A Software-Defined Platform for Current and Future Communications Systems

Bluetooth, WiMAX, cdma2000, ZigBee, GSM, EDGE, RFID -- the list of wireless and communications standards continues to grow at an unprecedented pace (see 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.

Figure 1. Demand for wireless networking and connectivity is creating a wireless and communications standards "log jam."

Driven by the race to release products, research and design are outpacing test. Manufacturers are releasing ZigBee and 802.11n devices before the standards have even been completed. Predefined standard test systems from traditional instrument manufacturers are nowhere in sight. This is mainly attributed 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 lengthy. Considering that this model is applied horizontally to dozens of standards releasing within the same time frame, test equipment manufacturers are faced with a tough decision -- either be late to market with compliant test equipment or potentially spend millions in R&D for a standard that doesn't take off. This is pushing engineers to seek flexible, out-of-the box solutions.

Adaptable Software-Defined Testers
One approach to keeping stride with RF and wireless advances in test is through software, where engineers model new channel coding and modulation techniques or algorithms. The logical solution is to take a software-defined approach to instrumentation by using coding and modulation software to generate and measure signals through modular, general-purpose RF instruments. This software-defined radio (SDR) approach to test is then completely application-driven and user-defined. A large proponent already pushing this strategy is the Department of Defense (DoD). "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). To illustrate this capability, Figure 2 demonstrates a simple digital communication link in National Instruments LabVIEW software. The included VIs 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. A real-world communication link also includes the hardware equipment and the physical channel over which the transmission occurs.

Figure 2. By developing with the NI Modulation Toolkit for LabVIEW, engineers can model their code after the digital communications systems they are designing.

Case Study: MIMO-OFDM Development at the University of Texas
A lead application of software-defined instrumentation was demonstrated in the development of a MIMO-OFDM system. The combination of multiple input, multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) are two new technologies behind many of the latest wireless and data standards emerging including 4G mobile cellular communications and 802.11n Wi-Fi data networking, which are designed to increase the number of subscribers and data throughput on cell phones and computers, respectively. OFDM benefits include resilience to RF interference, high spectral efficiency, and lower multipath distortion, while MIMO promises increased bandwidths using multipath signal propagation. The Wireless Networking and Communications Group (WNCG) at the University of Texas at Austin studied the characteristics of this system to validate the research and benefits of MIMO-OFDM. The study conducted by three WNCG members involved two main components -- simulation and full hardware integration -- and was completed in less than six weeks.

For simulation, the group used NI LabVIEW because of the data simulation and analysis functions (known as VIs). The LabVIEW environment also provides the NI Spectral Measurements Toolkit and the NI Modulation Toolkit for LabVIEW, two extensions designed specifically for communications system design, simulation, and analysis. With these tools, the group could directly control system parameters, including channel coding, power, and transfer rates, while adding fading and multipath interference to determine the system's immunity and response. The group also used the simulation to transfer live data and information to view the effects of adjusting physical layer parameters and channel characteristics, only possible with a software-defined system. In addition, the WNCG team could verify the effects to data throughput by adjusting the number of antennae and the algorithms processing the data received from the antennae. In this environment, WNCG members could effectively evaluate, through simulation, the benefits and weaknesses of MIMO-OFDM for next-generation data communications. Figure 3 demonstrates one of the interfaces WNCG used during the MIMO-OFDM study.

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Figure 3. The University of Texas WNCG Group's MIMO-OFDM interface used NI LabVIEW to compare the sent image to the received image.

Adapting Simulation Software for Full Implementation

The team then reused the software simulation code to develop the hardware-based MIMO-OFDM wireless transmit/receive system. They were able to use NI modular instruments to develop the system, including an arbitrary waveform generator module for baseband and IF frequency generation and an RF upconverter module to establish MIMO-OFDM system uplink. Similarly, WNCG members used the digitizer and RF downconverter modules for the wireless transceiver downlink. They then installed the modules in a PXI chassis with an embedded controller for high-throughput, real-time measurement processing. Figure 4 demonstrates this system, which includes the National Instruments PXI-5660 RF vector signal generator and the PXI-5670 RF vector signal analyzer.

Figure 4. This is the block diagram of a transmit-and-receive MIMO-OFDM system with two antennae.

Once developed, WNCG members were able to verify their hypothesis and simulation results with hardware-based transmit and receive systems. Because they simulated and designed the MIMO-OFDM transmitter and receiver system in software, WNCG members experienced a straightforward and efficient transition from a simulation to an actual wireless link using software-defined hardware in less than six weeks.

The transmitter design challenge for a software-defined instrument is assembling the desired waveform to achieve the correct modulation. The NI Modulation Toolkit for LabVIEW contains many of the building blocks necessary to modify a carrier signal with the message to achieve the correct modulation. The toolkit also provides common channel coding, equalization, and measurement functions to make creating and analyzing the waveform simple. Once you create the waveform -- configuring the rate, bandwidths, frequencies, and other parameters of the hardware -- downloading the waveform completes the generation.

Software-Defined Communication Systems Provide 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.

Ron Harrison
RF and Communications Product Marketing Manager
ron.harrison@ni.com

Download NI Modulation Toolkit for LabVIEW examples and other RF and communication resources.

This article first ran in the February 7, 2006, issue of NI News and the Q1 2006 issue of Instrumentation Newsletter.

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too much fluff
I had to read through the whole article just to figure out what NI offers to help with this problem; and then read the data sheets for the products to figure out how they might help. I don't think anyone would read the article to begin with if they weren't interested in designing or testing RF systems.
- Dan Lutes, Prophesi Technologies. dlutes@prophesi.com - Feb 8, 2006