In order to increase the data rate of cellular network, 3GPP defined a new set of radio frequencies for 5G NR and beyond. These radio frequencies are classified into two groups, FR1 and FR2 respectively. FR1 is also called sub-6 GHz band, which includes the traditional 3G/4G bands and new band in the 3.5 GHz spectrum. FR2 bands are 6 GHz and above, which also includes the mmWave band n261 in the North America at 28 GHz. For more details on 3GPP definition and evolution, please see reference [1].
The FR2 bands are designed to provide very wideband data transmission. For example, n261 provides a bandwidth of 400MHz, while n257 commonly used in the Asia Pacific region provides a total bandwidth of 800 MHz. Undoubtedly, in order to support the next generation high bandwidth and low latency applications, FR2 bands will play a very important role. However, radio propagation is restricted by path loss that is related to the carrier frequency. A doubling of the carrier frequency would theoretically lead to an increase of 6 dB in path loss. As a result of this loss, a 28 GHz 5G NR signal would require 20 dB more power than a 2.8 GHz 5G NR signal to reach the same coverage area. There are also more losses in terms of penetration and shadowing as the frequency increases.
Figure 1 shows an urban area of 400m by 400m served by a 28 GHz 55 dBm mmWave base station in the position of the red dot. The black areas are footprint of buildings, while the yellow dot is a smart repeater behind a building. Figure 2 shows the intensity of the base station in the coverage area. A typical working signal level is above -70 dBm, which corresponds to an orange intensity level. Most of the area at the lower half of the map would have spotty coverage if only the base station is transmitting data.
Next we shall evaluate the effectiveness of having a smart repeater to improve the coverage of the lower half of the coverage area.