R not to overlap with the first three LTE OFDM symbols that carry the control channels (PDCCH). This can be easily done with the mini-slots. In LTE, it is rather useful to have Multimedia Broadcast Single Frequency Network (MBSFN) subframes, where some OFDM symbols are left blank. This has enabled many new features in LTE. So, having the option to schedule devices in only a limited set of OFDM symbols in a slot will free those OFDM symbols for future use without breaking compatibility with other devices, and they will support a mini-slot from the beginning. Finally, this concept is forward-compatible toward an unlicensed spectrum operation. Resource grid Up to this point, we have analyzed the NR frame structure in the time domain. Now, let us focus on the frequency domain. In the frequency domain, a Physical Resource Block (PRB) in NR is composed of 12 subcarriers, like in LTE. A Resource Element (RE) corresponds to one OFDM symbol in one subcarrier. Waveform, numerology, and frame structure One Resource Block (RB) (12 subcarriers) of = e.g 15, 30, 60 kHz One Resource Element (RE) (One modulation symbol) Figure 4.23 - NR resource grid Note that a PRB in NR is not statically 180 kHz in the frequency domain, like in LTE. In NR, due to the numerology, as the subcarrier spacing changes, 12 subcarriers will occupy different amounts of bandwidth. max, u 15 kHz 30 kHz 60 kHz 120 kHz 240 kHz 480 kHz Figure 4.24 - Minimum and maximum resource blocks changing by numerology The preceding table indicates the minimum system bandwidth limited by the synchronization signals and the maximum number of PRBs, 275, which corresponds to 3,300 subcarriers. For the bigger 5G Air Interface and Physical Layer Procedures - Part 1 subcarrier spacing, the limiting factor is not the number of subcarriers but the component carrier bandwidth, which cannot be more than 400 MHz. The resource grid changes as the number of available subcarriers and the number of OFDM symbols change. 120 kHz (p = 3) PRB # o PRB # 1 60 kHz (p=2) PRB # o PRB # 1 PRB 