Abstract: We ll do that with the following agenda:

Transcription

1 Abstract: As the pace and cost of SATCOM missions continue to change, commercially available simulation tools can help system-level designers improve system planning while reducing development time. Modeling the orbital mission enables system engineers to capture dynamic channel impairments, and the resulting insights allow them to refine and capture design ideas in the software. This paper will show how to develop robust, cost-effective satellite electronics using system analysis and hardware design performed using a co-simulation link between software from Keysight and Analytical Graphics. We ll do that with the following agenda: 1

2 Applying System Analysis to SATCOM Mission Requirements and Channel Impairments is intended to help attendees understand the power of modern software simulation tools. Especially when considering the bridge Keysight provides between operational hardware and simulation. This can take the form of measurement assisted models like X-parameters, or functional loads into test equipment to create hybrid DUT/Test Equipment, systems. With the connectivity to AGI s System Tool Kit (STK), environmental (kinematic) modeling, a mission realistic simulation of the full operational system can be run. The added software simulated kinematics can then be implemented in Keysight s PropSim to allow developers to fly-before-they-fly. Simulation, modeling and hardware-in-the-loop have never been more powerful. These tools now allow engineers to move faster and more confidently than ever before. 2

3 So there are some MAJOR trends going on that changed the dynamic of the space industry 1. Huge reduction in launch cost that opened the door to new commercial business ventures 1. Launch cost divided by SmallSat complexity growing relatively cheap and easy to launch satellite networks 3. More Competition 2. As Consumers, we have grown more dependent on higher data throughput mobile data, global imaging 1. Use of our mobile devices, and the emergence of IoT means that there is a lot of wireless data that could be offloaded to satellites 2. New applications like global imaging will generate continuous streams of high resolution video images requiring large data down links. 3. RF frequencies are occupied little availability pushing us to higher frequencies and use of steered antenna arrays to minimize cross-talk 1. Oxygen absorption is not an issue between satellites in space, but focusing our RF beams can help combat losses. 2. As we look to 5G networks in the future it s entirely possible that there will be a mix of base stations, pico cells and satellite communications that will allow seamless transfer between data sources. 3

4 When it comes to space based systems, the distances and speeds involved make it very important to consider kinematics when modeling the electrical system. kin e mat ics ˌkinəˈmadiks/ noun the branch of mechanics concerned with the motion of objects without reference to the forces that cause the motion. the features or properties of motion in an object. Satellite kinematics: Even a GEO satellite that in theory is stable in space with respect to a point on earth, has some motion. That motion creates Doppler shift and changing delay. The transmitter or receiver can also be in motion. The atmosphere is a dynamic media which also causes fading. While link budget calculators create some static understanding of the losses, the motion dynamics or kinematics create dynamic losses which challenge a design in more ways that can be simply defined by individual measurements. Link Budget, short example: uplink path loss + transmitting antenna gain + transmit EIRP & operating flux density + uplink to noise ratio + satellite operating down link EIRP + satellite operating bandwidth + down link path loss + receiver antenna gain + receiver G/T + other considerations (down link rain attenuation (db), Receive antenna pointing loss(db) all provide insight on 4

5 Eb/No Required for certain BER and eventually link margin 4

6 A SATCOM mission has several challenges, such as latency, channel loss and possibly moving targets such as airplanes. In the case of moving assets the channel becomes a dynamic ever changing channel dependent on the mission kinematics. Capturing a dynamic channel properties such as delay, Doppler shift, loss and noise is critical in the early stages of the design. In this picture we look at Atmospheric absorption. Notice that moving up in frequency means more atmospheric attenuation. Satellite Uplinks are at the higher frequency, because you have power available on the ground. The downlinks have power constraints. The simulation techniques that we will discuss in this presentation will cover how to handle these channel dynamic and constraints. 5

7 In the past, so-called bent pipes were used where the signal is received by the satellite and converted to the downlink frequency and then re-transmitted. These designs have limited processing and flexibility, but the design can be less complicated. Many modern communication satellites are starting utilizing what s called a digitally regenerative payload. Digital data links provide a variety of advantages over analog links. The digital data link easily interfaces with digital computers, digital information compression schemes and high-speed packet switching. The digital data link also has the ability to transmit wide dynamic ranges with low RF signal-to-noise ratios. However, digital data links do require considerably more complexity than the traditional analog bent pipe. The key to achieving higher data rates is to improve SNR. In bent pipe architectures this implies focusing on the gain of antenna and amplifiers. In regenerative payloads processing can also provide gains. As we can see above, the signal travelling through the digital payload are modulated signals. While continuous wave (CW) tones can be used to test some of the components, such as power amplifiers (PAs), CW tones cannot be used to test the complete payload. Here modulation analysis becomes more important, and we ll discuss this in later sections. 6

8 Today, we will focus on the component test in the satellite payload, mainly including the PA. The PA determines the final power level that can be transmitted. And at the end of the paper, we will discuss modulation accuracy test on the system link. 6

9 There are a variety of modern techniques and considerations to get the most gain out of a satellite communication system. The first is operating a power amplifier outside of the linear regions. While this has issues, pre-distortion techniques can be utilized to improve the ability to operate even as the amplifier starts to go non-linear. Another technique is to use custom or specialized modulations such as those defined in DVB-S2X, where the ring ratios are chosen to compensate for the typical non-linearity of the amplifiers. Narrower beams or beam forming is another technique that is designed to provide more signal gain. Lastly for satellites that have digital systems there is processing gain that can be achieved when the signal is demodulated, but the kinematics such as Doppler, delay, fading and noise all need to be considered. 7

10 With Model Based Engineering Concepts Modeling and Simulation can be used to predict system performance as real data and technology refresh occurs Unit Level Data can be used for predicting system performance, trending, and reduction of costs through margin management Technology refresh or system optimization effects can be predicted by feeding back into the system model Hardware in the Loop can be used to mix new hardware with simulation earlier in the process Test plans can be developed and evaluated earlier by using software models Use of simulation to drive and discover issues early in process this will help reduce total cost of test / ownership and accelerate time to market or mission launch. 8

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12 The Simulation Software that we will be using is SystemVue from Keysight Technologies. SystemVue has two main Simulation engines: 1. RF System Analysis: This allows System Designer to do cascaded analysis 2. Data Flow (DF): Time Domain Simulator that contains features to analyze complex modulated type systems in the Aerospace and Commercial applications. We will be using both capabilities to analyze a Satellite Mission Scenario. 10

13 An overview of a full system, both with simulation and actual hardware. With ADS, SystemVue modeling and linkages to the industry standard kinematics tool STK, a high fidelity simulation can be built. This model then can be used throughout the satellite development life cycle discussed earlier in order to analyze the various tradeoffs that occur throughout the satellite program. Early in the program development, decisions and tradeoffs can be made through examining the high fidelity models. As hardware choices are made, measurements can be made on the actual hardware to determine updated parameters for the simulations. Test equipment can be substituted components that have not yet been decided on, including a PropSim to model the actual satellite link. During verification, any deviation from expected performance can be reexamined in the simulation. 11

14 System Design and Analysis starts very often from a vendor datasheets or System Engineering proposed specs for in house designs. A basic PA model can capture the AM/AM characteristics and allow designers to make first order predictions. This amplifier has a saturated power of 54 dbm (24 dbw).. Notice that the power sweep clearly shows the departure from linear behavior as the output power is increased and approaches the saturated level. This model assumes flat response after reach saturation and the AM/PM effects are not captured. The model has some obvious limitations but is commonly used in the early stages of system parametric requirement lineups. TWTA/SSPA Technical Brief: Amplifiers come in many shapes and sizes, especially as device technologies such as gallium nitride (GaN) have achieved high output levels in small packages. But in some cases, older technologies such as traveling-wave-tube amplifiers (TWTAs) are still in use and still useful solutions for many applications. There are clear differences between solid-state amplifiers and TWTAs in terms of output power, packaging, and power supplies, and understanding the differences between the two technologies helps to simplify the process of choosing the optimum amplifier for an application. 12

15 The microwave industry once relied heavily on TWTAs for amplification, although solid-state power amplifiers (SSPAs) have made great inroads over the last few decades, starting with silicon bipolar transistors, then extending at higher frequencies with gallium arsenide (GaAs) transistor devices, to a present-day expansion of applications relying on solid-state amplifiers that build upon GaN transistors and integrated circuits (ICs). Fans of solid-state amplifiers like to point out that they are not victims of the single failure point (the tube) as in TWTAs, although TWTAs typically deliver reliable operation and long operating lifetimes, even with the single failure point compared to SSPAs. TWTAs are still used in broadband applications, such as broadcasting, radar, and satellite communications (SATCOM) systems. Two types of tube devices are used in these amplifiers: broadband amplifiers are typically based on helix TWT devices while narrowband amplifiers may use coupled-cavity TWTs in amplifiers at higher power levels. 12

16 A refinement to the model can be done by adding AM/PM data so that in addition to the amplitude imperfections the phase compression characteristics are also included. This model can be obtained from either measured (such as VSA measurements) or from circuit design simulated data. 13

17 X-parameters are the mathematically correct superset of S-parameters, applicable to both large-signal and small-signal conditions, for linear and nonlinear components. We can measure using a NVNA. You can use X-parameters as a model for you device & simulate linear and nonlinear properties of you device or circuit. For more information please visit our website. 14

18 A much higher fidelity model can be implemented by using measured X- parameters. The 15 GHz CW results from the X-parameter model are compared to those in DVB Document A171-2: Digital Video Broadcasting (DVB) Implementation guidelines for the second generation system for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications; Part 2 - S2 Extensions (DVB-S2X) Pout and PhOut (green and blue traces respectively) are the suggested Amplitude and Phase response that should be used for testing DVB communications. Our target PA definitely has similar characteristics that match those suggested. Note that the measured X-parameters provide the best level of fidelity of the three models that we have seen and we will use for the rest of our System Analyses. 15

19 The X-parameters used in the model must be properly calibrated in order to ensure accurate, repeatable parametric results. The X-parameter calibration routine consists of three steps, including a vector (S-parameter) calibration, a phase Cal, and a power cal. Most calibration standards such as Keysight s ecal modules are highly stable. However, when measuring with high powers signals such as high power amplifiers, there is a much higher risk that a large signal will make it to the calibration standard without the proper signal conditioning elements (like an attenuator). Additionally, the wear and tear of connectors is a key contributor to instability and non-repeatable measurements. As a result, Keysight has developed a procedure to examine the connectors of every module that comes in for calibration physically, mechanically, and electrically. The connectors are thoroughly cleaned every time. If the connectors fail the examination, they are replaced. This ensures optimal, stable measurement results for X-parameters. Other tips to avoid errors during calibration are as follows Ensure proper connections of adaptors and cables, which can cause amplitude and phase errors Connect the short and through connections correctly; switching them introduces major phase errors To test for suspected calibration errors, compare the calibration results for 2 16

20 amplifiers cascaded together to a mathematical combination after each is calibrated separately If signal conditioning pre-amplifiers are used, try to calibrate with them in place to avoid noisy results, unless they are located between the RF source and reference coupler Do not damage the network analyzer. As a general rule-of-thumb, the power level should be at least 3 db (ideally 6 db) below the damage level of the instrument s connector 16

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22 The mission kinematics are simulated using STK from Analytical Graphics Inc. (AGI). This scenario includes a Geosynchronous Satellite, Ground Station for DTH (Direct To Home) analysis and a flying Aircraft. We will be analyzing both paths simultaneously. The purpose of this analysis is to capture complex channel dynamics from STK and incorporate them in our SystemVue analysis via the SystemVue & STK Link. 18

23 This figure shows the RF signal propagation from a high fidelity SystemVue transmitter model (Transmitter), through an RF channel (RF Channel) controlled by the STK Interface with time varying STK scenario parameters that are define in the STK environment, assets and targets, and into a high fidelity SystemVue receiver model (Receiver). A major benefit of this analysis flow is that it allows System Designers to incorporate industry standard mission Analysis Software, STK into the System Analysis. No longer is the System Analysis Engineer constrained to using spreadsheets and point tools that do not interact with each other. 19

24 This analysis is performed using the Spectrasys simulator in SystemVue. The spectrasys analysis allows System Analyst to have an overall view from the Transmitter thru the channel and receiver. Parameters such as DCP (desired channel power) or CGAIN (cascaded gain) can be obtained from such a simulation. 20

25 The System Analysis can also perform detailed analysis of certain group of components or subsystem. Here we are looking at the receiver. Typical receiver measurements are facilitated by the Spectrasys simulator, such as: CNF cascaded noise figure MDS minimum detectable signal COMP stage compression NDCP noise plus distortion C/(N+D) carrier to noise plus distortion 21

26 Graphical results of MDS and DCP. In theory the more these two results are separated the more margin or dynamic range in your receiver design. Notice that the horizontal axis is actually shows the components and node numbers in the schematic thru which the signal analysis was done. This capability shows graphically the performance of the entire cascaded analysis with the component identified. Analyses like this allow designers to quickly identify any problem spots. 22

27 Another results that user can obtain with Spectrasys is the spectrum at any node in the circuit. This Spectrasys chart shows the results for the output of the Low Noise Block Converter (LNB). Notice that the noise floor is getting shaped by the system filters. This kind of analysis allows Engineers to locate problematic spectral components that may distort the desired signal (spurs, intermod products, etc.) that fall in band. 23

28 SystemVue ships with a selection of Communication Standard Libraries (e.g. WLAN, LTE, etc.) and Communication Analysis capabilities. Custom modulation format can also be created by the user. In this presentation we will do a DVB (Digital Video Broadcast) communication system. A DVB-S2X library is available for SystemVue that is fully compliant with the standard. To evaluate the quality of the system we can use EVM (error vector magnitude) and BER (bit error ratio). 24

29 The HPA for the DVB-S2X Satellite transmitter is the one based on the high fidelity X-parameter model we examined earlier. This simulation show the CCDF results after the HPA. Note that the HPA nonlinearities show up in the results. For example PAPR (peak to average power ratio) is 3.63 db but the DVB modulation has a PAPR ~ 6.8 db. 25

30 In order to make things a bit more interesting the LNB for DTH and Aircraft are different. The story behind this is that different teams have elected to use different components depending on such things as performance and cost that are unavoidably always a tradeoff that designers must make. The differences are the LNA noise figure, RF filter ripple and IF filter bandwidth. The airplane team have favored lower cost and are willing to take a performance degradation hit for it. We will examine if this was a wise decision. 26

31 In this analysis we want to compare the Airplane and DTH performance. The blocks highlighted include all the necessary SystemVue elements that allow the simulation to talk to STK. This link is establish by the STK_Link component which is part of a service library (AGI_STK_Link_Parts). In essence the highlighted blocks include the STK channel information that also includes the Tx and Rx antenna behavior. 27

32 One of the component we have not looked at is the antenna. The airplane team decided they would use a phased array antenna to track the satellite as the airplane moves. This Antenna Array was designed using the Antenna Beamforming capabilities in SystemVue. 28

33 In this mission the antenna beam will rotate as needed to point at the satellite. For this analysis let s assume that the antenna control signal has an error with a normal distribution and a standard deviation of 1 degree in both azimuth and elevation. The plot shows a CW signal received instantaneous power at the aircraft receiver input would look like (orange trace). Notice that because STK includes all the complex mission parameters the instance that the aircraft turns is captured and we can clearly see this as a change in antenna azimuth pointing angle. 29

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35 The different components of the entire SATCOM system have been designed and analyzed in prior slides it is time to put the entire mission together. On this slide we see the SystemVue analysis for the complete Satellite mission. Included are the needed components to analyze both the DTH and Aircraft communication systems. The needed data collectors (aka sinks) have been added. Note that Spectrum Analysis (using a Spectrum Analyzer sink) and measurements such as BER and EVM are concurrently being done for the Airplane and DTH scenarios. 31

36 The results shown here are for a nominal transmit power. This analysis allows us to look simultaneously at the DTH and Airplane scenarios. The above chart shows the results for both. Recall that the airplane receiver designers had compromised on the LNB performance. Looking at these results and those that follow it is clear they made a good choice. 32

37 These are the results of varying the nominal transmit power. The top results show what happens when the power is reduced by 3 db. Constellation shows a tighter distribution of the points. The spectrum for the airplane shows a improved ACPR response even though it had a tighter if filter bandwidth. The reason is the elliptic filter in the IF section of the DTH LNB has a group delay response that shows up as this noise level increase. In the bottom plots simulation the Tx power was increased by 3dB. The results show a clear increase in ACPR and the constellation distortion is quite noticeable. 33

38 Recall that the airplane used an LNB which should result in worse performance. The results are obviously fairly similar between both applications. Why? The aircraft designers realized that in their case the airplane is flying at a height of 10 Km so it has an advantage that it will have somewhat less propagation loss and less atmospheric noise. Therefore the airplane design can afford to use a cheaper LNB. 34

39 Insights gained from simulations can greatly reduce time and uncertainty during the build and test phase of a satellite. Today s simulations combine high fidelity electrical modeling and kinematics, more closely representing the performance of the design once deployed. 35

40 R&D, Verification, Production PNA-X Network Analyzer TVAC Cal Pods PSA Spectrum Analyzer PSG, MXG Signal Generator AWG, Arbitrary Waveform Generator Infiniium, Oscilloscopes Infiniivision Oscilloscopes Advanced Design System (ADS) Optical Modulation Analyzer PTS Payload Test System Power Meters Phase Noise Tester PropSim Satellite Simulator GNSS Constellation Emulation 89601B VSA Software Operation and maintenance Field Fox, Hand Held Analyzer N6841A RF sensor 36

41 MXA Spectrum Analyzer Infiniivision Oscilloscopes Signal Surveyor 4D Hand Held Multi-Meter Power Supplies Power Meter 36

42 Thank you! 37