Basics of Software Defined Radio, Part 1

This book excerpt from Chapter 18 of RF & Wireless Technologies provides a sample of the comprehensive information available in this text. The book aims to be a complete desk reference and features input from numerous industry-leading experts. This part introduces software defined radio.

Part 2 examines architectures for SDR, focusing on the receiver.
Part 3 covers transmitters for software defined radio.

Over the last decade as semiconductor technology has improved both in terms of performance capability and cost, new radio technologies have emerged from military and R&D labs and become mainstream technologies. One of these technologies is software-defined radio.

Although much has been discussed in recent years, a good definition of software radio is difficult to generate. This is largely due to the flexibility that software-defined radios offer, allowing them to take on many different forms that can be changed to suit the need at hand. However, software-defined radios, or SDRs, do have characteristics that make them unique in comparison to other types of radios. As the name implies, an SDR is a radio that has the ability to be transformed through the use of software or redefinable logic. Quite often this is done with general-purpose digital signal processors (DSPs) or field programmable gate arrays (FPGAs), as discussed later in the chapter.

In order to take advantage of such digital processing, traditional analog signals must be converted to and from the digital domain. This is accomplished using analog-to-digital (ADC) and digital-to-analog (DAC) converters. To take full advantage of digital processing, SDRs keep the signal in the digital domain for as much of the signal chain as possible, digitizing and reconstructing as close to the antenna as possible, which allows digital techniques to perform functions traditionally done by analog components as well as others not possible in the analog domain. There are limits to this, however. Despite the fact that an ADC or DAC connected directly to an antenna is a desirable end goal, there are issues with selectivity and sensitivity that an analog front end can remedy.

The alternative to digitizing at the antenna is the use of a completely flexible analog front end (AFE) capable of translating a wide range of frequencies and bands to that which the data converters themselves can adequately process [1]. SDRs are ideal candidates to be used for multicarrier, single-carrier, single-band, multiband, and multimode transceivers. Some of these issues will be covered later. The key point is that SDRs have the ability to go beyond simple single-channel, single-mode transceiver technology with the ability to change modes arbitrarily because the channel bandwidth, rate, and modulation are all flexibly determined through software. These characteristics may be changed by direct input, floppy disk, over-the-air download, or through the use of careful signal analysis to determine analytically how the information is coded through a process termed cognitive radio [2].

Regardless of the means by which the radio is reconfigured, a fully implemented SDR will have the ability to navigate a wide range of frequencies with programmable channel bandwidth and modulation characteristics. The following list outlines some of the possible dynamic characteristics of an SDR:

• Channel bandwidth
• Data rate
• Modulation type
• Conversion gain