CMOS transceiver developed for UMTS

By Rainer Koller, RF Concept Engineer at DICE Danube Integrated Circuit Engineering GmbH & Co. KG, and Irina Prjadeha, Product Marketing Manager, RF Engine at Infineon Technologies AG



Universal Mobile Telecommunications System (UMTS) has become a truly global standard, with multiple frequency bands specified to completely cover requirements in Europe, Asia, North America, and Japan. The UMTS standard calls for more features and faster transmission rates along with smaller and significantly cheaper components. This represents a real challenge for all suppliers.



Infineon's SMARTi 3G is a CMOS single-chip RF transceiver specifically developed for W-CDMA applications. An advanced technology process along with innovative packaging allows extremely compact design, leading to a 5mm x 5mm footprint.



SMARTi 3G supports all six frequency bands for UMTS specified within W-CDMA UTRA FDD for operation in different parts of the world. Thus the UMTS phones incorporating SMARTi 3G can be used in Europe, Asia, North America, and Japan. Specifically, with support of bands I (2,100MHz), IX (1,700MHz) and VI (800MHz), and advanced HSDPA performance.This article explains some of SMARTi 3G's design approaches to fulfill the challenging requirements coming with true multi-band capability such as adaptive RX baseband filter, fully-integrated

fractional-N PLL´s for TX, as well as for RX and multiple, flexibly programmable front-end control outputs.



INTRODUCTION

After the launch of the UMTS in Japan and Europe, the demand for advanced W-CDMA transceivers is tightened again. Due to the full duplex nature of UMTS and since support of all bands requires multiple TX and RX signal paths integrated onto one chip, crosstalk minimization becomes very challenging.



SMARTi 3G is a low-current, six-band, fully-integrated single-chip W-CDMA/HSDPA direct-conversion transceiver for frequency division duplex (FDD) operation. It is implemented in Infineon´s 130nm RF-CMOS process C11RF. Following the successful single-chip, fully-integrated transceiver "SMARTi U" [1], SMARTi 3G reaches highest level of integration and thus allows reducing full radio system design complexity, manufacturing cost and shortens time to market for the high-end UMTS solutions.



SYSTEM OVERVIEW

The design of SMARTi 3G comprises three zero-IF (intermediate frequency) RX paths, three direct-conversion TX paths and two fractional-N synthesizers. All of the IC's functionality is controlled by a flexible, multi-standard compatible programming interface based on the 3-wire-bus concept, giving both backwards-compatibility and full read/write access.



Each receiver path includes differential inputs, the second low-noise amplifier stages (2nd LNAs), down-converter with CMOS Gilbert-type mixers followed by low-noise buffer amplifiers, a calibrated, analog active 6th order baseband filter, accompanied with an additional 2nd order programmable notch filter at 2.7MHz. All DC offsets are compensated by internal circuitry.



Figure 1 depicts two of the receiver characteristics, the composite error vector magnitude (EVM) and SNR over various values of input power for receiver operating in UMTS band I (European application).



The transmitter paths include a 3rd order, Butterworth-type analog active baseband filter, three direct-upconverters, VGA stages with >85dB gain control range for each path and high-power output driver stages (up to 7dBm). Adaptive biasing in the VGA stages guarantees minimum current consumption over the full output power range. Each direct-conversion transmitter path incorporates a fully-differential, programmable input buffer to handle different baseband input signals. The additional 3rd order, Butterworth-type baseband filter removes unwanted signal content such as far-off noise or spurious emissions of the baseband DAC while not distorting the wanted signal. The typical case for TX output spectrum for the central frequency of 1.95GHz is shown in Figure 2.



Both RX and TX use fully-integrated fractional-N synthesizers with on-chip loop filters and reference resistors, reducing the external component count as far as possible. To cover all operating bands and provide additional frequency margins for process tolerances, the differential VCOs with a wide tuning range are used. Operation in UMTS bands V and VI is possible by activating an additional by-2-divider at the VCO RF outputs. The appropriate VCO band is selected by an internal alignment algorithm which is triggered each time the PLL is started or a new frequency is set. Simultaneously, further calibrations minimize all aberrations in the PLL, such as the loop filter corner frequency spread. The RX synthesizer features initial cell search speedup (ICSS) in addition to allow very fast frequency switching and settling in a limited frequency band. This minimizes the necessary time to establish the link to a base station.



OPERATION ENVIRONMENT

SMARTi 3G operates at 2.7V to 3V supply in an ambient temperature range of -30 to +85°C and can be configured for various reference oscillator frequencies as well as different BB interface parameters (e.g. I/Q common-mode voltage) to ensure maximum compatibility. Several power-down modes ensure minimum current consumption for all operating scenarios. If neither RX nor TX functionality is used, a dedicated operating state called "sleep mode" can be activated which lowers the typical current consumption to a few microamperes. This is especially useful if the UE is used for non-radio purposes such as a PDA functionality which is standard in advanced end-user terminals to maximize battery life. In the sleep mode, the total current consumption can be kept below 10µA even at high temperatures of up to 55°C. All register settings are kept in a special RAM so that the IC remembers all settings at next wake-up.



PACKAGING

To achieve the optimum performance and smaller size the careful consideration of the package layout along with the IC layout is required. The overall size of just 5mm x 5mm is possible due to the revolutionary packaging approach. The PG-WFSGA, has a ball grid pitch of 0.5mm and a maximum height of only 0.8mm which allows extremely slim handset design.







HSDPA CAPABILITY

New services offered by UMTS, such as video signal streaming are the drivers to dramatically increased data rates. The 3GPP W-CDMA release 5 defines the data rates as high as 14.4Mbps (theoretical limit) by using the HSDPA. The HSDPA calls for new shared downlink channel, high speed downlink shared channel (HS-DSCH), new modulation techniques, and link adaptation for fast and spectrally efficient transmission, which all lead to more complex HW implementation. For instance, on the transceiver's TX side, the addition of HSDPA affects the ACLR performance and maximum output power, issues that need a careful consideration in the design process. Higher order modulation used to achieve the higher data rate implies strong overall performance particularly stressing the receiver linearity. On the RX side, the EVM is directly influenced, as increased data rates increase the SNR requirements. The EVM shall not be worse than 17.5 percent when the base station is transmitting a composite signal using only QPSK modulation. The EVM shall not be worse than 12.5 percent when the base station is transmitting a composite signal that includes higher order modulation. For SMARTi 3G, the maximum EVM is 12.5 percent, enabling usage in HSDPA category 7/8 capable networks.



Different HSDPA categories, defined by modulation technique, number of codes used and data rates supported, are listed in the table below [2]







Maximum data rate is determined by (referred to different HSDPA categories) modulation scheme and code rate (TFRC: Transport Format and Resource Combination) as well as number of codes used.



UMTS: Global Standard with different frequencies.

Japan is the first country in the world that has introduced the UMTS to its mobile users. The first commercial UMTS network went life in 2001 and is operated by NTT DoCoMo.



While UMTS in Europe is currently in the introduction stage and in Japan after four years of existence, it is already a fully mature technology. Many Japanese handset manufacturers are willing to address the market outside of the domestic and are highly interested in entering European market. Looking back to the 2G environment, it would imply two development projects running in parallel -- one in order to supply for the domestic market and another for the overseas. 3G is relaxing this situation -- as the standard is the same all over the world, with only exceptions in the frequency bands specified.



SMARTi 3G offers a chance for handset manufacturers to target the global market. Supporting all UMTS bands currently specified by the ITU it provides an opportunity to save the development time and resource, and with only one product being capable to operate in any part of the world.



The UMTS frequencies in Europe, Asia and Japan are assigned exclusively for 3G and no collisions with other services occur. The situation is different in North America, where the UMTS spectrum allocated by ITU is already used for 2G (the 1,900MHz range) and satellite communications. Even though there are activities devoted to freeing the spectrum, in the nearest future, the UMTS in North America will still have to share spectrum with existing 2G services. This posses additional challenges for the RF designers, and has been sucesfully solved for SMARTi 3G using an additional notch interstage filter which is activated in a "hybrid filter mode." An additional notch filter can be activated by software programming to fulfil the UMTS requirements for Bands II and III.







Supportingtotal of 6 bands SMARTi 3G can be configured to the 3 physical bands at the time: Low, Middle and High according to the table below:







The IC's operating band can be set separately for RX and TX by the three-wire bus using the appropriate combination of total six-band select / front-end control output pins.



SMARTi 3G provides the designers full flexibility to allow the development of multiple solutions from the same basic radio design. High-end triple band solutions as well as single band are fully supported by SMARTi 3G.



An example of a typical triple-band application is given in the Figure 5.



The typical PCB form factor for a triple-band UMTS solution using SMARTi 3G can be as small as 370mm˛ with only 74 components. Growing front-end complexity caused by multiband and multimode operation makes an efficient control for external components such as LNAs, PAs, switches, etc. necessary. Therefore, a very flexible, software-programmable front-end control is part of SMARTi 3G which handles these demands by event-triggered state switching of six dedicated output pins called RXBAND1..3 and TXBAND1..3. For future solutions, additional three general purpose outputs (GPO) can be switched by programming.



The basic front-end control feature is providing three arbitrarily programmed sets of output states for each band. With this feature e.g. the signal path in the front-end components can be selected. This feature is extended by customizable switching delays for the RXBAND output pins. The TXBAND outputs can additionally programmed to go to "low" output state when the TX path is turned off and returned to the selected set at turn-on.



Typical LNA modules available today feature a gain step for better power dynamics coverage. Since the point where the LNA is switched between low- and high-gain mode is dependent on the RX gain strategy used, the RXBAND2 and RXBAND3 pins can be independently programmed to output a signal corresponding to the actual RX gain setting.



MULTIMODE APPS

SMARTi 3G is able to serve as front-end control center with minimum current consumption if it is used in a multi-mode environment and active alternative transceiver (e.g. GSM transceiver). This is done by activating a special mode which turns off RX and TX and allows direct control of the TXBAND1..2 and RXBAND1..2 pins.



RX ICSS

After initial user equipment turn-on, the connection to the best base station should be established as fast as possible. Unfortunately, the optimum RX frequency is unknown and therefore has to be found by reception of control information and "scanning" all possible bearer frequencies. Since this frequency range is usually very limited due to network provider licensing issues, an extremely fast synthesizer settling after frequency switching can be achieve by omitting the calibration routines. This special RX PLL function can be activated by programming, and it shortens the settling time after each frequency jump by a typical value of eight which is a direct contribution to end-user comfort.



CONCLUSION

With UMTS market constantly growing, driving the need for s smaller and much flexible devices, SMARTi 3G receives more and more attention from mobile phone manufacturers and platform providers. Support of all currently defined UMTS bands, excellent technical characteristics combined with an extremely low overall size are understood as its key success factors. The cost effective and highly innovative SMARTi 3G is expected to gain a significant market share and continue the undoubted success of its predecessors.



[1] W. Thomann et al., "A Single-Chip 75-GHz/0.35-µm SiGe BiCMOS W-CDMA Homodyne Transceiver for UMTS Mobiles," IEEE Proc. RFIC 2004, Fort Worth, USA, pp. 69-72, June 2004.

[2] 3GPP TS 25.306: "UE Radio Access Capabilities"