Pre-processing Design And Performance Analysis Of Frame Synchronization Under Extremely Low SNR Condition

In deep space communication and strong electromagnetic countermeasure scene, the received signal tends to be very weak, or easily overwhelmed by background noise. Synchronous technology plays an important role in detecting useful signal under extremely low SNR condition.The detection performance of frame synchronization almost entirely depends on the design and application of good frame synchronization codes. A composition of the 13-bit barker code and the 48-bit preferred synchronization sequence is adopted as the frame marker, namely 624-bit composite sequence. The 624-bit composite sequence performs well in aperiodic autocorrelation characteristics.The square wave and raised cosine wave are used as baseband shaping waveform. According to the principle that energy of each symbol waveform is equal, the specific value of eight times signal bandwidth samples in one symbol wave of square wave and raised cosine wave are calculated.The detection scheme of frame synchronization code based on sampling values is proposed. Through adding pre-processing steps after sampling, every eight successive sampling values are pre-processed to improve the SNR of the input signal for correlation calculation.In this thesis, smoothing filtering and matched filtering are used as pre-processing methods for the extremely low SNR communication to accumulate the symbol waveform energy. The smoothing filtering is a method which calculates the average of every eight successive sampling values of received signal, while the matched filtering is a method which calculates the correlation of every eight successive sampling values of received signal and the eight sampling values of shaping waveform. In addition, simulation shows that more energy can be accumulated through the matched filtering method than the smoothing filtering method.The thesis then analyzes the probability of correct detecting of three frame synchronization schemes minutely, including non-pre-processing scheme, smoothing filtering scheme and matched filtering scheme. Firstly, for the square wave signal, both smoothing filtering scheme and matched filtering scheme can achieve the same detection performance which is better than that of non-pre-processing scheme under the same SNR and threshold. Secondly, for raised cosine wave signal, matched filtering scheme achieves the maximum probability of correct frame synchronization under the same SNR and threshold, followed by smoothing filtering scheme, and non-preprocessing scheme achieves the minimum probability.Finally, simulation experiments are designed to validate the performance of the three schemes. For square wave signal or raised cosine wave signal, when the threshold is 550, smoothing filtering pre-processing or matched filtering pre-processing has at least 7d B more SNR gain than non-pre-processing to achieve the same probability of correct frame synchronization. For raised cosine wave signal, when the threshold is 425, matched filtering pre-processing has at least 4d B more SNR gain than smoothing filtering pre-processing to achieve the same probability of correct frame synchronization.