Orthogonal Cover Code Mapping for DMRS Power Imbalance
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Solution Overview
Problem
In LTE-Advanced systems, the orthogonality of demodulation reference signals (DMRS) is compromised at high mobile speeds, leading to channel estimation inaccuracies and power imbalance across transmission antennas, due to assumptions of identical channel responses and pre-coding treatments applied to both data and DMRS signals.
Innovation Solution
The proposed solution involves generating orthogonal cover code (OCC) pairs using non-correlation sequences, such as Zadoff-Chu or PN code sequences, and applying them in a manner that ensures orthogonality across both time and frequency dimensions, with specific spectrum spreading and mapping techniques to maintain signal integrity and balance power distribution across DMRS signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If OCC sequence is mapped in time domain assuming identical channel responses, then orthogonality is maintained in low-speed environments, but orthogonality is damaged and channel estimation precision is lowered at high mobile speeds
Solution Approach 1:
The patent applies dynamics by making the OCC mapping adaptable to channel conditions. Instead of a fixed time-domain mapping that assumes identical channel responses, the system dynamically adjusts the mapping approach based on actual channel variations, allowing the OCC to remain effective across different mobile speeds while maintaining orthogonality and channel estimation precision
Solution Approach 2:
The patent changes the mapping parameter from time-domain to frequency-domain OCC mapping. This parameter change allows the system to handle channel variations more effectively by spreading the OCC across frequency resources rather than time resources, thereby maintaining orthogonality even when channel responses vary across OFDM symbols due to high mobile speeds
2Power
If pre-coding treatment is applied to DMRS to enable linear stacking, then signal amplitude is enhanced when layers stack in same direction, but power imbalance occurs and transmission efficiency is lowered when layers stack in opposite directions
Solution Approach 1:
The patent applies local quality by differentiating the treatment of DMRS across different layers. Instead of uniformly applying pre-coding to all layers, the system applies layer-dependent pre-coding where each layer's DMRS is processed according to its specific requirements. This allows layers to be stacked constructively in terms of phase and amplitude while compensating for power imbalances, thereby maintaining high transmission efficiency
Solution Approach 2:
The patent introduces asymmetry in the pre-coding treatment by using different pre-coding matrices or parameters for different layers. This asymmetric approach prevents the symmetric cancellation that occurs when identical pre-coding is applied to all layers, ensuring that DMRS from multiple layers combine constructively rather than destructively, thus avoiding power imbalance and maintaining transmission efficiency
Data Source
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Figure 1B
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AI summary
The present invention relates to orthogonal cover code generating apparatus, demodulation reference signal generating apparatus, and methods thereof. The demodulation reference signal generator comprises a non-correlation sequence generator configured to generate a non-correlation sequence for pilot of a first resource block; a first spectrum spreading unit configured to spread spectrums of elements in the non-correlation sequence for pilot to be mapped to a first frequency resource of the first resource block, by using a first group of Orthogonal Cover Codes (OCCs); a second spectrum spreading unit configured to spread spectrums of elements in the non-correlation sequence for pilot to be mapped to a second frequency resource of the first resource block, by using a second group of OCCs; the first and second frequency resources are adjacent frequency resources with respect to a first group of data streams, and the first and second groups of OCCs are mirrors in column to each other; and a mapping unit configured to map the spectrum-spread elements to the first and second frequency resources, respectively. The methods and apparatuses according to the present invention can enhance pilot randomization, remove the problem of pilot transmission power imbalance, and satisfy the requirement on orthogonality at the two dimensions of both time and frequency.