Many wireless short packet systems have been recently proposed because of new emerging use-cases in wireless communications. These competing schemes exhibit quite different design choices, at least from the physical layer perspective. An interesting question arising is whether there would be a particular waveform that could best fit the short length requirements in terms of transmission rate. In this paper, we focus on four short packet modulation schemes that are Ultra Narrowband (UNB), Direct Sequence Spread Spectrum (DSSS), Chirp Spread Spectrum (CSS) and CP-OFDM. We propose a discrete model for CSS modulation and compare the best achievable transmission rates of the four modulation