Energy-Efficient CPM Modulation for IoT: A Joint Index and Spread Spectrum Approach

The rapid advancement of Internet of Things (IoT) technologies demands low power consumption, high data rate, and massive connectivity. To meet these requirements, this paper proposes a novel modulation scheme, IM-CPM-SS, which integrates continuous phase modulation (CPM) and spread spectrum (SS) inspired by index modulation (IM). The design leverages the quasi-orthogonality of CPM-SS symbols to facilitate joint modulation and spreading, thereby enhancing performance. We analyze and validate the quasi-orthogonal properties of CPM-SS symbols and develop both coherent and non-coherent demodulation methods to support varying receiver capabilities. Approximate bit error rate (BER) expressions are derived, and the scheme is evaluated in terms of data rate and computational complexity. Simulation results show that IM-CPM-SS achieves superior BER performance under diverse conditions compared to conventional schemes with similar complexity. Moreover, it maintains the constant-envelope and continuous-phase features of CPM, enabling efficient spectrum usage and compatibility with nonlinear power amplifiers. These advantages make IM-CPM-SS a promising solution for energy-efficient, high-reliability IoT applications.