SDR – Software-Defined Radio

Software-defined radio is a modern wireless communication system in which both the hardware and the software aspects are defined by software. It is also sometimes called a virtual radio. In particular, this term refers to an architecture that uses digital signal processing (DSP) techniques for signal reception and transmission from or to antennas, together with general-purpose computers for processing instructions. In contrast, traditional hardware radios use separate analog circuits for both receiving and transmitting signals. In addition, SDR may include the use of high-performance digital signal processors (DSPs) running specialized programs. SDR-based radio development usually starts with a software-defined radio (SDR) platform, which may have additional hardware components, such as an interface to the host system’s processor. Routine operations are performed by programming these computers with the appropriate software. The intent is to use the radio transceiver to support communications systems that are sometimes called SDRs.

History of Software Defined Radio

The first SDR concept was proposed by David W. Carver at MIT Lincoln Laboratory in 1982 as a method to test radar cross-section of targets using computer simulation instead of real-world hardware targets. In his paper, Carver described the concept of a DSP-based receiver where all signal processing was done by software. In 1994, Michael Georgeff and John Vincent, researchers working for Harris Corporation designed a programmable transceiver that uses a DSP as the central component instead of the traditional analog approach.

In 1999, Wolfgang Wüster and Romuald Blanc of Swiss Federal Institute of Technology (EPFL) coined the term “software-defined radio”, as they published an article describing their recent work with what they called “programmable radios”. They used FPGAs to emulate signal processing blocks. In 2001, Van-Gin Ho at the University of Pennsylvania, along with Bernard Meyerson from the University of Virginia and Vinayak Mehra from Loyola Marymount University, developed a software architecture for a software-defined radio that provides a platform for wireless network deployment. In 2002, a European research program called “I18S” was launched under the leadership of Carsten Spörkel and David Cooke at the German Aerospace Center (DLR) to develop SDR technology. The I18S project developed a general framework based on Xilinx FPGAs which were used as development cards for software-defined radios. In 2003, an SDR system called “PRISM” was developed at the University of Bristol. It used a PC as a host and an FPGA board to carry out signal processing tasks. In 2005, an SDR architecture was developed at Stanford University called “Stanford Digital Radio Project” (SDRP) for use in wireless local area networks. In 2006, researchers from MIT Lincoln Laboratory designed an SDR system named WARP (Wireless Architecture for Reconfigurable Processing). In 2007, Intel released the XScale-based FPGA “Cortina” SoC, which was the first commercial SDR platform.

How does it work? 

In a software radio, the primary method of modulation is done in software—hence the name. In a conventional radio, modulation is normally implemented in hardware. Modulation is the process that turns a baseband signal into a radio frequency signal that can be transmitted over the air to a receiver. In a software radio, the carrier waveform used for transmission may be generated by switching samples from a lookup table in software, or by upconverting baseband signals using DSP algorithms to create much higher frequencies. 

In the case of an SDR system, much or all of the processing is done digitally in software on computers and digital signal processors (DSPs). 

Many SDR systems are based on the idea of “software-defined radio”. The software is designed to drive a combination of digital signal processing devices to process RF signals. This processing may be implemented in computer programs, FPGAs (Field Programmable Gate Array), or custom ASICs (Application-Specific Integrated Circuits). The main difference between traditional radio and an SDR is that in the SDR, the radio’s control and processing of radio signals and RF bandwidth are implemented with software running on a general-purpose microprocessor or high-performance digital signal processor (DSP), instead of being explicitly built into hardware logic. Thus, an SDR system can be reconfigured by changing the software parameters, such as filter coefficients or modulation schemes, without having to modify the hardware.

Benefits of Using Software Defined Radio

  • The main benefit of using programmable hardware instead of hardwired analog circuits is that adjustments can be made in real-time to improve performance without having to modify the hardware design. Also, programmability allows real-time adjustments for changing conditions without requiring human intervention or specialized personnel.
  • SDRs may be implemented with software running on general-purpose computers, or even in dedicated hardware using special-purpose chips. 
  • The software-defined radio is flexible and adaptable. The SDR can be reconfigured to use many different kinds of signaling and encoding without having to change the hardware of the SDR. In addition, new modulation techniques can be added at any time allowing for rapid changes in the wireless environment. For example, an SDR could easily change between digital signal processing (DSP) based modulation and spread spectrum modulation. If a frequency band becomes unavailable, an SDR can be programmed to switch to a new frequency band with little effort. Since software control is available over wide frequency bands, an SDR can also carry out multi-role functions such as radar and also wireless network functions simultaneously. 
  • The SDR can share resources among multiple radio standards, greatly simplifying system design. One conventional receiver may be replaced with an IF-based receiver that can receive multiple standards, whereas the SDR may be reconfigured for each standard with no need to make changes to the hardware of the system. A single software-defined receiver can thus implement all or part of a satellite navigation receiver (GPS), personal communications services (PCS) receiver, wireless local area network (WLAN) transceiver, AM/FM radio tuner, etc.

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