 So, having seen your FDMA and TDMA techniques, so somewhere around 95 or so, this spread spectrum techniques became popular, they existed before that, but somewhere around that time is when they became very popular. They used to be used mostly in the military earlier, because they have highly proof from jamming. So, what is this spread spectrum? So, this forms the basis of both CDMA and wireless LANs, both of them follow the same underlying mechanism. So, the technique is like this, basically the bandwidth W and the message bandwidth R are related as W is far greater than R. Typically, what do we want to do, we want to take the message and transmit it in as much short a bandwidth as possible, you want to use as little of the spectrum as possible, whereas in CDMA what we are saying is that it is actually counter-intuitive, saying that instead of trying to pack the message into as smaller bandwidth as possible, spread it out over as larger a spectrum that is available. So, it turns out that it achieves several desired objectives, enhanced capacity, interference, tolerance and so on. So, there are two types of spread spectrum systems, one is called frequency hopping, the other is called direct sequence. So, the direct sequence one is the more popular one. Frequency hopping is basically what it means is that you transmit on a particular frequency for a while, then you hop to another frequency, you transmit on that frequency for a while, then you hop to a third frequency and so on. The direct sequence one is the one which is more important. So, this basically illustrates frequency hopping. So, let us say you want to transmit over a certain amount of time, this is your width. So, you are basically transmitting in different frequencies. So, the sender and the receiver should know the hopping sequence, that pattern is called the hopping sequence. So, obviously I need to know the hopping sequence, otherwise you would have hop to one frequency, I will be trying to listen to you on a different frequency. So, that is the basic idea. It is not that popular as the direct sequence spread spectrum techniques. So, what happens in direct sequence? Basic idea is like this, I have some data which is shown by this shaded area here. Now, this data is spread over a large spectrum. So, what remains common, what remains constant? The area. So, the area is going to remain constant between both of the cases. So, how is this spreading achieved? This spreading is achieved by using what is called a spreading waveform. So, you take the data waveform. So, for each bit in the data waveform, you are going to explore it with your spreading sequence, what is called the spreading sequence. So, you take this bit, you explore it with the spreading sequence and this is the actual bit that is going to be transmitted on the wireless medium. Why is this, how is this useful? At the other end, obviously it is like you are just going to receive whatever you are hearing, you are going to receive everything. So, let us say you have received a whole bunch of stuff which includes your original data plus some amount of noise, noise either deliberate or unintentional. So, now that is what is called jamming and as a result of D spreading and demodulation, you are kind of able to recover your demodulated spectrum. So, that is the basic idea of such a system. This slide actually explains it in a more layman's terms which is what I am more comfortable with when it comes to signal processing. So, what we are saying is, this is the signal we have spread out because of the property of the signal. So, the key, where is the key here? What is the key thing that we have to get right in order for this to work? We have to choose our spreading sequence very carefully, not any sequence can be used as a spreading sequence because this property depends upon what are you using as your spreading sequence, correct? What is the code? What is the spreading code or the sequence that you are using, right? So, you have spread it using a sequence. What is the property of the sequence? If you XOR it again with itself, then you will get the data back, right? So, now here there is a bunch of noise that is going and the signal. So, there is very little difference between the signal and the noise as far as the wireless medium is concerned. Now, when you XOR it with the spreading sequence again, the property of the sequence is such that this signal is recovered whereas an XOR of the noise with the spreading sequence remains as noise, okay? So, that is the key property that is being exploited here. So, what it shows is, suppose there is an interference. Suppose there is a narrow band interference in my signal space, okay? I have spread my signal here. So, in the first case there was no interference, there was just noise but the signal and the noise were very close to each other. Even then you are able to recover that particular signal because it is a property of the spreading sequence. Now, if there is a narrow band interference here, what happens is, again an interesting thing happens that once I do the XOR, so what is happening in the receiver is basically an XOR again with the spreading sequence. Once I do that, my interfering signal being narrow band gets spread and my spread signal gets recovered. So, this basically shows that narrow band interference I am able to recover from and this one shows that suppose I have two of these spread signals, okay? Again because of the property of the sequence that is being used, different sequences are used for each of the two signals, okay? Because of the property of the sequence, one of the interfering signals remains a spread sequence while the original one is recovered as my narrow band signal at the other end, okay? So, this is the basic idea behind any CDMA or a wireless LAN kind of a system, okay? What is the implication of this idea? See, what is the implication of this one for example, okay? I have one spreading sequence here, another spreading sequence here, so let us ignore anybody who is intentionally trying to jam our system, yeah? So, basically what this means is, it has two, three implications. One implication is, I do not have to so much worry about frequency allocation which was a big thing for me in GSM, right? I do not have to worry about it at all because I am transmitting in the entire band, right? So, that is one thing. So, one problem of how to do frequency reuse, how to do all the channel allocation, everything, all that just goes away. The other thing is that there is very little difference between handling interference from other users and handling general noise, right? So, the handling of both of them is more or less the same. As far as the CDMA system is concerned, it does not matter if there is noise in the system or if there are many other users in the system. They are both treated in the same manner, okay? So, because again it is not very, you know, it is very easy to talk about it. But if you put yourself in the place when it was being invented, it is not very easy to envisage codes which have this beautiful property of being able to just recover that signal out of a whole bunch of noise, right? So, what are the various properties that it gives us? It gives us this one property that I can increase. So what is the key thing as far as my receiver is concerned? As far as my receiver is concerned, what matters is how much of this signal is above anything else that I see, correct? If I have a very good receiver, then what will happen? If I have a very good receiver, then I can do with less of this information, right? If I can do with less of this information, I can handle more users in my system, okay? So, that is the key difference between a GSM system and a CDM system. If you have to give just one key difference between the two systems, okay? That is that in a GSM system, interference is bad, right? In a CDMA system, it doesn't matter whether it is interference, whether it is other users, they are all treated in the same way. If I want to increase the number of users in a CDMA system, how do I do that? If I want to increase the number of users in a GSM system, how do I do that? I have to make smaller cells, I have to do more frequency reuse and all that thing, right? If I want to increase the number of users within a cell in a GSM system, how do I do that? If I want to increase the number of users within a sector in a GSM system, how do I do that? In a GSM system, if I want to increase, right? Once you take the smallest cell that you can have in a GSM system, what is the number 992, right? That is the maximum upper bound. You cannot increase 125 into 8 minus all the control channels, right? So, you cannot increase the number of users in a GSM cell beyond that number, okay? Whereas in a CDMA system, that number is not any hard and fast number, okay? It depends on the type of handsets that are there. It depends upon how much power control the base station is doing. So, if it is telling all the hands, if they are all very good quality handsets in that area, the base station can do very fine grain power control. It can transmit at very, you know, it can just kind of talk very softly. And it can hear also when the handsets are talking very softly, right? So, in that sense, more handsets can be accommodated in the area, okay? Is that making sense? Okay, so how do we arrive at some number for this, okay? So, we have to deal with a little bit of calculation in order to arrive at what is this number in a CDMA system, okay? So, let's say W is your spread bandwidth, R is your data rate, which is your data signal bandwidth in hertz, okay? So, W by R is your spread, okay? Processing gain, correct? So, S is your received power of the desired signal. J is the received power for undesired signals, right? So, J by S becomes your signal to interference ratio. And then these systems have a notion of EB by N0, which is EB is your received energy per bit for the desired signal. And N0 is the equivalent noise spectral density, okay? So, now the key equation here is like this. J by S is what you want to know. You want to know how much interference can I tolerate in such a system, correct? So, don't bother to write all this down. Basically what it means is, J now it turns out that J is N0 into W. So, once you go through all these things, it finally turns out that J by S, the maximum J by S that you can tolerate is W by R, which is your processing gain, okay? Divided by EB by N0 mean, okay? What does this mean? What this means is, we can see in this example, okay? So, here is an important point. In conventional systems, W by R is approximately equal to 1, okay? Like J SM, etc. What is the data rate and what is the available bandwidth? They are more or less the same, right? It's approximately equal to 1. So, what happens when W by R is approximately equal to 1 is, so when W by R is approximately equal to 1, this is a non-zero quantity. So, you find that your J by S becomes less than 1, in order for your system to operate satisfactorily, okay? J by S less than 1 means, yeah, signal to noise ratio, signal power should be more than my interference power, significantly more than the interference power. So, that is all the stuff which we saw in the morning, right? 200 kilohertz, why is it 200 kilohertz? Why is it not 100 kilohertz in a J SM system? All those things come from the need that my received signal strength should be much greater than by noise that I'm receiving, right? On the other hand, if you see in the CDMA system, so suppose R is 9600, W is some 1.2288 megahertz, okay? EB by N0 is typically 6 dB. These are values taken from a IS-95 CDMA system, okay? So it turns out that 1.2288 divided by 9.6 is something like, what's that, 128, right? W by R is, yeah, W by R is approximately 128, okay? And now if you convert it into dB, you get what is called your jamming margin as 32. See what that means is that that's kind of indication of the capacity of your system, okay? So if you decrease this EB by N0 value further, okay? So EB by N0 is saying what is the threshold at which you are able to receive, okay? So if you can decrease this EB by N0 further, then the number of users in your system can be higher, okay? So that is the basic idea behind the CDMA system, okay? So that's what I am saying here. So the jamming margin can be used to accommodate multiple users in the same band, okay? If EB by N0 min and the PG are fixed, then the number of users can be maximized if we are doing perfect power control, okay? Capacity is maximized by maintaining the signal to interference ratio at the minimum acceptable level, all right, all right? So some of the key factors, some of the key things in a CDMA system are this universal frequency use. There are one or two key points which one needs to understand, okay? So one was this universal frequency use. So many of the pain points in JSM have actually gone away when we go into a CDMA system, okay? So once you move away from the air interface, okay? So I don't really know. I know in theory how the system works. I mean, there is a notion of orthogonal Walsh codes which are used, there is a PN sequence which is used and all that, but there is no point in really telling you all those things. You can find all that in a book, okay? So just the key insights in a CDMA system are that the main problem with JSM of figuring out frequency, frequency reuse, all that, all that goes away simply because the universal frequency reuse applies not only to users in the same cell, but also in all the other cells, okay? So that is a big advantage of a CDMA system over a JSM type of a system. The other advantage is what is called the soft handoff. Now in JSM there is no notion of a soft handoff. What is a soft handoff? A soft handoff is basically I talk to two base stations at the same time, okay? So I am sending the signal through this base station, I am sending the signal through that base station and I add up the signal at the mobile in order to get a higher received power, okay? It's just like, you know, in order to tell you something there are two people who are whispering it very, very softly and somehow you are able to know both of them, you are able to match both of them and get a higher signal at the other end. So it is not at all intuitive, okay? But it can be done primarily because again of these property of these codes, okay? So what you do in a CDMA system is instead of talking to a mobile node through a single base station, okay? You can talk to a mobile node through one base station from here at very low power. You talk to the mobile node the same thing you said tell the mobile node through another base station from there at again at very low power and the signal can be added up at the mobile node, okay? In order to recover the original signal, okay? What is the advantage of that? Maybe in this cell there is a lot of users. So you want to decrease the transmit power in that cell, okay? Maybe in this cell also there are a fair number of users. So that is why you are trying to arrive at this point, okay? Okay? So here what happens is typically it is used at the time of handoff. So the mobile will start communicating with the new base station without interrupting on the old one, right? Same frequency assignment is used because there is no frequency reuse problem here, okay? This is basically what is happening here. So if I am talking through one MSC, so the MSC actually has to do more work because it has to send the same data along two different paths, okay? So the MSC has to send its kind of a multicasting of the data has to happen from that point. But at this point, at the air interface, but that is not a problem at the, in the background because these are wired links, okay? So the MSC sends the data along both the directions and this guy can get part of it from this BTS and also communicate with this BTS at the same time, okay? So typically at the time of handoff what the mobile does is that it for a while it communicates with both the BTSs till the received power of this BTS goes far below a threshold at which point it, it switches completely to communicating with the new BTS. Any comments, questions, I am sorry? Time hopping, does it fall under spread spectrum? What is time hopping? Yeah, I guess, I do not know, I have not come across anything specifically as time hopping. I can kind of understand what you are saying that you transmit in this slot first, you transmit in some other slot later, right? Perhaps, I do not know, I do not, I do not see really what would be achieved because the interference is actually frequency dependent. So unless we say that the interference depends more on the time within the same frequency, just pure time hopping I do not, I do not think it would be very effective, right? Time hopping combined with frequency hopping may be possible, but that is probably what frequency hopping does anyway. You hop in frequency as well as you hop in time slot, right? So you hop in both of them, okay? So having, although CDMA has some advantages over GSM, why is the GSM penetration more? Because GSM came in first, because GSM could do better roaming, okay? A lot of other issues are there, okay? So yeah, it is a standard and then also, see remember in a CDMA system, GSM you are guaranteed something, right? Once you get a slot, okay, you know that your traffic is going to go. In CDMA system, because the same thing, no interference power control can work against you. In fact, if you remember some of the earliest deployments of the CDMA systems, people were fine initially. After a while, everybody started complaining, saying I cannot hear anything, I cannot get anything going on this system, right? Because if it is the performance of the system, because if the operator gets too greedy, right? Then the users can suffer. In a GSM system, that cannot happen, right? In terms of voice quality, no, what I am saying is, if a given amount of resources, right, you are guaranteed a certain quality in GSM, okay, which need not be true in a CDMA system. In a CDMA system, it depends upon the handsets, significantly upon the handset that you are using, okay? How much, like, basically it comes down to how much of the signal it is able to extract from the interference, right? Sir, but call, without, without the, I guess so, so the call admission control mechanism in CDMA will not admit a call if it is not able to satisfy the QoS parameters, correct? Without disturbing the existing thing. So, but the point here is, if I, if I lower the thresholds, isn't it? If I lower the threshold of what is acceptable, see that is the key difference. In GSM, nobody can dictate what is acceptable, right? There is nothing variable. You are going to say, you are going to get, once your call is established, you are going to get, get 577 microseconds on the air, okay? In every frame. Now, if you have a better handset, you can do better codecs on that. Your quality can be improved. In CDMA, you are not guaranteed that you will get 577 microseconds on the air, right? There is no such notion. So, what happens is that you are basically saying that a certain signal to interference ratio is what you are trying to play around with, right? So, now this EB by NOR, 6 is the, 6 dB is what is used in IS 95. Suppose I make it less, okay? I could always support more users, isn't it? So, what is tolerable is the thing that moves around in a CDMA system. Yes, yes, you are governed by the authenticity of the call admission control protocol, yeah. That, yeah, that's a good way of putting it. In GSM, the time, power control is per time slot level, while in CDMA, it affects the entire BTS. I am not very sure if that is so, no, no, it can do that. But I thought, I don't recall that GSM actually does it at a power time slot level. Theoretically, it could, but I don't recall that it, so, correct, yeah. The size of the cell shrinks in CDMA because of the power control, yeah, but it's not so significant actually because we are doing a clear separation. So, the power control in GSM is not so critical, as in power control is critical, not so much also for the interference because you are already doing one level of interference control by separating frequencies and time slots, okay? So, the 200 kilohertz and all those kind of things you have already done in order to separate, in order to do some interference control. So, the power control is also for the device. So, if you are nearby, then there is no point in wasting power in shouting to me. That is the key reason I think, right? That's true, correct. So, you can't reuse your frequencies. So, you have to follow the appropriate equation i square plus j square plus ij, right, which we all know. So, you have to follow that equation in order to do the reuse, okay? So, let me do one thing. So, this is all again data, which I don't want to read out, okay? So, this is what is a kind of an architecture. So, somewhere towards the end of tomorrow, we will try to work out an application so that we go through an 802.11 network, through some other type of network and reach maybe a wired access node and see what happens at every hop in such a network, okay? So, there is one topic of, you know, how do we handle worldwide web and mobility? What I am going to do is I am going to put this topic also off and deal with it towards the end of tomorrow's session, okay? So, initially again this was because about a couple of years back, this was an important topic and so I had kind of put it up ahead, but nowadays the emphasis on that is decreasing. So, I don't really see the point in doing it at 6 in the evening on a long day. So, we will do it at 6 in the evening tomorrow, okay? We will see you tomorrow.