Software-defined radio architecture that accelerates wireless technology development and testing

Today, the field of 无线通信\'); companyAdEvent.show(this,\'companyAdDiv\',[5,18])"> wireless communication is full of diversity, with a dazzling array of wireless communication standards. At the same time, modern electronic products often integrate multiple wireless standards. These have brought unprecedented challenges to engineers in RF design and testing. How to cope with the rapid update of wireless standards and technologies has become a topic of common concern.

The purpose of this article is not to discuss the advantages and disadvantages of different wireless communication standards in depth, but to point out how engineers can test multiple standards through a unified software radio platform to keep up with the pace of technological development when multiple standards coexist in the market.

The growing wireless technology

From remote video conferencing, wireless tire pressure monitoring to wireless remote meter reading systems based on ZigBee or GSM, the application of wireless technology is now penetrating into all walks of life and even becoming an indispensable function. Some standards have been integrated into the design of circuits and systems before the corresponding manufacturers provide test solutions; there are also many emerging products that implement two, three or even multiple standards for data and voice communication on one device, such as Apple's latest iPhone, which integrates Bluetooth, Wi-Fi and GSM/EDGE functions at the same time. While it is convenient for users, it also brings huge challenges to the development and testing of wireless technology.

At the same time, the continuous influx of various wireless communication standards also makes people feel helpless. Each company and organization is developing and optimizing different standards and protocols according to the needs of their specific applications. This has led to a large number of new standards emerging like mushrooms after rain, and the life cycle of each standard has been greatly shortened. If the various standards are displayed through a timeline, the evolution process of many standards will become very clear, and the development of new standards is also proceeding at an unprecedented speed. As shown in the figure below, before 2000, there were only four or five coexisting standards, and they had a long lifespan. Today, this situation has been overturned by the endless emergence of various standards.

Here are some emerging wireless standards that are currently in the development phase:

OFDM (Orthogonal Frequency Division Multiplexing) – This technology is becoming increasingly popular and is being implemented in many new standards.

4G Cellular Technology

Cognitive radio - part of the 802.22 standard - searches for empty spectrum to use when there is a conflict or traffic is flowing. Traffic is then moved to other unused spectrum.

Ad Hoc and Sensor Networks

Multiple-Input Multiple-Output Systems (MIMO) – In these systems, multiple antennas are used to increase system capacity.

Ultra-Wideband (UWB) - On first generation devices (3.1 to 4.8 GHz), each channel uses the full 528 MHz and transmits data at 480 Mbit/s.

Software-centric wireless test platform

With all these new standards emerging and coexisting at the same time, equipment manufacturers, test engineers and designers are facing many challenges. The purchase cycle of RF equipment is usually 5 to 7 years, but the launch cycle of new standards and new technologies is every two years, and the purchased RF equipment will soon become obsolete. Faced with such challenges, more and more companies are adopting a software-centric platform with modular hardware to meet the evolving technology needs. Such a software-centric platform can help users test the latest standards at the first time and speed up the time to market of their products or solutions; and as long as the software is adjusted, it can meet the new standards, which has strong flexibility; and for engineers themselves, they can add their own intellectual property technology to the system to gain technical initiative, so that these technologies are no longer just in the hands of standard manufacturers or institutions.

Digital signal processing technology enables multiple wireless technologies

Let us first understand the main functional modules of a typical communication system, which will help us better understand the entire design and testing process.

Source encoding and decoding

Source coding is mainly used to compress data to facilitate subsequent transmission. Common source coding algorithms include JPEG compression, zip (a combination of LZ77 and Huffman coding algorithms), MP3 and MPEG-2.

Channel Coding and Decoding

Unlike source coding, channel coding can add or rewrite data bits to reduce the effects of noise and attenuation in channel transmission, and obtain a better original transmission signal after decoding.

Modulation and Demodulation

The strict definition of modulation is the process of changing one or more properties (amplitude, frequency, and/or phase) of an electromagnetic wave or signal. It is mainly divided into two categories: analog demodulation and digital demodulation. Modulation can be used to transmit the original low-frequency signal at a relatively high frequency.

Up-conversion and down-conversion

The frequency of the input signal can be changed using the up-conversion and down-conversion modules.

In the past, some important functional modules in the above process (such as coding and modulation) were often implemented by integrating digital signal processing ( DSP ) into circuits or ASICs. Such design, development and integration into communication systems usually took several months, which made it difficult for users to keep up with the development of new standards. A faster and simpler way to keep up with the pace of wireless technology development is to apply software radio technology. The coding and modulation process is completed in the software, and the modulated waveform is generated, and then the frequency is up and down converted through modular instruments. This software-centric test method is completely application-based and user-defined, which enables engineers to quickly meet the requirements of new standards by simply modifying the software, greatly accelerating the design and testing time and mastering the technical initiative. [page]

The figure below shows the National Instruments LabVIEW graphical language and NI's PXI platform, which reproduces the functional block diagram of the above communication system through the software radio method. In addition to completing the physical up and down frequency conversion process, the modular hardware completes the rest of the functions through software.

Software Radio Test Platform Based on PXI and LabVIEW

NI has been advocating the concept of "software-centric test and measurement architecture". Since the launch of its flagship software LabVIEW in 1986, NI has been helping engineers improve their work efficiency through this revolutionary graphical programming language. Later, NI launched the PC-based industry standard test platform PXI for the first time in 1997, once again leading the industry trend. In 1998, NI and other test and measurement companies jointly formed the PXI System Alliance. So far, the alliance has more than 70 member companies and more than 1,200 PXI products, with functions ranging from power supply, DMM to RF , allowing users to choose and combine according to their specific test needs.

As a solid, PC-based measurement and automation platform, PXI combines the electrical bus characteristics of PCI/PCI Express with the solidity, modularity and mechanical packaging characteristics of CompactPCI, and adds a dedicated timing and synchronization bus; its backplane bus bandwidth of up to 6GB/s can ensure that the intermediate frequency signal after up and down conversion can be digitized in real time and continuously to the PC for processing; PXI controller uses the most advanced dual-core processor and runs the Windows operating system, which can handle any complex communication algorithm. All these make PXI an ideal high-performance, low-cost carrier platform for wireless testing.

A software-based modular test platform requires a flexible software platform. LabVIEW is a graphical programming language designed specifically for engineers. In addition to seamless connection with PXI hardware, it also integrates up to 600 signal processing and analysis algorithms, as well as various toolkits such as modulation and demodulation, spectrum analysis, etc., for RF applications, it can complete a series of measurements such as power spectrum, peak power and frequency, in-band power, adjacent frequency power, etc. In the open software environment of LabVIEW, users can also implement RF algorithms with independent intellectual property rights to meet the needs of the growing wireless communication standards. (See the figure below)

Today, LabVIEW, PXI, and modular instruments have become essential tools for engineers and scientists to develop and test the latest technologies, including wireless standards. In the following two cases, we will see how researchers at the University of Texas at Austin used this technology to develop a 4G-based system in just 6 weeks; and how a local company developed the industry's first successful solution for testing 1C2G RFID systems.

User Solutions

User Solution 1: Prototyping a MIMO-OFDM 4G System

This is a representative example of how to quickly prototype and develop a system using this platform. OFDM (Orthogonal Frequency Division Multiplexing) is a multi-carrier digital communication modulation technology. It selects mutually orthogonal carrier frequencies as subcarriers and uses multiple subcarriers for parallel transmission. OFDM technology can overcome the problem of increased signal interference when supporting high-speed data transmission in CDMA, and has the advantages of high spectrum efficiency and simple hardware implementation. Therefore, OFDM is regarded as the core technology in the fourth-generation mobile communication system. MIMO (Multiple Input Multiple Output) uses multiple antennas to achieve multiple transmissions and multiple receptions, which can increase the channel capacity exponentially without increasing spectrum resources and antenna transmission power.

MIMO-OFDM combines the advantages of MIMO and OFDM, and can simultaneously improve the speed, range and reliability of wireless communication systems. It has now been written into the WLAN802.11n and WiMAX802.16 standards. The research on the 4th generation mobile communication, which has attracted widespread attention in the industry, is still in its early stages, and its basic functions and core technologies are still in the conceptual stage. MIMO-OFDM is also one of the popular solutions for building 4G systems.

The University of Texas in Austin developed a MIMO-OFDM 4G system. Under the guidance of Professor Robert Heath of the UT Wireless Network and Communications Laboratory, three students designed a prototype of a 4G system in six weeks.

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The lab chose a software-based modular test platform because the researchers had a good starting point with the readily available NI RF vector signal generator, RF vector signal analyzer, LabVIEW software, and modem toolkit, so they could focus on the core development. When completing the design process, it was necessary to build a prototype for the MIMO wireless communication system and provide practical verification for theoretical research. The traditional approach requires expensive dedicated hardware, which is time-consuming to program and difficult to maintain in the lab. Using integrated NI software and RF products, researchers at the University of Texas at Austin can quickly create a wireless communication system, including various elements such as modulation, synchronization, and equalization.

The University of Texas at Austin used this technology, which combines LabVIEW software and PXI hardware, to successfully achieve a 4G solution, and now researchers at the University of California, Berkeley are using the same equipment to conduct similar research.

User Solution 2: C1G2 RFID Tag Test Solution

The Class 1, Generation 2 (C1G2) RFID standard is the latest standard of the international RFID standards organization EPCglobal. This standard specifies the communication protocol between RFID tags and readers operating in the ultra-high frequency (UHF), that is, the frequency range of 860 to 960 MHz. C1G2 provides a faster, more secure, globally recognized and cheaper specification to deploy. So far, Europe and North America have accepted this standard.

C1G2 increases the tag reading speed to about 1,500 times per second in the United States and 600 times per second in Europe. If current technology is used, the tag reading speed is 100 to 300 times. When using C1G2, the writing speed is twice that of current products. This algorithm and the use of spread spectrum technology enable readers to selectively communicate with different tags at acceptable distances and different frequencies. In addition, the standard solves the problem of interference between readers, and in open spaces, the reading distance of UHF can reach 10 to 20 feet. In terms of protecting tag information and user privacy, C1G2 includes password protection read access and permanent lock memory content functions, and increases the password length from 8 bits to 32 bits. C1G2 uses a complex anti-collision algorithm, which can greatly improve the reader's ability to read a large number of tags at one time in the reading area. For example, when we check out at a large supermarket, we need to take out the goods one by one to read the barcode information, which often causes congestion at the checkout during busy hours. If C1G2 technology is used, all the product information in the cart can be read by simply pushing the cart through the sensing area. This process may only take an incredible one second!

While the C1G2 standard RFID brings many advantages, it also poses a problem for manufacturers in standardization testing. Since the standard is relatively new and the protocol is complex, it has high performance requirements for test equipment, especially the real-time response capability of RF. For semiconductor manufacturers that produce RFID products, this is undoubtedly a major obstacle to delaying the launch of products. When major companies have not yet launched test solutions on the market, a Chinese engineering company, Shanghai Juxing Instruments, has developed the world's first test equipment that supports all instructions of the C1G2 standard.

This test solution is based on NI RF modular instrument hardware. The intermediate frequency processing hardware is the software radio platform NI RF RIO (Reconfigurable I/O) with a powerful FPGA embedded in it. The software is implemented based on the LabVIEW graphical programming environment. Each RFID tag response communication time can be completed within 400-500 microseconds. Currently, this test solution is receiving verification from the RFID standardization organization.

Summarize

In summary, facing the emergence of more and more wireless application standards in the market, the software-based test platform uses high-performance modular hardware and flexible software platforms to provide engineers with a unified platform to test these standards and can easily meet the ever-changing market needs. On the one hand, it enables technology innovators to no longer be restricted by the limitations of test manufacturers, and on the other hand, it helps those small but strong companies to have high competitiveness in this rapidly developing market and become market pioneers.