  We propose a dual-mode (DM) time domain multiplexed (TDM) chirp spread
spectrum (CSS) modulation for spectral and energy-efficient low-power wide-area
networks (LPWANs). DM-CSS modulation that uses both the even and odd cyclic
time shifts has been proposed for LPWANs to achieve noteworthy performance
improvement over classical counterparts. However, its spectral efficiency (SE)
is half of the in-phase and quadrature (IQ)-TDM-CSS scheme that employs IQ
components with both up and down chirps, resulting in a SE that is four times
relative to Long Range (LoRa) modulation. Nevertheless, the IQ-TDM-CSS scheme
only allows coherent detection. Furthermore, it is also sensitive to carrier
frequency and phase offsets, making it less practical for low-cost
battery-powered LPWANs for Internet-of-Things (IoT) applications. DM-CSS uses
either an up-chirp or a down-chirp. DM-TDM-CSS consists of two chirped symbols
that are multiplexed in the time domain. One of these symbols consisting of
even and odd frequency shifts (FSs) is chirped using an up-chirp. The second
chirped symbol also consists of even and odd FSs, but they are chirped using a
down-chirp. It shall be demonstrated that DM-TDM-CSS attains a maximum
achievable SE close to IQ-TDM-CSS while also allowing both coherent and
non-coherent detection. Additionally, unlike IQ-TDM-CSS, DM-TDM-CSS is robust
against carrier frequency and phase offsets.
