The RFE consists of one or more transmit (Tx) and receive (Rx) channels operating over a wide tuning range from 0 to 18 GHz (with upgrades to 40 GHz) and a high instantaneous bandwidth of up to 3 GHz at the highest of market – bandwidth SDRs. The RFE is responsible for the initial gain, filtering, mixing and antenna coupling requirements to bring the signal to a manageable level.
High-end MIMO SDRs provide multiple Rx/Tx channels in the RFE. They interface with the digital back-end unit via independent analog-to-digital converters (ADCs) and digital-to-analog converters (DACs).
The digital back end is based on field-programmable gate-array (FPGA) technology with built-in digital signal processor (DSP) functions including modulation, demodulation, up/down conversion and data packetization. In addition, FPGAs can perform application-specific functions such as communication protocols, data storage solutions, and artificial intelligence/machine learning (AI/ML) algorithms.
The high level of FPGA reconfiguration allows the SDR to be easily updated to the latest radio protocols without hardware modifications. In addition, FPGAs provide the perfect framework for the parallel processing required in MIMO applications.
SDRs also use dedicated timing boards to generate, control, and distribute the clock to all modules, allowing the channels to operate synchronously. Oven-controlled crystal oscillators (OCXOs) are the core of high-performance timing boards, providing a very accurate and stable 10-MHz clock signal that optimizes phase noise.
Compared to traditional radio equipment, the advantages of SDR become clear. Analog radios are significantly bulky, requiring special hardware for each RF function that cannot be changed without physical circuit modification or complete equipment replacement.
SDRs, on the other hand, provide a flexible and modular solution that typically does not require hardware modification. Also, the modular, compact and unitary nature of SDRs allows them to be deployed for several size, weight and power (SWaP) requirements, which is critical in the satellite industry.
Additionally, while analog radios have limited accuracy in satellite applications, SDRs easily support several TT&C functions such as real-time kinematic (RTK) positioning. Finally, SDRs can be fully reconfigured on the fly to adapt to application requirements, making them ideal for easy resource allocation in service-based GS networks.
Although SDRs provide several advantages over analog systems, most of which are critical for SATCOM, there is a huge variety of devices on the market in terms of processing capabilities, power consumption, number of channels, tuning range, and other RF specifications. This means that not all SDR devices are suitable for beam steering applications in GS, so the designer must be careful when choosing a device.
Considerations when using phased array SDRs
The first thing a designer should consider when choosing an SDR is the antenna system itself. Satellite requirements determine the basic parameters of the antenna, such as geometry, size, number of elements and beamforming process. See the article : Elon Musk Offers $100 Million Carbon Capture Prize – Bloomberg. These requirements will determine the basic parameters of the SDR, such as MIMO capability and FPGA processing power.
Additionally, digital transmission requirements must take into account the amount of data that must be transferred to the host system to ensure that broadband raw data from multiple satellites is offloaded. A high-performance timing architecture is critical to phased array antennas, making essential timing boards with very low phase noise and high stability. High-end radio frequencies and robust digital backends are fundamental to overcoming some of the most challenging issues in GS, including bit error rate (BER), Doppler shifts, and co-channel interference (CCI).
Beam steering and other phased array antenna techniques require a tremendous amount of processing due to the many different beams and large tuning ranges. Therefore, the digital back-end of SDRs implemented in phased-array antennas must implement FPGAs with high-performance DSP capabilities to embed sets of parallel beam-steering functions (e.g., phase shifters and optimization loops). Thus, the FPGA specification is one of the most important criteria for SDR selection for phased array systems.
Another important specification is the phase coherence in the RFE channels. MIMO SDRs are essential in the design of electronic control systems, but they must also provide sufficient phase coherence to properly control the antenna array. The JESD204B standard defines a serial interface between the FPGA and DAC/ADC that provides highly deterministic latency between channels, greatly improving the phase coherence of MIMO SDRs.
Additionally, to comply with most SATCOM standards, the chosen SDR must be able to operate at Ka and Ku bands, which are popular frequencies in satellite applications – this means RFE with wide tuning possibilities. The designer must also address basic RF requirements such as gain specification, interference suppression capabilities, and host interface. Additionally, modular SDRs are desirable in critical SWaP implementations where the designer can customize the equipment to reduce overall complexity and satisfy system constraints.
ESA Modern Research
Several research studies are currently underway on electronically steered antennas (ESAs) for SATCOM applications. To see also : Global Small Satellite Market (2021 to 2026). As mentioned above, nanosatellite constellations are extremely useful for Internet industry and research applications, and cubesats are one of the most popular examples.
In this context, Sheldon et al1 proposed a phased array UHF ground station designed specifically for cubesat applications. They applied commercial off-the-shelf (COTS) components, including an SDR, to drive a four-element antenna array—using an array that can be extrapolated to an eight-element version that is cost-competitive with current cubesat ground stations.
However, they implemented one SDR for each element, which can significantly increase the cost of the overall design. MIMO SDRs can handle multiple channels using only one device for both Rx and Tx functions, which can reduce overall cost and complexity. Nevertheless, the study demonstrates the feasibility of using COTS equipment to create a low-cost GS for cubesats with high levels of flexibility and reconfiguration.
Adaptation is another important feature of ESA ground stations to prevent jamming. Reyes et al2 proposed an anti-jamming antenna system to address the increase in interference caused by the decreasing separation angle between GEO satellites as the demand for geostationary services increases.
The authors designed a square array of antennas applying the linear constrained minimum variance (LCMV) algorithm to adaptively control the signal of each antenna to minimize the gain in the jamming direction. The system (fig. 3) was simulated in the study, achieving deep nulls of less than −134 dBi at blanking angles while maintaining 38.3 dBi constant gain at the desired angle of arrival, with a signal-to-jitter-to-noise ratio (SJNR) of 9, 3 dB.
Comments are closed.