by Brendon McHugh and Simon Ndiritu, Per Vices
Introduction to SDR Systems
A SDR system consists of a radio front-end that handles analog signals and a digital back-end that operates on digital signals. The radio front-end performs signal transmit (Tx) and receive (Rx) functions and is engineered to operate over a wide tuning range, with the high performing SDRs operating in 0-18 GHz (upgradeable to higher frequency ranges as well).
The digital back-end of a SDR system features a field programmable gate array (FPGA) that is capable of performing various digital signal processing (DSP) operations. This device utilizes reconfigurable components for various DSP operations such as upconversion, downconversion, modulation and demodulation. The reconfigurability of the digital back-end makes SDR systems highly flexible and easy to upgrade. This flexibility also allows new DSP algorithms and radio protocols to be implemented with ease and at a low cost, by using software tools to perform various processing and/or loading new IP cores onto the FPGA.
SDR platforms offer multiple independent transmit and receive channels, making them ideal for implementing multi-function systems. Each channel features a dedicated digital-to-analog converter (DAC) or analog-to-digital converter (ADC) for signal conversion from one domain to the other. The architecture of a typical SDR system consists of various modular boards, as shown in Figure 1.
Radar and its Applications in Today’s Industries
RADAR (Radio Detection And Ranging) is a technology that utilizes radio waves to measure the angle, distance or velocity of an object that the signal is aimed at. First used in World War II, this technology is applied in a broad array of mission critical applications including aircraft monitoring systems and weather monitoring systems.
A typical radar system consists of a transmitter that generates and transmits an electromagnetic signal in the direction of a target. When the transmitted signal hits the target, a fraction of the signal is reflected back to the radar dish or antenna hooked up to the receive channel. From the receiver, the reflected signal is processed by the host computer. The host computer calculates the required measurements and displays them on a screen; for example, it might calculate an approximation of the target’s angle, distance, velocity or size.
Radar systems use a multi-functional antenna that is capable of performing both transmit and receive functions. A duplexer switch connects the transmitter and the receiver to the shared dish or antenna to switch between receive or transmit mode. Radar transceivers come in different forms and are mounted in different positions depending on the application. These transmitters/receivers are commonly found on the noses of planes, on the decks of ships, on military vehicles, within air traffic control towers and many other places. In aircraft and ships, multiple devices are used to support different services and applications.
Radar systems come in different forms and configurations to meet the diverse needs of today’s applications. Some of the most common types of radar include Synthetic Aperture Radar (SAR), Doppler radar and phased array radar.
Synthetic Aperture Radar (SAR) is a specialized radar system that is usually mounted in aircraft/spacecraft and used for creating high resolution two-dimensional images of three-dimensional targets. Creating an image from this radar entails transmitting multiple successive pulses of electromagnetic signals towards the target scene and recording the reflected signals. The recorded echoes are processed to reconstruct an image of the scene. The resolution of the reconstructed image is mainly determined by the size of aperture: the larger the aperture, the higher the resolution. SAR is commonly used for mapping and remote sensing.
A Doppler radar system utilizes the Doppler effect to calculate an approximation of the velocity of a target. This radar measures the frequency difference between the transmitted and the reflected signals and uses the variation to calculate an accurate estimate of the speed of the target. Doppler radars are usually lighter and are commonly used in sounding satellites, aviation, radar guns, meteorology and radiology.
A phased array radar utilizes the principle of interference to combine the radiation patterns of individual antenna elements in such a way to obtain the desired effective radiation pattern. This radar offers a fast response time and can be used for different functions. The impressive performance of this radar makes it a suitable choice for defense applications where a single radar system is used for multiple functions.
Radar is a core component in many mission critical systems such as military and defense, aviation, marine navigation, and meteorological systems. As shown in Figure 2, when combined with a software defined radio (SDR), radar systems have a broad array of capabilities and find applications in a variety of industries.
Today’s marine navigation and collision avoidance systems are heavily dependent on radar technology. Marine radar systems utilize C-, S- and X-bands and enable ships to detect other ships and land obstacles. These radar systems also provide ships with bearing and accurate distances.
In modern aircraft, radar is used in a broad array of critical systems including navigation and landing systems. A significant fraction of aviation bands are dedicated to radar-based systems, including non-directional beacon (NDB), distance measurement equipment, airborne collision avoidance, surveillance radar, radio altimeter, airborne weather radar and airborne Doppler radar systems.
Meteorology radar systems have dedicated frequency bands in S-, C- and X-bands. The S-band (2700–2900 MHz) is mainly used in systems for monitoring hurricanes, tornadoes and heavy rains in tropical and temperate climate areas. The C-band (5600-5650 MHz) is dedicated for use in regions where signal attenuation by large hail or heavy rain is a minor issue. Lastly, the X-band (9300-9500 MHz) is mostly used for short range hydrological and meteorological application uses. A common application of this meteorology band is in urban hydrology.
Integrating SDRs into Radar Systems
The hardware architecture of SDR systems allows them to be easily integrated into a broad array of complex modern radar systems. To begin with, the SDR architecture allows radar engineers to connect a duplexer and power amplifier using 50Ω SMA connectors. A simplified block diagram of a radar system based on a SDR system is shown in Figure 3.
A SDR platform offers multiple input multiple output (MIMO) channels, making it ideal for today’s complex radar systems. High performance SDR platforms offer up to 16 channels, thus allowing implementation of radars utilizing a multi-channel phased array system through a single device. A MIMO SDR is also ideal for implementing a complex radar system consisting of multiple radar antennas with different range capabilities.
The wide frequency range offered by high performance SDR systems makes them suitable for the most demanding radar applications. Apart from allowing enough channel spacing, this characteristic also enables a single SDR to cover all the frequency bands used for radar applications. In addition, these platforms offer impressive frequency and phase stability, making them suitable for use in today’s radar systems.
High performance SDR platforms are designed to offer impressive noise, dynamic range and Spurious-Free Dynamic Range (SFDR) characteristics. The SFDR defines the range between the amplitude of the fundamental carrier signal and the strongest spurious signal. This quantity is usually given in dBc. SDR platforms offer a high SFDR, making them ideal for sophisticated radar systems. In addition, SDR systems are engineered to deliver high sensitivity and a low noise figure. This, along with the high dynamic range characteristics, means that a single device is capable of capturing very weak and strong reflections in radar systems.
On top of the radio front-end hardware, a SDR’s digital back-end requires a high data backhaul for today’s demanding radar applications. Complex radar systems such as those used in modern air traffic control systems handle massive amounts of data which need to be passed to a host system. High performance SDR systems feature high data throughput 40Gbps qSFP+ transceiver ports which can be upgraded to offer up to 100Gbps.
Beamforming/Beam Steering for Phased Array Radar
Phased array beamforming is one of the techniques used to improve the performance of antennas in highly demanding applications. In a phased array system, the individual radiation patterns of an assembly of antenna elements are constructively combined to produce the desired effective radiation pattern (main lobe). In the case of undesired directions, destructive interference is used to produce side lobes and nulls.
Beamforming maximizes the energy that is radiated in the main lobe and minimizes the energy radiated in the side lobes. Adjusting the phase of the signal that is fed to the antenna elements allows the direction of radiation to be manipulated. The algorithm that performs phase adjustment is implemented in the FPGA. This algorithm can be modified to include more elements and functions by simply reconfiguring the FPGA.
Let us consider a linear array consisting of 8 antenna elements, as shown in Figure 4. For simplicity, we will assume that the delay in the feedline is negligible. As illustrated in Figure 4, we can draw a right-angled triangle that allows us to relate the phase shift and the deflection angle with applied phase shift. A dashed line is drawn perpendicular to each phase shifted beam and respective antenna element. The short side of the right angle (i.e. from the dashed line to the face of the linear array of antennas) is denoted x. The distance between the array of antenna elements is denoted d. Letting the desired beam steering angle be q, we can use basic trigonometry to determine the required phase shift y as follows:
In terms of wavelength (l), x is related to phase shift as shown in Equation 2:
Combining Equations 1 and 2, we obtain the expression for phase shift as shown in Equation 3:
Beamforming for phased arrays reduces the time required to reposition an array of antennas and allows multiple data streams to be transmitted. In addition, electric steering of arrays is more efficient and requires less maintenance compared to mechanical steering. On the flip side, beam steering for phased arrays results in more power dissipation.
Benefits of Using a SDR in Radar Systems
There are many benefits of integrating a high performance SDR platform into a radar system. For one, the capability of a single SDR device to handle many functions reduces the overall complexity of a radar system. Moreover, SDR platforms are highly flexible and versatile because they utilize reconfigurable components. Furthermore, these devices are highly interoperable and can be integrated into both legacy and newer radar systems. In addition, the flexibility of these platforms makes them suitable for use in new deployments as well as in service life extension programs (SLEP).
The digital back-end of a SDR system employs reconfigurable software-based components for various signal processing operations. This reconfigurability allows advanced algorithms to be tested and implemented easily and at a low cost since no hardware modifications are required. In addition, since SDR platforms utilize software-based components instead of hardware components, they are usually cheaper and lighter. The power consumption of these platforms is also lower compared to that of conventional radio systems used in radar.
The reprogrammability of SDR platforms means that a single device can be reconfigured to perform different functions. Since there is no need to buy multiple components, SDR platforms are more cost-effective compared to conventional radio systems. Apart from commercial off the shelf (COTS) platforms, leading manufacturers of SDR platforms can also provide custom solutions that are tailored to meet the specific performance needs of your application.
Per Vices offers high performance SDR platforms that feature multiple independent channels, custom FPGA builds, high tuning range, high bandwidth and high throughput, making them suitable for use in sophisticated multi-functional radar systems.