Optical Likelihood Generation Circuit Using Shared 1D/2D QAM LUTs
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Solution Overview
Problem
The increasing circuit scale of digital circuits in optical communication systems, particularly for high-order modulation methods like 128-QAM, due to the use of large-scale integration (LSI) circuits and the need to support various modulation methods such as QPSK to 128-QAM, poses a challenge in maintaining a manageable circuit size.
Innovation Solution
An optical transmission device with a likelihood generation circuit that includes one-dimensional and two-dimensional modulation lookup tables, shared to generate likelihoods for both low-order and high-order non-rectangular QAM, reducing the overall circuit scale by utilizing these tables to perform soft decision decoding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If high-order modulation methods (128-QAM, 256-QAM) are used to increase transmission capacity, then transmission capacity is improved, but circuit scale increases significantly
Solution Approach 1:
The likelihood generation circuit is segmented into multiple LUTs, each responsible for specific bits of the modulation signal. For 128-QAM, the circuit is divided into LUTs for different bit positions (b0-b6), allowing parallel processing while reducing the complexity of each individual LUT. This segmentation enables high-order modulation support without requiring a monolithic complex circuit.
Solution Approach 2:
The patent transitions from one-dimensional LUTs to two-dimensional LUTs to reduce overall circuit scale. By using 2D LUTs with shared resources, the patent achieves more efficient memory utilization and reduces the total number of storage elements required for high-order modulation likelihood generation.
2Reliability
If LUTs are used for likelihood generation in high-order modulation, then soft decision decoding performance is improved, but circuit scale becomes enormous
Solution Approach 1:
Adjacent LUTs are merged to share common resources such as multiplexers and logic circuits. For example, LUTs for different bit positions share multiplexing resources, reducing redundant circuitry while maintaining the ability to generate accurate likelihood values for soft decision decoding.
Solution Approach 2:
The LUT circuit is designed with universal components that can serve multiple functions. The same LUT structure and resources are used across different bit positions and modulation schemes, allowing the circuit to handle various modulation orders (QPSK, 16-QAM, 64-QAM, 128-QAM) without requiring separate dedicated circuits for each.
3Adaptability or versatility
If the same LSI is used to transmit and receive various modulation methods (QPSK to 128-QAM), then adaptability is improved, but circuit scale approaches LSI integration limits
Solution Approach 1:
The likelihood generation circuit is designed to be dynamically configurable through control signals that select which LUTs are active based on the current modulation scheme. This dynamic activation allows the same physical circuit to adapt to different modulation orders (from QPSK to 128-QAM) without requiring all LUTs to be simultaneously active, thus reducing the effective circuit scale at any given time.
Data Source
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AI summary
Provided is an optical transmission device including: a symbol demapping unit configured to convert a reception signal to a signal of an I-axis component and a signal of a Q-axis component on a phase plane, and a signal being a concatenation of the signal of the I-axis component and the signal of the Q-axis component; a likelihood generation circuit configured to generate likelihoods relating to the reception signal; and an error correction decoding unit configured to execute soft decision decoding. The likelihood generation circuit includes: a first one-dimensional-modulation lookup table configured to input the signal of the I-axis component as an argument to output a first likelihood; a second one-dimensional-modulation lookup table configured to input the signal of the Q-axis component as an argument to output a second likelihood; and a two-dimensional-modulation lookup table configured to input, as an argument, the signal being the concatenation of the signal of the I-axis component and the signal of the Q-axis component, to generate a third likelihood. The error correction decoding unit is configured to execute the soft decision decoding based on the first likelihood, the second likelihood, and the third likelihood.