Traditionally, the satellite industry has relied on geosynchronous earth orbit (GEO) satellites that take years to build and require very expensive launches to deliver them to orbit. Satellite networks using geosynchronous equatorial orbit (GEO) are effective at providing stationary coverage to a specific area, however, Latency issues due to the distance of these orbits limit the ability of these satellites to be used for real-time communications like voice or live video transmissions.
New technology is driving a wave of innovations and evolution to smaller micro-sats deployed in low earth orbit (LEO) with reusable rockets delivering multiple satellites at a time with a single launch vehicle reducing deployment costs. The attention of researchers is recently shifting to satellite networks employing the low Earth orbit (LEO) or very LEO (VLEO) mega-satellite constellations.
Unlike GEO satellite networks, LEO or VLEO satellite networks can achieve higher data rates with much lower delays at the cost of deploying more dense satellites to attain global coverage performance. These smaller satellites deployed in mega-constellation arrangements can provide voice, video, imaging, and data to commercial and military clients with higher data rates and lower latency than legacy GEO deployments.
NewSpace drives the changing of the business models in the space with shorter lifetimes, high performance, and a substantially lower investment compared with traditional satellites. These changes have been disruptive, affecting design processes, design, and verification test requirements, and cost of test.
Newspace test requirements
NewSpace networks and terminal vendors need to thoroughly test wireless communication systems and components to ensure uninterrupted operation and high quality of service. Additionally, manufacturers need to reduce test time in order to keep user terminal costs at a practical level.
This increase in the number of satellite uplink and downlink stations will require systems to be designed to reject real-world RF interference from other uplink and downlink transmitters, as well as constellation communications between satellites as part of the relay network.
Another change that we are seeing is the move towards the use of higher frequencies such as Ka Band. Increasing channel bandwidth not only enables higher data rates per client, but also extends the number of channels for higher system capacity. High-throughput satellites now use transponders with bandwidths up to 2.1 GHz to achieve required data rates at Ka-band. Most HTS typically file for 3.5 GHz bandwidth in Ka-band: 27.5 – 31 GHz for uplink and 17.7 – 21.2 GHz for downlink.
Newspace Satellite communications utilize large signal bandwidths and use higher-order modulation schemes to improve their spectral efficiency. However, wider bandwidths also gather more noise, and higher-order modulation schemes are sensitive to system noise. Both of these bring test challenges in signal generation and signal analysis.
The utilization of higher frequencies and wider bandwidths in satellites requires more complex testing and characterization to ensure that components and systems meet demanding space requirements. For a variety of test scenarios essential to the design, verification, and manufacturing of satellite components and systems, signal analysis and signal generation are the fundamentals of a robust test system.
The integration of such a test system requires a combination of software and hardware tools. You can use off-the-shelf instruments to build the system, but need to calibrate the frequency responses across the entire bandwidths of both the signal generation and analysis.
User terminal considerations
LEO constellations introduce a challenging set of requirements for user terminal test engineers when compared to legacy GEO VSAT terminals. For example, Iridium mobile phones deployed a compact circular polarization antenna that communicated directly with the satellite. Satellite and terminal engineers need to accurately characterize RF components and sub-systems to ensure their devices’ performance meets the design requirements.
NewSpace networks typically deploy fixed antenna user terminal designs that employ a parabolic or phased-array terminal solution that mounts to a fixed position or even on maritime or aero vehicles. These CPE-like devices then typically communicate via a commercial interface, such as Ethernet, to a commercial, off-the-shelf device, such as a server, base station or Wi-Fi router. The user terminal acts as a transparent relay to a preferred commercial user device.
Ground station considerations
Historically VSAT terminal testing focused mainly on static RF performance, ensuring that frequencies and power level tolerances fall within defined limits. While fading exists in both systems, high-rate Doppler issues that invoke complex handovers are inherent in the LEO constellations and require greater sophistication in design, integration and test.
Different NewSpace design approaches impact ground station functionality. Networks based on digital regenerative payloads implement much of the radio resource management on board the satellite. For bent pipe systems, the radio resource management controls reside in the ground station, which direct the user terminals regarding IP scheduling and radio control.
From a testing perspective, the ground stations require similar testing to the user terminals. The need for RF calibration and modem verification are similar, although the cost and test methodologies could differ depending on cost and complexity. Fewer ground stations exist in a network, significantly reducing handover burdens. However, the larger data throughput and combining of multiple return link signals adds significant complexity to the bandwidth and data handling. With the lower volume and higher responsibility, ground station modems and RF systems require longer test times, typically.
Some of the key components in the ground stations require extra considerations during testing. Antenna size is obviously dramatically larger. Gateways and TT&C terminals may use 10-meter dishes or large phased arrays. Over-the-air (OTA) testing solutions most likely require large indoor or outdoor test ranges and chambers. Small OTA chambers are not relevant for large ground antenna testing. Ground station power amplifiers require additional high power and wide bandwidth, compared to user terminal power amplifiers.
NewSpace applications require increased complexity in the radio resource management (RRM) system, which demands increasingly complex user terminals designs compared to legacy VSAT terminals. These solutions are more dynamic, with some constellation scenarios requiring several handovers per minute. This creates scheduling issues not typically experienced in legacy VSAT modems. Protocol stacks in LEO NewSpace systems require greater complexity. Consequently, user terminals resemble complex terrestrial mobile phones with perhaps even more stringent requirements on radio resource management relating to handovers and scheduling.
Ideally, engineers thoroughly test the terminal radios prior to satellite launch. In addition to the single satellite network emulation tools, LEO constellations especially require constellation simulators. Constellation simulators marry multiple network terminals with fading and ephemeris emulation models so the terminal under test can prove its functionality in a real-world environment. This scenario most closely resembles a functional, multi-satellite dynamic constellation.
While the single network emulator generally proves individual terminal modem and RF functionality, the constellation simulator adds increased complexity to the test models that most closely resemble actual working network conditions. Static GEO systems did not require such complex systems for verification. However, complex RRM intensive systems, such as LEO NewSpace constellations, require a constellation simulator to reduce the tremendous risk of failure that is extremely difficult to troubleshoot with orbiting satellites.
Test methods and Instrument
With strong demand for faster data throughput, satellite communications increase signal bandwidths and use higher-order modulation schemes to improve their spectral efficiency. However, wider bandwidths also gather more noise, and higher-order modulation schemes are sensitive to system noise. Both of these bring test challenges in signal generation and signal analysis. In addition, the complex modulation schemes do not only improve spectral efficiency but also need to minimize nonlinear amplification in the RF power amplifier.
Amplitude and phase-shift keying (APSK) improves nonlinear distortion for satellite communications; orthogonal frequency-division multiplexing (OFDM) increases spectral efficiency for high-throughput satellite communications. However, both modulation techniques bring in test challenges — generating and analyzing custom, proprietary modulation schemes.
For a variety of test scenarios essential to the design, verification, and manufacturing of satellite components and systems, signal analysis and signal generation are the fundamentals of a robust test system.
For signal generation, you need:
- a calibrated vector signal generator (VSG) to upconvert the baseband signals to the desired frequency,
- and a power meter or a vector signal analyzer (VSA) to calibrate the frequency responses.
For signal analysis, you need:
- a signal analyzer downconverting the input signal to a wideband IF signal,
- a high-speed digitizer or an oscilloscope to acquire the IF signal or the direct RF signal for analysis,
- and a comb generator or vector signal generator to calibrate the frequency responses.
Carrier-to-Noise Ratio Testing
Testing the performance of a satellite communications channel in the presence of noise and interference is critical, as a poor communications channel will limit the amount of data and range that can be communicated and the reliability of the communications. Modern modulation schemes significantly help reduce the impact of noise in the communications channel, but ultimately too much noise or interference can overpower the carrier and prevent the receiver from distinguishing it from noise.
Carrier-to-noise ratio is one key parameter of receiver testing along with energy per bit-to-noise power density (Eb/No) and carrier-to-interference (C/I), combined key ratios like this give a full picture of what levels of interference or noise a channel or receiver can tolerate. Testing system performance for these different parameters and optimizing for desired performance levels is time consuming and error prone due to the manual trial and error process of measuring levels and resetting generators.
Carrier-to-Noise Ratio Testing with the UFX7000A
Carrier-to-noise (C/N) ratio is one of the most common parameters to test when determining the performance of a satellite communications channel. The more noise a channel can tolerate, the better the quality of the link, making the system more reliable.
Noisecom UFX7000A instruments provide a flexible architecture to create sophisticated noise signals enabling satellite system designers to test the unique characteristics of their RF uplink and downlink paths in the presence of noise.
The UFX7000A is a precision, programmable broadband noise generator. When used in conjunction with a power meter or spectrum analyzer a test engineer can create a specific carrier-to-noise ratio to evaluate system performance and receiver tolerance to interference testing. In this example the power meter is used to measure the signal power level and based on the noise bandwidth, a test engineer or technician can calculate the noise power required to create the desired carrier-to-noise ratio.
The desired ratio is created by applying the test signal to the input of the UFX7000A and adjusting the noise attenuator to set the noise power. The combined test signal and broadband noise at the output of the UFX7000A will then be at the desired C/N ratio to test the receiver.
Amplifier Linearity – Noise Power Ratio
Amplifier performance is important in a system as non-linearities will reduce the dynamic range of the communications channel by limiting the lowest power level of a signal that can be received erroneously. Noise power ratio is a convenient way to test for non-linearities created in a system.
The performance of high power amplifiers with many carriers (>10) is normally tested using a noise power ratio (NPR) measurement. In this test white noise is used to simulate the presence of many carriers of random amplitude and phase. The white noise is first passed through a bandpass filter (BPF) to produce an approximately square spectral pedestal of noise of about the same bandwidth as the signals being simulated. This signal is then passed through a narrow band-reject filter to produce a deep notch at the center of the noise pedestal.
This noise signal is used to excite the test amplifier. Amplification will produce IMD products, which tend to fill in the notch. The amount that the notch fills in gives an indication of the non-linearities generated in the device or system under test and is used to determine the noise power ratio of an active component in the system or the system as a whole. The depth of the notch at the output of the amplifier can be observed with a spectrum analyzer, and is the measure of the NPR.
RF calibration and fast RF verification
RF calibration, or tuning and alignment for RF frontend modules, is common to both mobile phone and user terminal manufacturing. Terminals must compensate for nonlinearities in power amplifiers during the manufacturing process. Basically, this requires frequency and power measurements, sometimes using CW signals, and other times using calibrated modulated wideband signals.
Receiver Reference, Calibration and Performance Test
Noise is an excellent option to be used as a receiver reference or as built-in test equipment (BITE) to evaluate system performance. Switching in a noise source enables the receiver to collect spectral data of its front end and model the system.
Using noise for receiver reference, calibration or test involves having noise available to be switched in as needed. For receiver reference the system switches to the noise source and based on the known excess noise ratio (ENR) the received signal level can be determined. For receiver performance test the system switches in the noise source to verify the receiver part of the system is operational.
Swept power and frequency measurement techniques, coupled with advanced measurement algorithms remove the non-linearities, sufficient to allow terminals to pass RF measurement specifications. High-volume, low-cost terminals require these compensations as lower cost components increase non-linearities compared to legacy, more expensive terminals, with higher performance power amplifiers
Today, most mobile phone type-approval systems utilize conducted test strategies. Depending on the component or subsystem, these conducted test connections exist at either the IF or RF link. Conducted testing allows for simple and effective troubleshooting and isolating any performance problems. It also more easily allows design engineers to characterize both the early stage pre-compliance performance, as well as final performance verification.
Over-the-air (OTA) testing
Wireless enabled devices must pass a variety of industry certifications prior to commercialization. These certifications include regulatory and compliance testing such as EMC, conformance testing such as protocol, RF, RRM or LBS, as well as performance testing that is measured over-the-air (OTA).
Generally, conformance testing is performed at the conducted port(s) of the device under test (DUT) and is based on pass/fail tests. Traditionally the mobile phone industry uses conducted measurements, but for satellite user terminals with beamforming and tracking antennas, over the air RF measurements supplant conducted methods.
The wireless industry is still debating how best to accomplish this for satellite user terminals and next-generation 5G mobile phones. Traditional antenna test facilities can be used, but they are very expensive, usually exceeding one million dollars for each device commercialization. Many near fields and quasi near field chamber concepts are under investigation.
Complex beamforming, phased array antennas or multi-element passive antennas, operating at high frequencies and wide signal bandwidths, introduce new complexities which the industry is currently trying to understand better. Advanced techniques such as near field to far field transformations are increasingly important for reducing cost.
OTA test systems analyze and optimize the radiated device performance and provide a controlled physical environment to validate terminal radiated performance with industry, operator and internal company requirements. OTA systems verify the antenna patterns and the wireless system performance of the transmitter and receiver chain, such as TRP and TIS/TRS respectively. These measurements follow test plans and detailed test and setup procedures published by industry organizations such as CTIA and 3GPP
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