Testing for NewSpace

- Varanasi Pavan Kumar, Product Specialist – Signal Generators & Spectrum Analysers, Rohde & Schwarz India

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NewSpace constellations introduce new challenges for test engineers when compared to legacy VSAT terminals. Satellite engineers need to thoroughly test RF subsystems and components to ensure uninterrupted operation with a high quality of service. On the other hand, engineers need to reduce test time and cost while the total number of beams and links is increasing. These challenges require test solutions that provide high measurement performance and repeatability

Varanasi Pavan Kumar, Product Specialist – Signal Generators & Spectrum Analysers, Rohde & Schwarz India

Today’s satellite communication systems combine features from legacy cellular networks and emerging wireless technologies. New constellations are under development that attempt to provide ubiquitous mobility and internet networks via satellites, ground stations and user terminals. Each link in the supply chain presents unique challenges for R&D, production and deployment for both the components and system development.
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. NewSpace networks, on the other hand, deploy user terminals as agnostic interfaces between the satellite and the end-user communication device . These networks typically deploy fixed antenna user terminal designs which 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.

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. These challenges demand test and measurement solutions that provide high measurement performance and repeatability.

Testing requirements in NewSpace

Testing for NewSpaceRF verification versus modem verification: LEO user terminals require significantly greater complexity compared with point-to-point GEO VSAT terminals and require greater sophistication in design, integration and test. 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 the radio resource management relating to handovers and scheduling.

Legacy terrestrial mobile measurement techniques can offer assistance in translating these test needs into LEO terminal test requirements.

Network emulation test systems: Modems running commercial firmware implement dynamic algorithms that respond in real time when terminals measure network power, provide measurement reporting and respond to system scheduling. This dynamic behavior prevents controlled RF parametric testing. Consequently, test engineers are unable to parametrically characterize modems and RF systems under these conditions. In an ideal world, a test engineer could set the modem state machine to any condition and execute measurements in that fixed condition. Legacy terrestrial mobility engineering teams benefit from “callboxes”, which are network emulator test systems that control a mobile phone state machine, and subsequently make RF parametric measurements on a mobile phone in a controlled state, such as fixed power or frequency. Callboxes require a significant development NRE, which is typically offset by the large scale commercial ecosystem of mobile networks, mobile phone manufacturers and mobile chipset vendors (LTE, WCDMA, etc.). This is not an option for NewSpace constellations that implement custom air interfaces which do not scale economically like large mobile networks. So, how can NewSpace engineers realize the same features of a callbox, given the cost constraints?
NewSpace engineers can utilize one the following network emulation implementations:

❙ Golden radio
❙ Software defined radio (SDR)

It should be noted that golden radio and SDR test systems both require complementary test equipment such as vector signal analyzers, spectrum analyzers and signal generators, to fully characterize the RF performance of the terminal under test. They all work together to simulate a real network in order to stress the terminal in real world conditions.

Testing for NewSpaceOver-the-air (OTA) testing requirements: 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 field and quasi near field chamber concepts are under investigation.

RF calibration and fast RF verification added to production: The added complexity of modem testing also impacts production testing. While initial manufacturing volumes might not exceed hundreds or even thousands of units, networks anticipate full production volumes with yields in the millions of units. Manufacturers require advanced strategies for reducing cost through efficient test methods. 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. 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. In addition to RF calibration, all devices require basic RF and modem verification in manufacturing. Similar to calibration, fast test methods utilizing pre-programmed sweep methods significantly reduce overall test time by reducing legacy iterative and repetitive setup production processes, with the terminal under test and the test equipment. Just as early design considerations are critical for network emulation test methods, production calibration and verification test also require early consideration. The most important step concerns the chipsets and modem in the terminal radio. By implementing fast test modes in the system engineering design, chipset and modem designers can engineer these features early with technical ease and minimal cost.

Ground station considerations: 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.

Solutions at a glance

The testing of NewSpace user terminals and ground stations require rigorous performance design verification and manufacturing testing compared to legacy terminals. Introducing Radio Resource Management test methodologies, common in the mobile phone industry, creates a more complex verification and conformance testing requirement.

As components continue to integrate into smaller and cheaper packaging, the frequencies and test ports become a limiting factor. Traditionally manufacturers test terminals at the IF, RF and antenna stages. In some cases, especially in satellite ecosystems, no conducted contact points exist for testing at the IF or even RF stages. This section discusses both conducted and radiated testing solutions.

Conducted testing: 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.

OTA testing solutions: 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.

Testing for NewSpaceNetwork emulation for physical layer: The R&S SMW200A signal generator provides network emulation of the physical layer signal conditions . This enables layer 1 receiver testing via BER/PER/BLER measurements. The process of establishing receiver sensitivity testing in the absence of a closed link full protocol system requires the modem to be designed with test modes to operate in such a manner. This process is already common in commercial mobile LTE handsets for both integration and production. This allows for custom payloads to facilitate rapid signal/waveform development for many conditions.

Terminal transmitter analysis: Terminal transmitter analysis provides signal and spectrum analysis capability for terminal transmitter testing. Many NewSpace air interface signals work with existing legacy measurement personalities, such as LTE and DVB-S2X. These are easily modified to include updated air interface changes. The R&S FSW locks to specific air interface signals to allow for more exact filter and frequency measurements, which allows for tighter EVM, ACLR and other modulation quality measurements.

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