1 00:00:00,000 --> 00:00:14,800 Hi, my name is Murthy Upmaka. I am an Applications Engineer for Keysight Technologies. In this 2 00:00:14,800 --> 00:00:21,080 video I will discuss the most important considerations for a face-to-face system design. I used a 3 00:00:21,080 --> 00:00:27,040 simulation and modeling tool called SystemVue to illustrate various steps in a face-to-face 4 00:00:27,040 --> 00:00:32,760 system design. At the end, you will be able to download the SystemVue workspaces I used 5 00:00:32,760 --> 00:00:34,440 in the video. 6 00:00:34,440 --> 00:00:41,160 How is the far field of a phased array computed? Traditionally, if we know the complex voltages 7 00:00:41,160 --> 00:00:46,440 or current waveforms at the input of each of the radiators in a phased array, we can 8 00:00:46,440 --> 00:00:52,600 compute the far field by summing them at a faraway distance. We typically build the schematic 9 00:00:52,600 --> 00:00:57,760 with the number of RF channels equal to the number of array elements and simulate each 10 00:00:57,760 --> 00:01:04,000 channel individually. If we have a large array, simulating these voltages can take a significant 11 00:01:04,000 --> 00:01:10,240 amount of time. You can see how this time grows in the graph shown here with the size 12 00:01:10,240 --> 00:01:18,520 of the array. This translates to a 20-hour simulation for a 256-element phased array, 13 00:01:18,520 --> 00:01:24,200 scanning over plus-minus 20 degrees in azimuth and plus-minus 10 degrees in elevation with 14 00:01:24,200 --> 00:01:31,520 a 1-degree resolution. Fortunately, the tool used here has a technique that makes the simulation 15 00:01:31,520 --> 00:01:38,000 run several orders faster. To use this technique, we just need to construct one schematic and 16 00:01:38,000 --> 00:01:45,840 simulate any arbitrary-sized array. For example, even a 10,000-element phased array simulates 17 00:01:45,840 --> 00:01:53,080 in 2 seconds. With this kind of speed, the earlier 256-element array example will simulate 18 00:01:53,080 --> 00:01:59,000 in just 14 minutes. Having this technique makes the phased array design a lot more efficient 19 00:01:59,000 --> 00:02:08,200 and fun. To begin, let us first understand heuristically how a phased array works. If 20 00:02:08,200 --> 00:02:15,360 we place a set of omnidirectional radiators along a line and observe the waves emanating 21 00:02:15,360 --> 00:02:21,440 from them in all directions, we can easily notice that the waves add coherently in some 22 00:02:21,440 --> 00:02:28,120 directions, not so coherently in some other directions, and incoherently in other directions. 23 00:02:28,120 --> 00:02:34,600 This creates a distinct pattern at a faraway distance from the array where it becomes independent 24 00:02:34,600 --> 00:02:42,640 of distance. This pattern will have a main lobe, several side lobes, and a back lobe. 25 00:02:42,640 --> 00:02:49,080 The pattern is called far-field pattern. A three-dimensional far-field pattern of a 26 00:02:49,080 --> 00:02:57,040 20-by-20 rectangular array can be seen here. The far-field pattern and its characteristics 27 00:02:57,040 --> 00:03:05,320 are of utmost importance to baseband designers, RF designers, and antenna designers. 28 00:03:05,320 --> 00:03:12,400 In a phased array system design, the key parameters of a phased array architecture are the number 29 00:03:12,400 --> 00:03:18,240 of elements, distance between elements, which is usually less than lambda by two, the geometry 30 00:03:18,240 --> 00:03:24,720 of array, such as uniform linear, uniform rectangular, circular, etc., the frequency 31 00:03:24,720 --> 00:03:30,760 of operation, and finally, the far-field pattern of individual elements. By choosing these 32 00:03:30,760 --> 00:03:36,080 parameters judiciously, we can arrive at a desired far-field pattern. 33 00:03:36,080 --> 00:03:41,160 How does a far-field pattern affect overall system performance? Let us consider a simple 34 00:03:41,160 --> 00:03:48,040 example. A base station is sending signals to four users through four distinct beams. 35 00:03:48,040 --> 00:03:54,240 In this case, the base station is operating in a shared array mode where all four users 36 00:03:54,240 --> 00:04:00,480 are utilizing all the elements of the array. Let us measure the quality of the signal received 37 00:04:00,480 --> 00:04:07,240 by user 1 in terms of constellation and error vector magnitude. Since we made everything 38 00:04:07,240 --> 00:04:14,400 in this system ideal, we expect a very good constellation and low EVM, but that does 39 00:04:14,400 --> 00:04:21,400 not seem to be the case here. We wonder why this is so. 40 00:04:21,400 --> 00:04:27,960 Purely by virtue of the architecture of the array, a side lobe of user 2 is pointing in 41 00:04:27,960 --> 00:04:34,600 the same direction as the main lobe of user 1. Hence, user 1 is receiving not only the 42 00:04:34,600 --> 00:04:40,920 desired signal, but also an interferer from user 2. At this point, the problem can be 43 00:04:40,920 --> 00:04:46,880 fixed by increasing the signal-to-interference ratio, by increasing the power of the user 44 00:04:46,880 --> 00:04:53,880 1 signal, or reducing the side lobe level of user 2, or increasing the number of elements 45 00:04:53,880 --> 00:04:59,480 of the array, which will increase the gain of the array, as well as produce lower side 46 00:04:59,480 --> 00:05:03,920 lobes. As we have seen in this example, the far-field 47 00:05:03,920 --> 00:05:08,600 pattern has a significant influence on the performance of this system. 48 00:05:08,600 --> 00:05:16,400 Before we go further, let us break and then learn a few interesting facts. In the early 49 00:05:16,400 --> 00:05:28,000 days of World War I, they used acoustic waves to form a phased array, as you can see here. 50 00:05:28,000 --> 00:05:35,920 And in the modern world today, the largest phased array is a project called Space Fence 51 00:05:35,920 --> 00:05:45,800 for U.S. Air Force, and you can see that the transmit array has 36,000 antenna elements, 52 00:05:45,800 --> 00:05:53,880 whereas the receive array has 86,000 antenna elements. You can also see that the canopy 53 00:05:53,880 --> 00:06:02,240 covering the transmit and the receive buildings is actually a radome, and you can also observe 54 00:06:02,240 --> 00:06:09,400 the calibration tower to see how they calibrate these two huge phased arrays. If you are interested, 55 00:06:09,400 --> 00:06:16,800 you can go to this website and then probe further. I hope you like these facts. 56 00:06:16,800 --> 00:06:22,200 Having understood the importance of the far-field and its characteristics, we need to understand 57 00:06:22,200 --> 00:06:28,160 various factors that influence the far-field pattern as part of the design process. 58 00:06:28,160 --> 00:06:33,680 The subsystem characteristics and the integration of the subsystems can have profound influence 59 00:06:33,680 --> 00:06:39,280 on the far-field pattern. I am going to talk about several critical factors that influence 60 00:06:39,280 --> 00:06:47,640 the far-field pattern, AM to PM of the amplifiers in the RF electronics, the number of bits 61 00:06:47,640 --> 00:06:53,440 of the digital phase shifter and the digital attenuator in the TR module, active impedance 62 00:06:53,440 --> 00:07:00,960 of the antenna elements, which interestingly can be a function of frequency and scan angle, 63 00:07:00,960 --> 00:07:06,640 and finally, random antenna element failures. I am going to discuss about all these four 64 00:07:06,640 --> 00:07:10,960 factors and their influence on the far-field pattern. 65 00:07:10,960 --> 00:07:18,120 Let us first understand the AM to PM of the amplifier on the far-field pattern. As the 66 00:07:18,120 --> 00:07:25,160 input power to the amplifier is increased, the phase change the signal undergoes through 67 00:07:25,160 --> 00:07:33,680 the amplifier is no longer a constant. Most phase-to-array designs use amplitude distribution 68 00:07:33,680 --> 00:07:39,560 at the antenna elements to control the side lobe levels. This means the amplifiers in 69 00:07:39,560 --> 00:07:45,840 various channels feeding to the antenna elements will be operating at different power levels 70 00:07:45,840 --> 00:07:53,040 when amplitude tapering window is applied across the array. We see the far-field pattern 71 00:07:53,040 --> 00:08:00,760 with and without AM to PM effect here. The side lobe levels and their shape and the number 72 00:08:00,760 --> 00:08:08,520 all change with AM to PM. By plotting the 2D cut pattern, we can easily measure the 73 00:08:08,520 --> 00:08:19,000 side lobe levels. We can see a 5 dB raise in side lobe level and 1.6 dB drop in directivity. 74 00:08:19,000 --> 00:08:26,240 Next, let us study the influence of number of bits of the digital phase shifter and attenuator 75 00:08:26,240 --> 00:08:32,720 on the far-field pattern. The phase distribution and the amplitude distribution of the signal 76 00:08:32,720 --> 00:08:38,520 arriving at the antenna elements are achieved by programming these digital phase shifters 77 00:08:38,520 --> 00:08:46,440 and digital attenuators. The digital control word length is limited usually to 3 to 6 bits. 78 00:08:46,440 --> 00:08:52,240 It is interesting to see how the far-field pattern varies with the bit precision of these 79 00:08:52,240 --> 00:09:00,000 components. The plot here shows a comparison between unlimited bits, 5 bits, and 3 bits 80 00:09:00,000 --> 00:09:07,720 phase shifters and attenuators. The 3-bit precision clearly increased the number of side lobes 81 00:09:07,720 --> 00:09:14,760 and their levels. We can see more details in a 2D cut pattern. We can see that the 3-bit 82 00:09:14,760 --> 00:09:21,840 phase shifters and 3-bit attenuators narrowed the beam width by 5 degrees and increased 83 00:09:21,840 --> 00:09:29,200 the side lobe levels by almost 25 dB. The active reflection coefficient of the antenna 84 00:09:29,200 --> 00:09:36,520 is very important, especially if it mismatches with the electronics preceding each element. 85 00:09:36,520 --> 00:09:42,600 I mentioned earlier that the active impedance can change with scan angle as well as frequency, 86 00:09:42,600 --> 00:09:48,720 so we have to monitor the far-field performance while scanning the azimuth and elevation in 87 00:09:48,720 --> 00:09:56,000 a range in which the phase delay is expected to function. We see here both the 3D plots 88 00:09:56,000 --> 00:10:02,720 and the 2D cut pattern. It is clearly noticeable that the active impedance of the antenna mismatching 89 00:10:02,720 --> 00:10:08,320 with the electronics in front of it raises the side lobe level significantly. We also 90 00:10:08,320 --> 00:10:16,200 see that the way the pattern impacted depends on the scan angle. 91 00:10:16,200 --> 00:10:21,800 Before we conclude, let us go through an interesting and important design step. Phased arrays are 92 00:10:21,800 --> 00:10:28,600 attractive because even if a certain number of elements fail, the array will still function, 93 00:10:28,600 --> 00:10:34,560 though with a graceful degradation of performance. How do we know which element failures and 94 00:10:34,560 --> 00:10:41,920 how many failed elements are acceptable before the system is declared to be unacceptable? 95 00:10:41,920 --> 00:10:47,920 This can be a difficult question to answer without a simulation tool, but thankfully 96 00:10:48,040 --> 00:10:53,120 we can perform a Monte Carlo simulation by randomly failing a fixed number of elements 97 00:10:53,120 --> 00:11:01,040 or we can attribute a probability of failure to each element. We can easily see here that 98 00:11:01,040 --> 00:11:08,040 the side lobe levels degrade the performance with random failure of the elements. 99 00:11:08,040 --> 00:11:15,040 Finally, we gained an understanding of what impacts the far-field pattern, which in turn 100 00:11:15,160 --> 00:11:20,960 influences the system performance. We just learned that it is absolutely essential to 101 00:11:20,960 --> 00:11:27,080 be able to simulate the array very quickly as we have to explore a lot of design space. 102 00:11:27,080 --> 00:11:33,720 In addition, sometimes the array can be very large. Studying beamforming architecture and 103 00:11:33,720 --> 00:11:39,480 techniques is very interesting and I hope you enjoyed learning along with me in this 104 00:11:39,480 --> 00:11:46,480 video. You can download the system view workspaces I used from the link below for your own use. 105 00:11:46,480 --> 00:11:48,600 Thank you for watching this video.