DMRS Sequence Initialization for LTE-A CoMP
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Solution Overview
Problem
The LTE Rel-10 system faces challenges in managing the high overhead of maintaining downlink reference signals due to the large number of antenna ports, and the existing DMRS sequence initialization methods fail to support downlink CoMP transmission scenarios effectively, leading to conflicts between DMRS orthogonality and interference randomization.
Innovation Solution
A new method for dynamically selecting DMRS sequence initialization parameters using modulo-2 addition of nSCID and nNDI, allowing for decoupling of the DMRS sequence from the serving cell's identity, and combining these parameters to provide multiple candidates for initialization, thereby supporting CoMP scenarios and optimizing orthogonality and interference randomization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the base station broadcasts CRS to all mobile stations based on the largest number of MIMO layers, then channel estimation and channel quality measurement are supported, but the overhead becomes too high due to the large number of antenna ports
Solution Approach 1:
The reference signal is segmented into two types: CRS for channel quality measurement and DMRS for channel estimation. This segmentation allows CRS to be maintained only for essential channel quality measurement purposes while DMRS provides user-specific channel estimation, thereby reducing overall overhead while maintaining reliability
Solution Approach 2:
The DMRS functionality is extracted from the CRS structure. The DMRS is a separate, user-specific reference signal that is multiplexed with data, allowing the system to maintain CRS for basic channel quality measurement while using DMRS for efficient channel estimation, thus reducing the overhead burden of maintaining CRS on all ports
2Adaptability or versatility
If the DMRS sequence is tied to the serving cell's identity, then cell-specific reference is provided, but it fails to support downlink CoMP transmission scenarios effectively
Solution Approach 1:
The DMRS sequence initialization is made dynamic by introducing time-varying parameters (slot index, OFDM symbol index) and user-specific parameters (nSCID, nNDI). This dynamic initialization allows the same cell to provide different DMRS sequences for different users and transmission scenarios, enabling CoMP transmission while maintaining cell identity reference through the structured initialization formula
3Productivity
If multiple antenna ports are supported, then MIMO capacity is increased, but DMRS orthogonality and interference randomization become conflicting requirements
Solution Approach 1:
Orthogonality among multiple antenna ports is achieved by utilizing different dimensions: different cyclic shifts (time-domain dimension), different orthogonal cover codes (code-domain dimension), and different spatial layers (space-domain dimension). This multi-dimensional approach allows multiple ports to maintain orthogonality while supporting high MIMO capacity
Solution Approach 2:
The system dynamically changes DMRS parameters (cyclic shift, orthogonal cover code index) based on user assignment and transmission conditions. These parameter changes enable the system to maintain orthogonality across multiple antenna ports while providing interference randomization through dynamic parameter selection, thus supporting high MIMO capacity without sacrificing orthogonality
Data Source
AI summary
A new sequence initialization for demodulation reference signal (DMRS) sequence design used in the LTE-A system is provided. In some implementations, the modulo-2 addition of nSCID and nNDI, both of which are provided in a downlink control information (DCI) message, is used to dynamically select one of a plurality of parameter candidates and the dynamically selected parameter is then used for determining the initialization state of a DMRS sequence. In some other implementations, the modulo-2 addition of nSCID and nNDI is used for determining the least significant bit of the initialization state of the DMRS sequence.