A Synchronization Approach Based on Bidirectional Correlation Search for Aperiodic Chaotic Direct Sequence Spread Spectrum Signals

Abstract

The chaotic direct sequence spread spectrum (CD3S) is one of the distinguished techniques providing physical layer secrecy and low probability of intercept (LPI) in wireless communication. Due to the noise-like and aperiodic characteristics of chaotic signals, the synchronization of CD3S signals is a challenging issue. A synchronization approach for aperiodic CD3S signals based on bidirectional correlation search (BDCS) is proposed. Through a joint acquisition of forward frequency domain matched filtering (FDMF) and backward FDMF, the synchronization module aligns received CD3S signals with local chaotic sequences without aid of the pilot signal. The probability distribution of correlator output is derived with additive white Gaussian noise and narrowband jamming. The probability of detection, probability of false alarm, and bit error rate are analyzed mathematically. An approximate solution of the local optimal acquisition threshold coefficient is obtained. These theoretical analyses are verified by numerical simulations, the results of which demonstrate that the proposed synchronization approach enables the CD3S system to resist interception and adapt to the environment with low signal-to-noise and signal-tojammer ratios.

Acquisition of Time and Doppler Shift in WFRFT Encrypted Chaotic Direct Sequence Spread Spectrum System

“…x corresponding to the single message bit k b . In [17], the received signal is also divided into segments of N chips to facilitate parallel search.…”

“…First, each bit of the WFRFT-CD3S signal is spread by different WFRFTchaotic codes. Even if the Doppler frequency shift is not considered, the aperiodic WFRFT-CD3S signal cannot be acquired by a fixed WFRFT-chaotic sequence when the WFRFT-CD3S signal leads the local WFRFT-chaotic sequence [5], [17]. Second, the computational complexity of the time-frequency two-dimensional search is unacceptable since the WFRFT-chaotic sequence is aperiodic.…”

“…There is only a fixed phase angle offset between the differential received signal and the local differential spreading sequence, but no Doppler frequency shift [20], which will not cause attenuation of the correlation peak. Inspired by [17], the correlation search is implemented bidirectionally to ensure that the correlation peak can be detected regardless of whether the aperiodic received WFRFT-CD3S signal lags or leads the local spreading sequence. After the time acquisition, we coarsely estimate the Doppler frequency shift by analyzing the spectrum of the despread signal to obtain a relatively large estimation range.…”