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1.INTRODUCTIONHistorically reductions in cost per Gbit/sec have been achieved through increasing payload power or increasing satellite design life. A recent trend has also been towards multibeam missions with increased mission capacity through the application of frequency re-use in multi-beam antennas to provide high speed connectivity and broad coverage. The major bottlenecks for the adoption of these technologies are the increase of the size, weight and power consumption. Currently operators already require a 10-fold increase of total payload capacity (in the order of the Tb/s) delivered with today’s satellite platforms (named Ultra High Throughput Satellites – UHTS). As the operators push for more and more capacity it is well understood today that conventional RF and microwave technology cannot satisfy the explosion of capacity requirements. In that sense, the addition of two new technologies to the present multi-beam architectures could be the key to address the challenges of providing up to 10-fold capacity at similar size-mass-power envelop:
DAS Photonics in collaboration with SSL (a Maxar Technologies company) has developed a V/Q Broadband Photonic Converter Assembly (PCA). The PCA represents a single string configuration or single frequency conversion based on a single Local Oscillator (LO). The PCA is aimed to be a representative building block of a multi-string payload configuration or assembly performing multiple frequency conversions from multiple LOs. A PFM unit has been used in a hosted payload demonstrator denominated Single String Photonics Payload (SSPP) operating at Ka-Band. The DAS Photonics product will be used within the low power section of this SSL telecom payload to provide a generic solution for any SATCOM operating frequency enabling broadband multi-frequency conversion from L-band to V-band. This product should open the door to simplify the design and implementation of future telecom payloads, especially for ultra-high throughput satellites. The design has been optimized to cover up to V band at the RF input and Q band at IF output, although the frequency conversion is broadband in the sense that almost any RF, IF and LO frequencies can be supported covering the typical SATCOM frequency bands. The SSPP is understood as a broadband mixer also applicable to up-conversion. No channel filtering is included as part of the SSPP (either electrical or photonic), so external filters will set the final functionality of the string (up or down converter, and in which frequency). The Q-Band (40GHz) and V-Band (50GHz) frequency spectrums are being considered for satellite communication systems to help with issues related to an already crowded Ka-Band frequency spectrum placing limitations on achievable system capacity. To utilize the V and Q band spectrum for commercial satellite payloads, high performance, space-qualified, millimeter-wave components will be required 2.EUTELSAT 7C SATELLITEThis DAS optical hardware is part of a SSL Ka-band hosted payload in the SSL manufactured satellite for EUTELSAT, as EUTELSAT 7C. Its broadband capability was tested on ground up to the Q and V bands per de design requirements for an universal, broadband photonic frequency converter to be able to operate at any SATCOM frequency band from Ku to V band for future payloads with multiple LOs and frequency conversions. 3.PHOTONIC PAYLOAD CONCEPT FOR HTSSSL and DAS Photonics are pursuing the introduction of payload equipment based on photonic technology within the commercial satellite market and are developing an architecture concept named V/Q-Band Photonic Converter Assembly (V/Q-PCA), which is intended to fulfil the UHTS needs with the following features:
Figure 1– Concept of photonic frequency conversion with multiple LOs. Simplified scheme with 3 LOs modulated in three optical carriers, multiplexed (wavelength multiplexing), simultaneous mixing with the RF signal in an electro/optical mixer, demultiplexed and photodetected.  The product is a Broadband Photonic Frequency Converter from V/Q/Ka to Q/Ka band with photonic LO Module. The main modules of the product are:
The architecture described and the associated modules are what is named Multi-String Photonic Payload (MSPP) and the fundamental idea is to build generic photonic modules with RF interfaces able to cover either the future UHTS architectures (in V/Q and Ka to/from Ka/Ku band) and the more conventional ones (in Ka and Ku band), not requiring modification from one satellite to other (eventually minor changes). To do so, a first iteration is to demonstrate the key functionalities that are the core of the concept, let’s say electro-optical-conversion up to Q/V bands, frequency conversion, optical amplification and fiber-optics remote delivery, and the implementation of a single chain of the architecture that integrates all these functionalities and components in a simplified enclosure. A view of the Single String Photonic Payload (SSPP) is shown in the Figure 2, including internal block diagram and proposed mechanical enclosure. The SSPP is a distributed system composed of three assemblies interconnected by optical fiber. Each assembly has its own DC power and TM/TC interfaces, as well as specific optical and electrical ports. These assemblies are:
PhLO and PhDOCON are attached together to form a single unit, while the Ph-Receiver will be an independent unit. Photos of the SSPP PFM are shown in the Figure 3. 4.PHOTONIC PAYLOAD QUALIFICATIONThe photonic payload has been submitted to a qualification campaign according with the requirements for a 15-years GEO missions, which included the following tests at module level:
The hardware was constructed with Level 1 Hi-Rel EEE and RF parts. The photonics components were up-screened to build a set of photonic flight parts from lots of Telcordia-based parts in some cases, and from custom made components in other cases. The flight lot was qualified according to a program designed by SSL and DAS teams and in accordance with ECSS and NASA standards as well as SSL internal procedures. The test flow for the flight parts is shown in the Figure 5. The photonic parts up-screening test sequence based upon MIL, Telcordia and ECSS standards was the following:
The qualification of the flight lot included the following tests on a set of up-screened samples: 5.RF TRANSFER PERFORMANCEThe RF transfer parameters have been tested in Ka band for the final configuration implemented in the SSPP PFM to be hosted on E7C. The capabilities of the unit to be operated in different frequencies (Ka and Q/V) were assessed as well. More specifically, the Ka band tested frequencies were:
And the Q/V band frequencies tested were: 5.1Conversion GainThe following figure shows the gain tested at different LO and RF input powers, converting from minimum to maximum LO power (+8.5 to +10.5 dBm) and the minimum, maximum and absolute maximum (-31, -8 and -5 dBm) of the RF input power. The minimum gain at ambient in all the configurations was higher than -12.5dB and -12.1dB at hot. 5.2RF input power rangeThe gain compression has been measured for different LO powers and temperatures. The following figure shows the gain compression at 14°C (close to ambient) and -10°C (cold case). The gain (S21) is constant up to approximately -10dBm of input power and then the gain starts to compress, being the 1dB input compression point higher than 0dBm (around 2 dBm). 5.3Noise FigureThe following figures include the noise figure tested at different temperatures for the Ka-band and at ambient temperature at V-band. For the Ka-band, at soft cold, cold and hot temperatures this measurement was also carried out for different LO input powers, covering from minimum to maximum LO powers (+8.5 to +10.5 dBm). The maximum noise figure at ambient, soft cold, cold, soft hot and hot are 40dB, 40 dB, 41.1dB, 39.5dB and 40.5 dB respectively. The Noise figure tested at V-band input, Q-band output at ambient temperature was lower than 40 dB with LO input power of +10dBm. 5.4Third Order LinearityThe table below summarizes the measured third order linearity performance for the two Ka-bands flight LO frequencies and the RF input frequencies at -11dBm per carrier (dual carrier test). A screenshot of one test is shown in Figure 9: Table 1– Third order linearity at different RF input frequencies and temperatures for Ka1 & Ka2 frequencies.
6.CONCLUSIONThe first Photonic Local Oscillator and photonic frequency converter able to operate up to V/Q band has been designed, manufactured, tested and integrated into a commercial telecom satellite. Thought it was tested on the ground up to V/Q band, due to its broadband characteristics it will be operated in orbit at Ka-band. The photonic suite is able to work with LO frequencies beyond 35GHz and RF input/output frequencies beyond 51.4/42.5 GHz. The interconnection among the photonic modules (LO, DOCON, Receiver) is achieved through optical fiber, which replaces of segments of waveguides and/or coaxial cables in the payload, thus reducing the overall mass and footprint of the RF harness. The photonic units have been designed and constructed following the standard requirements for a 15-years missions in GEO orbit and has been qualified at module level with a qualification vehicle (qualification-like model). A specific process for up-screening and qualification of the photonic parts (not available in hi-rel, rad-hard version) has been designed and implemented. Functional test results have demonstrated the broadband operation of the photonic solution and its suitability for commercial telecom satellites, especially for HTS in which a large optimization of mass, size and power consumption is foreseen with respect to a traditional RF implementation. |
