Hello. In this video, we will learn millimeter wave system and its challenges. We will begin with understanding the Millimeter Wave System and the associated challenges in realizing a Millimeter Wave System, and lastly, we will summarize this video. As the existing spectrum are densely utilized, we need a new spectrum. Thankfully there is a lot of spectrum available in the millimeter wave frequency range starting from 30 GHz to 300 GHz. For example, a few of the selected bands are local multipoint distribution service at 28 to 30 GHz. License-free band at 60 GHz. E band at 71 to 76 GHz, 81 to 86 GHz and 92 to 95 GHz. The wireless system operating at Millimeter Wave Frequencies are referred to as Millimeter Wave System. Realizing a millimeter wave system poses several challenges, such as low range of communication that occurs because of free-space loss, atmospheric attenuation, and rain-induced fading. Apart from this, the penetration capability is also low that arises because of the material penetration, rain-induced fading and foliage attenuation. A detailed illustration of these challenges are summarized in the diagram here. In the diagram, other than the aforementioned issues, you can see the challenges like Doppler Effect and Propagation Mechanism. As you know that the millimeter wave system uses higher carrier frequency, the Doppler effect also increases with the carrier frequencies. Apart from this, the propagation mechanism such as refraction, diffraction, multi-path scattering, and reflection also plays a critical role in realizing a millimeter wave system. Now let us focus on Free Space Loss. Usually, the free space loss between two communicating notes is expressed as, where F is the carrier frequency in gigahertz and D is the distance between the transmitting and receiving antennas in kilometers. In the figure, the free space loss with respect to distance and different carrier frequency has been outlined. Here you can observe that as the distance increases, the free space loss also increases, and it also increases with increasing carrier frequency. However, as per Friis’ law, the relation between the received power (P_r) and the transmit power (P_t) is defined as this, where lambda is the carrier wavelength. D is the distance between the transmitting and receiving antennas. G_t and G_r denote the transmitting and receiving antenna gains, where G_t is a function of the effective aperture area of transmitting antenna and the carrier wavelength lambda. Similarly, the G_r is a function of the effective aperture area of receiving antenna and the carrier wavelength lambda. After substituting the expressions for G_t and G_r, the Friis’ law can be expressed in terms of the effective aperture areas of transmitting antenna and the receiving antenna, and the carrier wavelength lambda and the distance between the transmitting and receiving antennas. Interestingly, the antenna gains G_t and G_r are inversely proportional to the carrier wavelength. And, therefore, as the career wavelength reduces, the antenna gains G_t and G_r increase, which is true for a millimeter wave system. And, therefore, the propagation loss reduces. Let us understand this through an illustration, we consider an isotropic antenna having gain G_t and G_r, the transmit and receive antenna are separated by a distance D, P_t is the transmit power and P_r is the received power. Once we increase the carrier frequency, that is, we reduce the wavelength, the antenna gains G_t and G_r increase and their effect is something like this. Now, let us talk about Atmospheric Attenuation. Here in the diagram, the specific attenuation in dB per kilometer is plotted for oxygen and water vapor. The losses for 02 objections are at 60 GHz is around 15 dB/km, whereas it is 1.4 dB/km at 119 GHz. Similarly, the losses due to resonance of H20 molecules is 0.18 db/km at 23 GHz, whereas it is 28.35 dB/km at 183 GHz. And, therefore, we need to make a suitable choice of operating frequency so that atmospheric attenuation should be at lower side. In the same diagram, the blue lines represent the effect of light rain and heavy rain. We can see that if the rain is on the heavier side, the specific attenuation, is higher. Usually, the specific rain attenuation at a given rain rate R in millimeter per hour increases exponentially up to the critical frequency. The relation is given here, where kappa and alpha are functions of the operating frequency and the other factors like temperature, polarization, altitude, etc. The illustration here shows why the rain induced fading is critical for higher carrier frequency. We have considered here two different carrier frequencies, one at 3 GHz, the other at 60 GHz. The 3 GHz carrier frequency implies the wavelength as 100 mm, whereas 60 GHz carrier frequency implies the violence as 5 mm. As the droplet size is roughly of the order of 2 mm, as the droplet size is roughly of the order of 2 mm, the same droplet, what seems to be very small once we are operating at 3 GHz, whereas it became of the same order once we operate at 60 GHz where the wavelength is 5 mm and, therefore, it induces a fading effect. Now let us move to the Material Penetration Capability of the millimeter wave carrier frequencies. The table here shows the penetration losses of different materials in excess of free space loss in dB/kilometer. For example, once we use a mesh glass, the penetration loss at 2.5 GHz is around 24.1 dB/cm, whereas it is 39.1 dB/cm for 60 GHz system. Similarly, you can observe the penetration losses for dry wall and the whiteboard in the table here. Let us discuss the challenges involved in propagation mechanism. Similar to the rain-induced fading, thee small structural features that are comparable in size to the millimeter wavelength endure a rich diffused scattering. Here in the diagram, you can see that the incident signal is diffused into multiple directions. Therefore, each reflected ray has a different reflection direction leading to a scattered power reception and to a poorer smooth surface specular reflection. Next, we will discuss about the Doppler Spread. The Doppler spread results in frequency dispersion is caused by the mobility of the communicating nodes, which is proportional to frequency and speed. Thus, the Doppler spread at mmWaves is significantly higher than at frequencies below 3 GHz. For example, the Doppler spread at 60 GHz is 20 times higher than at 3 GHz. Last but not the least, is an interesting challenge related to the millimeter wave system that is Human Blockage. Whenever a millimeter wave signal traverses through a human body, it gets blocked and therefore it can reduce the network throughput by 10% given a human density of one person per 10 m square. Further, it was reported that only up to 42% of the signal power incident on the human skin is being reflected at 60 GHz. However, the good part is that the severity of human shadowing is not influenced directly by the number of blocking persons, but rather by the transmit and receive antenna configurations. In this video, we have learned the need for millimeter wave system and its realization challenges in detail. Thank you.