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TI Baseband Interface Devices Complement Its DSPs to Provide Powerful, Highly Integrated DSP Solutions for Wireless Communications

Texas Instruments (TI) is well known for its technical and market leadership in digital signal processors (DSPs), the processing engines that have enabled the rapid growth of digital wireless communications, as well as many other types of advanced applications. TI's TMS320 DSPs are now used in wireless handsets and base stations throughout the world, supporting various transmission standards for cellular and Personal Communications Systems (PCS) telephony.

Equally important is TI's leadership in providing the integrated circuits that connect DSPs with the analog world of audio- and radio-frequency signals. Referred to as baseband interface circuits, these mixed-signal devices simplify wireless system design, offload processing functions from the DSP, and reduce board space, power consumption and system cost. Using advanced process technologies and system-level design expertise, TI has created baseband interface ICs that fully complement its DSPs, providing powerful, complex, highly integrated DSP Solutions for digital wireless communications.

These and other developments are the fruits of TI's organizational commitment to the wireless industry. For nearly a decade, TI has coordinated its digital and mixed-signal product developments to furnish solutions for wireless communications. The creation of TI's Wireless Business Unit in 1994 signaled the company's intent to establish and maintain leadership in this fast-growing market. Since that time, the group has focused TI's vast resources on the design, manufacture and support of highly integrated DSP Solutions for wireless base stations and handsets used with different standards around the world.

Digital wireless phones especially require an efficient analog subsystem to interface with the real world, resulting in increased demand for analog baseband solutions. In 1996, TI shipped more than 17 million analog baseband components into the digital wireless phone market.*

Functions of Baseband Interface ICs

Of the three major subsystems in a digital wireless system, two handle system power and radio frequency (RF) transmission and reception. The third major subsystem processes the communicated signal at baseband frequencies -- hearing-range frequencies and higher, up to but not including RF modulation. At the heart of the baseband subsystem is the DSP, responsible for speech encoding and decoding, error correction, channel encoding and decoding, equalization, demodulation and encryption of the digital signal.

Linking the DSP to the microphone and speaker is the voice coder/decoder (codec), and to the radio frequency subsystem is the RF codec. With far more functionality than these names would suggest, these two baseband interface devices provide a sophisticated front end to digital communications systems. Baseband interfaces not only link DSPs with users' voices and RF signals, they also carry a great deal of processing workload -- maintaining signal quality, extending talk time and improving overall system performance.

A voice codec performs the dual functions of sending a user's voice to the DSP and allowing the received signal to be heard through the earphone speaker. In transmitting, the codec receives analog input from the microphone, filters out non-voice frequencies, converts the signal from analog to digital, then passes it along to the DSP. In receiving, the device accepts the digital signal from the DSP, converts it to analog, passes the signal through filters that correct and smooth it, then sends it to an amplifier for output through the speaker. The overall performance of the filtering and amplification circuitry directly affects the voice quality.

By contrast, an RF codec performs the functions involved in preparing a signal for output from the RF subsystem and converting a received analog signal for further processing by the DSP. Before sending an outgoing signal, an RF codec converts the digital data stream from the DSP into the modulation format appropriate to the transmission standard. The codec also routes the I and Q modulation signals from the DSP to the RF subsystem, where they are translated from baseband to radio frequency. When the RF subsystem has received an incoming signal and translated it to baseband frequencies, the codec extracts the I and Q signals, converts the signal to digital, filters the signal to shape it, then sends it to the DSP.

Baseband Interface Integration

Baseband interface ICs must achieve a remarkable balance among requirements, providing at the same time flexibility, feature-rich functionality, ease of design, very low power consumption and economy. The devices must easily accommodate a variety of system configurations, so they must connect straightforwardly with microphones, speakers and other components.

Today, TI offers voice products that are either stand-alone or integrated with the RF codec. For example, the TCM4400 integrates both the voice and RF codecs on the same silicon for cellular handsets supporting the worldwide GSM standard. TI's voice-band audio processor (VBAP™) is popular with designers due to the design flexibility it offers, particularly in wireless telephone systems, but also in a variety of telecommunications, digital audio and multimedia products that require digital processing of voice signals. An Advanced RF Cellular Telephone Interface Circuit (ARCTIC™) provides an RF codec tailored for cellular applications supporting the North American IS-136 standard. Other standards TI plans to support with baseband interface products include the varied standards appearing in the newly-allocated PCS frequency range and IS-95, a North American standard based on spread-spectrum technology.

These baseband interface devices, used in conjunction with TMS320 DSPs and TI-supplied software, provide highly optimized DSP Solutions for digital baseband systems. TI supplies the VBAP and ARCTIC, together with a ROM-coded DSP, as a system solution. The TCM4400 (GSM analog baseband) is similarly designed to be used as a TI DSP Solution. The high level of integration in these baseband products reduces chip counts, saves board space and wiring, lowers costs and helps to simplify design for faster time to market.

In addition, by designing the codecs around its DSPs, TI is able to partition functions effectively to enhance system performance. For example, some filtering functions can be performed in either analog or digital circuitry. By offloading such filtering from the DSP, a codec can save processor MIPS that can be used for other types of functions to improve signal quality, increase compression ratios or add features.

Minimizing power consumption is also an important design constraint in wireless systems. In the TI baseband products, functional blocks in every device are designed to be powered on and off independently to help conserve battery power. In the RF codecs, the receive and transmit circuitry are powered-on only when the system is in receive or transmit mode respectively. Another design strategy takes advantage of the fact that when the system is in standby mode, most of the baseband subsystem can be powered down. With some standards, just a small part of the RF codec can be used along with the RF receiver to monitor the airwaves for a paging signal, indicating an incoming call. The resulting power savings are significant: for example, in idle mode, the GSM analog baseband chip consumes only 6% of the power it consumes while in active mode.

TI also recently announced a new family of power management devices that are designed to reduce design cycle times for cellular telephones and other wireless communications systems by integrating a complete baseband power supply and audio system on one chip. The first device in this family, designed the TPS9104, has three low dropout voltage regulators which provide all of the power supply needs for the baseband portion of a cellular phone. The regulators' low dropout of 200 milli-volts maximum, coupled with their micropower operation, extends the life of the batteries significantly by continuing to regulate the system voltage even as the battery voltage approaches the regulated system operating level. The regulators are capable of delivering 100 milliamps of output current.

Strategies such as these have made TI's baseband solutions among the most power-efficient in the industry, allowing manufacturers to use smaller batteries in their systems and helping to prolong standby and talk times for users. In addition, TI continues to improve its process technologies to minimize power dissipation. Analog CMOS technology, pioneered by TI and used today, is far more cost-effective and power-efficient than older bipolar technologies. TI's continual refinement of its analog CMOS process also lowers noise, so that components can achieve the same signal clarity while consuming less power.

Future Goals of Integration

By combining the voice and RF codecs on the same silicon, the TCM4400 illustrates one next-generation solution for higher integration in digital wireless systems. However, there are alternatives, some of which may offer advantages for different types of systems. The voice codec functions are more or less generic from one application to another, so they might be integrated along with power management functions, depending on design constraints. The TI Wireless Communications Business Unit is able to address these types of alternative solutions through the depth and breadth of its DSP solutions for wireless communications.

On the other hand, different transmission standards require different implementations of RF codecs. As a result, these codecs may be combined with some of the components of the RF subsystem, which also vary in implementation from standard to standard. The RF codec shares with the RF subsystem the process of modulating the baseband signal up through intermediate frequencies to the final radio frequency for transmission, and the opposite process for reception. Therefore, it makes sense for TI to look for ways to integrate many of the functions relating to these processes from both its RF codecs and its RF subsystem components, especially when integration can help eliminate some of the intermediate modulations and their resulting power consumption.

Another integration goal is the capability to integrate the entire baseband section -- the codecs and the DSP, along with program and data memory and the microcontroller that controls system-level operations. At present, the most cost-effective integration solution is to separate the analog and digital functions within process technologies that are optimized for analog and digital respectively. Nevertheless, the evolution towards single-chip digital cellular phones will require analog and digital integration. In preparation for these future requirements, TI offers processes that allow analog and digital integration to be realized at a very cost-effective and power-efficient level.

Enabling Future Wireless Communications

TI has shown its technical and organizational commitment to the wireless industry through the development of DSP Solutions for base stations and handsets supporting major standards around the world. In addition to the DSPs themselves, these solutions encompass the baseband interfaces that link the processors to the analog world of audio and RF signals. Along with TI's RF solutions and its power management ICs, these highly integrated baseband ICs provide complete system-level solutions for digital wireless communications. In the future, TI's products, process strengths and design expertise will continue to lead the way to even more highly integrated solutions, helping to enable the ubiquitous wireless communications of the next century.

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* Market figures are based on TI proprietary information