Software-defined radios (SDRs) date back to the 1970s and were initially used primarily in military applications.


Then, as FPGA and DSP signal processing technologies advanced, IC-based wireless transceivers were developed, and small cell wireless networks evolved, the introduction of SDRs began to increase rapidly in the 2000s.

In this article, we will look at the basic concepts of SDR, its flexibility compared to traditional RF architectures, and key use cases.

Additionally, as wireless connectivity becomes an essential aspect of our lives, we introduce new applications that can make useful use of SDR.

■ Wireless use in every aspect of life

Wireless connectivity has been unknown since Marconi first used a spark transmitter to send a Morse code message across the Atlantic.As you can see, it has been developing continuously.

Today, 122 years later, wireless communication is used in every aspect of our lives.

Wireless communications play a fundamental role in connecting intelligent devices, operating cellular networks, and transmitting images from other planets to our televisions here on Earth.

RF (radio frequency) engineering has always been a specialized and traditionally analog discipline.

In the early days, wireless communication was mainly used for voice communication and sending and receiving messages using Morse code.

To transmit information, whether by voice or Morse code, it was necessary to modulate the transmitter frequency.

Broadcasters began using AM (amplitude modulation) for medium wave and long distance shortwave transmission for local broadcasts.

FM (Frequency Modulation) became popular for very high frequency (VHF) broadcasting for local and national broadcasts.

The wireless receiver and transmitter designs were entirely analog.

Digital data transmission converts binary to analog domain using a radio modulator and demodulator (modem) unit.

Initially, frequency shift keying (FSK) was used as a modulation technique for these applications.

As we will see later, SDR is bringing about fundamental changes in wireless system design. Before looking into how SDR works, let's look at some use cases for SDR.

Amateur radio enthusiasts were early adopters of SDRs.

SDR-based transceivers provide a convenient, lightweight and portable means of emergency communications in disaster situations.

In a recent example, these communications were used to coordinate rescue efforts when a volcano erupted in the Azores.

SDRs are also being used extensively in astronomical research. Using SDRs, the receiver's center frequency can be controlled, the bandwidth can be adjusted, and radio signals from distant stars can be displayed on a spectrum waterfall display.

As cellular networks advance, small cell open radio access networks (ORANs) are particularly well suited to using SDR. Telecom and network operators are increasingly deploying SDR in next-generation 5G and 6G cellular networks and base stations.

■ What is SDR?

Although the configuration of SDR receivers and transmitters may vary slightly, it is clear that many of the functions of traditional analog circuits are being replaced by software-based digital signal processing techniques.

The core functionality of an SDR receiver will still use analog circuitry.

The wireless front end detects extremely low voltage radio frequency signals received from an antenna.

However, in later stages, signal processing, mainly consisting of demodulation functions, is performed using software.

The software can run on a dedicated embedded programmable processor or on a laptop or desktop computer.

Figure 1 shows the main functional blocks of a traditional AM superheterodyne radio receiver.

A weak signal received from an antenna is passed through a bandpass filter to limit the bandwidth of the detection signal to the required bandwidth, and then amplified and passed to a mixer circuit.

The mixer combines the received signal with the output of a variable frequency oscillator to produce a fixed intermediate frequency (IF).

The receiver can be tuned by adjusting the local oscillator (LO) frequency. The IF amplifier greatly increases the signal level, and the filter removes unwanted components due to the mixer function. Next, demodulation and amplification of the audio signal are performed.

Figure 2 shows how SDR receivers differ.
The analog portion of an SDR receiver is limited to the RF front end.

The filtered signal is passed to an analog-to-digital converter (ADC) for subsequent processing in the digital domain.

The exact architecture of an SDR receiver can vary from design to design.

With the increasing adoption of SDR designs, the need for intermediate frequencies has been eliminated.

The Zero-IF (ZIF) approach, also known as direct conversion, takes the output from the RF front end directly to baseband digital processing functions and processes them in software.

It includes demodulation and filtering functions.

This article briefly explains the concept of SDR and its differences from existing RF techniques.

For readers who want to learn more about SDR, I recommend Software-Defined Radio for Engineers, an excellent resource available for download from the Analog Devices website.

Here we only looked at the receiver architecture, but the same can be said for the transmitter.

■ SDR Design used

SDR platforms, characterized by their flexibility, come in a variety of shapes and sizes.

A simple USB power dongle can cost anywhere from $25 to a full-featured SDR transceiver costing $6,000.

A variety of SDR evaluation kits and development modules are available, priced from $100 to $200.

Many popular SDR platforms combine commercially available RF transceiver ICs with FPGAs.

In addition to choosing an SDR hardware platform, you must also decide how to program it.

GNU Radio is a free and open source DSP programming toolchain entirely for designing SDR applications.

Originally developed purely for educational purposes, it is now widely used in radio research and development, amateur radio, and radio astronomy research.

GNU provides a set of functional blocks including filters, graphical displays, demodulators, signal generators, mathematical operators, channel models, and Fourier analysis functions. Individual functions can be brought into the workspace and connected using visual flow graph style programming (Figure 3).
Another popular toolchain for SDR engineers is Matlab and Simulink with their DSP and SDR extensions.

Also, as another SDR development ecosystem, Pothosware is based on GNUradio and includes the SoapySDR framework.

By leveraging free, open source SDR applications like SDR Console, you can reduce development time by eliminating the need to develop from scratch.

Lime Microsystems, a leading supplier of FPGA-based field programmable radio frequency (FPRF) transceivers, offers several SDR platforms based on its LMS7002 series of transceiver ICs. The LMS7002 is a highly integrated dual transceiver IC capable of full-duplex operation from 100 kHz to 3.8 GHz.

This IC is suitable for a variety of applications such as SDR prototyping, small cell base stations, satellite communication networks, and configurable wireless IoT networks. Figure 4 shows the architecture and main functions of the LMS7002.

Lime Microsystems took a groundbreaking approach to developing an SDR platform based on the LMS7002 series.

We decided to do crowdfunding through Crowd Supply.

The LimeSDR mini board adopts the LMS7002 and Intel Altera MAX10 FPGA to provide a comprehensive SDR solution in a USB-powered dongle-style PCB (Figure 5).

MyriadRF is an online community dedicated to helping adopt the LMS7002 series as an open source hardware and software SDR project, providing a complete ecosystem including development tools, resources, and project examples.

For developers familiar with the Grove Studio platform and the Raspberry Pi, the CS-LIME-10 Grove Starter Kit includes the LimeSDR mini board.
/> ADALM PLUTO from Analog Devices is a self-contained RF learning module that combines Analog Devices' AD9363 RF transceiver IC with Xilinx's Zynq 7000 FPGA.

It is powered via a USB connection to a host computer and operates at 325 MHz to 3,800 MHz in half-duplex and full-duplex modes.

Includes GNUradio, MatLab/Simulink, and Pothosware as SDR development support for PlutoSDR.

■ Future outlook

This article introduces the basics of Software Defined Radio (SDR). SDR is a new technology that is useful to embedded developers, analog designers, and RF engineers, and allows them to design wireless systems in a new way.

What's interesting about SDR is that it can be reconfigured on the fly, which allows for greater flexibility and agility in RF design. Rather than being limited to a single RF data communication technique, such as sub-GHz LPWAN LoRa, it is possible to design flexible transceivers that operate across LoRa, cellular, and Wi-Fi.

This will increase the functionality of the design, reduce the bill of materials (BOM) cost, and simplify the procurement of related parts.

Imagine if your home automation router could be designed to adapt as new wireless protocols emerge.

For example, as we move from 5G to 6G, SDR-based routers can be reconfigured to take full advantage of new cellular infrastructure through over-the-air (OTA) upgrades.

However, future use cases may require changes to the antennas and analog front end of this router.

Therefore, when designing the front-end, you should consider future operating frequencies and filtering requirements when designing the initial design.

Designing wireless communications with this level of flexibility is only possible using SDR.

SDRs allow customers to protect their investments over the long term by preparing for future changes.


※ Contributor
Mark Patrick Mouser Electronics