Rate-Adaptive Precoding for Constellation Density Changes
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Solution Overview
Problem
Trellis Coded Modulation (TCM) systems face challenges due to increased line turn-around time and incompatibility with Decision Feedback Equalizers (DFE) in Time Domain Duplexing (TDD) systems, leading to power discontinuities and errors when constellation density changes, especially in systems like DSL and ReachDSL.
Innovation Solution
Implementing truncated Trellis decoding, rate-adaptive precoding, and multi-dimensional Trellis coding with symbol interleaving to reduce delay, maintain constant power levels, and adapt to changing constellation densities, ensuring seamless transitions and accurate error metric calculations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Trellis Coded Modulation (TCM) is used to provide performance gain, then error performance is improved by 3-6 dB, but line turn-around time is significantly increased due to lengthy Viterbi decoder delay
Solution Approach 1:
The transmitter pre-calculates and transmits reference vectors before the receiver needs them for DFE operation. This preliminary transmission of reference information allows the receiver to immediately begin equalization without waiting for Viterbi decoder completion, thereby reducing line turn-around time while maintaining TCM error performance benefits
Solution Approach 2:
The patent segments the TCM system into separate reference vector generation and data decoding paths. By extracting reference vector generation from the main Viterbi decoding process, the system can prepare reference information in parallel, reducing the critical path delay that determines line turn-around time
2Reliability
If conventional precoder is used in TDD systems, then intersymbol interference is handled, but power discontinuities occur when constellation density changes causing errors
Solution Approach 1:
The precoder is designed with dynamic power scaling capability that automatically adjusts to match the instantaneous constellation density. When constellation density changes, the precoder power scale factor is updated in real-time to maintain constant output power, preventing power discontinuities and enabling seamless adaptation between different modulation rates
Solution Approach 2:
The system employs feedback mechanisms where the receiver detects constellation density changes and communicates this information back to the transmitter. The transmitter then adjusts precoder parameters accordingly, creating a closed-loop system that maintains power continuity during constellation density transitions
3Reliability
If Viterbi decoder delay is used for Trellis decoding, then decoding accuracy is maintained, but the channel is out of service during the delay period
Solution Approach 1:
Reference vectors are generated and transmitted in advance before the receiver requires them for data decoding. This preliminary action allows the receiver to begin processing incoming signals immediately without waiting for the complete Viterbi decoding process, thereby maintaining channel availability while preserving decoding accuracy through the use of pre-prepared reference information
Data Source
AI summary
One embodiment comprises a transmitter precoder operating on an input signal to produce an output signal for modulation. The output signal is transmitted using a constellation with a constellation density. The transmitter precoder comprises scaler logic, precoder logic and selection logic. The scaler logic scales the input signal to maintain constant power on the input signal independent of the constellation density. The precoder logic precodes the scaled signal to produce a dither signal limited to a range of values. The range is inversely proportional to the constellation density. The selection logic is operable in two states. In the first state, the selection logic combines the scaled signal with the dither signal to produce the output signal. In the second state, the selection provides the scaled signal as the output signal.