Modem Symbol Timing via FFT and Correlation
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Solution Overview
Problem
Current multi-band tactical radios face performance issues due to excessive errors in acquiring frequency offset and phase error during communication, leading to data loss, especially in UHF SATCOM systems, where false alarms occur during the non-random preamble portion of the waveform.
Innovation Solution
A communications device that processes digitally modulated signals using a modem with a demodulator and processor to generate initial frequency offset and phase error estimates through FFT, correlates sample blocks with shifted sequences, and iteratively refines these estimates to improve symbol timing and reduce false detections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If FFT is used to detect the waveform and estimate frequency offset and phase error, then the acquisition process is enabled, but excessive errors in acquisition estimates lead to data loss
Solution Approach 1:
The patent applies preliminary action by performing initial frequency offset and phase error estimates using FFT on the preamble portion of the signal before processing the entire waveform. This preliminary estimation allows the system to compensate for frequency and phase errors early in the acquisition process, preventing excessive errors from propagating through subsequent processing stages and causing data loss.
Solution Approach 2:
The patent implements feedback by using the initial frequency offset and phase error estimates to adjust and refine subsequent processing of the waveform. The system continuously refines these estimates by comparing expected signal characteristics with actual received signals, making iterative corrections to improve acquisition precision and reliability.
2Productivity
If the modem processes the waveform in each DAMA slot, then communication is enabled, but false alarms occur during the non-random preamble portion
Solution Approach 1:
The patent applies segmentation by dividing the waveform processing into distinct segments: the preamble portion and the message portion. The system processes the non-random preamble separately to establish initial estimates, then uses these estimates to guide processing of the subsequent message portion. This segmentation allows the system to handle the inherently problematic non-random preamble without compromising overall detection accuracy.
Solution Approach 2:
The patent implements local quality by applying different processing strategies to different portions of the waveform. The preamble section receives initial estimation processing with relaxed criteria, while the message section receives refined processing with stricter detection thresholds. This localized approach to quality control reduces false alarms in the preamble while maintaining high detection accuracy for the message portion.
Data Source
AI summary
A communications device includes a signal input for receiving signals such as a binary phase shift keyed (BPSK) communications signal having a repeated preamble bit or symbol pattern. A modem processes the communications signal and includes a demodulator and processor that generates an initial frequency offset estimate and phase error estimate by processing such as with a Fast Fourier Transform (FFT), that detects the repeated preamble pattern for a block of samples within the communications signal, correlates two halves of the block of samples with a plurality of different shifted sequences and determines a maximum correlation value for the shifted sequence that provides the maximum correlation value to establish a symbol timing estimate based on the known timing alignment of this shifted sequence. Radio circuitry is operative with the modem for processing communications data obtained from the communications signal.