Wireless technology is finding a home in an ever-increasing number of diverse applications. The latest consumer electronics devices provide untethered functionality unheard of a few years ago. For example, cameras now feature wireless LAN (802.11x) functionality for immediately uploading images from the camera to the Internet via a Wi-Fi hotspot – no cables required. New alarm clocks provide connectivity to backyard weather stations via remote wireless sensors providing up-to-the-minute weather details and onboard algorithms generating forecasts based on the collected data. Wireless connectivity has eliminated the need to continually adjust the household clock; these new clocks synchronize to the National Institute of Standards and Technology (NIST) low-frequency atomic clock broadcast on the NIST radio station WWVB.
This diversity, along with increased market pressure, breeds varying implementation details, rapidly changing requirements, increasing complexity, and decreasing time available for design cycles. These conditions challenge engineers as they design, test, and manufacture these devices.
The National Instruments software and hardware platform presents an intuitive way to address these challenges. This virtual instrumentation platform offers a tools-based, modular approach that not only tailors to fit your exact requirements but also facilitates change, allowing growth as your requirements change. You also benefit from commercial off-the-shelf component evolution. By building around a standard PC, you can upgrade components and immediately see benefits such as increased processor speed, resulting in increased throughput.
Tuning in to Software Radio
In many cases, new wireless applications have become practical because of advances in digital signal processing (DSP) and computational power. These advances have fueled the idea of a software-defined radio (SDR) architecture that addresses modern design challenges by using reconfigurable software rather than fixed hardware for processing signals prior to transmission or preparing a signal for transmission.
The general idea behind an SDR is using wideband radio frequency (RF) hardware to acquire or transmit waveforms that are manipulated as much as possible in software. The RF hardware might consist of one or more conversion stages where an intermediate frequency (IF) signal is converted to or from an RF frequency. The IF signal is converted to or from the digital domain by wideband analog-to-digital converters or digital-to-analog converters, respectively. Software running on a general-purpose processor manipulates the sampled IF signal, performing multiple steps that either prepare a signal for transmission or extract useful information from a received signal.
Figure 1. Digital Communications System Transmitter (Top Row) and Receiver (Bottom Row) Components
SDR as an Architecture
Figure 1 depicts elements of a digital communications system that you could implement with an SDR. It highlights some of the common software- and hardware-based jobs that you might see in a single-channel, single-carrier digital communications system. The top row represents functions commonly found in a transmitter: source coding, channel coding, modulation, upconversion to an intermediate frequency, digital-to-analog conversion, and upconversion to an RF frequency. The bottom row depicts the receiver side, with blocks that represent functions that serve to undo the work done by the transmitter and account for signal degradation. Receiver-side functions include downconversion, demodulation, channel decoding, and source decoding.
NI Platform Features for Software-Defined Radio
National Instruments offers software and hardware that is the basis for a modular, reconfigurable platform for SDR. With NI products, you can build custom applications for rapid prototyping, design validation, and test of SDR implementations. The modular nature of the NI platform makes it possible for you to choose exactly what you need to fit the specifics of your applications. It also helps you work at a high level on entire SDR systems or on a finer level with the SDR subcomponents.
On the hardware side, the NI platform is built around PXI and includes a variety of options that might be useful for SDR. The most relevant include:
- NI PXI-5660 RF signal analyzer – Digitally controlled wideband RF digitizer that can serve as the acquisition front end for an SDR receiver (features 20 MHz real-time bandwidth within a frequency span of 9 kHz to 2.7 GHz).
- NI PXI-5670 RF signal generator – Digitally controlled wideband RF signal generator that can serve as the signal generation front end for an SDR transmitter (features 22 MHz real-time bandwidth within a frequency span of 250 kHz to 2.7 GHz).
- NI reconfigurable I/O options for PXI/CompactPCI – Modules that facilitate signal processing and generation that require real-time execution; possibilities include PRBS bit generation, channel coding, modulation, and channel simulation.
The basis for the software portion of the platform is NI LabVIEW, a graphical development environment that is extensible, interactive, and easy to learn and use. LabVIEW is a general-purpose graphical programming language with functions for measurement, signal processing, instrument control, and other engineering tasks. Toolkits and modules are available to enhance LabVIEW with application-specific functionality for communications and RF-related jobs associated with SDR.
National Instruments software options most relevant to SDR include:
- LabVIEW 7.1 graphical development environment – Software for building custom applications for rapid prototyping, design validation, and test of SDR systems and subsystems
- Modulation Toolkit 3.0 for LabVIEW – Toolkit that extends LabVIEW with functions for digital and analog communications and features high-level and building-block functions that implement many tasks that are common to SDR transmitter or receiver signal chains. Highlights from the latest version (3.0) of this toolkit include functions for PRBS bit generation, channel coding/decoding, modulation/demodulation, feed-forward equalization, addition of impairments, channel models, and modulation domain measurements/visualization. You can use this toolkit for prototyping an SDR subsystem, generating test signals for SDR subcomponents, acquiring modulation domain measurements, and many other communications tasks.
- LabVIEW Digital Filter Design Toolkit 7.5 – Toolkit that extends LabVIEW with functions for design and implementation of fixed- and floating-point digital filters that are commonly applied in multiple places throughout the SDR Tx/Rx signal chain. It offers tools for both floating- and fixed-point designs with the ability to automatically generate ANSI C code, integer LabVIEW code, and single-cycle timed-loop (SCTL)-optimized LabVIEW FPGA code.
- LabVIEW FPGA Module – Module to graphically program reconfigurable FPGA hardware on NI reconfigurable I/O devices. You can use this module for rapid development of signal processing, analysis, and generation that requires real-time execution and to implement SDR-related jobs such as PRBS bit generation, channel coding, modulation, and channel simulation.
An Emerging Wireless Technology
The idea of SDR proves useful as a framework that outlines elements often found in emerging wireless digital communications technologies such as ZigBee. Based on the IEEE 802.15.4 standard, ZigBee promises a low-power, low-cost, and low-complexity wireless network focused on the remote monitoring and control market. What makes ZigBee different from current technologies such as 802.11x or Bluetooth? Whereas the latter two wireless technologies provide high data rates and a power profile ranging from a few hours to days, ZigBee offers a low-data-rate alternative and low-power consumption maintaining battery life for years.
ZigBee proponents are promoting it for applications such as building automation. Imagine the time and expense saved by eliminating the need to run miles of copper wire for heating, ventilation, and air conditioning (HVAC) and lighting control. A ZigBee receiver on the lighting ballast and a transmitter on the light switch deliver a wireless switch that you can locate anywhere. With such setups, you can move HVAC controls to a room or office for convenience and comfort. This highlights one of many use cases for this emerging technology.
With the advent of new technologies comes the need to develop and test devices to particular standards. These compliance tests ensure that devices perform to specifications and do not violate FCC regulations. National Instruments Alliance Partner SeaSolve Software has developed compliance test software according to the ZigBee standard. With this software, you can perform all 14 physical-layer RF tests specified by the standard on your device with pass/fail results.
SeaSolve ZigBee compliance software performs various RF tests using the National Instruments PXI-5660 vector signal analyzer, PXI-5670 vector signal generator, LabVIEW 7.1, and the Modulation Toolkit 3.0 for LabVIEW. These tests include:
- Spectrum PSD mask
- Error vector magnitude
- Transmit center frequency tolerance
- Maximum transmit power
- Symbol rate
- Maximum input power
- Receiver sensitivity
- Adjacent channel jamming resistance
- Alternate channel jamming resistance
- Energy detect (ED)
- Link quality indicator (LQI)
- Clear channel assessment (CCA)
- Tx-Rx turnaround time
- Rx-Tx turnaround time
Learn more about National Instruments products for communications applications.
Senior Product Manager, LabVIEW Signal Processing and Analysis
Joseph E. Kovacs
RF Product Marketing Manager