Trellis Coded Chirp Spread Spectrum Modulation for Massive Machine Type Communications

In this paper, we propose a new PHY layer trans-mission scheme for the uplink of a Single Input Multiple Output (SIMO) massive Machine Type Communication (mMTC). Our scheme is based on a Chirp Spread Spectrum (CSS) modulation at the transmitter side and a joint Maximum-Ratio Combiner (MRC) and Successive Interference Cancellation (SIC) decoder. One of the key characteristics of CSS signals is the linear variation of the frequency carrier over time, with symbol-dependent cutoff points. In the frequency domain, this temporal behavior leads to a peak power with magnitude and carrier-frequency position that are symbol-dependent. Indeed, adjacent carriers to the peak-carrier also hold a substantial amount of power. In a multi-user context, the non-homogeneous power-peak distribution across the different symbols biases the in-terpretation of the spectrum at the receiver side. Indeed, in a noisy environment, the high-power on the neighboring carriers of the peak-one induces erroneous decision. These two issues are further exacerbated by the SIC decoder as errors propagate from one device to another, creating a bottleneck. To address these challenges, we propose to (i) balance the power at the peak-carriers regardless of the symbol; (ii) refine the position of the peak-power index using a trellis-coded modulation (TCM). Our results show that the power balance combined with the TCM refinement of the peak-power index enhances the error performance rate of the SIC decoder.