# Radiator Enablers for Wireless Communication Evolution
## 1 Introduction
### Key Performance Indicators and Formulas  
These use cases are related to various 5G/6G key performance indicators (KPIs) whose ever-increasing target levels are the driving forces of the mobile technology evolution. A not-complete list includes throughput (peak data rates from 20 Gb/s to 1 Tb/s), spectrum efficiency (from a peak value of 30 b/s/Hz to 60 b/s/Hz), network energy efficiency (to b/(1 pJ)), area traffic capacity (from 10 Mb/s/m\({}^{2}\) to 1 Gb/s/m\({}^{2}\)), connection density (from 10\({}^{6}\) to 10\({}^{7}\) devices/Km\({}^{2}\)), latency (from 1 ms to 10\(\upmu\)s), and reliability (from 10\({}^{-5}\) to 10\({}^{-9}\) for the frame error rate (FER)) (ref. [5]). Of course, different use cases have different needs on different KPIs, with eMBB focusing on throughput and capacity (see Formulas (1) and (2)), URLLC on latency and FER (see Formulas (3) and (4)), and mMTC mainly on connection density and energy efficiency (see Formulas (5) and (6)). For future reference and to organize our study, we include in Table 1 critical formulas that interrelate those KPIs and help clarify the steps needed to evolve the wireless communication networks further. Thus, we enlist the area traffic capacity (bit/s/Km\({}^{2}\)); the spectral efficiency (=capacity(=bit-rate[b/s])/ BW = n \(\times\) BW \(\times\) log2(1 + SINR)/BW [(b/s)/Hz], drawn from [PERSON]'s law and generalizing assuming n independent parallel channels of power gain H); the latency (in time units); the reliability (dimensionless number); the device density per angular sector (dimensionless natural number); and the energy efficiency (=capacity/P = n \(\times\) BW \(\times\) log2(1 + SINR)/P = (n \(\times\) BW \(\times\) log2(1 + SINR)/BW)/(P/BW) [b/J]).  
BW is the available spectrum; n is the independent parallel channels due to MIMO; SINR is the signal to interference plus noise ratio (with SINR = S/(I + N), where S is the signal power, I is the interference power, and N is the noise power); FER is the frame error rate; BER is the bit error rate; L is the number of bits in the frame; and P is the power of the used source. Q is the usual tail integral of the standard normal distribution function and is used in BER = Q((2 \(\times\) H \(\times\) SINR)\({}^{1/2}\)), which holds for BPSK but also uses similar formulas for other digital modulation techniques. It is important to note that the above list of wireless network governing equations (which plays a pivotal role in scheduling and resource allocation) is neither exhaustive nor in full detail, rather serving as a clear reference for conclusions that will be drawn in the following sections. Especially, these are needed to assess the use case (eMBB, RLLC, and mMTC), challenges, and the enabling technologies presented in the next section.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**KPI** & **Critical Formula** & **Equation Number** \\ \hline Area Traffic Capacity & Area Traffic Capacity = & (1) \\  & (Cells/Km\({}^{2}\)) \(\times\) (BW) \(\times\) Spectral Efficiency & (3) \\ Spectral Efficiency & Spectral Efficiency = n \(\times\) log2(1 + H \(\times\) SINR) & (2) \\ Latency & Latency = processing latency + propagation latency & (3) \\ Reliability & Reliability =FER = 1 \(-\) [1 \(-\) BER]\({}^{\text{L}}\) = 1 \(-\) [1 \(-\) & (4) \\  & Q((2 \(\times\) H \(\times\) SINR)\({}^{1/2}\))]\({}^{\text{L}}\) & \\ Devise Density per & Device Density per angular sector = Device Density \(\times\) & (5) \\ angular sector & 360/(angular sector) & (6) \\ Energy Efficiency & Energy Efficiency = Spectral Efficiency/(P/BW) & (6) \\ \hline \hline \end{tabular}
\end{table}
Table 1: KPIs and respective critical formulas.