Signal type identification and demodulation of digitally-modulated unknown signals

Abstract

The main objective of this dissertation work is to develop a signal monitoring system which is able to identify the modem constellations and demodulate the transmitted data of unknown digitally-modulated signals. In order to achieve to identify modem constellations of unknown signals, the important modulation parameters such as symbol rate and carrier frequency should be known accurately to the signal monitor in general. The input signals are considered in this thesis are restricted to PSK (Phase Shift Keying) and QAM (Quardature Amplitude Modulation) signals taking into account-its popularity in high-speed digital communication systems. However, the main study area of this dissertation work is divided into the studies for symbol rate detection, carrier frequency detection, modem constellation identification, and data demodulation of unknown digital communication signals. First, in order to detect the symbol rate accurately, two detector schemes are proposed based on the spectrum of signal envelope which may be obtained from analytic signal of complex envelope. Here, two nonlinear transforms such as SLR (Square-Law Rectifier) and AVR (Absolute-Valued Rectifier) are used in order to obtain the signal envelope from complex signal. It is found out that the detector using AVR has much more robust detection capability than that using SLR especially for the signal having small excess bandwidth. Furthermore, it is presented that the proposed symbol rate detector can be used as a passband symbol timing synchronizer and then its jitter performances are obtained from a simulation study. Second, complex fourth-power transform of complex envelope is used to detect a peak frequency component which gives the deterministic information for the carrier frequency of unknown signals. It is shown from simulation results that the carrier frequency can be detected accurately even at low SNR (Signal-to-Noise Ratio). Third, in order to identify the modem constellation type, the propos...