How does a radio channel change with frequency?
6G wireless connectivity will span a diverse range of frequency bands, including the currently used sub-6 GHz and millimetre-wave spectrum, along with newly explored bands in the 7-15 GHz centimetric range and the 100-300 GHz sub-THz range. But how and why does the radio channel, or actually the underlying propagation channel of electromagnetic (EM) waves, vary across different frequenciesBy frequencies we mean here the radio frequencies used for wireless communications and similar applications.? In radio communications, the signals are transformed into EM waves and radiated by a transmit antenna; they propagate through a channel constituted by the physical environment and are finally captured by a receive antenna. Antennas affect the polarization of waves and attenuation of signals in angular and frequency domains. Let us neglect the antenna effects in the following discussion and focus on the propagation channel.
The energy of a wave radiated by transmit antennas cannot increase in the channel, but it can only attenuate. Moreover, the wave can change polarization and propagation direction and experience delay due to the limited phase velocity (speed of light). Moreover, the energy carried by EM waves does not vanish. It either converts to other forms (e.g. heat) or becomes dispersed/re-directed to other directions than the observation point, i.e., the receive antenna. A wave propagates in principle to all directions outwards from the transmit antenna. Even without any interactions or other losses, this causes a reduction of the power intensity, which is proportional to the propagation distance squared. This phenomenon is the distance-dependent transmission loss, which itself is not frequency dependent. However, by including antennas, we get the well-known Frii’s transmission formula, which, on the other hand, indicates path losses increasing proportionally with the frequency squared.
Other sources of attenuation and also depolarization and re-direction of waves are so-called wave-material interactions. They are reflection, diffraction, scattering, shadowing from/by obstacles, and interaction with atmospheric gases and particles. Gases are, e.g., oxygen and water vapour, and particles are, e.g., raindrops and ice crystals. Atmospheric interactions are frequency-dependent by two different mechanisms. When the frequency is high, and consequently the wavelength is small and comparable to, e.g., a water drop size, the main mechanism is scattering that re-directs waves from the original propagation direction, causing attenuation of the received wave. When the wavelength is large as compared to the particles and especially to gas molecules, the main mechanism is absorption. Gas molecules have different energy levels that can be excited by EM waves of different radio frequencies. Hence, the absorption in propagation through such substances has a strong variation across the frequency – forming an absorption spectrum. Most influential in practical applications is the oxygen absorption around 60 GHz and the water vapour absorption having significance at close to 300 GHz and above. The absorption loss increases exponentially (linearly in decibel units) with increasing propagation distance.
Reflection, diffraction, and scattering take place in boundaries of the propagation medium, e.g., on various obstacles, where involved materials have different EM properties. Material parameters themselves, such as permittivity and conductivity, are not strongly frequency-dependent on practical radio frequencies. Reflection from smooth surfaces, which are large with respect to the wavelength, is almost frequency-independent. Diffraction from edges of large obstacles, on the other hand, decreases with increasing frequency. As an ultimate example, the visible light practically does not diffract at all around corners. Scattering from rough surfaces relates to radio frequencies by the level of roughness in wavelengths. The higher the roughness, i.e., height variation of non-smooth surfaces, the stronger the scattering phenomenon, and the more the wave becomes dispersed in various directions. On the other hand, with increasing frequency, the distance-dependent attenuation of scattered waves becomes higher due to the spreading of scattered waves.
There is also another aspect that makes the mentioned wave-material interactions frequency-dependent. Namely the size of obstacles in wavelengths, which has a two-fold impact. Firstly, with increasing frequency even small obstacles may cause strong reflection and scattering, and on the other hand, severely shadow the pathway by blocking the wave. Secondly, again, with increasing frequency, even a small clearance between the obstacles is sufficient for propagation between the obstacles without excess attenuation by shadowing. Sizes of sufficient clearances and of significant obstacles depend on the propagation distance and the wavelength, and so-called Fresnel zones can approximate it. Also, the transmission loss through obstacles increases strongly with frequency; hence, waves penetrating through, e.g., building walls, become very weak on higher frequencies. Even tree leaves and the foliage in general can cause substantial attenuation.
As a summary, we can say that many aspects of the propagation channel are clearly frequency dependent, though in multipath propagation the dominant paths are often the same regardless of the frequency. With increasing radio frequency ↑:
• Distance-dependent path loss increases ↑
• Reflection loss remains unchanged ↔
• Diffraction loss increases ↑
• Penetration loss increases ↑
• Scattering is quite complicated and has no clear trend ↔?
• Atmospheric losses increase, with strong variation due to the absorption spectrum around 60 GHz and above 300 GHz ↑~
About the authors
Pekka Kyösti
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Postdoctoral Researcher