To meet the needs of next-generation cellular phone designs for more features, multiple modes, and operating frequencies, engineers must look for ways to increase RF front-end integration. By adopting the latest integration solutions in the CMOS process, they found the answer to this challenge.

Consumer demand for more features in smaller, cheaper handsets and handheld devices, as well as high-speed wireless data services and multi-radio technology (multi-mode), is driving the growth of the mobile phone market. Advances in 2.5G networks (GPRS and CDMA 1xRTT) and 3G networks (UMTS/W-CDMA and cdma2000) enable high-speed wireless data services while adopting appropriate silicon processes and key building blocks such as integrated RF transceivers It can reduce the size and cost of mobile phones and handheld devices.

Figure 1: Common wireless subsystems common in multimodal platforms.

Mobile phone manufacturers integrate multiple technologies to deliver the best-selling solutions they offer to specific markets. For example, handsets that support GSM, GPRS, EDGE, and W-CDMA enable users to access multiple high-speed networks with a single device, which is the most basic application mode for multi-mode technology. GPS, Bluetooth, and no WLAN are other common wireless features that may be used in cellular phones and handheld devices.

Most multi-mode platforms today have multiple independent wireless subsystems built on the same platform, as shown in Figure 1. For example, multi-mode handsets supporting GSM, W-CDMA, Bluetooth, and GPS may be GSM/W-CDMA baseband, application processor, power management IC, memory IC, GSM radio transceiver, and discrete components that make up the W-CDMA transceiver. A single-chip Bluetooth system, a two-chip GPS subsystem, and a multi-mode RF front-end and passive components are built to support a variety of wireless functions.

In this mobile phone example, the single-chip Bluetooth and dual-chip GPS chipsets are connected to the application processor, and their respective drivers are embedded into the operating system that controls the entire platform. In addition, because they are separate "stand-alone" systems, the Bluetooth and GPS subsystems can work in parallel on a network call established by the handset.

While the "system-level" integration of multiple wireless features makes sense for some applications, it does not provide a solution optimized for the lowest cost or smallest form factor handsets. The final integration of multi-mode functions is performed at the component level of the RF front end, baseband and transceiver.

Integrated RF front-end system

Cellular phones based on the GSM standard and operating on time-division duplexing make their RF front-end systems only need to be implemented with switches. The simplest GSM phone works in single-band mode and requires only a single-pole double-throw switch, a receiver filter and matching network, and a power amplifier. However, the demand for more feature phones in today's market has raised the requirement for GSM phones to support up to four frequency bands. Therefore, a quad-band GSM handset may contain up to four transmit channels and four receive channels.

The transmit channel requires at least two amplifiers: one for the GSM850 and GSM900 bands and one for the DCS-1800 and PCS-1900 bands. Adding the filters and passive components required for the receive channel, there are a total of six channels, increasing design complexity and device count.

When a second wireless system, such as an 802.11b WLAN, is added to the same platform to form a multimode device, the challenge of providing more functionality without increasing design complexity or device cost is multifaceted. Since GSM and 802.11b operate in different frequency bands, their front-end devices cannot be shared. Therefore, both modes require a set of their own power amplifiers (PAs), switch networks, receiver matching circuits, and filters on the PCB. The best way to implement these front-end functions is to use pre-integrated modules and packages.

PA modules in the form of multi-chip modules containing power amplifiers and power control logic have been used in multi-band handsets and multi-mode 802.11a/g WLAN applications. Similarly, RF front-end modules including switch networks and receive filters are also available. In the future, as long as the market needs, there may be an RF front-end subsystem that integrates cellular and WLAN.

Baseband split

Many of today's cellular baseband chips are highly integrated CMOS system-on-chip (SoC), and either a chip with both digital and analog functions, or one chip for analog and digital baseband. The choice between these two options is influenced by a number of factors, including future integration options.

The two-chip solution is the most competitive integration because the analog baseband functions are isolated from the "pure" digital circuits that make up the digital baseband. In this way, digital basebands can be scaled down to smaller and smaller CMOS geometries in accordance with Moore's Law, which is difficult to implement with analog circuits.

Another advantage of this splitting scheme is the ability to integrate other digital CMOS platform devices (such as application processors, image processors, and memories) into one of the SoCs. With the advent of digital RF interfaces, such as the interfaces defined by the current DigRF standards organization, analog circuits may disappear completely in the baseband. This approach advocates the definition of a standard high-speed digital serial interface between the radio and the cellular baseband.

Similar efforts are being made by WLAN vendors as their JEDEC 61 organization defines standard serial interfaces. Once the digital serial interface is standardized, the cellular baseband functionality can be more cost-effectively integrated with complementary digital functions or other modes of wireless baseband circuitry.

RF transceiver integration in CMOS

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