VSB Timing Error Detector Using I and Q Channels
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Solution Overview
Problem
VSB receivers face performance degradation due to residual carrier phase offset and jitter, which conventional symbol timing recovery methods cannot effectively address, as they rely on the I channel alone and are unable to cancel out phase offset terms.
Innovation Solution
A timing recovery system that includes an A/D converter, phase splitter, carrier recovery system, Timing Error Detector (TED), loop filter, and interpolating Square root Raised Cosine filter, which generates accurate timing errors regardless of carrier phase offset, using a VCXO or NCO to adjust sampling instants and compensate for constant quarter symbol phase offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional timing recovery methods use only the I channel signal, then the device complexity is reduced, but the measurement precision of timing error degrades due to carrier phase offset
Solution Approach 1:
The patent transitions from using only the real I channel signal to utilizing both the real I channel and imaginary Q channel signals in the timing error detection process. By incorporating the Q channel (which is 90 degrees phase-shifted relative to I), the system creates a two-dimensional signal space that allows cancellation of carrier phase offset effects through appropriate signal combination, thereby improving timing error measurement precision without excessive complexity increase
Solution Approach 2:
The patent introduces an intermediary processing step where the Q channel signal is used as a mediator to eliminate the harmful carrier phase offset term. By combining I and Q channel signals in a specific manner (using the relationship between them), the phase offset component cancels out, leaving only the pure timing error information that can be accurately detected
2Measurement precision
If conventional timing recovery methods rely on I and Q channels simultaneously, then the measurement precision improves by canceling carrier phase terms, but this approach is ineffective for VSB systems where Q channel is a Hilbert transform of I channel
Solution Approach 1:
The patent applies local quality by making the timing error detection method specifically adapted to VSB modulation characteristics. Instead of using a universal I-Q combining approach that fails for VSB, the invention tailors the signal processing to exploit the specific relationship between I and Q channels in VSB (where Q is the Hilbert transform of I), creating a detection method that is locally optimized for this modulation type
Solution Approach 2:
The patent changes the fundamental parameter of signal combination from linear I-Q mixing to a method based on the energy relationship between I and Q channels. By utilizing the fact that for VSB, the sum of squared magnitudes of I and Q channels eliminates phase offset, the invention transforms the detection approach to work with the specific statistical and algebraic properties of VSB modulated signals
3Reliability
If carrier recovery is performed before timing recovery, then the carrier phase offset is reduced, but residual jitter and imperfections still degrade timing recovery performance
Solution Approach 1:
The patent applies preliminary anti-action by designing a timing error detector that proactively compensates for carrier phase offset effects before they can corrupt the timing measurement. The detector structure includes mechanisms that anticipate and counteract the phase offset influence, effectively neutralizing it in advance of the timing error calculation, thus protecting against both residual jitter and imperfections from carrier recovery
Solution Approach 2:
The patent converts the harmful carrier phase offset into a beneficial effect by exploiting its mathematical properties. The phase offset term, while harmful to conventional detectors, contains predictable structure that can be eliminated through the proposed detection method. By using the relationship between I and Q channel phase components, the system transforms the phase offset from a source of error into a canceling term that actually helps isolate the pure timing error
Data Source
AI summary
The present invention provides a novel symbol timing recovery method for VSB receivers. Systems are described that comprise a timing error detector (TED) that produces an exact symbol timing error even in the presence residual carrier phase offset, loop filter that controls the characteristics of acquisition and tracking of digital PLL loop, Voltage/Numerically Controlled Oscillator (VCO/NCO) that adjusts the sampling instant and phase, A/D converter that samples a continuous VSB input signal, and a interpolating squared root raised cosine filter that performs both matched filtering and a compensation of constant timing offset of quarter symbol caused by the invented TED. The timing error detector in this invention comprises an envelope detector, band pass filter, squaring block, high pass filter, and decimator. It uses both in-phase and quadrature-phase component of received VSB signal, is operated at twice of a symbol frequency F, and guarantees consistent symbol timing error signal resulting in the improvement of receiver's performance.