Constellation identification using the Radon transform

The problem of identifying the type of signal constellation in use by a data modem without the active cooperation of the transmitting modulator is considered. This problem arises in so-called `multi-drop´ networks in which nodes may independently enter and leave the network. It is shown that Radon transform techniques can be used to recognize the type of rectangular or cross constellation in use, and to determine its orientation, even if the received signal-to-noise ratio is quite poor. This method is successful even when there is incomplete equalization and some residual carrier remains. With these estimates a decision-directed demodulator can be approximately initialized. The technique itself and examples of its performance over a range of constellation size, noise power levels, and number of input samples are described