Case Study: Software-Defined Radio Architecture for Communications Test

Welcome to the Designing Next Generation Test Systems Developers Guide. This guide is collection of whitepapers designed to help you develop test systems that lower your cost, increase your test throughput, and can scale with future requirements. This whitepaper provides a case study for building a modular, software-defined RF platform. To read the entire developers guide, you can:

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RF and Communication Industry Trends

Bluetooth, WiMAX, cdma2000, ZigBee, GSM, EDGE, RFID -- the list of wireless and communications standards continues to grow at an unprecedented pace, as shown in Figure 1. At the same time, viewing football highlights on V CAST and obtaining location data from Google Earth are becoming commonplace, fueled by the likes of Microsoft, Vodafone, and Google. Given this insatiable demand for more data bandwidth and the fact that wireless communications are now outpacing land communications in many countries, the large challenge ahead for mobile communications becomes meeting this demand effectively.
In addition to the demand for multiple wireless standards, industry is driven by the ever-present pressure to quickly get new products to market, and research and design are outpacing test. Manufacturers release ZigBee and 802.11n devices before the standards are complete. Predefined standard test systems from stand-alone instrument manufacturers no longer exist. This is attributable to the fact that the traditional cycle of releasing a wireless standard, prototyping devices among lead users, and developing test equipment for mass commercial use is too time-consuming.

The demand for devices that incorporate multiple standards and the pressure to release new products before the competition are two major reasons many engineers work with more than one wireless and communications standard. In fact, the data gathered by a National Instruments “Instruments Study” survey in Figure 2 illustrates that almost two-thirds of engineers designing and testing devices with wireless and communications functionality use more than one standard, with the following percentage breakdown:

• 37 percent use one standard
• 30 percent use two to three standards
• 33 percent use four or more standards

Traditionally, you would need a separate stand-alone instrument for every communications standard to be tested. Each instrument has vendor-defined functionality for a particular standard. The communications measurement algorithms for the standards exist as firmware running on the embedded processor in each instrument, which means they are not user-accessible or customizable. Purchasing a new stand-alone instrument for each standard that you need to test is not productive or cost-effective. This is pushing engineers to seek flexible, out-of-the-box solutions.

Flexible Software-Defined Communications Test

One way to keep stride with wireless and communications advances is through software. You can take a software-defined approach to instrumentation by using coding and modulation software to generate and measure signals through modular, general-purpose RF instrumentation. This software-defined radio (SDR) approach to test is completely application-driven and user-defined. You can use it to leverage the software modeling and simulation software used in research and design for test and measurement. The Department of Defense (DoD) already supports this strategy.

“For the military, SDR is a transformational technology that allows the development of a truly interoperable family of radios that can communicate in any theater of operation with any allied force at any time,” said Colonel Steven MacLaird, director of the Joint Systems Program and program manager for the JTRS Joint Program (SDR Forum, August 2003).

A Typical Communications System
By stepping through a simplified functional block diagram of a typical communications system, you can see how to combine communications software with modular, general-purpose RF instrumentation to create a test system that supports multiple standards. Figure 3 represents the major functional blocks in a typical communications system. You can use these blocks for source coding, channel coding, modulation, and upconversion on the transmit side and the reverse of this process on the receive side. A real-world communication link contains a physical channel across which the transmission occurs. Physical channel examples include air (wireless), fiber-optic, and copper.

Source Coding and Decoding
The primary function of source coding is to represent your message in as few bits as possible to minimize resources. Source coding is similar to data compression; the smaller the message, the faster the transmission time, which translates into more efficient use of precious resources and spectrum. With source coding, you can send more information using the same bandwidth. Some of the more common source-coding algorithms include jpeg compression, zip (a combination of the LZ77 and Huffman coding algorithms), MP3 (part of MPEG-1 for sound and music compression), and MPEG-2 (used in DVDs).

Channel Coding and Decoding
Unlike source coding, channel coding can add bits to the data, which increases the message size. Added or reworked bits ensure that the original message can better withstand the effects of any channel impairments, including noise and fading, for proper decoding to obtain the original transmitted message. Many channel-coding algorithms balance the need to correctly encode and transmit data while minimizing message size.

Modulation and Demodulation
Modulation is the process of varying one or more properties (amplitude, frequency, and/or phase) of an electromagnetic wave or signal. You can use modulation to transmit information that originates at a low frequency signal to a signal operating at a higher frequency. You may wonder why you would want to transmit at a higher frequency as opposed to a lower frequency. Transmitting a baseband audio signal (from 20 Hz to 20 kHz) in a wireless fashion would require an antenna, power source, and electronic equipment of substantial proportions, which is impractical because of the large wavelength that is inversely proportional to the frequency. Therefore, if you transmit this same signal at a higher frequency, the wavelength becomes smaller, and you can reduce the size of the equipment and the amount of power you need. This fact signifies the prevalence and importance of modulation. With modulation, you can piggyback your baseband signal on a higher-frequency signal. The lower-frequency signal that contains the information or message you want to transmit is the modulating signal. The higher-frequency signal is referred to as the carrier signal because it “carries” the baseband information. The resulting combined signal is called the modulated carrier signal.

You also can use modulation when you want several signals to share the same channel or if you want to transmit more information without increasing the signal bandwidth. You achieve more efficient bandwidth use because more information can be carried in the same amount of space. You can choose a specific modulation format depending on the application and the amount of data you need to transmit. In addition to standard modulation formats, by performing modulation and demodulation in software, you can develop custom formats, which is particularly useful for proprietary and/or military applications that require custom formats.

Upconversion and Downconversion
You can use an upconverter and downconverter to shift an input frequency either up or down, respectively. The primary component of upconversion and downconversion is a device called a mixer. Mixers “multiply” two signals with different frequencies to produce a sum and difference signal.

Figure 4 illustrates the earlier functional block diagram of a typical communications system with National Instruments LabVIEW graphical code. The functions are for source coding, channel coding, modulation, and upconversion on the transmit side and downconversion, demodulation, channel decoding, and source decoding on the receiver side. The software is particularly suited for a PXI system, which provides the modular, general-purpose RF instrumentation required to both generate/upconvert and downconvert/acquire the communications signals.
There are many reasons why the PXI platform is ideal for software-defined communications test. Most importantly, it is PC-based. The functionality of PXI instruments is defined in software so a single PXI RF instrument can test multiple communications standards by simply changing the software running on the system controller. PXI controllers employing the latest dual-core processors can easily process the most complex communications algorithms.

As communications standards continue to scale the amount of data transferred, it is important to base a communications test platform on a high-throughput bus to transfer the data. PXI is based on the PCI and PCI Express buses, providing up to 6 GB/s of system bandwidth and up to 2 GB/s of bandwidth to a single instrument. With this throughput, you can use PXI to perform long-duration recording of communications signals for offline analysis and the playback of previously recorded or simulated signals.

Also, with the modular nature of PXI, you can upgrade a single component of a system. For example, you can increase the performance of all of the instruments in a PXI system by upgrading to a controller with a higher-performance processor. This type of upgrade is not possible with stand-alone instruments where the embedded processor is not user-accessible or upgradable. Moreover, because PXI is a multivendor platform, the modular components of a system can come from multiple vendors. You are not locked into a single vendor, and, because all PXI products must adhere to the PXI hardware and software specifications, interoperability among different vendors is guaranteed.

Most systems that test communications must also test other device functionality and include other instruments, such as digital multimeters (DMMs), programmable power supplies, and switching. The PXI platform is general-purpose and offers instruments for most applications and measurements. More than 1,000 PXI modules are available from the more than 68 members of the PXI Systems Alliance (PXISA).

Conclusion: Software-Defined Communications Systems Provide a Future-Proof Platform

The trend toward software-defined communications test systems will continue to grow. Organizations have embraced the movement because it helps them develop test systems in conjunction with standards development. Software-defined test offers the solution for current communications systems, but, more importantly, it provides a paradigm and platform for emerging and future communications systems.

References

[1] R. Harrison, A Software-Defined Platform for Current and Future Communications Systems, Instrumentation Newsletter, Q1 2006.

[2] J. Kovacs, LabVIEW and PXI Enhance Communications Design and Test, Instrumentation Newsletter, Q2 2006.

[3] Colonel S. MacLaird, Software Defined Radio Takes Step Closer To Wide-Scale Military Use, SDR Forum, August 2003.

Relevant NI Products and Whitepapers

National Instruments, a leader in automated test, is committed to providing the hardware and software products engineers need to create these next generation test systems.

Software:

• NI TestStand Test Management Framework
• LabVIEW Graphical Programming Environment
• Signal Express Interactive Measurement Software
  

Hardware:

• Modular Instruments (Oscilloscopes, Multimeters, RF, Switching, and more)
• Multi-function Data Acquisition
• PXI System Components (Chassis and Controllers)
• Instrument Control (GPIB, USB, and LAN)
  

Whitepapers
NI offers a Designing Next Generation Test Systems Developers Guide. This guide is collection of whitepapers designed to help you develop test systems that lower your cost, increase your test throughput, and can scale with future requirements. To read the entire developers guide, you can: Download the PDF (90+ page) version or view the web-version of the Designing Next Generation Test Systems Developers Guide.

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