OFDM Subcarrier Randomization for LPD and PAPR Reduction
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Solution Overview
Problem
OFDM communications systems are susceptible to time-variations, carrier frequency offsets, high peak-to-average power ratio, intermodulation, and interference due to their nature of transmitting multiple subcarriers, which complicates Low Probability of Detection (LPD) and Low Probability of Interception (LPI) requirements, especially under frequency selective fading and jamming conditions.
Innovation Solution
The system employs Symbol-Based Randomization (SBR) and frequency domain spreading using a Walsh Transform to distribute data across multiple orthogonal subcarriers, reducing power per frequency, minimizing peak-to-average power ratio, and enhancing instantaneous signal-to-noise ratio while maintaining Low Probability of Detection and Interception by frequency hopping and spreading subcarriers over the frequency domain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If OFDM transmits multiple subcarriers simultaneously, then spectrum efficiency is improved, but peak-to-average power ratio increases and susceptibility to interference worsens
Solution Approach 1:
The patent segments the OFDM signal into multiple orthogonal subcarriers and applies frequency domain spreading to each subcarrier using Walsh functions. This segmentation allows the total power to be distributed across many frequency components, reducing the peak-to-average power ratio while maintaining high spectrum efficiency through parallel transmission.
Solution Approach 2:
The patent introduces frequency domain spreading as an additional dimension of signal processing. By spreading each subcarrier across multiple frequency components using orthogonal Walsh functions, the signal energy is distributed in the frequency domain, reducing peak power while maintaining data transmission efficiency.
2Stability of the object's composition
If OFDM uses fixed frequency subcarriers, then transmission stability is improved, but vulnerability to frequency selective fading and jamming worsens
Solution Approach 1:
The patent applies dynamic frequency hopping to the OFDM subcarriers using pseudo-random sequences. The frequency positions of subcarriers change over time according to the hopping pattern, which provides diversity against frequency selective fading and jamming while maintaining transmission stability through the orthogonal structure and coherent detection.
Solution Approach 2:
The patent converts the vulnerability to frequency selective fading into a benefit by using frequency domain spreading with Walsh functions. The orthogonal spreading codes allow the receiver to selectively combine signals from multiple frequency paths, transforming the harmful fading effects into diversity gain that improves transmission reliability.
3Power
If OFDM concentrates power on specific subcarriers, then signal strength is improved, but detectability and interceptability increase
Solution Approach 1:
The patent applies frequency domain spreading to distribute the signal power locally across multiple frequency components using orthogonal Walsh functions. Each subcarrier's energy is spread over a wider bandwidth, reducing the power spectral density at any single frequency while maintaining total signal strength, thereby reducing detectability and interceptability.
4Productivity
If OFDM transmits high symbol-rate streams, then data rate is improved, but sensitivity to multipath propagation worsens
Solution Approach 1:
The patent segments the high-rate data stream into multiple parallel low-rate subcarrier transmissions. Each subcarrier transmits at a lower symbol rate that is less sensitive to multipath effects, while the overall data rate is maintained through the parallel transmission structure and frequency domain spreading.
Data Source
Figure 1A~1B
Figure 2A~2C
Figure 3A
AI summary
A device and system communicates data and includes a modulation and mapping circuit that modulates and maps data symbols into a plurality of multiple sub carrier frequencies that are orthogonal to each other to form an Orthogonal Frequency Division Multiplexed (OFDM) communications signal based on a fixed or variable OFDM symbol rate. A pseudo-random signal generator operative with the modulation and mapping circuit generates pseudo-random signals to the modulation and mapping circuit based on an encryption algorithm for frequency hopping each sub carrier at an OFDM symbol rate to lower any probability of interception and detection, reduce power per frequency (dB/Hz/sec), and lower any required transmission power while maintaining an instantaneous signal-to-noise ratio. A frequency domain spreader circuit is operatively connected to the modulation and mapping circuit for spreading the multiple sub carriers over the frequency domain.