What after all is millimeter wave? Well, millimeter wave is nothing but a specific range along the frequency spectrum. It is that range where the wavelength of your radio or wireless signal is on the order of a few millimeters. Consider the formula we had seen in Module 1, if you do the math, you will conclude that it is roughly the frequency range above 24 GHz. That is when the wavelength of your wireless signal begins to go as small as a few millimeters. That is the reason why this frequency spectrum is called millimeter wave. 5G millimeter wave is nothing but when 5G is deployed in millimeter wave spectrum. Once again, it is underlying the same technology, 5G, just that there may be some improvements or optimizations possible on top of the principal technology. That is why 5G millimeter wave garners as much attention as we see it doing. Now, as I said, the principle operating frequencies for 5G millimeter wave are around 28 GHz and 39 GHz, which happen to be the most popular frequencies in North American cellular markets. But on an experimental basis, higher frequencies, such as a few [inaudible] above 52 GHz, are also being considered for some of the advanced uses of 5G. Here is a qualitative map of the 5G spectrum. Here are the sub six frequencies under 6 GHz and over 24 GHz would be some of the millimeter wave frequencies. Once again, sub-6 millimeter wave are merely two frequency bands in which the same technology, 5G, could be deployed. Now, why do we even consider a millimeter wave given that it is such a high frequency range and path loss is going to be remarkably high. Well, despite high path loss, there are certain undeniable advantages to millimeter wave. We will elaborate upon some of the technical advantages shortly. But the most fundamental advantage of millimeter wave is that currently millimeter wave is largely unused. Hardly any technologies that exist today have the capability to design, build, and operate hardware that can successfully function in those frequency ranges. Currently, only about satellite communication: Satellite radio, satellite TV, etc. Those are the systems in consumer domain that are capable of utilizing these frequency bands, but pretty much no other technology. Because those bands are not occupied by any other technology, ample amount of spectrum tends to be available in millimeter wave bands. As I mentioned earlier, you could have as much as 400 MHz or even 800 MHz of spectrum available for a single cellular network operator in millimeter wave frequency spectrum. This enormous bandwidth directly tells us that a millimeter wave has the foremost potential to meet the aggressive throughput requirements of the upcoming data hungry 5G applications. Think about eMBB use case on a count of 800 MHz bandwidth. Imagine the kind of throughput and network capacity your network will be able to achieve and that is the principle benefit for which we will venture into this uncharted territory of millimeter wave. But before we draw on and on about the advantages of millimeter wave, let's try to get a reality check and ask ourselves whether there are any challenges that a millimeter wave propagation could face. The answer is, absolutely. There are plenty of challenges, some of which are the salient ones are listed on the top of this slide. First is something we have already discussed in that we know path loss is roughly proportional to the frequency of operation. Higher the frequency of operation in millimeter wave, it will mean that the path loss a millimeter wave wireless signal will encounter will be substantially higher. However, beyond the most obvious one, there are a few other challenges as well. The second is listed here called building penetration loss, also known as BPL, as engineers call it. What is building penetration loss? Well, in most cases, are cell phone based stations or the gNodeB, as we'll call it in 5G, are located outdoors on top of the buildings or in open fields, where as many users may be located indoors. Maybe they are sitting in their offices, in their homes, shopping malls, restaurants, movie theaters, etc. Whenever a wireless signal from the base station has to reach that user, that wireless signal has to propagate through one or more walls of the building in which that user is located. As it happens, a lot of the wireless signals power can be dissipated, absorbed, or reflected by those intervening walls. It is possible that as much as 90 percent or even more of the energy of the wireless signal can be absorbed or dissipated in one or more of these buildings wall. That is nothing but building penetration loss. Because much of energy is dissipated while penetrating the building, so to speak, that's why deep indoor coverage becomes a challenge. Because building penetration loss is also a function of frequency, one can summarize that having a millimeter wave signal go from a base station outside to somewhere inside a building penetrating its thick brick or other walls is going to be challenging so providing deep indoor coverage is another obstacle in millimeter wave road. Another challenge and this is something that some of you may have experienced already, severe attenuation due to rain and foliage. Because of the way millimeter wave frequencies interact with humidity, water, and other elements in the atmosphere, elements such as rain, snow, moisture, or even tree foliage tend to absorb or dissipate much of the energy contained in a millimeter wave signal. Case in point, if some of you have satellite radio or satellite TV, compare how the service works on a bright and sunny day verses how the service works under a thick cover of cloud or worse yet, when it is raining or snowing heavily. You may have experienced that when it rains or snows heavily, your satellite radio or satellite TV services are interrupted. Even if you have some service, the quality of the service won't be very good. That is because satellite services operate in nearby bands as a millimeter wave and those frequencies tend to get severely attenuated because of rain and foliage. These are some of the fundamental challenges that millimeter wave signals face. But despite these challenges, there are several salient advantages because of which millimeter wave deployment becomes commercially viable. Those are some of the advantages that we are going to learn about next.