Femto Cell Frequency Synchronisation via Macro Reference Weighting
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Solution Overview
Problem
Femto cell communication units face challenges in achieving accurate frequency synchronisation due to the presence of both macro and femto cell reference signals, leading to unreliable frequency lock and potential signal decoding failures, especially with inexpensive VCTCXO crystals used in femto cells.
Innovation Solution
A communication unit with receiver circuitry and signal processing logic that differentiates between reliable and unreliable timing sources by decoding system information, disregarding or applying weighting factors to RF signals from restricted access femto cells, ensuring synchronization only with reliable macro cell sources for precise frequency lock.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If femto cell units use inexpensive VCTCXO crystal oscillators for frequency reference, then cost is reduced, but frequency stability and synchronisation accuracy deteriorate due to frequency drift from temperature variations and aging
Solution Approach 1:
The patent implements a feedback mechanism where the femto cell unit continuously monitors frequency drift by comparing its operating frequency with reference frequencies from macro cell base stations. The system measures the frequency error and automatically adjusts the VCTCXO oscillator to correct the drift, maintaining synchronisation accuracy without requiring expensive high-stability oscillators.
Solution Approach 2:
The patent replaces the mechanical/physical solution of using expensive high-stability crystal oscillators with an electronic/software-based frequency correction mechanism. The system uses signal processing and control algorithms to compensate for oscillator drift, substituting hardware quality requirements with intelligent frequency management.
2Adaptability or versatility
If femto cell units synchronise with multiple reference signals including other femto cells, then the number of available timing sources increases, but synchronisation reliability deteriorates because femto cell references are less accurate than macro cell references
Solution Approach 1:
The patent applies different quality weights to different reference sources based on their reliability. Macro cell base station references are assigned high weight due to their superior frequency stability, while femto cell references are assigned low or zero weight. This local quality differentiation allows the system to utilise multiple sources while maintaining high synchronisation accuracy by prioritising reliable references.
Solution Approach 2:
The patent dynamically changes the weighting parameters assigned to different reference signals based on their source type and measured quality. The system adjusts these parameters in real-time, increasing weight for stable macro cell references and decreasing weight for less stable femto cell references, thereby optimising overall synchronisation accuracy.
Data Source
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AI summary
A communication unit comprises receiver circuitry for receiving radio frequency (RF) signals from at least one wireless serving communication unit, and a signal processing logic module. The signal processing logic module is arranged to decode system information within received RF signals transmitted by the at least one wireless serving communication unit, determine whether the wireless serving communication unit supports a restricted access communication cell based on the decoded system information, and decide whether RF signals received from that at least one wireless serving communication unit are suitable as a timing signal source when synchronising an operating frequency of the communication unit based at least partly on the determination.