GMSK Modulator Transition Smoothing Circuit
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Solution Overview
Problem
The transition between GMSK and 8PSK modulation types in EGPRS systems causes zero-crossings in the IQ-plane, leading to power amplifier nonlinearity and performance deterioration, which existing solutions attempt to mitigate by reducing power amplifier levels rather than addressing the transitions themselves.
Innovation Solution
A dual-mode modulator with transition smoothing circuitry that determines a safe point in the IQ-plane and jumps to it before switching modulation types, avoiding zero-crossings by traversing a quarter circle to the starting point for the new modulation type.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the modulator is reset instantaneously when switching between modulation types, then the switching speed is improved, but zero-crossings occur in the IQ-plane causing power amplifier nonlinearity
Solution Approach 1:
The patent applies preliminary action by determining a safe point in advance before the actual modulation switch occurs. The system calculates the safe point based on the current modulation state and target modulation type, then transitions to this pre-determined safe point before switching completes. This prevents zero-crossings by ensuring the transition path avoids the origin of the IQ-plane, thereby maintaining power amplifier linearity while enabling fast switching between GMSK and 8PSK modulation types.
2Object-affected harmful factors
If the power level of the power amplifier circuit is reduced to mask transitions, then the harmful zero-crossing effects are mitigated, but the signal power is attenuated
Solution Approach 1:
The patent extracts and eliminates the root cause of the problem by removing the instantaneous reset operation that causes zero-crossings. Instead of reducing power to mask the issue, the system extracts the harmful transition path and replaces it with a safe transition path that goes through a predetermined safe point. This approach eliminates power amplifier nonlinearity without requiring power reduction, thereby maintaining full signal power levels.
3Reliability
If the modulator trajectory is changed to avoid zero-crossings, then power amplifier performance is improved, but the transition complexity increases
Solution Approach 1:
The patent applies parameter changes by modifying the modulation trajectory parameters to pass through a predetermined safe point instead of the origin. The system calculates appropriate safe points based on current and target modulation states, adjusting the I and Q parameters to follow a safe transition path. This parameter adjustment eliminates zero-crossings and improves power amplifier performance while keeping the transition logic manageable through standardized safe point determination.
Data Source
AI summary
To minimize abrupt changes in modulated signal amplitude when switching between modulation types in a multi-modulation system, a “smoothing” circuit is used. When the modulation type is changed in a multi-slot modulation, the last symbol of the previous slot is overwritten and modulation is controlled by a smoothing circuit during the time of the final symbol of the previous slot to effect the trajectory smoothing. The IQ-plane is divided into two (or more) zones, each with an associated safe point. After the next-to-last symbol is modulated, modulation is made to jump to the safe point associated with the zone and modulation continues to a starting point. From the starting point, the next slot can be modulated using the new modulation type.