Sub-THz mesh backhaul
The long-term limitations of 5G standards are stressed already by the telecommunication industry and community research , for e.g. the delivery of ultra-low-latency broadband services, or the emergence of ubiquitous intelligence . Next-generation wireless networks are imagined to be faster (1 Tbps for instance), more reactive (sub-ms latency), ultra-reliable and denser, thus allowing for very accurate positioning, highly-immersive experiences, smarter autonomous objects, etc. As for the previous mobile network generations, the exploitation of new and wider bandwidths at higher frequencies is an obvious and promising solution towards significantly increased data rates and capacity in beyond-5G or 6G communication systems. The “sub-THz” spectrum from 90 to 300 GHz is definitively identified as a key enabler. An aggregated bandwidth of 58.6 GHz was identified in  as possibly available for terrestrial radio-communications between 90 and 200 GHz.
Elaboration of future sub-THz systems is facing many challenges in particular at the PHY layer such as the strong propagation losses, or the increased phase noise (PN), which are both considered in this paper. Due to the strong propagation constraints, the short-range connectivity is a relevant sub-THz target application. However, the huge available bandwidth can also serve the backhaul transport network. It may offer the future capacity required by cloud-RAN and massive front-haul data streams (e.g. for massive MIMO, cell-free MIMO and ultra-dense networks), ubiquitous AI (artificial intelligence), mobile video gaming, etc. That is why the authors explore the feasibility, reliability and achievable data rates of such a backhaul solution, with specific focus on the propagation impact.
The BRAVE project is designing robust communication from both receiver and transmitter perspectives. The optimum symbol detection criterion and the corresponding probabilistic demapper is derived for PN channel upon the maximum likelihood (ML) decision rule. We also propose a PN robust modulation scheme defined upon an efficient and structured constellation, adaptable to any signal-to-noise ratio (SNR) and PN variance.
The propagation channel properties are characterized and modelled to achieve a realistic evaluation of the proposed modulation. Only few sub-THz channel sounding campaigns have been published yet, as the equipments are new, complex and costly. Those recently realized inside various indoor environments do confirm the clear line-of-sight predominance, the channel sparsity and strong attenuations. The digital simulation is a convenient solution to complement this characterization, and produce on-demand channel samples for any kind of scenario. The Volcano ray-based model has been extended up to the sub-THz frequencies. It is employed in the BRAVE project to predict the propagation in two backhaul use cases: in-street outdoor, and inside a large venue. The performance of the proposed modulation scheme has been assessed considering this propagation data combined with highly-directive antennas and different phase-noise conditions.
The results of this research has been published at the EuCAP conference 2020; they are summarized in the picture below. The simulations have demonstrated the feasibility of sub-THz mesh backhaul networks, using a PN-robust modulation scheme, and considering real propagation constraints.
 M. Latva-aho, and K. Leppänen, Key drivers and research challenges for 6G ubiquitous wireless intelligence, White paper, Sept. 2019.
 Y. Corre, G. Gougeon, J-B. Doré, S. Bicais, B. Miscopein, M. Saad, J. Palicot, F. Bader, E. Faussurier, “Sub-THz Spectrum as Enabler for 6G Wireless Communications up to 1 Tbit/s”, 1st 6G wireless summit, Levi, Finland, March 2019.