# Fixed Wireless Access in Flexible Environment: Problem Definition and Feasibility Check
## 2 Related Work  
The work reported in [3] describes the availability of routes in hybrid networks but does not provide the overall availability of a connection considering possible failover alternatives, no system-level availability provided. While the authors of [10] calculate combined wired-wireless availability, the calculation is limited to primary and secondary paths only, in addition to assuming no redundancy in the wired network. Several research papers discuss the availability of wireless radio and wireless optical links [11; 12; 13; 14]. The WOBAN paper [15] considers the availability of wireless mesh networks and studies multipath routing as well. In contrast to these works, the present study considers dynamicity and even the concept of software defined networking (SDN) [16]: the available resources may change in the existing network and the traffic added can be dynamic consisting of multiple streams.  
Recent papers and actual network deployments have shown that fixed radio links operating in _E_-band (71-86 GHz) [17] can reach 'fiber-like' throughputs of 10 Gbps and even beyond. Available wide radio-frequency (RF) bandwidths combined with new technologies (e.g., dual-band and cross-polarization links, adaptive code rates using Quadrature Amplitude Modulation (_n_-QAM), Automatic Transmit Power Control (ATPC) and error correction coding) make even 20 Gbps throughputs available for short links [6; 7; 18]. There is already intensive research in the domain of sub-THz radio communication [19; 20]. As _E_-band is used more and more frequently, it is expected that with the launch of 6G this band may also become saturated. Additional wireless communication alternatives are also required. Perspective wireless candidates are in the radio _D_-band (130-175 GHz) and free space optics (FSO) [19; 20; 21; 22; 23; 24; 25]. Note that the term bandwidth refers to the spectrum that a signal occupies at radio frequencies (in MHz), and at the same time also often used for the maximum amount of data transmitted (in Mbps) in networks over a digital connection. To avoid confusion, for the rest of the paper the term throughput will be used for bandwidth in networks.  
Finally, we mention that several microwave planning tools exist, e.g., PathLoss, IQ-Link, Aircom/Asset [8; 9; 26]; they are limited to radio link design, typically calculating frequencies up to the _E_-band only; _D_-band is discussed only in research papers [20; 21; 22; 23]. In general, radio planning tools do not support e2e availability calculation for meshed networks including FSO and fiber-optical sections. Excellent optical design tools exist (e.g., Optiwave/OptiSystem, Synopsys/OptSim, etc., [27; 28]), but they lack radio link calculation capabilities. Generic optical system design tools support mainly power budget calculations and geographic visualization for fiber-optical networks, such as PON (Passive Optical Network) or FTTH (Fiber-to-the-Home) systems. Furthermore, optical software tools focus mainly on fiber-link design and rarely handle FSO links. Only a few tools can handle both fiber-optic and radio technologies, e.g., Spider (non-commercial tool), Forsk/Atoll [29]. According to the authors' current information, there is no commercially available planning tool that supports combined optical and radio anyhaul planning, including e2e availability calculations and the same time considering dynamic traffic load.