# Key Roles of Plasmonics in Wireless THz Nanocommunications--A Survey
## 4 Beyond 5G Networks: Towards to THz Band Communications  
At the moment, wireless traffic in the access 5G networks exploits wide radio bands such as the mmW frequencies and systems, respectively. The short term roadmap for 5G and beyond communication would anticipate the establishment of many high-rate small cells forming the access link, operating in the mm wave spectrum, which are in the order of several Gb/sec, thus, in total, the aggregation capacity of the fronthaul/backhaul link should be several times higher, so as to guarantee reliable and fast data delivery from multiple users, which are connected to the small cell. Hence, in such circumstances, the THz band available bandwidth resource would be highly appreciated, while the high propagation loss of THz band fronthaul/backhaul links would be compensated by the extremely high antennas directivity [79]. In order to accommodate the continuously increasing wireless traffic demands of 5G communications and even beyond, researchers have been focused on taking advantage of higher regions in the radio spectrum (above 300 GHz), pointing to the THz band communication and infrastructure, thus enabling efficient operation of bandwidth hungry applications, that are not feasible for these systems at the moment. Hence, in the case of THz band communication, the supplied bandwidth required, would be one order of magnitude above current mmW systems, offering faster data transfer and download speeds, lower latency, and higher link directionalities [80] with non-line-of-sight (NLoS) propagation [81], as THz waves could penetrate thin objects, able to carry the data, such as the data transmission/reception by a smartphone in a pocket. Despite the fact that current research works have been focused on various 5G scenarios based on photonic assisted wireless communication systems with very high data rates, the capacity demand of THz wireless systems has not been achieved yet. Uni-travelling photodiodes (UTC-PDs) and comb sources, [82; 83], in the W-band (75-110 GHz) incorporating optical polarization division multiplexing (PDM) [84], and spatial multiple-input-multiple-output (MIMO) techniques, are considered as state of the art photonic sources, achieving data rates of 100 Gbit/s and beyond [85; 86]. However, as mentioned, higher regions in the radio spectrum at higher operating rates are required, in order to reach THz communication system goals, seeking for brand new drastic solutions. Reference [21] is a survey dedicated to hybrid radio frequency, FSO systems with THz/O links, considering it as a viable approach for equipping future THz wireless communication. Integrated  
Figure 5: In-vivo wireless nanosensor networks (iWNSNs) for healthcare applications. microwave photonics (IMWP) in the THz range [87], have been also proposed as an enabling approach for equipping 5G wireless systems, by providing optical signal generation and distribution of mm waves towards antenna terminals [88], dynamic filtering [89], optical control of antenna arrays [90], and many more other functions [91]. 5G applications, however, pose very stringent requirements on the speed of IMWP circuits, for processing mm waves or even sub-THz frequencies, in order to access the multi-GHz bandwidths required for high data rates [91]. In practice, there are a few more challenges a designer has to consider, when applying these suggested photonic based technologies in THz wireless communication systems. Specifically, given that free space path loss increases as the square of the frequency, directive antennas and directional transmitters focused on individual users, such as high gain steerable phased arrays, will be required in order to compensate for the large free space path losses of EM waves. Beam steering techniques [92] and massive MIMO antenna array elements arranged on terminal devices [93] may be promising solutions for increasing antenna directivity gain.