Switchable Time Constant Loop Filter for TDMA-TDD Demodulation
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Solution Overview
Problem
In TDMA-TDD systems, existing demodulation methods face challenges in minimizing jitter and achieving quick pull-in during transitions from asynchronous to synchronous states due to the use of a single time constant for loop filters, which affects frequency deviation compensation and bitclock recovery in synchronous detection systems.
Innovation Solution
A radio communication apparatus with a demodulation section that uses a filter with switchable time constants, including small, intermediate, and large values, based on the reception state, to optimize frequency deviation compensation, carrier recovery, and bitclock recovery, minimizing jitter and achieving quick pull-in.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single time constant is used for the loop filter, then the circuit structure is simple, but the demodulation performance deteriorates during transitions from asynchronous to synchronous states due to inability to achieve both quick pull-in and low jitter
Solution Approach 1:
The patent applies the dynamics principle by making the time constant adjustable rather than fixed. The time constant switching section dynamically changes the time constant value based on the reception state (synchronous or asynchronous), allowing the system to adapt its filtering characteristics to current operating conditions and achieve both quick pull-in and low jitter performance
Solution Approach 2:
The patent implements parameter changes by switching between multiple time constant values (first, second, and third time constants) depending on the reception state. This allows the loop filter to have different time constants for different operational phases, optimizing performance for both asynchronous and synchronous states without requiring separate filter circuits
2Speed
If a small time constant is used for quick pull-in in asynchronous state, then the pull-in speed is improved, but the jitter after convergence increases
Solution Approach 1:
The system dynamically switches time constants based on reception state. During asynchronous state, a small time constant is used for quick pull-in. After synchronization is achieved, the system transitions to using a larger time constant to reduce jitter, thus adapting the filtering characteristics to the current operational phase
Solution Approach 2:
The patent employs periodic switching of time constants corresponding to different reception states. The time constant is periodically adjusted based on whether the system is in asynchronous or synchronous state, allowing the system to alternate between fast acquisition mode and stable tracking mode
3Measurement precision
If a large time constant is used for low jitter in synchronous state, then the convergence accuracy is improved, but the pull-in speed decreases
Solution Approach 1:
The system dynamically selects time constant values based on reception state detection. When synchronous state is detected, a large time constant is applied for accurate tracking and low jitter. When asynchronous state is detected, the system switches to a smaller time constant for faster pull-in, thus optimizing both speed and accuracy at different operational phases
4Reliability
If multiple filter circuits are provided for different time constants, then the demodulation performance is optimized, but the circuit size increases
Solution Approach 1:
The patent merges multiple filter functions into a single loop filter circuit by making its time constant adjustable. Instead of providing separate parallel filter circuits for different time constants, the invention combines them into one filter whose time constant can be switched between multiple values, thus reducing circuit size while maintaining optimized performance for different reception states
Solution Approach 2:
The loop filter is designed with multi-functionality to serve different operational requirements. By equipping the single filter circuit with the capability to switch between multiple time constant values, it can perform both fast acquisition and accurate tracking functions that would otherwise require separate dedicated circuits
Data Source
AI summary
A demodulation section 13 receives a TDMA-TDD based phase-modulated burst signal of mobile communications and demodulates the burst signal by a synchronous detection system (or a quasi-synchronous detection system). The demodulation section 13 includes a frequency deviation compensation section and a carrier recovery section each having a loop filter 14 with three or more stages of time constants. The time constants are switched by a selector switch 15 based on a control signal from a demodulation control section 16. This achieves quick pull-in and jitter after convergence is minimized, thereby allowing highly efficient performance of frequency deviation compensation, etc. that is required for synchronous detection (or quasi-synchronous detection) without increasing the size of circuit.