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Design a single-chip, six-band UMTS transceiver for global connectivity

Posted: 01 May 2007 ?? ?Print Version ?Bookmark and Share

Keywords:UMTS standard? UMTS frequency band? UMTS phone? multiband?

By Irina Prjadeha, Infineon Technologies AG
?????Dr. Rainer Koller, DICE GmbH & Co. KG

Universal Mobile Telecommunications System (UMTS) is one of the technologies emerging for the third-generation (3G) mobile communications system that is being developed as a successor to the existing GSM system. UMTS has become a truly global standard, with different frequency bands specified to completely cover requirements in Europe, Asia, North America and Japan. To take advantage of this technology, the designers of UMTS mobile phones are increasingly calling for multi-band support to allow global roaming, more features and faster data transmission rates, along with smaller and significantly less expensive components. This presents a real challenge for all communications semiconductor suppliers.

To meet this challenge, chip designers are focusing on three major features when designing their next-generation RF transceivers: small size, low power consumption and support for multi-frequency bands. Creating such a solution requires integrating a true multi-band capability into a single chip, including an adaptive receive baseband filter, fully integrated fractional-N PLLs for both transmission and reception, and multiple, flexibly programmable front-end control. Additionally, the manufacturing process for the chip should be one that results in a footprint that meets the extremely compact cellular phone form factor requirements.

The ideal solution is a single-chip, low-power CMOS RF transceiver that supports all the UMTS frequency bands currently specified within the W-CDMA-based UTRA FDD (UMTS Terrestrial Radio Access Frequency Duplex Division) access scheme. UMTS phones incorporating such a chip would be able to be used in Europe, Asia, North America and Japan, ensuring easy access in the areas of the world currently most widely served by cellular services.

UMTS: Global standard with different frequencies
While it is intended to be a global standard, the different frequency bands specified by the UMTS can make it difficult to predict support for which physical band will be required by the market over the next few years (Table 1 shows the different UMTS frequency bands and the regions in which they most widely used). Currently, the UMTS frequency band combination for which cellphone designers most often ask is Bands I, II and V, which allows worldwide roaming. These combinations may change, however, due to market demands and the expectations and preferences of mobile network operators and end-users.

Table 1: Different UMTS frequency bands

With this high degree of uncertainty as to long-term requirements, multi-band support, offering optimum design flexibility, is a key success factor for semiconductor suppliers. A single-chip multi-band transceiver supporting most UMTS bands currently specified by the ITU, and adapted to deal with the unique situation in North America (where the 1900MHz range spectrum allocated by the ITU is already used for 2G mobile network and satellite communications), helps to cut design time and resources, allowing handset manufacturers to offer a single device capable of operating anywhere in the world.

System overview
As exemplified by Infineon Technologies' SMARTi 3G, it is possible to achieve true multi-band operation with a single-chip transceiver IC that supports all of the currently requested UMTS bands and offers up to three physical bands for flexible multi-band applications. The design of such a transceiver comprises zero-IF (intermediate frequency) RX (receive) paths, direct-conversion TX (transmit) paths and fractional-N synthesizers. It also includes a notch interstage filter that is activated in a "hybrid filter mode" to allow adaptation to the unique North American band allocation, and allows an additional notch filter to be software-activated to meet the specific UMTS requirements for Bands II and III.

The direct conversion receiver offers a fully differential signal path for each band. The signal filtering in the receive chain is accomplished by a calibrated, active BB (baseband) filter, accompanied by an additional second-order programmable notch filter at 2.7MHz. All DC offsets are compensated by internal circuitry.

The receiver in general offers a very linear design, and has outstanding sensitivity, one of the key parameters. The RX PGC (programmable gain control) is also highly linear, so only a very few calibration points are needed in the handset production cycle, which directly translates into time savings and lower cost of ownership.

Figure 1 illustrates two key receiver characteristics, the composite Error Vector Magnitude (EVM) and the SNR, over various values of input power for a receiver operating in UMTS Band I.

Figure 1: Receiver performance characteristics, UMTS band I. Composite EVM and SNR over input power. (Click to view full image)

The transmitter paths include a third-order Butterworth-type active BB filter, direct up-converters, VGA stages and high-power output driver stages. 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. An additional third-order Butterworth-type BB filter removes unwanted signal content, such as far-off noise or spurious emissions of the BB DAC, while not distorting the wanted signal.

Overall, the TX is described by its high linearity, leading to an outstanding Noise Figure characteristic, and excellent output power values. Figure 2 compares the power of the FDD signal (blue line) in defined offset from the carrier (for Band I, TX 1950MHz) to the spectral mask (red line) specified by the 3GPP (3rd Generation Partnership Project). This figure illustrates the performance of a transceiver that has enough margin for the complete system to fulfill the 3GPP requirements.

Figure 2: Transmitter spectrum mask compared to the 3GPP specification. (Click to view full image)

Both RX and TX use fully integrated fractional-N synthesizers with on-chip loop filters and reference resistors, reducing the external component count as much as possible. To cover all operating bands and provide additional frequency margins for process tolerances, 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 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.

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 backward-compatibility and full read/write access. A standard analog interface implementation offers yet another benefit for mobile handset manufacturers and platform providers by enabling interoperability with multiple 3G baseband processors. This permits selecting and matching different products from different vendors to achieve the very best component combination.

Triple-band design considerations
To ensure support for all the UMTS bands and a number of band combinations a designer may desire, three physical bands, Low, Middle and High, can be configured at the same time. The IC's operating bands can be set separately for RX and TX by a 3-wire bus using the appropriate combination of six-band select/front-end control output pins. An example of a typical triple-band application is shown in Figure 3.

Figure 3: Typical triple-band application example. (Click to view full image)

The typical PCB form factor for a triple-band UMTS solution can be as small as 370mm? and require only 74 components. This represents a reduction of 50 percent compared to other solutions that only support a single band.

Growing front-end complexity caused by multi-band and multi-mode operation necessitates an efficient control for external components, such as power amplifiers and switches. This need can be addressed by a highly flexible, software-programmable front-end control that handles these demands by event-triggered state switching of six dedicated RXBAND and TXBAND output pins.

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

HSDPA capability
New services offered by UMTS, such as high-quality video signal streaming, fast downloading of music content and interactive gaming, are the drivers of significantly increased data rates. Release 5 of the 3GPP W-CDMA standard defines data rates as high as 14.4Mbps by using the High Speed Downlink Packet Access (HSDPA) technology. UMTS networks upgraded with HSDPA are currently deployed across Europe, with data speeds between 1.8-3.6Mbps. HSDPA category 8 (with a speed of 7.2Mbps) is expected to become the mainstream by the end of 2007. PC data cards are seen as the first application in which HSDPA will be supported, with handsets gaining more and more importance as the technology migrates to them. While the networks are still in the development phase, RF and BB cellular components supporting HSDPA category 8, including Infineon's SMARTi 3G RF transceiver, are already available.

HSDPA calls for a new shared downlink channel, High Speed Downlink Shared Channel (HS-DSCH), new modulation techniques and link adaptation for fast and spectrally efficient transmission, all of which lead to more complex hardware implementations. For instance, on the transceiver's TX side, the addition of HSDPA affects the ACLR (Adjacent Channel Leakage Ratio) performance and maximum output power, issues that require careful consideration during the design process. The higher order modulation used to achieve the higher data rate implies strong overall performance that can particularly stress the receiver linearity. On the RX side, the EVM is directly influenced by HSDPA, as increased data rates increase the SNR requirements.

Different HSDPA categories, defined by modulation technique, number of codes used and data rates supported, are listed in Table 2.

Table 2: HSDPA categories1

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

CMOS as preferred UMTS transceiver process
Although multi-band capability and HSDPA support are clear transceiver trends targeted by all transceiver vendors, a no-way-less-important trend is the emergence of CMOS technology, which is becoming a technology of choice for transceiver design implementation, replacing the SiGe and BiCMOS processes. Almost all of the leading transceiver vendors are now using CMOS in their current designs.

CMOS offers a direct cost advantage over such traditional RF technologies as BiCMOS and SiGe because it requires fewer masks and processing steps. CMOS technology features faster transistors and low size area, resulting in a very small design footprint. Such cost reductions, along with enhanced integration possibilities, are among the forces driving the move of next-generation RF designs to CMOS.

As exemplified by the Infineon SMARTi 3G transceiver, CMOS allows for a high overall percentage of digital logic to be implemented. This leads to flexibility in integration of various compensation techniques, such as DC offset and filter calibration, and also makes possible such features as a short locking time and fast frequency settling for the fractional-N PLL. Some specific features, such as RAM readback that allows keeping the pre-programmed register settings in a special RAM in a sleep mode so that the IC remembers them at next wake-up, are only and exclusively possible with a CMOS process.

Because CMOS is used by semiconductor manufacturers in many other products, the same generic technology process and production line are generally available for monolithic integration of the digital, RF and mixed-signal elements of a mobile phone on the same die, leading to even smaller sizes and a much higher integration level.

According to research firm iSuppli Corp., RF CMOS technology will enjoy increasing growth over the next few years, reaching around 40 percent of the market share by 2009.?

Conclusion
The need for an increasing number of multi-band handsets is driving the demand for high-performance, cost-competitive RF transceivers that are capable of supporting different frequency bands. A truly multi-band single-chip transceiver, such as Infineon's SMARTi 3G, provides designers full flexibility in developing multiple mobile handset solutions from the same RF platform design. Due to the high re-use factor, systems based on such a transceiver can be easily adapted to the various band-combination needs of future mobile applications with a minimum of software and hardware effort. Full compliance with HSDPA requirements for higher data rates gives system designers an easy migration path to next-generation UMTS handsets based on the same architecture, further saving costs and design time.

References:
1 3GPP TS25.306, "UE Radio Access Capabilities Definition," version 5.8.0.
? "CMOS use rises in mobile handsets, says iSuppli," EETimes, 09/07/2005.

About the authors
Irina Prjadeha
is in product marketing for the RF Engine Business Unit Communications Solutions for Infineon Technologies AG.

Dr. Rainer Koller is a member of RF Concept Engineering at DICE (Danube Integrated Circuit Engineering) GmbH & Co. KG.




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