Femto Cell Synchronization Using UE Time Offset Measurements
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Solution Overview
Problem
Indoor small cells, such as femto cells, face difficulty in synchronizing with umbrella macro cells due to weak GPS signal strength indoors and cost constraints that prevent GPS receiver installation, hindering efficient frequency spectrum reuse in heterogeneous networks.
Innovation Solution
User Equipment (UE) measures time offsets between macro and femto cell signals, reporting these to the enhanced NodeB (eNB), which adjusts the femto cell's timing to synchronize with the macro system, using training signals and dedicated synchronization access slots to ensure accurate timing alignment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If GPS receivers are installed in indoor small cells for synchronization, then synchronization accuracy is improved, but device cost increases
Solution Approach 1:
The patent introduces User Equipment (UE) as an intermediary to perform time offset measurements between macro and femto cells. Instead of equipping femto cells with GPS receivers, the UE (which already has GPS capability) acts as a mediator to measure and report time offsets to the eNB, which then coordinates synchronization with the femto cell through backhaul communication.
Solution Approach 2:
The system utilizes the existing GPS receiver in the User Equipment to perform synchronization measurements. The UE leverages its own GPS capability to measure time offsets, eliminating the need for separate GPS infrastructure at femto cells. The eNB then uses this information to coordinate timing between macro and small cells.
2Reliability
If GPS signals are used for synchronization indoors, then synchronization reliability is improved, but signal strength decreases
Solution Approach 1:
The patent shifts the measurement dimension from direct GPS signal reception at the femto cell to time offset measurement performed by the UE. The UE measures the time difference between macro cell signals and femto cell signals in the air interface domain, rather than relying on GPS signal strength in the spatial domain. This dimensional shift allows synchronization to work effectively indoors where GPS signals are weak.
3Measurement precision
If time offset measurement is performed, then synchronization precision is improved, but measurement complexity increases
Solution Approach 1:
The patent implements a feedback mechanism where the UE measures time offsets and reports them to the eNB. The eNB then uses this feedback information to coordinate with the femto cell through backhaul communication, adjusting timing as needed. This closed-loop feedback system achieves precise synchronization while distributing the measurement complexity across multiple network elements rather than concentrating it in one device.
Data Source
AI summary
Example embodiments are directed to a method including transmitting, by a small cell, a pilot signal to a user equipment (UE) based on a first training signal received from the UE, and receiving, by the small cell, a second training signal from the UE. The second training signal is offset by a time based on the pilot signal transmitted by the small cell. The time offset represents a difference in time between the UE receiving a reference signal transmitted by a macro cell and the UE receiving the pilot signal transmitted by the small cell. The small cell adjusts a local reference timing based on the second training signal.