BRAVE revisit the PHY-layer by looking back on single-carrier (SC) modulations, thus allowing for improved spectral efficiency and reduced power consumption (i.e. from lower PAPR). Indeed the line-of-sight transmission and the use of large antenna arrays make the propagation channel favourable to SC.
The deﬁnition of an efﬁcient physical layer (PHY-layer) in the considered frequency band is very challenging due to radio frequency impairments, as described previously, and complexity. The PHY-layer can be revisited by looking back on single carrier (SC) modulation for the waveform, and to investigate techniques for the transceiver combined with approaches close to analog systems. Similar studies were conducted in the 80’s and some concepts, forgotten, could be resurrected. For instance, non-coherent modulations inherently robust to phase noise effect could be a good candidate for sub THz communication.
Multicarrier systems were justiﬁed in frequency selective channels, in order to simulate a great number of ﬂat small channels. The propagation above-90 GHz is often characterized by a dominant path, obtained either from LoS situation or antenna beam alignment. Therefore probability for the frequency-selective situation is very low. The special channel characteristics make single carrier modulation schemes, as opposed to the popular multicarrier schemes, potentially more appropriate for communication systems operating in such frequency bands above 90 GHz.
It is well known that single carrier signal has much lower PAPR. Recent researches for mmW communications are studying SC modulations as the Continuous Phase Modulated Single-Carrier Frequency Division Multiple Access (CPM SC-FDMA) which is a generalization of SC-FDMA using samples of a Continuous Phase Modulation (CPM) signal as generalized coded modulation input symbols. This scheme can provide a low PAPR signal with very low spectral side lobes. Another scheme is the Constrained Envelope Continuous Phase Modulation (ceCPM-SC) which is a true single carrier scheme generalizing CPM by allowing controlled small envelope variations.