Preliminary verification of UWB and WMAN radio systems

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Preliminary verification of UWB and WMAN radio systems [Copy link]

Wireless local area networks (WLANs) using an orthogonal frequency division multiplexing (OFDM) physical layer are now being deployed commercially around the world, primarily based on the IEEE 802.11a/g OFDM radio standards. Designers are now using multi-mode IEEE 802.11a/b/g chipsets for mobile wireless platforms to increase IC integration for affordable wireless Internet access. Emerging IEEE 802.15.3a Ultra-Wideband (UWB) and IEEE 802.16d Wireless Metropolitan Area Network (WMAN, also known as WiMAX) standards will drive IC design toward higher data rate OFDM applications. The industry's focus on developing WLAN OFDM products over the past three years has revolutionized the design and verification process, increased design efficiency by emphasizing early verification, and accelerated the introduction of new OFDM products in the competitive consumer market. Zero-IF CMOS RFICs have revolutionized WLAN design, making low-cost products economical. These products typically include baseband and RFICs, which integrate off-chip components such as power amplifiers, filters, and antennas in a board-level module or package, as shown in Figure 1. The RF and baseband ICs are often integrated into a prototype reference radio to perform system-level OFDM radio certification testing before mass production of the ICs. For example, RF IC defects can be reduced using programmable baseband IC compensation algorithms, which must be verified during system-level reference radio testing. The benefit of integrating simulation and verification tools is that WLAN radio system designers do not have to wait until late integration to find problems in the RFIC or baseband algorithms, but can verify important OFDM radio specifications early in the design process. In the integrated design process, simulation of the RF and baseband ICs must be performed at the beginning to verify important specifications before tape-out, thereby shortening the design cycle. During the simulation process, various virtual measurements should be performed, that is, the candidate OFDM design is analyzed to obtain the error vector magnitude (EVM) of the transmitter and perform debugging. The test signal synthesized during simulation is downloaded to the instrument to generate OFDM test signals to evaluate the prototype IC. As for the signal acquisition and bit error rate (BER) testing of the receiver, it must be performed under the condition of including transmitter distortion, channel defects and unwanted interference signals. To simplify the design process, the design can be evaluated based on the transmitter spectrum, EVM and BER equivalent energy measurement standards using pre-set test and verification settings.

OFDM radio systems follow the IEEE 802.15.3a standard for UWB wireless personal networks (WPANs), using carrier frequencies below 11 GHz and data rates of 110 to 480 Mbps, to provide short-range (<30 feet) applications such as wireless USB and streaming video in the home office. Wireless metropolitan area networks (WMANs) follow the IEEE 802.16d standard, using carrier frequencies below 11 GHz and data rates up to 75 Mbps, to provide broadband wireless Internet access (BWA), or "last mile" Internet access, for 4 to 6 miles. As spectrum managers, standards-setting organizations, industry groups, and associations push for IEEE standards, it is unclear which standard will win. It is generally believed that the efforts of various WLAN product manufacturers to achieve interoperability of consumer products will help the IEEE 802.11a/b/g standards become the global wireless network standards. As for the emerging WPAN/WMAN standards, it will take several years to complete. During this period of competition for new standards, OFDM radio chip designers need flexible design and verification tools to set new simulation and test solutions for different standards.

Accelerating the Development of Emerging OFDM Products Verification of mixed-signal system-on-chips (SoCs) ensures that designs meet performance specifications based on simulated behavioral and component models before IC tape-out and production delivery. Currently, the production process for 0.1 um ICs is long and the required mask costs are approximately $1 million. SoC designs with tens of millions of gates often require hundreds of billions of binary test patterns (test vectors); generating and simulating countless input combinations can ensure that the design will never reach an unrecoverable state. Engineering teams may spend 50% to 70% of the total design time performing verification work. One of the biggest challenges in producing mixed-signal SoCs is the development of appropriate test benches for analog and digital signals to control and observe internal IC signals in an automated design and verification process. As SoCs become more complex, verification becomes particularly important and challenging during the design cycle. OFDM design engineers integrate system-level simulation and verification design flows to perform early pre-production verification of complex IC designs in simulation and to connect these flows to production flows and manufacturing test procedures. This allows designers to use design intellectual property (IP) throughout the product lifecycle to achieve cost reduction goals. Three key strategies can make new OFDM products more predictable and profitable: ? Perform upfront verification ? Integrate design, verification, and manufacturing test ? Perform flawless RF simulation As products enter the final stages of development, the financial and schedule impact of design problems becomes increasingly apparent. Late integration issues and RFIC failure to meet performance targets are two of the most common causes of project delays and cost overruns, and can be addressed by integrating design, simulation, and test tools and methods. Several key success factors in developing IEEE 802.11 WLAN OFDM products include: ? Using OFDM communication model libraries to perform standard-compliant reference radio simulations. ? End-to-end simulation and verification of reference radios, where each system block was developed at different times. ? Performance checks were performed as each design team moved individual IP blocks to the next stage of development. ? Consistent measurement and analysis algorithms were used between simulation and physical instrumentation. ? Design and test stations across sites and units used the same performance measurement standards. 　 Figure 1. OFDM reference radio.

OFDM Product Development Lifecycle The wireless product development lifecycle begins with emerging system design specifications and radio architecture concepts and ends with profitable product mass production. Trade-offs made at the system level initially determine how to meet critical radio system specifications while achieving economical hardware implementation. System and circuit level simulations during the design phase provide an upfront prediction of whether the proposed radio architecture and associated reference radio will meet critical specifications. Simulations validate the reference radio against critical performance specifications for each OFDM standard using detailed component models, RFIC process design kits (PDKs), and RF board/module package parasitics. Faced with tremendous time-to-market pressures, design engineers must use efficient simulation tools to export design IP for reuse later in the lifecycle, thereby shortening design cycles. As radio complexity continues to increase, designers must validate performance as early in the lifecycle as possible. Validating product performance at every stage can help prevent risks by identifying critical issues early through simulation, rather than waiting until late in the development cycle to correct them, which can be time-consuming and costly. Integrated Design, Verification and Manufacturing Test - Connected Solutions Connected Solutions combine simulation software with test and measurement instruments to form flexible solutions that provide new design and verification capabilities that can be re-defined for different OFDM radio standards. The combination of simulation software and measurement instruments means that signals, measurements, algorithms and data can be shared to solve special problems that cannot be solved using EDA tools or instruments alone. New measurement utilities provided by the integration improve the design process and extend the capabilities of test instruments. In recent years, data communications (compared to voice) have made a great contribution to the establishment of wireless communication IC design flow. Connected Solutions has indeed met many flexible verification needs during the development of various WLAN standards in the past few years. The large number of WMAN OFDM subcarriers and higher output power levels (compared to WLAN) increase the difficulty of meeting the EVM specifications of power amplifiers/transmitters. Figure 2 shows the functional diagram and signal flow of the WMAN 802.16d radio designed and verified using ADS Advanced Design System and Agilent test instruments. The 802.16d test signal is downloaded from ADS to the Agilent E4438C ESG signal generator, then passed through the WMAN power amplifier device under test (DUT), and analyzed using the Agilent 89641A VSA vector signal analyzer to verify the EVM specification. Figures 3 and 4 show the results of measurement and simulation, respectively. The measurement values are displayed in the VSA display diagram, and the simulation results are displayed in the ADS Advanced Design System data display format. UWB OFDM frequency hopping makes it more difficult to design oscillators and receivers to meet the requirements of low phase noise and frequency hopping. Figure 5 is a circuit diagram that can generate UWB OFDM Mode 1 frequency hopping (3 hops); Figure 6 is the simulation result displayed by ADS2004A. 　 Figure 2. WMAN Connected Solutions test bench. 　 Figure 3. Measured WMAN (a) spectrum vs. time, (b) OFDM star diagram and symbol/error table, and (c) error vector spectrum vs. time. 　 Figure 4. Simulated WMAN (a) output star diagram and (b) output spectrum. 　 Figure 5. Simulated UWB Mode 1 frequency-hopping signal source. 　 Figure 6. Simulated UWB (a) time burst and spectrum, (b) symbol and LO frequency versus time, and (c) OFDM spectrum.

Move IP into production—Use the same algorithm from simulation to test In the integrated design and verification process, design IP for specific OFDM algorithms (developed in design tools such as C, ADS, MatLab(TM), HDL or VerilogA) can be reused. The ability to quickly move OFDM signals, data, and test vectors between simulation and test operations helps to increase the speed of debugging, determine the relationship between test results, and speed up the verification process. Similarly, IC tester machines can also share signals generated during simulation and shared during manufacturing testing to accelerate the development, characterization, and communication of production test plans. A well-known mobile phone manufacturer has reduced the communication between ATE automated test systems and tests from several months to two weeks.

Conclusion Integrating simulation tools with instruments allows the same analysis and verification algorithms to be shared throughout the OFDM design lifecycle. Early verification helps shorten design cycles because design problems can be detected from simulation before IC tape-out, which is absolutely necessary to achieve the ultra-low price target for OFDM radio chips. At each design stage, system certification tests can be performed according to emerging UWB and WMAN standards, and automated test benches can be generated and reused for verification at all important stages. Standardized verification processes apply to the entire lifecycle, so correlation can be sought for data obtained from the initial design to the production test stage. (The author of this article works in the Agilent EEsof EDA department)