Quadratic Phase Modulation for Low PAPR OFDM Preambles
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Solution Overview
Problem
OFDM systems face challenges in managing high Peak-to-Average Power Ratio (PAPR), which leads to increased in-band and out-of-band noise, bit error rate, and adjacent channel interference, especially when the number of subcarriers changes, and existing PAPR reduction techniques are inefficient or require significant processing power.
Innovation Solution
A novel low PAPR preamble is introduced, which achieves a typical PAPR of 2.6 dB regardless of the number of subcarriers, using quadratic modulation and evenly spaced subcarriers, allowing for efficient channel estimation and improved signal-to-noise ratio, and can be easily implemented in hardware using IFFT/FFT circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional OFDM preambles are used with variable subcarrier configurations, then system adaptability is improved, but PAPR increases causing clipping and noise
Solution Approach 1:
The patent applies parameter changes by modifying the phase parameters of subcarriers in the preamble sequence. Specifically, a quadratic phase function is applied to the subcarrier indices, transforming the standard preamble into a low-PAPR version while maintaining its identification and synchronization functions. This phase parameter modification ensures that the signal peaks are reduced regardless of the number of subcarriers active in the system.
2Reliability
If PAPR reduction techniques are applied to OFDM signals, then in-band noise and bit error rate are reduced, but signal distortion and processing complexity increase
Solution Approach 1:
The patent implements preliminary action by pre-modulating the preamble sequence with a quadratic phase function before transmission. This low-PAPR preamble sequence is designed and prepared in advance at the transmitter, eliminating the need for complex real-time PAPR reduction processing during signal transmission. The receiver simply correlates with the known low-PAPR preamble pattern, avoiding complex iterative processing while achieving reliable detection and synchronization.
3Manufacturing precision
If power amplifier back-off is increased to avoid clipping, then signal distortion is reduced, but transmit power and spectral efficiency decrease
Solution Approach 1:
The patent converts the harmful high-peaked waveform into a beneficial low-PAPR waveform by applying quadratic phase modulation to the preamble sequence. This transformation inherently reduces the peak amplitude without requiring power amplifier back-off, allowing the PA to operate at higher efficiency while maintaining signal fidelity. The low-PAPR characteristic of the modified preamble sequence directly enables higher transmit power utilization without clipping-induced distortion.
Data Source
AI summary
A communications device includes communications data and a training sequence corresponding to a preamble. A modulation and mapping circuit modulates the communications data and training sequence into a plurality of multiple subcarriers that are orthogonal to each other to form an orthogonal frequency division multiplexing (OFDM) communications signal having modulated subcarriers carrying the communications data forming a data payload and modulated subcarriers forming the preamble. The modulation and mapping circuit applies a quadratic modulation to the subcarriers carrying the training sequences to produce a low peak-to-average power (PAPR) preamble of approximately 2.6 decibels (dB) independent of the number of subcarriers.