Baseband Phase Correction for Local Oscillator Pulling in TDD Radios
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Solution Overview
Problem
In LTE TDD base stations, local oscillator (LO) pulling and pushing due to impedance changes and external interference degrade signal quality by affecting the phase noise and lock time of the TX LO phase locking loop, making it challenging to switch between transmit and receive modes efficiently.
Innovation Solution
A method that iteratively determines and corrects phase errors in the digital domain using adaptive phase correction algorithms or look-up tables, allowing for online adaptive compensation of LO pulling or pushing, thereby improving signal modulation quality and reducing hardware overhead.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the TX LO is switched to matched load at uplink receiving intervals to minimize leakage to the receiver, then the transmitter OFF power spectral density is reduced, but the impedance change causes the PLL to relock resulting in LO pulling
Solution Approach 1:
The patent applies preliminary action by pre-adjusting the VCO control voltage before impedance switching occurs. The system predicts the upcoming impedance change (e.g., TX LO switching to matched load) and proactively modifies the VCO control voltage to compensate for the expected frequency pull, preventing signal quality degradation before it happens
Solution Approach 2:
The patent implements feedback by monitoring the actual frequency deviations caused by impedance switching and using this information to adjust the VCO control voltage dynamically. The system continuously detects LO frequency pulls and applies corrective voltage adjustments to maintain stable frequency despite impedance changes
2Reliability
If the TX LO is kept continuous to avoid LO pulling, then signal quality is maintained, but the leakage to the receiver increases
Solution Approach 1:
The patent changes the parameter of VCO control voltage dynamically based on the operational state. Instead of simply switching the LO on/off or to matched load, the system adjusts the VCO control voltage to different levels that correspond to different impedance states, enabling smooth transitions that maintain frequency stability while allowing impedance switching for leakage reduction
3Manufacturing precision
If a narrow bandwidth PLL is used to get good phase noise, then phase noise performance is improved, but the lock time becomes much longer than the 20us switching time
Solution Approach 1:
The patent applies preliminary action by pre-adjusting the VCO control voltage anticipating upcoming impedance switches. This preparation reduces the frequency deviation that would otherwise require lengthy PLL relocking, enabling the narrow bandwidth PLL to maintain good phase noise while responding quickly to TDD switching requirements
Data Source
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AI summary
Disclosed is a method and a radio network node for compensating for local oscillator pulling or pushing. The method comprises determining, in a digital domain, a correction phase for the local oscillator to offset a phase error caused by the local oscillator pulling or pushing. The method also comprises correcting a phase of the baseband signal in the digital domain using the correction phase to compensate for the local oscillator pulling or pushing With the proposed method and radio network node, the phase error caused by the local oscillator pulling or pushing could be diminished due to phase correction in the digital domain.