After having looked at some of the core network components in Module 3, and then having studied some of the air interface or RAN-specific features such as millimeter-wave and massive MIMO, it is now time for us to take a detailed look at 5G RAN over here. We'll study the overall evolution of a cellular RAN that has brought us to the current generation, which is what 5G will most likely use. We will take a magnifying glass to that specific design and see what are some of the principles of RAN design followed by a 5G. As always, we'll try to see what are some of the benefits of what we are going to do here. Let's get started with 5G RAN evolution. No matter the generation, RAN basically has two components; they may be called differently in different generations of cellular communications, but their essence is pretty much the same, in that one can be called BBU or the baseband unit, and the other would be called RRH, or remote radio head, or simply RU, meaning the remote unit. Although generation-wise names are different, the essence is very similar. For us to understand what the two components of the RAN are as stated here, let's take a minute to understand what is a baseband, so to speak. In order to understand what BBU is, I'd like to take you back, jog your memory a little to one of the earlier modules wherein we saw the schematic of what happens inside your phone and your mother's phone when you try to talk to each other. I had mentioned that at the high level there are multiple elements to your phone, such as modulator, up-converter, power amplifier, and antenna, so to speak. Although this example is for a phone, I'd like to remind you, and this chapter is about RAN, as you may have guessed already, your phone and the 5G RAN happen to share some basic elements of functionality regardless of their role in the network. The [inaudible] also happens to have some of the similar components, such as modulator, up or down converter, power amplifiers, antennas, etc. We can safely look at the schematic. Now, we have to understand two components baseband unit and RRH or remote radio head or radio unit. Baseband effectively is representative of whatever you see on the screen of your phone, be it a picture, be it an email, that in the end is composed of bits and bytes, and those bits and bytes are handled by your phone or the [inaudible] in a certain way before those are up-converted or radiofrequency. Every portion of the data before it is converted into radiofrequency can be thought of as baseband in technical terms. In this figure, up to this point, every module beneath modulator, modulator included would be classified as a baseband. Whereas the moment that signal gets converted into carrier or radio frequency, that is when we will be talking about the realm of radio unit or remote radio head. Everything above this point roughly would be a part of RU or RRH on the [inaudible]. Baseband once again is bits and bytes and associated data that you see on the phone and in the form that your phone handles it, that is called baseband, whereas once your baseband signal is converted into higher radio frequencies, that is when it begins to be in the realm of remote radio head or remote unit. This was just a quick reminder for you to understand what baseband unit is versus what remote radio unit is. Now, we're ready to look at a high-level, different generations of RAN evolution. At the earliest, what we had was called D-RAN or Distributed RAN. In that, every base station, let's say that this particular box is one base station and similarly, all these boxes would be different base stations. Every base station would have its own dedicated BBU and its dedicated RRH. If you as a network operator, had to build a new base station, you would have to buy one integrated box that would contain both BBU and RRH, which was okay as far as legacy technologies and legacy applications were concerned. But then an optimization came along on top of this D-RAN or Distributed RAN that optimization being C-RAN or Centralized RAN. Over here, we still have the logical division between BBU and RRH, which will now be called RU, but essence is the same as RRH. But instead of having a one-to-one correspondence between BBU and RRH like the older generation, this generation allowed you to combine several BBUs into what is known as one single BBU pool, and that BBU pool would be responsible for handling multiple RUs. You would no longer need to have a dedicated BBU co-located with your base station, you could have one common BBUs across multiple base stations. The base station would still have the antennas, which would be a part of radio unit, but the backend BBU wouldn't necessarily be located with the base station, it would have been located at another place. However, one thing that stayed the same as the previous generation is that the connectivity between BBU and RRH, also known as the fronthaul as we are going to see shortly, that fronthaul has some very specific requirements with respect to the type of data that it is supposed to carry. Throughput and latency, reliability and availability on that specific fronthaul just like we have end-to-end throughput, latency, reliability, etc., metrics, 5G networks also have specific throughput, latency, and reliability metrics on this wire that goes from BBU to RU that is called for fronthaul. In both the generations, the fronthaul continue to have some stringent requirements because it carried essentially similar data between BBU and RU. However, that substantially changes in the current generation of radio access networks and that is known as vRAN or Virtualized RAN. Although this is not a mandate, 5G is well in a position to utilize this aspect of RAN design rather than some of the original older aspects. In this aspect, we still continue to have a BBU and a radio unit, but not only is that BBU pooled across multiple base stations, but that BBU pool itself can be offloaded into a cloud. Network operator wouldn't have to host this BBU pool on their own premise anymore just like we saw several network functionalities can be offloaded onto cloud in one of the earlier modules. This functionality, i.e. BBU, can also be safely offloaded to the cloud. 5G is increasingly getting in a position where it will follow this architecture wherein some of the BBU functionalities will be residing in the cloud rather than on operator premises. We continue to maintain this one-to-many correspondence between BBU and your radio units, just that your BBU now resides in a cloud and is able to handle requests from multiple radio units.
