DMRS Density and SRS Collision Avoidance in LTE Uplink
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Solution Overview
Problem
In 3GPP LTE systems, machine-type communication (MTC) devices face challenges with signal strength, leading to performance deterioration when base stations estimate channels using demodulation reference signals (DMRS) due to potential collisions with sounding reference signals (SRS) from legacy UE, affecting uplink channel transmission.
Innovation Solution
Increasing the density of DMRS for demodulating PUSCH by reusing existing DMRS and introducing additional reference signals (extended DMRS) in specific OFDMA symbols, while applying cyclic shifts and orthogonal cover codes to manage multiplexing and interference, and optimizing DMRS mapping to prevent collisions with SRS.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the base station estimates the channel using DMRS transmitted by MTC devices, then channel estimation is performed, but performance deteriorates due to signal weakness at MTC device locations
Solution Approach 1:
The patent combines DMRS and SRS transmissions in the same resource elements by applying different cyclic shifts to the reference signal sequences. This allows the base station to simultaneously receive both MTC device channel estimation signals and legacy UE sounding signals without resource collision, merging previously separate transmission resources into a unified structure that serves both purposes.
Solution Approach 2:
The patent changes the cyclic shift parameter of the reference signal sequence to enable multiplexing of DMRS and SRS. By dynamically adjusting the cyclic shift value, the system can distinguish between different reference signals transmitted on the same time-frequency resources, thereby improving channel estimation performance for MTC devices while avoiding collisions with legacy UE SRS transmissions.
2Productivity
If SRS is transmitted by MTC devices, then uplink channel sounding is achieved, but collisions occur with SRS transmitted by existing general UE
Solution Approach 1:
The patent applies different cyclic shift values to DMRS and SRS transmissions, creating local differentiation in the signal structure. This allows both signal types to coexist on the same time-frequency resources without collision, as the base station can distinguish them through their unique cyclic shift characteristics. The local quality change in the reference signal sequence enables simultaneous support for MTC and legacy UE operations.
Solution Approach 2:
The reference signal resource elements serve dual purposes: they function as DMRS for MTC device channel estimation and as SRS for legacy UE channel sounding. By making the same physical resources multi-functional through cyclic shift differentiation, the system eliminates the need for separate dedicated resources for each function, thereby improving overall resource utilization and preventing collisions.
3Measurement precision
If DMRS density is increased for MTC devices, then channel estimation performance improves, but resource overhead increases
Solution Approach 1:
The patent merges DMRS and SRS transmissions into the same resource elements, allowing the system to achieve enhanced channel estimation for MTC devices without proportionally increasing total reference signal overhead. The combined transmission structure enables both functions to share the same physical resources, reducing the cumulative resource consumption compared to providing separate dedicated resources for each signal type.
Data Source
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AI summary
One embodiment of the present specification provides a method by which a wireless device, to which coverage enhancement (CE) is applied, transmits an uplink channel. The method can comprise a step for receiving, by the wireless device, sounding reference signal (SRS) configuration information for other wireless devices within a cell. The SRS configuration information can comprise information on SRS transmittable subframes. The method can comprise a step for transmitting, by the wireless device, the uplink channel on symbols except for a final symbol on subframes according to a reduced format, when the wireless device transmits the uplink channel on the subframes directed by the subframe information within the SRS configuration information for other wireless devices.