Receiver AGC Loop for Unbalanced Constellation Gain Balancing
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Solution Overview
Problem
Current receiver automatic gain control (AGC) systems for asymmetrical or unbalanced constellations are inadequate as they destroy the integrity of signals like BPSK and 4ASK by re-normalizing I and Q power imbalances, and are not suitable for high-integration pluggable coherent modems due to complexity and noise introduced by external digital AGC loops.
Innovation Solution
Implementing a common AGC loop for both I and Q tributaries, with an IQ Gain Balancing function to set both tributaries to the same gain using an offset gain control signal, allowing for constant total I and Q power control and tracking of fast optical power transients, and optionally using hardware or firmware-based implementations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If independent AGC loops are used per tributary, then RF power can be controlled at ADC input, but asymmetrical constellation integrity is destroyed due to re-normalization of I and Q power imbalance
Solution Approach 1:
The patent merges the independent AGC loops into a single shared AGC loop that controls the total RF power for both I and Q tributaries together. This is achieved by summing the detected powers of I and Q channels (P_I + P_Q) and using this combined signal for gain control, preventing the re-normalization that would destroy asymmetrical constellation integrity while still maintaining total power control at the ADC input.
2Ease of operation
If external digital AGC loops are used, then gain control can be implemented, but device complexity and noise increase
Solution Approach 1:
The patent replaces external digital AGC loops with an integrated analog AGC loop implemented directly in the TIA circuitry. This analog implementation eliminates the need for complex digital signal processing and external components, reducing device complexity and interconnect noise while maintaining effective gain control capability through voltage-based feedback control.
3Ease of operation
If external AGC loops are used, then gain control is possible, but integration and noise performance deteriorate
Solution Approach 1:
The patent nests the AGC loop functionality directly within the TIA circuit by integrating the detector, loop filter, and gain control elements into the same hardware block. This nested integration eliminates external interconnects that would introduce noise and integration limitations, allowing the AGC system to be tightly coupled with the signal path while maintaining low noise performance.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution preserves the asymmetrical signature of I and Q power ratios, enhances integration and flexibility in hardware, and reduces noise and complexity, effectively maintaining signal integrity and tracking capabilities.
Implementation Method 1
a loop filter operable for integrating a voltage difference to provide the offset gain control voltage
Data Source
AI summary
An automatic gain control system for a receiver for an asymmetrical and/or unbalanced constellation, the system including: an automatic gain control loop adapted to be coupled to both a first transimpedance amplifier coupled to a first analog-to-digital converter forming a first tributary and a second transimpedance amplifier coupled to a second analog-to-digital converter forming a second tributary; wherein the automatic gain control loop is operable for providing an offset gain control voltage to gain balance a transimpedance amplifier voltage and a power associated with the first tributary and a transimpedance amplifier voltage and a power associated with the second tributary. The automatic gain control loop includes an analog automatic gain control loop. The automatic gain control loop is implemented in hardware or firmware.