  LoRa is a widely adopted 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 spectral and energy efficiency (EE) levels, making it
amenable to a plethora of Internet-of-Things (IoT) applications that rely on
LPWANs. However, a primary drawback of LoRa is its relatively low data data
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, proposed between \(2019\) and
\(2022\). A total of fifteen alternatives to LoRa are analyzed. 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.
