CDMA Signal Statistical Randomization for Low Probability of Intercept
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Solution Overview
Problem
Modern CDMA communication systems face challenges in maintaining low probability of intercept (LPI) and low probability of detect (LPD) due to exploitable statistical characteristics of modulated signals, allowing unauthorized parties to detect and demodulate data streams even without access to the original pseudo-noise sequence.
Innovation Solution
The implementation of a code division multiaccess (CDMA) communications system using dictionaries with long sequences of independent, identically-distributed complex samples from Gaussian random processes, which are pseudo-random or derived from unrepeatable physical processes, along with a one-time pad encryption system, to remove statistical characteristics except for incoherent power detection, thereby enhancing security.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional CDMA modulation methods are used, then communication efficiency is maintained, but statistical characteristics of the modulated signal become exploitable allowing unauthorized detection and demodulation
Solution Approach 1:
The patent applies parameter changes by modifying the modulation process to multiply the spreading sequence by complex exponential terms (e^jθ) where θ varies randomly or pseudorandomly. This transforms the traditional real-valued CDMA modulation into a complex-valued modulation scheme with time-varying phase and amplitude characteristics, making the signal statistically indistinguishable from noise while maintaining coherent communication capability for authorized receivers possessing the spreading sequence.
2Reliability
If signal power is increased to improve detection reliability, then communication reliability improves, but probability of intercept and detect increases
Solution Approach 1:
The invention changes the signal parameters by introducing complex exponential modulation with random or pseudorandom phase variations. This allows the signal to maintain adequate power levels for reliable communication while its statistical characteristics become indistinguishable from noise, preventing unauthorized detection and intercept.
3Adaptability or versatility
If pseudo-noise sequences with desirable statistical properties are used, then multiple communicators can operate simultaneously, but the sequences become exploitable by unauthorized parties
Solution Approach 1:
The patent transforms traditional pseudo-noise sequences into complex-valued sequences by multiplying with complex exponential terms containing random or pseudorandom phase components. This modification preserves the orthogonal properties needed for multiple access while destroying the predictable statistical structure that makes traditional PN sequences exploitable by unauthorized parties.
Solution Approach 2:
The invention creates a composite spreading sequence combining traditional pseudo-noise codes with complex exponential phase modulation. This composite structure maintains the code division multiple access capability through the PN sequence orthogonality while the complex phase component adds statistical randomness that prevents exploitation by unauthorized receivers.
Data Source
AI summary
A code division multiaccess (CDMA) communications system with low probability of intercept, low probability of detect (LPI/LPD) includes at least one data dictionary stored on a storage device of a sender subsystem and a recipient subsystem. The at least one data dictionary includes at least one data predetermined start time and date, at least one data predetermined end time and date based on a mission length or a predetermined wrap time and date, a CDMA chip rate, and a complex zero-mean independent and identically distributed (iid) sequence where each complex number in the complex zero-mean iid sequence represents a CDMA chip stored on the storage device of the sender subsystem and the recipient subsystem. The system includes a tangible, non-transitory, machine-readable medium comprising machine-executable instructions which, when executed by at least one processor of a machine, cause the at least one processor to: receive a message, convert the message to symbols with corresponding phasors, determine a date and time to send the message, look up a data spreading vector for each corresponding phasor by providing a mutually agreed number of chips per phasor stored on the storage device of the sender subsystem and the recipient subsystem and by matching the date and time the message is to be sent to the at least one data predetermined start time and date and the at least one data predetermined end time and date. Each data spreading vector is multiplied by its corresponding phasor to create a data spread vector for each data spreading vector. The sender subsystem is configured to sequentially transmit each chip of each data spread vector as a signal.