Femto Cell Frequency Synchronization via Transmit Power Analysis
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Solution Overview
Problem
Femto cell communication units face challenges in achieving accurate frequency synchronization due to the instability of inexpensive VCTCXO crystal oscillators, which are less accurate than those used in macro cell base stations, leading to potential signal decoding failures and frequency drift issues in combined macro cell/femto cell networks.
Innovation Solution
A communication unit with a signal processing logic module that decodes transmit power information from received RF signals to differentiate between reliable macro cell and unreliable femto cell timing references, allowing selective synchronization and fine frequency locking only with stable macro cell sources, thereby correcting frequency drift and maintaining high accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If inexpensive VCTCXO crystal oscillators are used in femto cell units, then cost is reduced, but frequency stability deteriorates leading to drift outside receiver lock range
Solution Approach 1:
The patent introduces a frequency synchronization mechanism that uses transmit power information as an intermediary to identify and select macro cell base stations as reference sources. This mediator allows the inexpensive VCTCXO oscillator to be corrected and stabilized by external high-stability references without requiring expensive oscillators in the femto cell unit itself.
Solution Approach 2:
The system implements feedback by continuously monitoring transmit power information from received signals and using this feedback to determine whether to synchronize with a particular base station. The frequency offset estimator provides continuous feedback about frequency drift, enabling dynamic adjustment and correction of the oscillator frequency to maintain lock within acceptable ranges.
2Reliability
If frequency synchronization is performed with every received base station signal, then frequency lock is achieved, but synchronization accuracy deteriorates due to femto cell signals having lower stability
Solution Approach 1:
The patent applies local quality by making the synchronization process selective rather than uniform. Transmit power information is used to locally identify macro cell base stations as high-quality reference sources, while femto cell signals are excluded. This creates different quality zones in the synchronization process, where only signals meeting the transmit power threshold are used for frequency correction.
Solution Approach 2:
The system changes the parameter used for selection from signal strength to transmit power information. By decoding and comparing transmit power values against a threshold, the system dynamically changes which base stations are eligible as synchronization references, ensuring only stable macro cell signals are used regardless of received signal strength variations.
Data Source
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AI summary
A communication unit comprises a receiver for receiving radio frequency (RF) signals from at least one wireless serving communication unit, and signal processing logic module arranged to decode information within the received RF signals is the at least one wireless serving communication unit. The signal processing logic module is further arranged to decode transmit power information in a received RF signal from the at least one wireless serving communication unit, and determine from the decoded transmit power information whether a transmission from the at least one wireless serving communication unit is suitable for use as a timing reference. If it is determined that the transmission from the at least one wireless serving communication unit is suitable for use as a timing reference, the communication unit synchronises an operating frequency to the received RF signal of the at least one wireless serving communication unit.