As the Internet-of-Things (IoT) has expanded, multiple solutions have attempted to address the issues of the Low Power Wide Area (LPWA) networks physical layer. In a previous work, we proposed the Turbo-FSK, a constant envelope modulation with orthogonal alphabet that allows the receiver to operate at very low levels of power (high sensitivity performance) and very low levels of energy per bit E$_b$. The scheme was demonstrated to approach Shannon's limit as close as 0.29dB. However, the scheme lacks of flexibility in terms of spectral efficiency (always lower than 10$^{−1}$ bits/s/Hz), especially compared to the recently standardized Narrow-Band IoT (NB-IoT) solution. In this work, we propose an evolution of the initial scheme, so-called Coplanar Turbo-FSK (C-TFSK). In order to increase the spectral efficiency of the system, two new features are introduced: a modulation combining linear and orthogonal properties where only subsets of the alphabet are orthogonal and a puncturing mechanism. Several aspects of the scheme are then studied under asymptotic hypothesis, such as the influence of the linear component of the alphabet and the effects of puncturing. The high flexibility in term of spectral efficiency, the short distance to Shannon's limit and the constant envelope property make the C-TFSK a serious contender for the physical layer of the IoT.