but the overhead increases and coding rate increases
Solution Approach 1:
The patent transitions from increasing reference signal density in frequency domain to increasing density in time domain. Specifically, it uses multiple adjacent uplink subframes (e.g., 2, 4, or 8 subframes) to transmit reference signals, thereby achieving improved channel estimation accuracy through temporal repetition rather than frequency domain density increase. This dimensional shift resolves the contradiction by maintaining low overhead while improving measurement precision.
Solution Approach 2:
The patent implements periodic transmission of reference signals across multiple uplink subframes. The reference signals are transmitted periodically with a configured period (e.g., every 2, 4, or 8 subframes), allowing the receiver to accumulate channel estimation information over time. This periodic temporal repetition improves channel estimation accuracy without requiring increased frequency domain density, thus avoiding overhead increase.
2Measurement precision
If the density of reference signals is increased in time domain, then the accuracy of channel estimation is improved, but the resource overhead and coding rate increase
Solution Approach 1:
The patent introduces dynamic configurability of the reference signal period, allowing the system to adapt the density of reference signals in time domain based on channel conditions and service requirements. The period can be dynamically adjusted (e.g., 2, 4, or 8 subframes) to balance between channel estimation accuracy and system throughput, resolving the contradiction by making the reference signal density flexible rather than fixed.
Solution Approach 2:
The patent changes the temporal parameter (period) of reference signal transmission to optimize the balance between channel estimation accuracy and system throughput. By adjusting the period parameter, the system can adapt to different SNR conditions and service types, achieving improved measurement precision when needed while maintaining high productivity when channel conditions are good.
3Reliability
If MTC UEs are supported in current LTE release with low SNR, then the coverage is extended to low SNR UEs, but the existing DMRS configuration is not suitable and requires modification
Solution Approach 1:
The patent segments the uplink transmission timeline into multiple subframes for reference signal transmission. Instead of using a single DMRS configuration, the system divides the transmission into periodic reference signal instances across multiple subframes, each carrying channel estimation information. This segmentation allows low SNR UEs to accumulate sufficient signal energy for reliable channel estimation without requiring complete redesign of the DMRS configuration.
Solution Approach 2:
The patent performs preliminary channel estimation by transmitting reference signals in advance across multiple uplink subframes before actual data transmission. This preliminary action allows the receiver to build up channel state information over time, improving the reliability of low SNR UE support while maintaining compatibility with existing LTE DMRS configurations.
Data Source
AI summary
Embodiments of the present disclosure relate to uplink data transmission in a wireless communication system. In an embodiment of the present disclosure, there is provided a method for uplink data transmission in a wireless communication system. The method comprises transmitting an indication for new-type reference signals to a user equipment, wherein the new-type reference signals have a reduced density of reference signals in frequency domain, and resource elements not used by reference signals in symbols for the new-type reference signals are used for coverage enhancement; receiving the new-type reference signals; and performing channel estimation based on the new-type reference signals. With embodiments of the present disclosure, it may reduce the density of reference signals in frequency domain, which means the overhead will not be increased substantially and besides saved resource element may be used to implement the coverage enhancement. Thus, the number of retransmission of MTC UE in low SNR could be further reduced, which in turn results in the improvement of the system throughput. Accordingly, it is possible for a UE with a low SNR to be used in LTE networks.