COFDM Subcarrier Randomization for LPD/LPI Enhancement
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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 from jammers, which compromise Low Probability of Detection (LPD) and Low Probability of Interception (LPI) characteristics due to the required minimum signal-to-noise ratio per subcarrier.
Innovation Solution
The system employs Symbol-Based Randomization (SBR) and frequency hopping with a Walsh transform to spread subcarriers over the frequency domain, reducing average power, increasing bandwidth, and enhancing LPD/LPI by selectively turning ON/OFF subcarriers and using a pseudo-random frequency hopping algorithm, thereby reducing Inter-Carrier Interference (ICI) and Inter-Symbol Interference (ISI).
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional OFDM transmits multiple subcarriers simultaneously to achieve high spectrum efficiency, then data transmission rate is improved, but average power increases making the system vulnerable to detection and interception
Solution Approach 1:
The patent implements periodic frequency hopping where subcarriers are selectively activated and deactivated in a time-varying manner according to a pseudo-random sequence. This periodic on/off action reduces the time-averaged power while maintaining high instantaneous data rates when subcarriers are active, thereby resolving the contradiction between transmission rate and average power consumption
Solution Approach 2:
The system dynamically adjusts which subcarriers are transmitted at any given time based on frequency hopping patterns and channel conditions. This dynamic activation/deactivation of subcarriers allows the system to maintain high spectral efficiency when needed while reducing average power consumption by leaving some subcarriers idle, thus resolving the power-rate tradeoff
2Productivity
If OFDM uses closely spaced subcarriers to achieve high spectral efficiency, then bandwidth utilization is improved, but Inter-Carrier Interference (ICI) increases due to frequency offsets and Doppler shifts
Solution Approach 1:
The patent introduces dynamic frequency hopping that continuously changes the frequency positions of active subcarriers over time. This time-varying frequency allocation prevents stationary interferers and jammers from consistently affecting the same subcarriers, thereby reducing ICI while maintaining spectral efficiency through opportunistic access to clean frequency slots
Solution Approach 2:
The system changes the frequency parameters of subcarriers dynamically according to hopping patterns and channel state information. By adapting subcarrier frequencies in response to channel conditions, the system maintains orthogonality and reduces ICI even with closely spaced subcarriers, thus preserving spectral efficiency while mitigating interference
3Reliability
If OFDM transmits with high average power to maintain signal-to-noise ratio, then bit error rate is reduced, but Low Probability of Detection (LPD) and Low Probability of Interception (LPI) characteristics are compromised
Solution Approach 1:
The patent employs periodic frequency hopping with pseudo-random patterns that cause the signal to appear discontinuous and unpredictable in the frequency domain. This periodic switching among different frequency sets reduces the time-averaged power spectral density, making the signal harder to detect and intercept while maintaining adequate instantaneous SNR for reliable communication through adaptive modulation and power control
Solution Approach 2:
The system uses frequency hopping patterns and spreading sequences as intermediaries to distribute the transmitted energy across a wide frequency band over time. This spreading action reduces the power spectral density at any individual frequency, enhancing LPI/LPD characteristics while the coherent processing at the receiver maintains the effective SNR needed for low bit error rates
4Difficulty of detecting and measuring
If frequency hopping is used to enhance LPD/LPI and reduce average power, then detectability is reduced, but device complexity increases due to pseudo-random sequence generation and synchronization requirements
Solution Approach 1:
The patent uses identical pseudo-random frequency hopping sequences at both transmitter and receiver, allowing the receiver to replicate the hopping pattern and synchronize with the transmitted signal. This copying approach enables the complex frequency hopping operation to be performed efficiently using the same algorithms on both sides, reducing the net complexity increase while achieving enhanced LPI/LPD characteristics
Data Source
Figure 1A~1B
Figure 2A~2C
Figure 3A
AI summary
A system and method communicates and generates a Coded Orthogonal Frequency Division Multiplexed (COFDM) communications signal as a plurality of subcarriers. Selected subcarriers are turned ON and OFF while also frequency hopping and spreading the selected ON subcarriers over the frequency domain based on a fixed or variable sample rate to reduce average power for enhanced Low Probability of Interception/Low Probability of Detection (LPI/LPD) and also allow for more transmit power within a spectral mask.