Users are increasingly demanding smaller, more affordable phones that offer faster service and advanced features in handheld devices. This growing demand is pushing the industry to develop innovative solutions while reducing costs to bring products to market quickly. As a result, manufacturers are re-evaluating their technologies to meet these challenges effectively.
Silicon-based technology and integrated key components like RF transceivers have significantly reduced both the size and cost of mobile devices. However, the strict performance requirements of cellular standards such as GSM previously limited the integration of RF transceivers, leading to the use of alternative technologies like SiGe BiCMOS or bipolar. Now, with the rising popularity of GSM/GPRS CMOS transceivers, the choice of CMOS RF technology has become more mature and reliable.
Despite the many advantages of CMOS RF transceiver design, engineers still face significant challenges when developing highly integrated multimode transceivers for WCDMA, EDGE, GPRS, and GSM standards. However, investing time and effort into CMOS-based RF development for multimode applications is well worth it, as the market response has been overwhelmingly positive.
**Multi-mode Development Trend**
To meet the needs of global operators and different cellular architectures, handset manufacturers are integrating multiple wireless technologies into a single device. This approach allows them to target specific markets with optimal features. For example, the number of handsets supporting EDGE has increased, and they remain backward compatible with GSM/GPRS services. In the future, 3G phones will support WCDMA alongside existing technologies like EDGE, GPRS, and GSM. Global roaming requires support for five frequency bands: GSM-850MHz, E-GSM-900MHz, DCS-1800MHz, PCS-1900MHz, and UMTS-2100MHz. Mobile phone designers must address all these requirements while delivering low-cost, compact devices that meet user expectations.
Silicon and module integration play a key role in enabling multi-mode functionality. Most multimode platforms combine separate wireless subsystems, such as a WCDMA transceiver alongside GSM/GPRS transceivers, along with RF front-end and passive components to support multiple modes and bands. This method is practical because GSM/GPRS and WCDMA operate on different reference clock frequencies (13MHz/26MHz and 19.2MHz, respectively). A typical GSM/GPRS transmitter structure cannot be directly used for WCDMA, so higher integration and innovative RF technology are needed to reduce component count and cost.
[Figure 1: Typical 3G multimode/multiband RF design with separate 3G and 2G wireless technologies]
[Figure 2: Block diagram of a single 4-band GSM/GPRS CMOS transceiver]
**Multi-RF Front-End System Integration Solution**
The RF front-end design for a 4-band GSM/GPRS system, as shown in Figure 2, uses a highly integrated single-chip CMOS transceiver. The antenna switch module connects the transmit and receive paths, while each GSM band includes a receiver SAW filter and matching circuitry. The transmit path typically requires two power amplifiers (PAs): one for the 850MHz and 900MHz bands, and another for the 1.8GHz and 1.9GHz bands.
Many integrated RF front-end modules help reduce component count and simplify design. These include PA modules with power control logic and transmitter modules that integrate PA and switching functions. At the receiving end, the design includes a SAW filter unit and an RF front-end module with a multiplexer and receive filter.
Compared to the simpler GSM/GPRS system in Figure 2, the more complex front-end design in Figure 1 supports both 2.5G and 3G RF signal transmission. Adding a multiplexer is necessary because WCDMA operates on frequency division duplexing, requiring simultaneous switching between transmitter and receiver. Despite this complexity, economic scaling continues to drive front-end component integration.
Today’s cellular base station architectures fall into two main categories: either splitting baseband functions into discrete analog and digital baseband chips or using monolithic CMOS SoC devices that integrate multiple functions. Choosing between these approaches depends on factors like long-term integration goals and cost efficiency.
While the monolithic approach saves PCB space, using separate analog and digital baseband chips is often preferred for better isolation of analog and digital circuits. This two-chip solution also allows the digital baseband to scale to smaller CMOS geometries while integrating other components like application processors and memory.
A growing trend in baseband architecture is eliminating the analog baseband chip to optimize digital baseband functionality and simplify the interface between wireless and baseband chips. This involves using a high-speed digital interface, which can be serial or parallel. Serial interfaces reduce pin count but increase transistor density, while parallel interfaces increase pin count and package size but are more silicon-efficient.
The DigRF Standards Body now defines a standard high-speed serial interface for 2.5G systems (see Figure 3). Supporting digital interfaces adds complexity to wireless design, as it requires analog-to-digital and digital-to-analog conversion, as well as interface logic for baseband communication. These functions are easier and more cost-effective to implement in CMOS technology.
[Figure 3: 2G DigRF interface between wireless and digital baseband]
[Figure 4: Single-chip 4-band CMOS transceiver block diagram]
**CMOS Advantages**
Implementing transceivers in CMOS technology offers several benefits:
- CMOS has a lower wafer cost compared to SiGe BiCMOS at similar process sizes.
- It allows fabrication across multiple manufacturers using standard processes.
- CMOS designs can scale down according to Moore's Law.
- It enables RF-enabled digital circuits, resulting in highly programmable, compact, and robust designs.
- CMOS transceivers can integrate additional functions like DigRF interfaces or digital baseband, forming a single RF and baseband component.
- CMOS has proven successful in producing a wide range of radios, including GSM/GPRS, WLAN, and Bluetooth.
- For multimode GSM/GPRS/EDGE/WCDMA handsets, choosing the right transceiver architecture is critical. High integration in monolithic CMOS transceivers—such as VCOs, frequency synthesizers, loop filters, and DCXO—is essential for ensuring good wireless performance by shielding critical functions from external noise.
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