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25 | mmWaves Moving to mmWaves poses challenges in terms of the characteristics of the radio channel and the fact that small-wavelength waves experience a higher rate of attenuation, e.g., due to atmospheric conditions. However, by exploiting shorter wavelengths with smaller antenna array elements that can be packed together more tightly, it's feasible to implement antenna arrays with tens or even hundreds of antenna elements in the mmWave band. This approach creates antenna systems with high antenna gains and narrow beams that not only compensate for the higher losses experienced by mmWaves but may reduce interference through their narrow beams. Throughput Optimization To achieve higher throughput, NR exploits the following techniques: • Higher spectrum utilization. The spectrum use defines the ratio of the transmission bandwidth to the channel bandwidth with guard bands on the edges. In Long-Term Evolution (LTE) communication, spectrum use is 90 percent; in NR, spectrum use can reach 98 percent. • Higher post–fast Fourier transform (FFT) subcarrier occupancy. In LTE, the number of occupied subcarriers after FFT is about 60 percent of the FFT size. For example, a 20MHz LTE system uses a 2,048-point FFT, of which 1,200 subcarriers are occupied. In NR, the number of occupied subcarriers increases by 25 percent in addition to the increase resulting from higher spectrum use. The channel bandwidth then increases by 25 percent for the same FFT size and subcarrier spacing. • Larger FFT size. Advances in integrated circuit (IC) technologies make it feasible to support larger FFT sizes. NR supports a maximum FFT size of 4,096, which is double that of LTE. • Higher subcarrier spacing. In LTE, there is single subcarrier spacing for data transmissions of 15 kilohertz (kHz). In NR, the maximum subcarrier spacing is 120kHz, which increases the channel bandwidth by a factor of eight without the need to increase the number of subcarriers. In FR1, the allowed subcarrier spacings for data transmissions are 15, 30, and 60kHz; in FR2, the allowed subcarrier spacings for data transmissions are 60 and 120kHz. Table 2 provides two examples for FR1 and FR2 and shows how the preceding techniques increase the throughput. 5G NR Physical Layer Aspects 5G NR is an orthogonal frequency-division multiplexing (OFDM)–based air interface, where the channels and signals in transmission are allocated to individual LTE Baseline at 20MHz NR at 100MHz (FR1) NR at 400MHz (FR2) FFT Size 2,048 4,096 4,096 Spectrum Use 90% 98.28% 95.04% Subcarrier Occupancy 58.6% 80.0% 77.3% Number of Subcarriers 1,200 3,276 3,168 Subcarrier Spacing 15kHz 30kHz 120kHz Transmission Bandwidth 18MHz 98.28MHz 380.16MHz Channel Bandwidth 20MHz 100MHz 400MHz Increase in Throughput for the Same Modulation and Coding Scheme 1 5.46x 21.12x Table 2: Throughput Increase in NR for FR1 and FR2

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