LDM Interleaving for Subcarrier Spacing and Doppler Robustness
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Solution Overview
Problem
Existing multiplexing methods like TDM and FDM are not efficient for sharing a medium, and legacy LDM systems face challenges in maintaining subcarrier spacing and robustness against Doppler spread, especially when serving both stationary and mobile receivers simultaneously.
Innovation Solution
Implementing separate interleaving stages for LDM compound cells carrying cells of more than one layer and those carrying cells of only one layer, with up-sampling and specific interleaving processes to maintain subcarrier spacing and enhance Doppler robustness, using techniques like row-column and pseudo-random interleaving.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional interleaving techniques are applied to LDM compound signals, then data transmission reliability is improved, but subcarrier spacing is no longer maintained and Doppler robustness is reduced
Solution Approach 1:
The interleaving process is segmented into two distinct stages: time interleaving followed by frequency interleaving. This segmentation allows the time interleaver to operate without disrupting subcarrier spacing, while the frequency interleaver is applied afterward to provide diversity without affecting the previously established spacing relationships.
Solution Approach 2:
The time interleaver acts as an intermediary between the LDM compound signal generation and the frequency interleaving process. It provides a buffer that allows the signal to be reorganized in time without immediately applying the frequency permutation that would disrupt subcarrier spacing, thus mediating between reliability needs and spacing preservation.
2Reliability
If up-sampling is applied to the upper layer, then subcarrier spacing is increased and Doppler robustness is improved, but receiver complexity increases
Solution Approach 1:
Up-sampling is applied partially - specifically to the upper layer signal with factor M>1 - rather than to the entire LDM compound signal. This partial application provides the necessary Doppler robustness for mobile receivers while avoiding the excessive complexity that would result from up-sampling the complete signal, including the lower layer that doesn't require such treatment.
3Ease of manufacture
If TDM or FDM is used for multiplexing, then implementation is simplified, but medium sharing efficiency is reduced
Solution Approach 1:
The patent employs a composite multiplexing approach that combines LDM (Layered Division Multiplexing) with OFDM (Orthogonal Frequency Division Multiplexing). This composite structure allows multiple services to be multiplexed in both time and frequency domains simultaneously, achieving superior medium sharing efficiency compared to pure TDM or FDM, while maintaining reasonable implementation complexity through the structured combination of established techniques.
Data Source
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AI summary
The present invention relates to a technique for broadcasting digital data, and in particular to layered-division multiplexing (LDM) in connection with orthogonal frequency division multiplexing, wherein the upper layer data is modulated only on every M-th OFDM subcarrier in order to reduce inter-carrier interferences. The invention provides separate interleaving stages for LDM compound cells carrying cells of more than one layer and for LDM compound cells carrying cells of only one layer. In this manner, time- and frequency interleaving can be performed on an LDM compound signal while maintaining subcarrier spacing for compound cells carrying cells of more than one layer.