Let's now turn our attention to what is called scalable OFDM based air interface. Now, many of these words are maybe unfamiliar to some of you. Air interface is something we have already seen. Air interface is nothing but wireless channel. OFDM stands for orthogonal frequency division multiplexing. Understanding the technical or operational details of OFDM is somewhat beyond our reach right now. Let me try to help you understand OFDM at a higher conceptual level. Imagine you design a wireless system that can only operate on wideband channels. We have previously seen there are two types of channels: Wideband and narrowband. Imagine, for example, to improve the performance of your wireless system, you design your wireless system so that it can operate on a wideband channel. But then two questions arise; What if your wireless system is deployed in a spectrum where wideband channels are not available? On the other hand, even if wideband channels may be available, what if the application that is running on top of your wireless device doesn't require a wideband channel, what if it only requires narrowband channel? Or for any other variety of reasons, what if the network for a momentary instance doesn't have wideband resources for you to operate on? If you have designed your wireless system to be able to only operate on a wideband channel like this, then although the peak performance of your system maybe good, there are many situations wherein such design maybe sub-optimal and best, and that is where the essence of OFDM comes into picture in that by using its engineering magic, OFDM allows you to send data not on wideband channels, but rather on multiple narrowband channels. Keep in mind, in total you will have used a similar amount of bandwidth, but you will have used that in terms of different chunks are different modules. For example, if at a certain instant the network was able to give you the entire wideband channel, well and good. You can for example, utilize all four narrowband channels. But if the network did not have enough resources for you, if it could only give you half the bandwidth, in a wideband only design you wouldn't be able to operate. Whereas in an OFDM based design, which is flexible and modular in this context, that design can still help you operate by using the first two narrowband channels because only that bandwidth is available. This is among others, one of the principal benefits of OFDM in that it makes the wireless channel related operation of a system fully modular so that depending upon the resource availability or application requirements the system can determine in real time what is the amount of bandwidth that can be most profitably used and the system can precisely use that bandwidth because the operation has been modularized. How can we understand OFDM in terms of our transportation analogy? Well, that can be this modular road design. Once again, let's imagine a road that currently has only two lanes because the traffic requirement maybe low. What if the traffic requirement goes up in the future and you need a four lane road? On the other hand, what if the traffic requirement goes down and you don't need a four or two-lane road anymore? You might do well with a single lane or a three lane road. What if you have the ability to dynamically add and subtract individual lanes from a road precisely depending upon the requirements of the oncoming traffic, because that is pretty much what OFDM allows you to do depending upon the resource availability and requirements of the applications running on top. OFDM allows you to use precisely the amount of bandwidth that you require and you have available and that it makes the whole system modular. Let's quickly look at how it might appear in a real life system. At the beginning you have a two lane road, but if traffic demand goes up, you can have a four lane road. If there is less traffic demand, you can have only a single lane road. How it can be accomplished is imagine that each individual lane is made of identical pieces that fit with each other, something like Legos. Just like most pieces of Legos look identical or very similar to each other but just by arranging them or stacking them differently, you can make entirely different shapes and objects. Just like that, imagine each individual lane is like a piece of the Lego puzzle. If you have moderate traffic demand, you can fit two of those pieces together to form a two-lane road. If there is more demand, you can fit together four of those lanes and form a four-lane road. If there is little demand, you can take one of those lanes away and only provide the single lane that is necessary for the low flow of traffic that you are expecting at that point in time. Now a couple of points to be compared and contrasted with what we saw previously. In our previous analogy, the road width would remain the same. What will change is the width of individual lanes at runtime. Whereas in this example, what is going to be fixed is the width of individual lanes and what is going to change is the width of the road itself. For example, in each of these figures, the lane width is identical. But what is differing is width of the road, so you will utilize more resources depending upon the requirements of your oncoming traffic, just like how OFDM allowed you to use 1, 2, 3, or 4 narrowband channels instead of one wideband channel in order to precisely meet the requirements of your applications or the situation of resource availability. This is how at a high level, scalable OFDM based air interface relates to a hypothetical road design where you can construct lanes out of Legos so to speak and you can add or subtract lanes on a given road precisely, depending upon the requirements of the oncoming traffic.
