Accelerate wireless device development and testing using software-defined radio architecture-EEWORLD

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As the number of wireless applications now grows—from video chats between two friends miles apart to PDA -controlled warehouse climate and lighting—the number of wireless standards is also growing. Every industry is entering the wireless communications space, yet each industry has its own requirements and specifications that require standards and protocols to be optimized for specific applications. As a result, the number of wireless and communications standards is rapidly increasing, not to mention the many proprietary protocols. Figure 1 shows the "crowd" of standards. Before these standards are fully defined, they are integrated into circuit and system designs, long before test equipment vendors can provide test solutions. Another proof of this problem is the fact that many new devices that users have come to accept and rely on actually use two, three, or more standards for data or voice communications. For example, Apple's new iPhone integrates Bluetooth, Wi-Fi , and GSM /EDGE. How can engineers meet the testing needs of standards in a short period of time?

Understanding Communications Signal Processing

To determine the answer to this question, first consider the technology behind the communication system. Bandwidth, power, coding complexity, redundancy, impairment resistance, and cost are all key factors that must be considered in order to achieve the goals of a specific wireless application. For example, ZigBee is ideal for monitoring and controlling sensors that need to operate for several years after installation. Therefore, the important design choices for ZigBee need to minimize power consumption and cost, while other parameters such as bandwidth are less important. The situation is almost the opposite for the next generation of Wi-Fi , which supports data rates up to 540Mb/s.

Figure 2 shows the major functional blocks of a typical communications system that engineers would use to optimize design choices. For more information on the different building blocks of communications systems, see the sidebar below on “Understanding Communications System Blocks.”

Figure 1: The ever-increasing demand for data has created a “crowd” of wireless and communications standards.

Figure 2: This diagram shows the main functional blocks in a typical communication system.

Implement multiple and emerging standards with flexible software-defined signal processing

For the most important functional modules of the communication system introduced above, digital signal processor ( DSP ) chips or ASICs have always been used to implement them, which take months to design, develop and integrate into a communication solution. However, in order to implement multiple standards in a short period of time, the solution now sought is to implement these standards quickly and simply. A method introduced in communication testing that keeps pace with the development of wireless and communication technologies is through software. Introducing software-defined methods in test instruments, engineers use general-purpose RF instruments to generate modulated waveforms and test signals through coding and modulation software. This software-defined radio ( SDR ) method for testing is completely application-driven and user-defined. It allows engineers to use software modeling and simulation software used in research and design for measurement and testing.

Figure 3: Communications software, such as NI LabVIEW , running on a PXI system provides a flexible platform for communications testing.

Figure 4 shows an early functional block diagram of a typical communication system using National Instruments' LabVIEW graphical code. The functions included are for source coding, channel coding, modulation, and up-conversion at the transmitter, and down-conversion, demodulation, channel decoding, and source decoding at the receiver.

Figure 4: A typical PXI system with a controller and peripheral expansion slots.

PXI—An Ideal Platform for Software-Defined Communications Testing

PXI is a modular hardware platform for instrumentation that has many units for implementing a software-defined communications test approach. More importantly, it is PC-based. The functionality of a PXI instrument is defined in software. Therefore, engineers can use a single PXI RF instrument to test multiple communications standards by simply changing the software running on a Windows-based controller. PXI controllers use the latest dual-core processors that can easily handle the most complex communications algorithms. Figure 4 shows a typical PXI system with a controller and instrument cards.

A major factor in achieving the same good signal processing performance on PXI hardware using software as can be achieved on a DSP or ASIC is the ability to continuously generate (or acquire), refresh (analyze), and output waveforms from the controller. PXI is based on PCI and PCI Express buses, which can provide up to 6GB/s of system bandwidth and 2 GB/s of bandwidth for a single instrument. This throughput, coupled with dual-core technology, enables long-term signal acquisition and waveform generation.

Reconfigurable hardware platform

Another new platform emerging in communications is based on field programmable gate array (FPGA) logic and integrated analog-to-digital converters (ADC) and digital-to-analog converters (DAC). The main difference between a simple PXI-based system and a system using FPGA technology is the location of signal processing. In a PXI-based system, most of the processing occurs in a software program running on the host controller. In contrast, in an FPGA -based system, the logic and processing modules are downloaded to the FPGA in the form of firmware. This essentially transforms the FPG into a custom communications processor.

The National Instruments PCI-5640R dual-channel IF input, dual-channel IF output board, which uses a Xilinx FPGA, is a good example of this architecture. The PCI-5640R provides a PCI bus interface and includes four DMA channels to transfer streams between the main CPU (PC) and the Xilinx FPGA. Digital up-conversion and digital down-conversion are implemented through ADC/DAC, offloading processing tasks from the Xilinx FPGA.

Figure 5: National Instruments PCI-5640R block diagram.

The NI PCI-5640R is ideal for prototyping a variety of communication links. By programming the onboard FPGA using NI LabVIEW software, engineers can test existing and emerging communication standards using a variety of coding and modulation algorithms. The module is also an ideal teaching tool for software radio, communication system design, and coding and modulation method concepts.

Now, the commonality between FPGA-based instruments (which can be some form of PXI instrument) and PXI systems is that both systems are software defined. This means that communication systems built on either architecture can be adapted to new communication protocols.

Future Development Trends of Software-Defined Communication Testing

The demand for equipment that supports multiple communication standards and the pressure to bring new products to market faster will certainly increase in the future. Software-defined communication test and platforms, including PXI, off-the-shelf general-purpose instruments, or some new FPGA-based architecture, form a flexible approach that can help test engineers meet these requirements now and in the future.