Software Defined Radio Use Case for Radars

Software Defined Radios (SDR) have become the best option for many radio applications due to their precision, flexibility, reliability and simplified design process. In this article, we will discuss the application of SDR in radars. Although composed of mainly analog circuits in the past, modern radars rely on powerful digital signal processing to operate. Aviation and marine navigation, weather/meteorology tracking, ground penetration assessments, military missile guidance and Earth's surface mapping are some examples of applications. Moreover, radar technologies consist of a variety of different configurations, such as Continuous Wave (CW) Radar, Doppler Radar, Pulse Radar, Frequency Modulated Continuous Wave (FMCW) Radar and Phase Array Radar. Despite the diversity of technologies, all radar configurations require large amounts of signal processing to work properly, and as such, SDR works as the backbone of these devices.

Radar Principles

1. Basic Operation

A stone thrown into a pond generates ripples that propagate through the water. If these ripples find an obstacle, reflected waves will travel back to the origin point, carrying information about the obstacle. Radar (Radio Detection and Ranging) uses the same basic principle, but with electromagnetic (EM) waves: the antenna sends RF radiation to the surroundings and measures the reflected EM fields. With this information, the radar can evaluate the size, speed and distance of objects. For instance, because light speed is constant, the distance between the receiver and the target can be estimated with the delay between pulses. Therefore, radar is used in any application that requires remote detection and monitoring of objects.

Modern radars are composed of three basic parts: the antenna, the hardware front-end and the signal processing module. The antenna is responsible for sending the interrogation EM signal and measuring the reflected waves. The size, shape and location of the antenna is highly dependent on the application: antennas can be found mounted on ships, inside the nose of airplanes, integrated into radar guns and on top of air traffic control (ATC) towers. The hardware front-end generally performs the signal transmission, (adaptive) impedance matching, amplification, anti-aliasing filtering and signal digitalization. The signal processing module extracts the required information from the received signal and displays the measured results. This step varies greatly with the radar technology being used: for instance, in Doppler Radars the frequency deviation of the received signal can be correlated with the speed of the detected object (Figure 1). Therefore, having a flexible processing unit is desirable for multi-purpose radar.

Figure 1: A diagram of the Doppler Radar is shown. The frequency of the signal reflected from stationary targets remains unchanged, whereas moving targets shift the frequency of the reflected signal. If the target is moving towards the radar the frequency increases, while the opposite occurs if the target is moving away. The frequency shift can be used to measure the target speed and to discriminate moving targets from the static cluster.

2. Frequency Bands

Radars operate in different frequency bands. Each band has its own characteristics and limitations: for instance, high frequencies allow the use of smaller antennas, so they are preferable in airborne applications. On the other hand, low frequency applications are less prone to attenuation, which results in larger covered areas but requires much larger antennas. RF bands are controlled by regulatory agencies, so radars are allowed to operate within a pre-defined slice of the frequency spectrum. The main radar frequency bands are shown in the table below.

Marine applications are typically able to implement large antennas, so they use X, C and S bands, which gives a good compromise between antenna size and propagation loss. Meteorological radar bands are limited by the trade-off between attenuation and antenna size: S band (2.7 - 2.9 GHz) is used in regions with high propagation loss, such as tropical areas with heavy rains, C band (5.6 - 5.65 GHz) is applied to monitor regions with smaller attenuation rate, and the X band (9.3 - 9.5 GHz) is limited to short-range hydrological and meteorological monitoring, such as urban and mountain valley hydrology.

Aviation bands are implemented in different spectrums for different applications, such as primary/secondary radar for navigation or instrument landing system (ILS). Airborne radars include Doppler radars, weather radars and ground mapping radars. The frequency bands used are Ka (31.8 - 33.4 GHz), Ku (13.25 - 13.4 GHz for Doppler and 15.4 - 15.7 GHz for weather radars), X (8.75 - 8.85 GHz for Doppler and 9 - 9.5 GHz for weather radars) and C (5 .50 - 5.47 GHz). Land radars are also implemented in avionics to monitor traffic and weather, using the S (2.7 - 3.3 GHz for meteorological radars) and L bands (1.215 – 1.4 GHz for Primary Surveillance and 1.02 - 1.04 GHz for Secondary Surveillance). K band is typically not used due to water absorption.

The UHF band is used for very far distance surveillance, such as Over the Horizon (OTH) radars, which are used to monitor areas with hundreds to thousands of kilometers. In these cases, larger wavelengths are required to prevent attenuation, which means extremely large antennas and powerful rotating systems.

3. Radar Types and Limitations

Continuous Wave Radar
This type works by using a continuous EM wave in the transmission. The main advantages are the reduced manufacturing complexity, cost and maximization of the total power sent to the target. CW radars are used by the military in semi-active homing for missile guidance. Frequency modulated continuous waves (FMCW) is a type of CW radar that applies FM modulation to measure distances precisely. It is used in altimeters, early warning radars and proximity sensors. Unmodulated CW radars are unable to measure distances. Also, because transmitter and receiver are always operating, proper isolation between the antennas is required and the output power of the transmitter must be low enough to prevent jamming. Consequently, the detection distance is limited.

Pulsed Radar
Pulsed radars transmit the interrogation signal in short pulses. After reaching a target, the EM pulse echoes back to the receiver, and the echo delay can be used to measure distance. There are several advantages of using pulsed signals. First, one antenna can be used for transmission and reception, using a duplex switch to alternate function. Pulsed radars also transmit power more efficiently, which means increased signal-to-noise ratio and range capability with a lower power consumption. Moreover, they are less prone to external detection and signal jamming, especially when using very short pulses. However, compared to FMCW radars, pulsed signal provides lower range resolution and suffers from a blind spot: a distance where the received signal echoes back during the OFF period of the receiver. Therefore, there is a minimum distance for detection.

Moving Target Indication and Doppler Detection
Moving target indication (MTI) is a technique that uses destructive and constructive combinations of the received pulses to discriminate a moving target from the stationary background. To avoid range ambiguities, low repetition frequencies are used. MTI is used to detect small targets at long distances. Doppler radars are also used to detect moving targets, measuring the speed using the Doppler shift. Different from the MTI, Doppler radars use high pulse repetition frequency to increase the resolution. It also provides better information about speed data, but with a smaller distance range.

4. Important Radar Parameters

1. Pulse Power: the amount of power contained in one pulse. Increasing the amount of output power increases the signal-to-noise ratio and range, but also increases the device weight due to power amplifier.
2. Carrier: the main frequency of the emitted signal. It influences several parameters, such as antenna size, frequency band, range and signal attenuation.
3. Pulse Width: the time duration of a pulse (Figure 2). In other words, the period in which the antenna is transmitting the signal.
4. Pulse Repetition Interval (PRI): also called pulse repetition time (PRT), it is the total interval between two pulses (Figure 2). Therefore, it can be calculated as the sum between the pulse width and the "silent period" used for signal reception.
5. Duty Cycle: defined as the ratio between the pulse width (PW) and the PRI, typically expressed in percentage (Figure 2). For example, using a PW of 100 microseconds with a PRI of 1 millisecond, the duty-cycle is 10 %. It can also be defined as the ratio between the average transmitted power and the pulse power: for a constant average power, decreasing the duty-cycle means increasing the pulse power.
6. Pulse Repetition Frequency (PRF): the inverse of PRI, and expressed in pulses per second.
7. Pulse Compression: technique to increase range resolution and signal-to-noise ratio. It applies frequency and/or phase modulation to the carrier within a single pulse, adding another layer of information that can be correlated with the received signals. It requires more complex signal processing, but increases information quality and radar capabilities.
8. Range Resolution: the minimum distance between two objects that can be distinguished by the radar. The degree of range resolution depends on the width of the transmitted pulse, the types and sizes of targets, and the efficiency of the receiver.
9. Signal-to-Noise Ratio: the ratio between the signal power and the background noise. It can be increased by increasing the output power, filtering, signal modulation, etc.
10. Jamming: defines both intentional and accidental interference by an external signal. This signal can be used to saturate the receiver or provide false information, jeopardizing the radar function. As explained before, CW can suffer from unintentional jamming if isolation between transmitter and receiver is insufficient. Moreover, malicious transmitters can detect radars and cause intentional jamming, which is extremely problematic in aviation and military applications.

Figure 2: The relation between Pulse Repetition Frequency (PRF), Pulse Repetition Interval (PRI), Pulse Width (PW) and Duty-Cycle (DC) is shown.

SDR Applied in Radar Systems

1. Software Defined Radio

SDRs are composed of two main parts: the radio front-end (RFE) and the digital backend. The RFE consists of all analog circuitry necessary to transmit and receive a quality signal. The RFE includes the antenna impedance interface, the receiver (Rx) and the transmitter (Tx). The RFE must receive signals in a wide bandwidth. As discussed before, higher frequency bands result in smaller antennas, so high bandwidth SDRs are desirable in compact and portable radars.

The digital backend typically contains an FPGA armed with on-board digital signal processing (DSP) capabilities. It can perform parallel signal processing, such as modulation, demodulation, up-conversion, down-conversion and filtering with very low latency. Also, because the circuit configuration is programmable, the board can be upgraded and customized for specific tasks, new radio protocols and algorithms.

Digital-to-analog and analog-to-digital converters (DACs and ADCs) are the interface between the frontend and backend. One SDR contains several independent Tx and Rx channels, that can be easily integrated into already existing radars. Because SDR embedded several functions for signal processing, communication, control and data storage into one device, the overall design complexity is significantly reduced.

2. Integrating SDR to Radar Equipment

The Tx and Rx channels of SDR systems can be directly connected to the power amplifier and antenna dish of any radar system, with minimum adaptations. SDR with multiple independent channels can be used in MIMO (Multiple Input Multiple Output) applications, such as multichannel phased array radars or multiple antennas for different ranges. MIMO SDRs are crucial. Moreover, to comply with different radar frequency bands and provide channel spacing, SDRs with wide bandwidth are required. The MIMO SDR Cyan (with High Bandwidth expansion), provided by Per Vices, has a tuning range from 0 to 18 GHz, that can be upgraded to 40 GHz, and up to 16 independent Tx and Rx channels. The instantaneous bandwidth of this device is 3 GHz, which makes it one of the highest bandwidth SDR on the market.

Besides speed, low noise and high dynamic range are extremely important to provide precision and robustness. The noise figure defines the sensitivity and the minimum detectable signal, which limits the ranging precision and the maximum detectable distance of a radar. The dynamic range, on the other hand, is the difference between the maximum signal and the noise floor. A large dynamic range is required to prevent radar saturation by a close and/or large target. Another important figure of merit is the spurious-free dynamic range (SFDR), which is the difference between the maximum and the minimum detectable signal. The SFDR is a good indicator of the receiver’s flexibility, linearity and noise.

Radar applications require more than signal integrity: high data throughput is crucial to process the massive amount of information obtained from the receivers, especially in MIMO radars. Data throughput as high as 40 Gbps (upgradeable to 100 Gbps) can be found in the Per Vices Cyan model, which implements four qSFP+ ports. The Cyan line (which includes versions with high-bandwidth and storage solution) also provides deterministic phase coherency and latency in all radio chains. Phase and latency coherence provides consistent operation and preserves the phase information.

3. Digital Signal Processing

FPGA processing provides very low latency and high data throughput. Complex computations can be performed on both the transmitted and received signals. For transmission, SDR provides a powerful and flexible waveform generation based on signal digitizers. By generating the signal digitally, the SDR can transmit arbitrary waveforms to suit different radar protocols without changing the hardware. Signal modulation and compression can be easily programmed for each channel independently. This also allows the SDR to control the pulse width and carrier frequency on the fly, making it suitable for adaptive radar applications.

Beam forming and beam steering are powerful tools, using multiple antennas to control the beam shape and direction by changing the phase between channels. Both techniques require extremely precise phase and trigger control of the outputs, so the SDR should have a deterministic phase coherence, frequency stability and powerful triggering control.

Sensitivity time control (STC) and sensitivity gain control (SGC) provide robustness to the radar. They allow the radar to prevent saturation caused by nearby clutter, such as buildings, trees, mountains and clouds or approaching objects. It consists on automatically reducing the signal gain, eliminating saturation. This requires an intelligent system that only SDR based radars are able to provide.

4. Advantages of Software Defined Radars

The most obvious advantage of SDR is the integration of all the required signal processing, channel management and control functions into only one piece of equipment, thereby drastically reducing the size and weight of the device. The Cyan SDR, for instance, has a length smaller than 50 cm, and weighs only 6.2 kg. If volume is critical, the Crimson SDR (Per Vices) provides a more compact solution. This is extremely important in portable radars.

Besides volume, SDR reduces the design costs and complexity. Therefore, there is no need to integrate, configure and test several different components before operation. As well, Per Vices devices provide interoperability with older/legacy radars, so the SDR can be used to upgrade existing systems without replacing the whole hardware.

One other key advantage of SDR is flexibility: it provides a configurable amount of output and input radio ports, allowing MIMO operation for any device. Furthermore, almost any signal parameter can be programmed, which increases tremendously the applicability range of the radar. Due to its programmable nature, the device can be upgraded constantly with state-of-the-art algorithms and protocols, so it will never become obsolete, since new technologies can always be developed and tested. Per Vices offers SDR solutions that can be customized to the user’s needs, providing customizations such as increased bandwidth and storage capabilities, and so no radar application remains unsupported.

Conclusion

Radars are radio-based systems able to detect a target's distance, angle and speed. They have a wide range of applications, including defense, aviation, navigation and weather monitoring. Different types of applications require different frequency bands and techniques. Due to the high frequencies, real time operation and the vast range of different techniques, radars require powerful signal processing and design flexibility. SDR provides a strong digital backend that integrates all processing functions into one system, reducing costs, complexity and volume. Modern SDR offers MIMO operation, frequency stability, phase coherence, high data throughput and easy signal control, suiting even the most demanding applications.

Click here to browse Per Vices SDR products listed on everything RF.

This is the second article in a series covering SDR applications. The previous article was on the Software Defined Radio Use Case for GPS/GNSS.