Software radio detection method for software radio products

1 Introduction

Software-defined radio technology was first proposed by American Joe Mitola in the early 1990s [1]. He proposed that software-defined radio (software radio for short) is a radio that uses software for channel modulation and demodulation. The few radio frequency parameters of such a digital communication system are determined by the hardware radio frequency front end, and the actual communication protocol is mainly defined by software. In other words, the system can select appropriate coding and modulation methods according to actual channel conditions and user needs to achieve adaptive communication capabilities. This software-defined radio theory has brought near-infinite flexibility to military communications, so the enlightened communication system designers received high-intensity R&D funding from the beginning. With the gradual commercialization of high-performance intermediate frequency devices at the end of the 20th century, mainly the commercialization of high-speed AD, DA and intermediate frequency processing devices, designers of wireless communication terminal products and designers of test and measurement instruments have gradually bathed in the shower of software radio. This has led to the development of software-defined radio frequency instruments and software-defined wireless communication products.

2. The application of software radio in mobile phones and measuring instruments

In the field of wireless communication, mobile phones and their testing equipment coincidentally use software-defined radio technology. Figure 1 and Figure 2 are the functional block diagrams of a typical detection system and a mobile phone, respectively. Software radio brings flexibility to both test and measurement instruments and wireless communication equipment being measured. As far as the instrument is concerned, the DUT with different communication protocols can be detected by calling different software. For communication devices, different communication protocols can be used to communicate through different software. The software can have different bearing platforms. The software can be installed on the computer hard disk or flash disk, and can be on the DSP or FPGA. For mobile phones, most of them operate in the latter form. For instruments, most of them use the Microsoft-Internet computer platform.



Figure 1 Block diagram of a typical software radio detection system



Figure 2 Block diagram of the typical structure of mobile phone electronics

In terms of instruments, it is divided into two genres: closed structure and open structure. The closed structure instrument provides a complete wireless test and measurement function with a certain number of protocols. The user can complete various tests on the DUT by calling program-controlled commands, such as GPIB commands. The internal structure of the instrument is not open to users. Protocol expansion or test function expansion is completely dependent on the instrument supplier. This genre is represented by Agilent and Rhode and Schwarz, with the famous 8960 and CMU200 instruments. Such traditional instruments are software-defined within the instrument supplier, and they are predefined for the user. The open structure instrument is built on a public standard bus structure, and each instrument module (also called a modular instrument) is controlled by a public computer function. The software developed by instrument suppliers, third-party software developers, or users themselves can implement various tests on the DUT by calling these function combinations to cooperate with digital signal processing and test and measurement algorithm software. The extension of the protocol and test functions can be developed by anyone with the ability. This genre is represented by National Instruments, and the PXI instrument platform is what it advocates.



Table 1 Comparison of closed and open structure communication testing instruments

At present, only PXI and VXI are available for the open structure bus. Because these two buses cannot support the nanosecond-level precision response synchronization required by many wireless communication protocols, and because of the technical difficulty of developing a signaling protocol stack, only a few protocols such as WiFi, Bluetooth, RFID, etc. are seen in open systems. As for the signaling test system, there is no analog call test system for common protocols such as GSM and CDMA. To this point, the conditions can be provided by enhancing the real-time processing capability of the instrument's intermediate frequency, or the signaling test can be simply cancelled to solve the problem. The former originates from test and measurement suppliers, and the latter originates from manufacturing plants. The trend of industry development this year has led to the reduction or cancellation of signaling testing as a basic requirement for production testing.

3. From call test to parameter test

Function test of analog call

First review the traditional mobile phone wireless detection. Traditionally, we use a radio frequency detection instrument with a base station signaling protocol stack. This instrument establishes an analog call with the device under test through the air interface of the wireless protocol, and detects the performance of the mobile phone while simulating the call. Usually we call this instrument a comprehensive tester. The air interface here is an interface between the wireless transceiver of the base station and the wireless transceiver of the terminal (such as a mobile phone) in the communication protocol. In actual use, it is through the air, but it can also be connected through a cable during detection. This kind of detection method that uses a comprehensive tester to simulate a call utilizes the protocol instructions prescribed by the standard, so that the radio frequency detection has nothing to do with the internal design of the product, and can be adapted to any mobile terminal designed and manufactured by any manufacturer under a specific protocol. The testing method of analog calls has monopolized the wireless testing industry. Until recent years, the profits of wireless terminal product manufacturers have become increasingly difficult to support the production costs brought by this testing method.

The pressure of test and measurement instruments on production costs comes from the apportionment of the depreciation of the instruments to the cost of each test piece. The instrument cost includes the instrument upgrade cost necessary to adapt to different DUT protocols. Assuming that various instruments have a close lifespan, the production cost can be broken down into three main factors, instrument price, test speed, and upgrade cost. Low price, high speed, low upgrade cost, low production cost.

At the end of the 20th century, the control of production costs was achieved through the reduction of instrument prices and the parallelization of test procedures. Due to the rapid increase in mobile phone production, tens of thousands of dollars of instruments are no longer purchased one or two, but dozens or hundreds of purchases. Therefore, the design and manufacturing cost of the comprehensive tester can be reduced, resulting in a significant reduction in the actual selling price. On the other hand, the radio frequency inspection process is also upgraded from one instrument to one DUT to one instrument to 2-4 DUTs, so that the start of the DUT and the non-RF inspection time do not take up the time of the comprehensive tester. So the test time is also reduced.

However, wireless communication protocols have developed rapidly in the past 3-5 years. The third generation (3G) communication has become a reality, and the third generation (4G) communication is about to emerge. At the same time, the integration of multiple protocols for a single terminal has also become a development trend. A mobile phone not only supports GSM, EDGE, CDMA, but also supports WiFi and Bluetooth. Mobile phones that support the third-generation communication protocol must also be compatible with GSM and EDGE. At the same time, the upgrade of the comprehensive tester seems to be over. The test of WiFi, Bluetooth, WiMax and other wireless communications requires the purchase of additional wireless test equipment. For a moment, how much investment is needed for wireless production equipment in 3-5 years becomes unknown. Large-scale production makes it impossible to build on the unknown, so each wireless production plant began to look for a good strategy to lock in production costs.



Figure 3 Block diagram of the mobile phone detection by the comprehensive tester

RF parameter test

The way to control the cost of production and testing of wireless products is to use radio frequency parameter testing instead of signaling call testing. Logically speaking, it is to separate radio frequency detection and digital circuit detection. For software radio products, since the modulation and demodulation are all done by software, the radio frequency detection should only need to detect the parameters of the radio frequency front end of the product, and it has nothing to do with protocol signaling. The digital circuit can be realized by software checksum and digital port reading and writing. Technically, RF parameter testing can be divided into two categories, RF calibration parameter testing and RF performance parameter testing. Among them, the former measures the transmit power control parameters, receiving sensitivity control parameters, frequency response equalization parameters, etc., related to the wireless terminal transceiver control parameters of the tested device. The latter measures the emission spectrum curve, sideband response, intermodulation distortion, spurious spectrum radiation, frequency accuracy and other parameters related to the quality of the RF front-end of the DUT. In fact, RF calibration parameter testing has existed in production plants for many years, but it has not been until recent years that performance parameter testing can replace call testing has gradually become an industry consensus.

In terms of testing methods, this is the transition from function testing to performance parameter testing, and also the transition from black box testing to white box testing. Technically speaking, it is a transition from closed design structure to partially open design structure. From the mobile phone test principle block diagram, the parameter test uses the digital communication port to control the DUT and command the DUT to enter a specific test mode, so that the DUT can transmit test signals, or receive test signals and receive signals or signals. The parameters are sent back to the test instrument through the communication port. Figure 4 is the principle block diagram of the radio frequency parameter test, which can be compared with Figure 3. We use digital port control mobile phone (or other wireless communication equipment) instead of air interface control. Among them, the digital port is often a serial communication interface for low-voltage logic. Refer to Table 2. The designer of the wireless terminal must disclose some digital control commands to the test system R&D engineer, and integrate part of the test code in the terminal R&D process into the terminal test mode software, so that it can run on the DUT during production testing.

The openness of test instructions by wireless terminal designers makes it possible to test radio frequency parameters, and the outstanding performance of software-defined radio frequency instruments has contributed to the parameter test. These modular instruments represented by the PXI bus are sometimes called virtual instruments or synthetic instruments. The modular PXI radio frequency instrument shares the local bus of 132M bytes/sec, and has a strong trigger synchronization function, so the test of some projects can be accelerated exponentially. In the past, it was tens of seconds or one or two hundred seconds to calibrate a mobile phone. Now, with a new calibration signal mode, the time to calibrate a GSM channel can be less than 1 second.



Figure 4 Block diagram of the mobile phone radio frequency parameter detection principle



Table 2 Comparison of function test and parameter test



Table 4 Measurement signal and analysis of receiving performance parameters of DUT

4. Examples of wireless test systems for software radio

The software-defined radio framework test system has been used in the production and testing of SCDMA mobile phones in my country as early as 2003, see Figure 5. The configuration of a typical PXI-based radio frequency detection instrument is shown in Figure 6. After years of development, on the NI PXI platform, Shanghai Juxing has realized the detection of multiple radio frequency protocols such as GSM, EDGE, CDMA, WCDMA, TD-SCDMA, WiFi, BP, and GPS. Part of the sample program interface is shown in Figure 7 and Figure 8. National Instruments plans to launch a new generation of software-defined radio modular instruments based on PXI and PXIe in 2008. These modules will provide a larger RF frequency range, wider real-time bandwidth and stronger real-time signal processing capabilities on PXI and PXIe bus platforms. , I believe that they are communication testing products that are worth looking forward to.



Figure 5 SCDMA production tester based on PXI software radio platform



Figure 6 The principle block diagram of a typical NI RF detection system based on the software radio architecture



Figure 7: WiFi data framing interface of Juxing Instrument Wireless Test Toolkit



Figure 8: Example interface of WiFi 54Mbps 64QAM OFDM demodulation of the wireless test kit of Juxing Instrument

  5 Conclusion

The theory of software radio has a long history, and wireless terminals and testing instruments based on the software radio framework have also become the mainstream of the market. However, due to technical confidentiality and other reasons, most production inspections, especially those of OEM manufacturers, still use traditional functional inspections that simulate actual communication signaling. Fortunately, under market conditions, parameter detection based on the software-defined radio framework is still gestating. More and more instrument suppliers open instrument baseband data to users, and more and more third-party RF test and measurement software appear on the market. This transition of wireless terminal testing from black box to gray box testing, in addition to bringing economic benefits to the production plant, is bound to increase the technical content of wireless production. The open-structure software radio modular instrument played a key role here.