What is a Software Defined Radio?

A Software Defined Radio (SDR) is a radio communication system that employs reconfigurable software-based components for processing and conversion of digital signals. Unlike traditional radio communication systems, these radio devices are highly flexible and versatile. This is an emerging technology used to connect our ever increasing wireless world.

As shown in Figure 1 below, a typical SDR system consists of an analog front-end and a digital back-end. The analog front-end handles the transmit (Tx) and receive (Rx) functions of a radio communication system. The highest bandwidth SDR platforms are designed to operate over a broad range of frequencies; usually near DC-18 GHz.

Whereas the front-end of an SDR system handles signals in the analog domain, the back-end processes signals in the digital domain. An analog-to-digital converter (ADC) and a digital-to-analog converter (DAC) are used to convert signals from one domain to the other. The digital back-end features a field-programmable gate array (FPGA) and utilizes reconfigurable logic gates for different functions. This FPGA has various onboard digital processing capabilities including modulation, demodulation, upconversion and downconversion. The reconfigurability of this digital back-end allows new algorithms and protocols to be implemented easily and without modifying the existing hardware.

The flexibility of the SDRs make them a suitable choice for a broad array of markets. This includes various mission critical applications such as radar, test and measurement, magnetic resonance imaging (MRI), global navigation satellite system (GNSS), low latency links and spectrum and monitoring.

Figure 1: This is a simplified block diagram of a software defined radio system.

The architecture of a typical SDR platform consists of the following boards: power, digital, time, receive (Rx) and transmit (Tx) modules. The boards are connected using high speed cables to ensure fast transfer of data from one board to another. The function of the power board is to supply power to the daughter boards of an SDR system.

The clock distribution network of a typical SDR platform is centered on its time board. This module provides a clean and stable clock to the modules of an SDR system. For the Crimson TNG SDR platform manufactured by Per Vices, the time board uses an oven-controlled crystal oscillator (OCXO) as a source for its internal reference clock. This reference source provides an accurate and stable (5 ppb) 10 MHz signal. This high performance SDR platform is also engineered to support an external reference clock.

The receive (Rx) board of an SDR platform consists of multiple independent receive channels. Each receive channel is capable of performing the receive functions and handles signals in the analog domain. Analog signals from the Rx board are channeled to an independent chain consisting of amplifiers, downconverters, various filters and an ADC for conversion to digital domain. Just like the receive (Rx) board, the transmit (Tx) board features multiple independent transmit channels. Each Tx channel is capable of performing transmit functions and sends signals in analog format from the DAC, upconverter, filter and amplification stages. The transmit and receive chains of an SDR system are as shown in Figure 2.

Figure 2: This is a high level overview of the various components within Receive (Rx) and transmit (Tx) chains of an SDR system

Benefits of SDRs and their applications in today's markets

As shown in Figure 3, SDR systems are suitable for a wide range of markets including radar, test and measurement, medical, high frequency trading, spectrum monitoring and low latency markets. There are many benefits of using an integrated SDR platform instead of a traditional radio communication system. To start with, the modular architecture of the SDR platform allows the size and configuration of the boards to be modified to meet the form factor requirements of the chassis. This includes modifying the number of Rx and Tx channels and DSP chains.

The high flexibility of an SDR system comes from its onboard FPGA. This reconfigurable circuit makes an SDR highly interoperable and easy to integrate with a broad array of systems including legacy systems. Moreover, this flexibility and reconfigurability of the SDR platforms makes them suitable for applications involving multiple RF functions. An SDR platform can be used for a totally different function by simply reprogramming its FPGA. In addition, SDR platforms are designed to work with open source digital signal processing software development toolkits, such as GNU Radio, right out of the box. This capability allows users to do tests and simulations easily and quickly.

In terms of tuning and bandwidth, the multiple independent transmit and receive radio chains offered by an SDR system can be tuned to any frequency within the range supported by a platform. High performance SDR platforms provide capturing over a wide bandwidth and are engineered to operate over a broad range of frequencies.

Figure 3: SDRs play a central role in various markets

Test & Measurement: The flexibility and performance characteristics of SDRs make them an unrivaled choice for the test and measurement (T&M) applications. To begin with, the wide frequency range supported by SDRs allows devices operating at HF, VHF, UHF or any other frequency band within an SDR’s frequency range to be tested using the same equipment.With the flexibility offered by SDR technology, a system can be reconfigured to perform a totally different T&M function without modifying the hardware. This means that a single SDR system can be configured to function as a vector network analyzer, a spectrum analyzer, or any other test and measurement equipment easily and cheaply.

Radar: Radar technology is used in a wide range of critical applications including air traffic control, weather forecasting and navigation. The performance characteristics of SDRs make them ideal for use in today’s most demanding radar applications. For one, the flexibility of SDR means that a platform can be tuned to operate in C, L or S band depending on the needs of an application.

An FPGA used in an SDR system is capable of supporting advanced digital processing techniques used in radar such as beamforming. Furthermore, FPGA based DSP techniques help to increase sensitivity and the overall performance of a radar system. Additionally, FPGAs used in SDR systems are capable of generating a variety of pulses making them ideal for use in modern radar systems.

Since SDR platforms offer multiple channel count, it is possible to use one platform for multiple radar applications. This capability makes SDRs particularly suitable for radar applications where multiple signal chains are involved. For instance, one SDR can be employed in a naval radar system where two or more bands are used to monitor different types of threats at different ranges. In addition, the highest channel count SDR platforms offer many transmit and receive channels making them suitable for use in the most demanding radar applications such as air traffic control systems.

Spectrum Monitoring and Recording: Interference can significantly degrade the quality of services and it is necessary for regulators to monitor sources of interference and illegal use. A rapid growth in the number of frequency users has made spectrum monitoring more challenging than ever before. To monitor today’s congested spectrum, advanced spectrum monitoring equipment with recording capabilities is required.

Spectrum monitoring professionals use various techniques such as time difference of arrival (TDOA) and angle of arrival (AOA) to identify sources of interference in critical areas such as airports. This kind of spectrum analysis requires monitoring devices with high data throughput and multiple input multiple output (MIMO) capability. These performance requirements make a MIMO SDR a suitable choice for wide spectrum analysis. Highest throughput SDR platforms with MIMO capability are designed to meet the needs of the most challenging spectrum monitoring assignments as they are capable of capturing very wide bandwidth instantaneously as well as sweeping of very large bandwidths at incredible speed

Medical Devices: The number of medical devices based on RF technology is increasing every day. These devices include magnetic resonance imaging (MRI) systems, microwave imaging systems and IoT implantable medical devices. Designing and prototyping RF-based medical devices involves a lot of test and simulation work. The reconfigurability of SDR platforms allows engineers to perform tests on new protocols faster and at a lower cost. This flexibility makes SDR platforms an ideal choice for testing and simulating new protocols for medical devices.

The FPGAs used in SDR systems are capable of generating different types of waveforms. This makes these platforms a suitable choice for medical applications where waveforms are required, such as in MRI imaging.

The quality of images produced by most medical imaging systems is greatly dependent on noise. For instance, a high signal-to-noise ratio is required to ensure high quality image reconstruction in microwave imaging systems. With an SDR system, it is possible to achieve high SNRs required by medical systems.

Low Latency: Today’s high frequency trading (HFT) platforms utilize complex algorithms to transact orders. These systems are designed to execute many orders within a second and require very low latency links. High performance SDR systems are capable of delivering very low latency making them ideal for applications such as HFT. Their use in Transatlantic links enables faster exchange of market information between firms, financial institutions, stock exchanges and so on.

In a radio communication link, the main causes of latency include radio chains, FPGAs, converters, packetization, buffering and network transmission. Optimization of these components and processes helps to minimize the overall latency of a radio link.

Since SDR systems can tune to different frequencies, they are capable of selecting the least congested band. This capability helps to minimize the overall latency of a communication link. In addition, filtering can be implemented on these platforms to improve the signal to noise ratio (SNR). The lowest latency HF radio is used in communication links for use in applications with the most demanding latency performance.

GNSS Markets: Satellite constellations such as GALILEO, GLONASS and BeiDou operate at different frequencies. The capability of SDRs to tune to different bands without requiring any modification to the hardware makes them a suitable choice for use in Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) applications. With an SDR platform, the same equipment can operate in L1, L2 and L5 bands depending on the frequency of operation of a satellite constellation. In addition, since SDR platforms offer multiple transmit and receive channels, one can tune to all bands simultaneously using a single device.

SDR platforms are highly flexible and this allows engineers to test and debug satellite constellations easily and without requiring separate equipment for uplink and downlink tests.In addition, unlike conventional radio communication systems, SDRs are highly interoperable and can be programmed to work with a wide range of systems including legacy ground stations. The versatility and impressive performance characteristics of an SDR system makes it an unmatched radio for GPS/GNSS simulation.

Selecting a company to design a high performance SDR solution

One of the most important factors to consider when selecting an SDR solution is the performance needs of your application. Some applications have unique performance requirements and demand custom SDR solutions. Developing and optimizing a custom SDR system requires experienced professionals. It is therefore important to work with an SDR company that has expertise in developing high performance SDR solutions.

Producing a custom SDR solution is a collaborative process that involves multiple stages, as shown in Figure 4. These stages include initial discussion, sharing of performance requirements, trial with stock solutions, statement of work, delivery of final product, product certification and discussion on volume product sales. Working with an SDR company that has expertise in high performance SDR solutions ensures that this multi-stage process is smoother and faster.

Figure 4: The Per Vices collaborative process will result in a customized solution

SDR systems are capable of processing massive amounts of data and require a host capable of meeting their throughput needs. High performance SDR companies have the expertise to build such host systems.

Setting up a complex system consisting of an SDR solution is a task that requires a team of expert engineers; from FPGA designers to electrical engineers to software developers. Similarly, support from these expert engineers is required to ensure that the system is running optimally and with as few downtimes as possible. High performance SDR companies have experienced individuals who can visit your site to set up systems or provide technical support in case there is an issue.

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A Software Defined Radio (SDR) is a radio that can be programmed to operate a particular frequency. The frequency of the radio can be tuned via software (or firmware). Traditional radios that are hardware based are usually designed to operate at specific frequency ranges or for specific applications. The frequency of hardware based radios can not be changed easily as the radio transceiver and components in the Front End are optimized to operate at a particular frequency. This is not the case in Software Defined Radios, these radios can be programmed to operate at a specific frequency. 

Traditional radios are implemented using hardware like mixers, filters, amplifiers, modulators, demodulators etc. SDR's use an ADC and DAC to convert a signal from Analog to Digital and then from Digital to Analog without the need for the various hardware components.

So an SDR that has a frequency range from DC to 6 GHz can be used for Wi-Fi (2.4 GHz and 5 GHz), ISM Band Applications (400 MHz, 900 MHz, 2.5 GHz) and various other applications within the DC to 6 GHz frequency range.

The concept of software-defined radios has been around for some time, however, their application has been limited. With higher frequency and higher resolution ADCs and DACs available, these radios are starting to become more popular and we are seeing them being used more and more.

SDRs usually have a broad frequency range by default and can be programmed to operate at a specific frequency within that range.