DRAFT 4) Hyper Connected: This scenario corresponds to people enjoying state-of-the-art connectivity, e.g., at least 4G and possibly 5G cellular connectivity, and/or with high speed fixed DSL or FTTH or FTTN connectivity. These subscribers typically live in urban areas, and it is extremely difficult to find them in rural areas, due to the numerous barriers discussed previously. This paper surveys the literature aiming to provide basic connectivity to the unconnected population, or aiming to increase the connectivity level of those with basic connectivity ("Under Hence, the challenges for FSO in space are less critical, and indeed significantly better performance can be achieved, especially due to the fact that the optical signal is not traversing the atmosphere. In 2008, an optical inter-satellite link achieved a data rate transmission of 5.6 Gbps for distances up to 5, 000 km in the European Data Relay satellite System (EDRS). Newer terminals can transmit at a data rate of 1.8 Gbps for distances up to 45, 000 km. The intersatellite Ka band link achieved 300 Mbps [44]. In EDRS, LEO satellites relay the data from ground stations to GEO satellites using FSO for the GEO-LEO satellite communication, and RF bands for communication with the ground stations (similarly to the satellite-satellite Ka link, the satellite-ground Ka link achieved 300 Mbps) [44]. Some of the recent satellite constellations discussed in Section II-E, e.g., Starlink, are also expected to use FSO for its inter-satellite communication [45,46,47].
D. HAPs/Drones/UAVs/BalloonsTo solve the coverage problem in large rural areas without relying on the deployment of costly wired infrastructure for backhaul, HAPs and UAVs are generally recommended [48].UAVs are proposed in [49] to provide backhaul connectivity for ground 5G base stations. A steering algorithm to allow the antennas of the UAV and the ground BS to be dynamically steered towards each other is proposed in [49], and tested via an experimental setup. In[50], a multihop network of HAPs is proposed for providing backhaul connectivity to TVWS deployments in rural areas. The HAPs communicate with each other using mmWave frequencies, and with the ground stations using FSO.An interference alignment scheme to maximize the sum-rate capacity of HAPs communicating with GSs is proposed in [51]. The scheme assumes M HAP drones and N GSs, with no CSI at the HAPs. A tethered balloon with (M − 1) × (N − 1) antennas is used as a decode and forward (DF) relay with half-duplex operation: The HAPs transmit their data to the GSs first, then the tethered balloon relays the data after performing pre-coding [51].Aerostats filled with lighter than air gas are tethered to the ground and used to provide connectivity to rural areas in [52]. Although these tethered aerostats can be used to provide backhaul connectivity by communicating with each other using directive antennas, they were also used in [52] to provide fronthaul WiFi access by using high gain omnidirectional antennas.