1 00:00:00,012 --> 00:00:04,620 In this video we're finally going to use everything that we've learned in the 2 00:00:04,620 --> 00:00:07,689 course. And we're going to learn how communication 3 00:00:07,689 --> 00:00:12,519 systems actually work, and figure out the basic elements of communication system 4 00:00:12,519 --> 00:00:15,545 design. First let's recall what the fundamental 5 00:00:15,545 --> 00:00:20,228 model of communication is. There is a message that someone wants to 6 00:00:20,228 --> 00:00:26,416 send to some destination, and the problem is that doing that is not easy because of 7 00:00:26,416 --> 00:00:30,517 the channel. Nothing good happens in the channel. 8 00:00:30,517 --> 00:00:34,433 Signals get attenuated, sometimes filtered. 9 00:00:34,433 --> 00:00:39,922 There's noise, interference, all those kind of issues pop up. 10 00:00:39,922 --> 00:00:46,103 So to best combat how the effects the channel has on transmissions, is that the 11 00:00:46,103 --> 00:00:52,791 transmitter and receiver are, are there. They're systems which produce signals 12 00:00:52,791 --> 00:00:59,028 related to the message, which makes it easier to get the message through the 13 00:00:59,028 --> 00:01:04,592 channel and to bring back the original message as well as possible. 14 00:01:04,592 --> 00:01:10,898 Like I said, we know all the tools already to figure out how transmitters and 15 00:01:10,898 --> 00:01:15,811 receivers work. They involve modulation, and filtering, 16 00:01:15,811 --> 00:01:21,183 and we already know how to do that. The thing we need to learn is so we've 17 00:01:21,183 --> 00:01:26,623 figured out which tools to use, are the characteristics of the channel, that 18 00:01:26,623 --> 00:01:30,938 governs everything. We need to understand the structure of the 19 00:01:30,938 --> 00:01:34,977 message, and the channel. Well, in this video we're going to talk 20 00:01:34,977 --> 00:01:38,779 about the wireless channel, electromagnetic radiation. 21 00:01:38,779 --> 00:01:44,805 And we're going to see that it has several interesting characteristics, both good and 22 00:01:44,805 --> 00:01:45,801 bad. Okay. 23 00:01:45,801 --> 00:01:54,303 So everything in the world of antennas, wireless communication is governed by 24 00:01:54,303 --> 00:01:58,854 Maxwell's equations. And here I show them. 25 00:01:58,854 --> 00:02:06,047 We're not going to talk about how to solve them for any particular setup. 26 00:02:06,047 --> 00:02:12,624 What I'm more interested in are the properties of Maxwell's equations, and 27 00:02:12,624 --> 00:02:18,331 that's what I want to talk about. So interesting properties. 28 00:02:18,331 --> 00:02:24,910 First of all, those equations are linear. That is very, very important. 29 00:02:24,910 --> 00:02:32,074 So, the idea is that suppose there are two transmitters, one sending the signal S1 30 00:02:32,075 --> 00:02:39,233 another sending signal S2, and they're coming out of these two antennas, and here 31 00:02:39,233 --> 00:02:46,273 we have our cell phone sitting down here. What that antenna gets, on the cell phone, 32 00:02:46,273 --> 00:02:51,445 is it's going to get both signals. And it looks like superposition. 33 00:02:51,445 --> 00:02:54,958 So what' you're going to get are both signals. 34 00:02:54,959 --> 00:03:01,011 You know, it could turn out that it's the transmitter sending signal 1, is what the 35 00:03:01,011 --> 00:03:05,520 cell phone, person using this cell phone really cares about. 36 00:03:05,521 --> 00:03:13,259 However S2 is going to come in also. So the idea is, how do you block this 37 00:03:13,259 --> 00:03:21,150 signal you really don't want? Well the signal you don't want is called 38 00:03:21,150 --> 00:03:27,802 interference. Interference is primarily produced by 39 00:03:27,803 --> 00:03:33,131 humans. It's not necessarily a natural phenomenon. 40 00:03:33,131 --> 00:03:39,292 Well it could be in some cases. So we have to figure out how you design a 41 00:03:39,292 --> 00:03:46,468 communications system as a whole so that the receiver can figure out what's coming 42 00:03:46,468 --> 00:03:50,504 in and pull out the signal it wants. Well, okay. 43 00:03:50,504 --> 00:03:54,517 Here's another, side to the linearity thing. 44 00:03:54,517 --> 00:04:00,941 And that is, suppose again, S1 is trying to send a signal to that cell phone. 45 00:04:00,941 --> 00:04:07,379 Well, let's suppose the second antenna's trying to send a signal to that cell 46 00:04:07,379 --> 00:04:12,010 phone. Well, they're going to pick up both, S1 is 47 00:04:12,010 --> 00:04:19,202 going to be picked up by that cell phone and S2 is going to be picked up by that 48 00:04:19,202 --> 00:04:23,845 cell phone. So the upshot of this is no privacy. 49 00:04:23,845 --> 00:04:31,234 Once you send a signal over an antenna, a wireless signal, any, any other system 50 00:04:31,234 --> 00:04:37,297 that has an antenna can pick it up. So this means you have to be a little bit 51 00:04:37,297 --> 00:04:42,618 careful if privacy is a concerned, about how you send that message. 52 00:04:42,618 --> 00:04:46,585 Okay. So another thing that's important, is that 53 00:04:46,585 --> 00:04:51,402 waves electromagnetic waves propagate at a speed of c. 54 00:04:51,402 --> 00:04:55,666 So, let me, illustrate what propagation is. 55 00:04:55,666 --> 00:05:00,745 Suppose I draw a axis. And it's a little interesting. 56 00:05:00,745 --> 00:05:07,667 Suppose this is space, and this is time. And in some particular location, I send 57 00:05:07,667 --> 00:05:16,830 out a pulse that's of this width and time. What propagation means is that sometime 58 00:05:16,830 --> 00:05:24,188 later, that same pulse appears at a different position x. 59 00:05:24,188 --> 00:05:32,703 So, the distance here, let's call it delta x, and the difference in time, delta t, 60 00:05:32,703 --> 00:05:40,160 the speed of propagation is delta x over delta t, whatever that may be. 61 00:05:40,160 --> 00:05:47,792 Well, for wireless signals, that speed of propagation is the speed of light. 62 00:05:47,792 --> 00:05:54,766 And, if you go back to Maxwell's equations, it's given by 1 over the square 63 00:05:54,766 --> 00:06:00,902 root of epsilon mu, and we're not going to go into the details of how that's 64 00:06:00,902 --> 00:06:06,471 calculated. But in free space the well-known result is 65 00:06:06,471 --> 00:06:13,149 that the speed of propagation is 10 to the, 3 times 10 to the 8th meters per 66 00:06:13,149 --> 00:06:17,059 second. That's the number we're going to use for 67 00:06:17,059 --> 00:06:22,733 wireless systems. Now, there's another aspect of propagation 68 00:06:22,733 --> 00:06:28,221 that applies for sinusoids. So, let's suppose I have a time axis, and 69 00:06:28,221 --> 00:06:33,919 I have a sinusoid that's coming out of an antenna, so it's propagating. 70 00:06:33,920 --> 00:06:42,648 So this time difference is the, that's the period, and we know that's 1 over f, from 71 00:06:42,648 --> 00:06:48,784 all we've already learned. However, I can also plot this as a 72 00:06:48,784 --> 00:06:54,278 function of space, same signal because it propagates. 73 00:06:54,278 --> 00:07:00,300 And now, the period in space is called the wavelength lambda. 74 00:07:00,300 --> 00:07:07,477 Lambda has units of distance, and it turns out for propagating waves, the wavelength 75 00:07:07,477 --> 00:07:12,399 times the frequency is equal to the speed of propagation. 76 00:07:12,399 --> 00:07:21,419 Very important result to keep in mind. So suppose we do a little example here. 77 00:07:21,419 --> 00:07:30,882 Suppose we have a frequency of 1 gigahertz, and that's 10 to the 9th. 78 00:07:30,882 --> 00:07:37,213 So that gives a wavelength of 0.3 meters, which is 30 centimeters. 79 00:07:37,213 --> 00:07:44,818 So the wavelength is still pretty significant compared to the size of your 80 00:07:44,818 --> 00:07:52,540 cell phone, if it's communicating at a frequency of 1 GHz, which is pretty close 81 00:07:52,540 --> 00:08:00,380 to what is used these days. Now, another less understood result is 82 00:08:00,380 --> 00:08:09,060 that because of Maxwell's equations and the properties of the atmosphere and other 83 00:08:09,060 --> 00:08:17,864 things, and the properties of antennas, is that low frequency waves do not propagate 84 00:08:17,864 --> 00:08:22,148 well at all. In fact, as we'll see a little bit later, 85 00:08:22,148 --> 00:08:28,823 the United States government does not regulate the transmission of signals below 86 00:08:28,823 --> 00:08:32,477 about 10 kHz. You can do anything you want. 87 00:08:32,477 --> 00:08:38,096 So if you try to send an audio signal from one place to another directly using an 88 00:08:38,096 --> 00:08:42,588 antenna it's not going to work very well, because it has no range. 89 00:08:42,588 --> 00:08:48,268 Low frequency waves just don't propagate. So in wireless systems, somehow we have to 90 00:08:48,268 --> 00:08:53,614 shift the frequencies of the message signal up to higher frequencies so they do 91 00:08:53,614 --> 00:08:56,915 propagate. We'll see how that works a little bit 92 00:08:56,915 --> 00:08:59,414 later. All right, more properties? 93 00:08:59,415 --> 00:09:04,832 And there's the conservation of power issue, which means that the amplitude of a 94 00:09:04,832 --> 00:09:10,546 signal transmitted by our wireless has to decay with distance from the transmitter. 95 00:09:10,546 --> 00:09:17,837 So here's a little diagram. And here we have a antenna, that let's 96 00:09:17,837 --> 00:09:23,567 assume, sends out a short pulse, just for fun. 97 00:09:23,567 --> 00:09:30,944 If you look at a specific distance R, let's say that from the antenna, the 98 00:09:30,944 --> 00:09:39,602 power, if you inverted the power over that sphere, right, we're in 3 dimensions here, 99 00:09:39,602 --> 00:09:45,047 that has to be the total power radiated by the antenna. 100 00:09:45,047 --> 00:09:50,003 If you go out a bit further, that power cannot be unchanged. 101 00:09:50,003 --> 00:09:55,815 We're essentially assuming that the wireless meeting is lossless. 102 00:09:55,816 --> 00:10:02,170 So there can't be a power change. However, this is a bigger surface area out 103 00:10:02,170 --> 00:10:06,591 here than in here. Consequently, the power at a given 104 00:10:06,591 --> 00:10:10,412 distance from the transmitter has to go down. 105 00:10:10,412 --> 00:10:16,606 You must have the result that the power any given distance from the transmission, 106 00:10:16,606 --> 00:10:22,559 transmitting tower times the surface area of the sphere has to be a constant. 107 00:10:22,559 --> 00:10:29,231 So that leads to the fundamental result that the power that a receiver gets must 108 00:10:29,231 --> 00:10:34,873 obey an inverse square law. So that the more distant the receiving 109 00:10:34,873 --> 00:10:40,470 antenna is from the transmitter, the smaller the powers might be. 110 00:10:40,470 --> 00:10:47,233 Or maybe said a little better, the amplitude has to go down like 1 over r. 111 00:10:47,233 --> 00:10:51,203 It's the fact of life, fundamental physics. 112 00:10:51,203 --> 00:10:58,535 Now the other thing is that materials both common and natural, can absorb and reflect 113 00:10:58,535 --> 00:11:02,764 waves and this depends on their characteristics. 114 00:11:02,764 --> 00:11:07,978 So let's suppose we're in a city environment now, where we have tall 115 00:11:07,978 --> 00:11:14,488 buildings and there's a radio tower trying to send a radio signal to my little car 116 00:11:14,488 --> 00:11:19,297 down here. Well, if there's a direct path, that's 117 00:11:19,297 --> 00:11:26,061 fine, but it turns out these buildings can reflect, and in most cases do. 118 00:11:26,061 --> 00:11:31,496 Buildings in the cities do reflect the electromagnetic energy. 119 00:11:31,496 --> 00:11:37,748 And actually this antenna will see both the direct and reflected paths. 120 00:11:37,748 --> 00:11:43,306 This turns out, causes a little bit of problem in communication systems. 121 00:11:43,306 --> 00:11:49,332 More importantly, suppose there's another vehicle over here, which I'm going to draw 122 00:11:49,332 --> 00:11:53,352 very badly, which is trying to receive the same signal. 123 00:11:53,352 --> 00:11:59,663 Well, that signal is blocked by the buildings, and this antenna won't receive 124 00:11:59,663 --> 00:12:03,726 anything. This is why driving in cities cell phone 125 00:12:03,726 --> 00:12:09,094 signals and radio signals can go in and out, depending on the height of the 126 00:12:09,094 --> 00:12:15,320 buildings and how many are, where their positioned and where their transmitter is. 127 00:12:15,320 --> 00:12:20,762 It's a real problem. Well it also turns out natural entities 128 00:12:20,762 --> 00:12:25,128 can also affect electromagnetic transmission. 129 00:12:25,128 --> 00:12:31,146 I'm thinking here of a tree. And you may wonder, why would a tree block 130 00:12:31,147 --> 00:12:36,808 a signal transmission? And the reason is because the leaves are 131 00:12:36,808 --> 00:12:41,138 loaded with water. That's their primary constituent. 132 00:12:41,138 --> 00:12:46,946 And water is a very good conductor of electricity, and essentially the tree 133 00:12:46,946 --> 00:12:52,138 shorts out the electromagnetic transmission, and our poor little car 134 00:12:52,138 --> 00:12:56,655 here, the only thing it can get is a signal by a reflected path. 135 00:12:56,655 --> 00:13:03,699 So if you're going through even a, a rural area, where there are lots of trees, you 136 00:13:03,699 --> 00:13:11,145 can have your transmission sorely affected by trees and other naturally growing 137 00:13:11,145 --> 00:13:15,805 things. Now, more, let's talk about that direct 138 00:13:15,805 --> 00:13:20,237 path a little bit more. And here we have what's called 139 00:13:20,237 --> 00:13:25,331 Line-of-sight communication. And that means that the transmitting 140 00:13:25,331 --> 00:13:30,835 receiver antenna, and the receiving antenna can quote on quote feed each 141 00:13:30,835 --> 00:13:34,070 other. That's why it's called Line-of-sight. 142 00:13:34,070 --> 00:13:38,366 There is a direct straight line path connecting the two. 143 00:13:38,366 --> 00:13:44,213 Because of the curvature of the Earth, this is, doesn't go on forever. 144 00:13:44,213 --> 00:13:50,719 If you're over here somewhere, there is no line of sight path to this receiving 145 00:13:50,719 --> 00:13:54,625 antenna. And, in essence you cannot send a signal 146 00:13:54,625 --> 00:14:01,042 from, directly from this transmitter over to the other one, because the Earth gets 147 00:14:01,042 --> 00:14:04,882 in the way. So it's very interesting to know what is 148 00:14:04,882 --> 00:14:11,116 the line of sight transmission distance. Well, simple geometry using the 149 00:14:11,116 --> 00:14:17,226 Pythagorean theorem, it's pretty easy to derive, shows that the line of sight 150 00:14:17,226 --> 00:14:23,034 distance is given by this formula. And this is the radius of the Earth. 151 00:14:23,035 --> 00:14:26,661 So let's suppose we have a little example here. 152 00:14:26,661 --> 00:14:32,346 Suppose we have a 100 meter tall antenna. That's a pretty tall antenna. 153 00:14:32,346 --> 00:14:36,928 And you can plug it into the formula just as well as I can. 154 00:14:36,929 --> 00:14:43,121 The line of sight distance is about 71 and a half kilometers. 155 00:14:43,121 --> 00:14:51,555 Now that's the total distance here. If you were on the ground level, let's say 156 00:14:51,555 --> 00:15:00,069 holding your cell phone, then that distance let's say from here over to the, 157 00:15:00,069 --> 00:15:05,033 from the transmitter is going to be half of that. 158 00:15:05,033 --> 00:15:15,588 So 35 or so, 37 kilometers is the total distance that a cell phone can be reached 159 00:15:15,588 --> 00:15:20,477 by a 100 meter tall antenna. So it's pretty limited. 160 00:15:20,477 --> 00:15:25,830 And certainly not small compared to the circumference of the Earth. 161 00:15:25,830 --> 00:15:31,170 This was a big problem when they were trying to develop trans-oceanic, 162 00:15:31,170 --> 00:15:35,284 communication. When Marconi was trying to figure out how 163 00:15:35,284 --> 00:15:39,722 to send wireless telegraph signals across the Atlantic Ocean. 164 00:15:39,723 --> 00:15:45,218 And it turns out he did this experimentally, and it turned out it 165 00:15:45,218 --> 00:15:52,154 worked, because he discovered, and, and quickly was explained that there is an 166 00:15:52,154 --> 00:15:57,526 ionosphere up there, which really helps. So here is my Earth. 167 00:15:57,526 --> 00:16:01,566 And let me complete the circle as well as I can. 168 00:16:01,566 --> 00:16:06,875 And suppose I'm over here and I'm trying to send over here. 169 00:16:06,875 --> 00:16:11,930 Well, there is no line of sight path. It's too far away. 170 00:16:11,930 --> 00:16:18,391 What[unknown] has discovered is that the ionosphere acts like an, a mirror. 171 00:16:18,391 --> 00:16:25,901 Signals can come up and be reflected back down to Earth, enabling communication 172 00:16:25,901 --> 00:16:32,530 between distant places. Now, I deliberately showed it over here as 173 00:16:32,530 --> 00:16:37,893 being a little iffy, and that turns out to be true. 174 00:16:37,893 --> 00:16:44,573 It turns out, at some frequencies, the ionosphere Is transparent, and radiation 175 00:16:44,573 --> 00:16:48,951 goes straight through. At other frequencies, it acts like a 176 00:16:48,951 --> 00:16:51,672 mirror. And so you have to understand the 177 00:16:51,672 --> 00:16:57,705 frequency properties of the ionosphere. It's similar to a transfer function. 178 00:16:57,705 --> 00:17:02,578 This is more of a reflectivity function that's a function of frequency. 179 00:17:02,578 --> 00:17:09,272 Then there's another phenomenon, and that is that the transmission reflection 180 00:17:09,272 --> 00:17:16,202 characteristics depend on the time of day. It turns out, solar radiation from our sun 181 00:17:16,202 --> 00:17:23,552 interacts with the ionosphere, and during the daytime, it does not look much like a 182 00:17:23,552 --> 00:17:27,663 mirror. Those same frequencies, where at night it 183 00:17:27,663 --> 00:17:32,506 looks like a mirror. And I don't know if you've ever tried 184 00:17:32,506 --> 00:17:39,418 this, but at night using classic AM radio, you can receive stations from a very long 185 00:17:39,418 --> 00:17:43,817 distance away. However if you tune into that same station 186 00:17:43,817 --> 00:17:49,233 during the daytime, it's impossible. You will not receive it at all, and that 187 00:17:49,233 --> 00:17:54,984 is because the ionosphere changes its characteristics depending if it's the sun 188 00:17:54,984 --> 00:17:58,776 is, is sending radiation to it, that affects it, or not. 189 00:17:58,776 --> 00:18:05,452 Well, how about satellite communication? Now here, almost drawn to scale the idea 190 00:18:05,452 --> 00:18:12,387 in satellite communication is that suppose there's a transmitter here, and here's our 191 00:18:12,387 --> 00:18:18,130 little satellite, up here. And the way satellite communication works 192 00:18:18,130 --> 00:18:24,550 is that a transmitter sends a signal up and that gets re-radiated back to Earth. 193 00:18:24,550 --> 00:18:32,582 And, basically it's a coverage, basically about a third of the Earth's surface, 194 00:18:32,582 --> 00:18:36,958 let's say. Well, now, as the Earth rotates, okay, 195 00:18:36,958 --> 00:18:44,271 what you want for the geosynchronous orbit is that it rotates a greater distance, but 196 00:18:44,271 --> 00:18:50,657 in the same time, so again, it appears that by the time the antenna gets over 197 00:18:50,657 --> 00:18:55,620 here, our little satellite is directly overhead still. 198 00:18:55,620 --> 00:18:59,823 So you don't have to change the transmitter at all. 199 00:18:59,823 --> 00:19:06,644 That's called a geosynchronous orbit. And so, what happens in a geosynchronous 200 00:19:06,644 --> 00:19:13,079 orbit is that the time it takes to go around once has to be exactly one day. 201 00:19:13,079 --> 00:19:20,913 And when you do the physical calculations from Newton's equations, what you get, is 202 00:19:20,913 --> 00:19:27,917 that means that the radius of that orbit from the center of the Earth has to be 42 203 00:19:27,917 --> 00:19:31,543 million meters. That's a long, long way. 204 00:19:31,543 --> 00:19:36,670 And in fact, it turns out there's a significant time delay. 205 00:19:36,670 --> 00:19:43,845 If you take the speed of light, and try to figure out what the time delay is, the 206 00:19:43,845 --> 00:19:50,586 time delay in sending a signal up to the satellite and back is at least about a 207 00:19:50,586 --> 00:19:55,587 quarter of a second. And if you're, have to do this several 208 00:19:55,587 --> 00:20:01,791 round trips, so to get around the earth let's say, that it's just going to add. 209 00:20:01,791 --> 00:20:07,966 And so that's the big delay that you encounter in satellite communication 210 00:20:07,966 --> 00:20:12,145 systems. I know in the old days we used to have a 211 00:20:12,145 --> 00:20:16,515 telephone call from places like India from the States. 212 00:20:16,515 --> 00:20:22,587 They used satellite communication ,and it greatly affected the quality of the 213 00:20:22,587 --> 00:20:26,739 conversation, the delay was instantly noticeable. 214 00:20:26,739 --> 00:20:31,200 Very interesting. So anyway, those are the characteristics 215 00:20:31,200 --> 00:20:35,889 of wireless channels. It, there is a, just from fundamental 216 00:20:35,889 --> 00:20:42,153 physics, an inverse square law governs the power that a receiver, an receiving 217 00:20:42,153 --> 00:20:47,633 antenna can get from a transmitter. And the speed of light may be fast, but 218 00:20:47,633 --> 00:20:51,687 it's not infinite. And you can get significant delays between 219 00:20:51,687 --> 00:20:55,845 transmission and reception. It depends on distance, of course. 220 00:20:55,845 --> 00:21:00,299 And this really affects the satellite communications scenario. 221 00:21:00,299 --> 00:21:08,054 More importantly, Maxwell's equations are linear and superposition applies, which 222 00:21:08,054 --> 00:21:12,373 has, as I pointed out, good and bad properties. 223 00:21:12,373 --> 00:21:19,390 No privacy, but this enables broadcast. So one radio antenna can send to lots of 224 00:21:19,390 --> 00:21:23,779 receiving radios. However radio is now faced with the fact 225 00:21:23,779 --> 00:21:28,954 that all the other stations are sending at the same time, and you somehow have to get 226 00:21:28,954 --> 00:21:33,080 rid of the ones you don't want. And if you think about it for a second 227 00:21:33,080 --> 00:21:37,447 what this means is filtering. We'll talk about that a little bit later. 228 00:21:37,447 --> 00:21:42,913 And the big down side in wireless channels is that lots of interference is possible, 229 00:21:42,913 --> 00:21:47,457 and it's all because of the linearity and superposition principle. 230 00:21:47,457 --> 00:21:51,764 We're next going to talk about wireline channels, and you'll see that the 231 00:21:51,764 --> 00:21:54,546 characteristics are very, very different.