## 2 The Fifth Generation of Cellular Communications
To summarize, it is evident that the very large capacity gains expected in International Mobile Telecommunications 2020 (IMT-2020) will be attained as a result of the use of different techniques, among which mm-wave, beamforming, and massive MIMO are considered the most important. Note that IMT-2020 corresponds to the future 5G version standardized by the International Telecommunications Union (ITU). Regarding mMTC communications [26], the ITU has defined strict requirements in terms of the number of connected devices and autonomy. More precisely, the number of connected devices should be one million per square kilometer, and each device should have up to 10 years of autonomy or more. URLLC [8] will enable the development of large sensor networks where the nodes can communicate with very little human interaction. The ITU has defined very ambitious requirements for URLLC in terms of latency. More concretely, the radio interface latency should be just 1 ms, which involves a tenfold reduction compared with the 10 ms achieved in 4G communications. In that context, multiple access will be redesigned in 5G to have higher flexibility [27]. Figure 4 summarizes some of the performance requirements of 5G, showing a comparison with 4G communications. As in 4G, the multiple access of 5G communications is based on orthogonal frequency division multiple access (OFDMA) for both the downlink and uplink directions. However, contrary to what happens in 4G, the OFDMA design is very flexible, since different use cases with different requirements should be addressed. As will be described in Section 3, NOMA is viewed as a strong multiple access candidate for the new releases of 5G due to its increased spectral efficiency. One of the new features of 5G NR is the scalable frequency spacing of OFDMA subcarriers. More precisely, although the subcarrier spacing was \(\Delta f=15\) kHz in 4G, the subcarrier frequency in 5G is given by \(\Delta f=2^{\mu}\times 15\) kHz, where \(\mu\) can vary from \(\mu=0\) to \(\mu=5\) (the maximum subcarrier spacing being \(\Delta f=480\) kHz). Under these conditions, the transmitted waveform can be adapted to the channel conditions. This is known as 5G numerology, and it is very important in 5G since, as the carrier frequency can vary substantially (i.e., both microwave and mm-wave spectrums will be used), the channel conditions can also vary. For instance, when the multipath is strong, a lower subcarrier spacing is desirable. However, when the phase noise is large (which happens frequently in mm-wave communication), a large \(\Delta f\) is desirable. A variable \(\Delta f\) is also important to support communications where the latency is critical (i.e., for the URLLC use case), since the duration of the OFDMA symbol is inversely proportional to the subcarrier spacing; that is, \(T_{OFDM}=\frac{1}{\Delta f}\). The duration of the physical resource block (PRB), which is the smallest quantity of time-frequency that can be granted to a given user, is 12 subcarriers and one time slot. When \(\Delta f=15\) kHz, there is only one slot per sub-frame (whose duration is always 1 ms). However, when the subcarrier spacing increases, the duration of each OFDMA symbol decreases, which allows for accommodating more slots in a given sub-frame. The PRBs are managed by the base station regularly so that system capacity and performance are balanced, and spectral efficiency is maximized. Table 1 shows some cases of 5G numerology. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{\(\Delta_{f}\)} & \multirow{2}{*}{**PRB (Frequency)**} & \multirow{2}{*}{**PRB (Time)**} & \multicolumn{2}{c}{**Slots Per**} & \multirow{2}{*}{**Sub-Frame**} \\  & & & **Sub-Frame** & & **Duration** \\ \hline
15 KHz & 180 KHz & 1 ms (eMBB) & 1 & 1 ms \\
30 KHz & 360 KHz & 0.5 ms & 2 & 1 ms \\
60 KHz & 720 KHz & 0.25 ms & 4 & 1 ms \\
120 KHz & 1440 KHz & 0.125 ms (URLLC) & 8 & 1 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 1: 5G Numerology examples. Figure 4: Performance requirements of 5G new radio (NR).