Chirp Spread Spectrum-Based Waveform Design and Detection Mechanisms for LPWAN-Based IoT: A Survey

The Long Range protocol, commonly referred to as LoRa, is a widely adopted and highly regarded method of utilizing chirp spread spectrum (CSS) techniques at the physical (PHY) layer to facilitate low-power wide-area network (LPWAN) connectivity. By tailoring the spreading factors, LoRa can achieve a diverse array of spectral and energy efficiency (EE) levels, making it amenable to a plethora of Internet-of-Things (IoT) applications that rely on LPWAN infrastructure. However, a primary drawback of LoRa is its relatively low data transfer rate. Despite this, there has been a dearth of research dedicated to enhancing the data transfer capabilities of LoRa until recently, when a plethora of CSS-based PHY layer alternatives to LoRa for LPWANs was proposed. This survey, for the first time, presents a comprehensive examination of the waveform design of these CSS-based PHY layer alternatives, which have been proposed between 2015 and 2022. A total of fifteen alternatives to LoRa are analyzed and compared. Other surveys on LoRa focus on topics such as LoRa networking, the deployment of LoRa in massive IoT networks, and LoRa architectural considerations. In contrast, this study delves deeply into the waveform design of alternatives to LoRa. The CSS schemes studied in this study are classified into three categories: single chirp, multiple chirps, and multiple chirps with index modulation, based on the number of activated frequency shifts activated for un-chirped symbols. The transceiver architecture of these schemes is thoroughly explicated. Additionally, we propose coherent/non-coherent detection mechanisms for specific schemes that have not been previously documented in the literature. We also provide some key insights and recommendations based on the performance of the schemes. The performance of the schemes is evaluated based on metrics such as EE, spectral efficiency, the bit-error-rate (BER) in additive white Gaussian noise, and BER in the presence of phase and frequency offsets. Finally, we highlight some open research issues and future research directions in this field.