A simple watermarking system

In a broadcasting system, the transmitter would be constrained to use a constellation, which can be demodulated by all the users and especially the users having the lowest signal to noise ratio (SNR). Choosing one type of modulation, would limit the reach of the broadcasting system. In this paper, we propose to use a simple watermarking system, to generate two classes of transmission on the same channel and using the same frequency plan. Basically, we will have like two channels running at the same symbol rate but with different constellation sizes: a slow channel and a fast channel. A slow channel is used to carry information at a lower constellation (QPSK for example) to users who have low SNR. The fast channel is used for the users who have high SNR. In fact, the channel, which uses the higher constellation, can be embedded on the QPSK signal without damaging the performance of the QPSK receiver. The obtained results agree with the statement of Costa on "writing on the dirty paper".