# Radiator Enablers for Wireless Communication Evolution
## 2 Challenges, Enabling Technologies, and Antenna Developments
### Challenges and Enabling Antenna Technologies
#### 2.1.1 The eMBB Challenge  
For the eMBB (the cellular wireless mobile telephony), the technological breakthrough is summarized in the following: increase significantly (multiplied by \(\times\)1000 (ref. [6])) the capacity-throughput-spectral efficiency subject to maintaining the degree of coverage constant. Let us elaborate on the constant (between 5G to 6G) coverage issue. Since there are 5G BSs (in the FR1 band (410 MHz-7.125 GHz)) already deployed, it would be very advantageous (in terms of 6G commercial penetration) and cost effective to use the same cell sites for 6G by just retrofitting the 6G components (mainly antenna systems). Even more advantageous would be for the 5G and 6G components to coexist on a common mast (with the latter placed rather higher above the former), at least as long as 5G networks are useful as legacy networks (ref. [7]). In this context, for the eMBB evolution to immersive communications, the key parameter is the achieved capacity. To grasp the capacity-throughput-spectral efficiency challenges, we should resort to the notion of network throughput. The network throughput (ref. [8]) (bits/(s*km2)) is the product of three terms, as shown in Equation (1): the cell density (cells/km2) (which is related to coverage), available spectrum (Hz), and spectral efficiency (bits/(s*Hz*Cell) (which is related to MIMO). Since the first term should not vary, one can use either a higher frequency band or push the limits of the MIMO technology further. The maximum spectrum used in each generation of mobile cellular technology is given by the following list (generation, spectrum (MHz)): (3, 5), (4, 20), (5, 100), and anticipated (6, 400). Thus, in this way, we achieve a \(\times\)4 improvement of the relative KPI (network throughput). Let us now turn our attention to MIMO and examine the MIMO evolution across the generations. (Generation (max) Number of Digital Ports): (3, 1(No-MIMO = SISO)), (4, (2)4--legacy MIMO)), (5, 16), (5.5, 64--massive-MIMO), (6, 256 g-MIMO). In these data, the \(\times\)4 (spectrum improvement factor) \(\times\)256 (MIMO improvement factor) = 1024 \(>\) 1000 increase in 6G capacity (or equivalent spectral efficiency and network throughput) is indeed achieved! Let us now turn our attention to constant coverage constraints. For the cell sites to remain constant, this means that practically, the ratio of received power on the receiver to the transmitted power (given by the Friis formula in the case of free space propagation) from the transmitter should not alter. This is possible if the increase in frequency is accompanied by an equivalent increase in the \"total\" antenna gain of the receiver, which, in turn, is possible if more smaller antenna elements are squeezed in the initial host area (refs. [9, 10]). The previous arguments mean that all the accessible spectrum, up to, say, THz, can be used to deliver extremely high data rates if highly directive antennas are used. Of course, extreme channel hardening (electromagnetic fields propagate like optical beams unable to penetrate opaque obstacles) and a lack of technological advances to provide adequatepower sources (increasing the frequency results in decreasing the output power (ref. [11]) and increasing on-chip distribution losses (ref. [12]) render FR2 (24.25-71 GHz), mmWave, and (sub)THz unsuitable for exploitation for wide-area mobile communications. With the above reasoning, it is accepted that the mid-band spectrum (FR1 + FR3 (7.125-24.25 GHz)) has enormous potential in terms of coverage and capacity. Nevertheless, one can also envision fixed wireless access (FWA), non-terrestrial networks (NTN), and wide area sensing. As the frequencies rise, one can again have at his disposal capacity hotspots (like avenues and dense urban), FWA, NTN, and local sensing options at mmWaves--FR2, and extremely FWA (like data center communications) in THz. Before we proceed, let us summarize that for the eMBB, one should use (1) and (2).