Читать книгу Advanced Antenna Array Engineering for 6G and Beyond Wireless Communications - Richard W. Ziolkowski - Страница 24

To date, every new generation of mobile wireless communication has been allocated its own dedicated spectrum. This is again true for 5G networks. Given the fact that the radio spectrum is a worldwide limited resource, the mobile wireless communication industry has been “forced” to start using the mm‐wave spectrum to accommodate some portion of its 5G networks, known as 5G mm‐wave. Application examples include small cells for data‐hungry hot spots and fixed wireless access services where line of sight (LoS) propagation is easier to be guaranteed. Moving forward to 6G, it is expected that some airborne and satellite systems will also embrace the mm‐wave spectrum. Compared with the microwave frequency bands, the propagation of mm‐waves is negatively impacted by higher attenuation rates and severe weather.

To emphasize this issue, Figure 1.9 shows the attenuation of electromagnetic waves from DC into the low terahertz (THz) range as functions of the propagation distance, altitude, and weather conditions. Notice that there are some windows in these spectra where the atmospheric attenuation is high, such as around 60 GHz, and, conversely, much lower. The former are clearly not suitable for long‐distance communication. The latter are targeted for many applications. Also notice that the propagation losses are reduced at higher altitudes where the air is thinner. Examining Figure 1.9 more closely, it is little wonder that the current “first choice” for commercial 5G rollouts of mm‐wave systems is at the lower end of the mm‐wave range, i.e., around 28 GHz.

Figure 1.9 Specific atmospheric attenuation (dB/km) at the indicated altitude h and for several exemplary weather and air conditions.

Source: Based on [16] / IEEE.

Certain important advantages for 5G operations are offered by mm‐wave systems. One is that high‐gain mm‐wave antenna arrays can be realized over physically small areas because the associated wavelengths are small (recall that the gain of an aperture antenna – Gain = 4π Area/λ²). In fact, given the inherent high propagation losses of their radiated fields, high‐gain antennas are needed for virtually all mm‐wave communication systems. As a result, it has become imperative to develop mm‐wave beamforming networks to support multi‐beam mm‐wave antennas. In the current 3GPP standards for 5G mm‐wave, for example, user equipment (UE) or terminals are required to have an array antenna with between 8 and 64 elements [17].