Dual-Mode Time Domain Multiplexed Chirp Spread Spectrum
Ali Waqar Azim, Ahmad Bazzi, Mahrukh Fatima, Raed M. Shubair, Marwa Chafii
Abstract
In this work, we introduce dual-mode (DM) time domain multiplexed (TDM) chirp spread spectrum (CSS), which offers improved spectral and energy efficiency for low-power wide-area networks (LPWANs). Previous work on LPWANs has proposed DM-CSS modulation using even and odd frequency shifts (FSs) to enhance performance over classical approaches. However, its spectral efficiency is only half that of the in-phase and quadrature (IQ)-TDM-CSS scheme, which employs IQ components with both up and down chirps, resulting in a four times higher SE of Long Range (LoRa). IQ-TDM-CSS only supports coherent detection and is sensitive to the carrier frequency and phase offsets. It makes it less practical for low-cost battery-powered LPWANs in Internet-of-Things (IoT) applications. In contrast, DM-CSS utilizes either an up-chirp or a down-chirp and consists of two chirped symbols that are multiplexed in the time domain. One of these symbols is chirped using an up-chirp and comprises even and odd FSs. The second chirped symbol is also composed of even and odd FSs, which are chirped using a down-chirp. We demonstrate that DM-TDM-CSS achieves a maximum achievable SE close to IQ-TDM-CSS while supporting both coherent and non-coherent detection. Moreover, unlike IQ-TDM-CSS, DM-TDM-CSS is robust against carrier frequency and phase offsets, making it more suitable for low-cost, battery-powered LPWANs in IoT applications.