Let's now learn a little bit about two of the most basic impediments that affect a wireless signal and transit noise and interference. When information is sent over any wireless channel in a typical wireless system, noise and possibly interference may be introduced. Now, both noise and interference are a form of unwanted signals or unwanted energy but there is a crucial difference between the two. Noise is unwanted energy that is not deliberate in nature, in that the wireless noise or the electromagnetic noise so to speak is always present in most devices and most communication links. In fact, some of the noise can be one generated within the communication device itself, also known as thermal noise. An easier way to understand this electronic or thermal noise is to compare that with ambient noise that is always present around us. Even if you are sitting quietly in a room there is always some form of ambient noise that's reaching you. For example, a hum of a refrigerator in the kitchen, or a rustling of the leaves outdoors, or chirping or birds, or cars passing on the street. All those are examples of ambient noise and even you are sitting quietly by yourself, those sources of noise have their own ways of finding you. There is only so much that you can minimize that noise too. The ambient noise which is always present and cannot be minimized beneath a certain threshold is a good way to understand this thermal or electromagnetic noise that is always present in most wireless communication media or most wireless or electronic devices. It is really not possible to minimize that noise beneath a certain threshold. On the other hand, interference is also unwanted form of energy but unlike noise, interference is a deliberate form of disturbance energy that presents disturbance in transmission. What is interference? Well, when two nearby transmitters and nearby is the key, when they use the same radio frequency at the same time, they create interference. For example, let's say that this is your intended transmitter and you are the receiver. When the transmitter is sending to you when you are located here, there is no source of interference in this region so your transmission will be quite good. However, if you move from this point to this point wherein there is another transmitter, but keep in mind that you're only talking to this transmitter. The signal coming in from this transmitter is your desired signal, whereas the signal that is transmitted by this transmitter which is not intended for you, it may be intended for someone else. But because you simply happen to be within the wireless range of the transmission sent by transmitter 2, that signal will happen to reach you. In that case, this will be your desired signal and this will be a form of interference. Noise and interference have a tendency to reduce the quality of the signal that is ultimately received by the user because as I said, just like crumpling of the letter or ray in dirt and snow change the appearance of the letter you are sending to your friend, noise and interference have a tendency to change some of the fundamental attributes of the electromagnetic carrier wave so that it becomes little difficult for the intended receiver to demodulate and extract the intended information from the signal that has slightly been changed in form because of noise and interference. To summarize, both noise and interference are forms of unwanted energy in addition to your intended energy. But noise is natural in sources, it is always present. You cannot minimize that beneath a certain threshold whereas interference is deliberate in nature and can be mitigated by careful planning of the frequency use by different entities in the network. For example, if you could institute certain rules saying that this transmitter cannot transmit at the same frequency or cannot transmit at the same time as transmitter 1, then you can tell that the interference shown here will be eliminated or greatly reduced if the transmitters follow those basic rules. It is possible to mitigate or altogether eliminate interference but not so much with noise. Now that we know fundamentals of noise and interference, let's try to look at a practical example of how exactly noise and interference can affect your wireless transmission. Let's say that you are planning to send a famous picture from the transmitter to the receiver, if there is no noise or interference which is idealistic. But under such utopian scenarios, the copy of the received signal will be the exact content that you sent on the transmitter because there are no impediments in the wireless channel but that is idealistic. In reality there will always be some form of noise or interference present in the channel and or the device. If that noise or interference is limited to a small extent, sure, your incoming picture will be affected to certain extent. But if we try to interpret this picture by human eyes, we are nonetheless able to make an educated and successful guess that this is indeed the famous picture even though it is slightly impaired by noise or interference. However, if you have an additional amount of noise or interference in the channel, like this, then it becomes a little difficult to recognize this picture. For someone who hasn't seen this picture before, it would be very difficult to correctly guess what this picture is. Beyond a certain extent, if noise or interference gets so high that it becomes almost impossible to recover your wireless transmission, as you can see in this example, it is nearly impossible for anybody to recognize what this picture is. This goes to show how the noise and interference at various levels adversely impact your wireless transmission. They gradually degrade the quality of the incoming or the received signal and make it gradually difficult for the receiver to successfully demodulate that transmission, so to speak. That is the adverse impact that noise and interference can have on your transmission. As the bullet says here, in case that noise or interference are very high, it is quite possible that your transmitted signal cannot be recovered by the receiver and that transmission will have been lost. Given that noise and interference play such a crucial role in a typical wireless system. It is imperative for us to have a mathematical way to quantify how much noise and interference is present in the medium relative to the amount of desired signal because that ultimately enables us to estimate what is the likelihood at which this receiver will be able to successfully decode or demodulate the transmitted data. To that end, engineers have two metrics that they live by. They are pretty much reflections of each other. The fundamental one is called SNR, or signal-to-noise ratio. If there is no interference present in the channel, if there is interference in the channel, that metric will be slightly modified to account for signal to interference and noise ratio. But regardless of which one you look at, what is the essence of both the metrics? Well, in the numerator, you have the amount of desired signal which is what you would want to demodulate. In the denominator, you have all the impediments, either noise alone or noise plus interference, which make decoding or demodulating your signal increasingly difficult. As you can tell, in case of little noise or interference, the SNR or SINR may be very high. That is the point that a wireless system aims to operate at. Whereas if you gradually increase the amount of noise or interference relative to the strength of the desired signal, you can tell that mathematically, SNR or SINR will begin to gradually drop. That is the example that we saw on the top half of this slide. Wherein gradually increasing quantities of noise or interference made demodulation increasingly difficult in that. As we added noise and interference in these pictures example, we were looking at a system that was fine to operate at gradually reducing amounts of SNR or SINR. These two metrics are quite important for any wireless system. It will be also important for us to keep those in mind because later, during this class, these two metrics will offer on the hand, when we try to examine benefits of certain schemes that 5G uses, as compared to some of the legacy technology. This is a bit about noise and interference. If you have understood those concepts so well, it should be possible for you to control some of the real-life situations or examples. Wherein you may have experienced some wireless interference. Pause the video here if you would like, and think for yourself, what kind of wireless systems have you used in the past. Whether you have experienced a situation wherein transmission from one of those systems happened to interfere with the transmission of another system. Pause the video here for a minute and think if you can come up with such a scenario. [MUSIC] If you have given this a little bit of thought, one example may have come to your mind. If not, do this experiment. If you have Bluetooth-enabled wireless headphones. Wear those headphones pair those with your phone or your tablet, and start streaming some music from your tablet to your wireless headphones. In the beginning, you will be able to hear that music without any interference but now, do one thing. Start walking towards the Wi-Fi router that you have at home. You will notice that as you get closer and closer to your Wi-Fi router, the music streaming in your Bluetooth headphones begins to get choppier and choppier. If you stand right over your Wi-Fi router, it is possible that your music quality will be so choppy that you won't be able to hear anything. The reason it happens is because of wireless interference. Coincidentally, your Wi-Fi router and your Bluetooth headphones operate in same or nearby frequency channels. At such, when both of them try to use the same or nearby channels near each other in each other's vicinity, that is when they begin to interfere with each other. That is the primary reason why it becomes difficult for you to smoothly stream your music and the music begins to get choppier and choppier. That is one real-life example of radio interference. Now that we have seen about noise and interference, some of the basic impediments that impact wireless transmissions, let's take a deeper look and learn about some of the fundamental resources that a wireless system will need.
