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AUTHOR = Roman Robles

Posted: 16 Feb 2004 ?? ?Print Version ?Bookmark and Share

Keywords:wireless? dsp? modem? air interface? asic?

Speed, power crunch looms for wireless

Engineers developing products for the wireless market face a daunting challenge: provide all the complex enhancements required to support new air interfaces plus full support of legacy - and do it at a price, per equivalent voice channel, that's significantly lower than today's cost of supporting the legacy functions alone. But for infrastructure designers there is an additional demand: accommodate the certainty of change in an environment of evolving standards as 3G redefines itself and 4G starts to take its first strokes toward land.

Despite the proliferation of new air-interface standards, the market continues to demand support for the legacy standards and their enhancements in addition to providing new functionality offered by 3G and beyond. Besides traditional cellular telephony, additional opportunity lies in untethered networking markets such as Wi-Fi, Bluetooth, 802.15 and ultrawideband, among others.

The expectation has been that all the enhanced capabilities would come at a cost below that of today's products. DSP will drive this migration. DSP for the wireless market describes a technology, not a device category.

An often-quoted aphorism holds that "If the only tool you have is a hammer, you treat everything like a nail." Unfortunately, this is an accurate description of the rationale for solutions proposed by some vendors attempting to address baseband processing.

Recognizing the dichotomy in the two diverse categories of baseband, most designers prefer to use the right tool for the job rather than forcing a one-size-fits-all approach.

If we focus on DSSS air interfaces, such as W-CDMA and cdma2000, it is easy to divide baseband processing into two general categories of tasks. One subset of these processing tasks is irregular functions that call for complex decision. When designing modems for these markets, the irregular, complex functions have been and will probably continue to be served by DSPs.

The other subset covers more repetitive, compute-intensive, relatively simple jobs. Here, the chip-rate correlation tasks are the most prominent elements; and traditional DSPs have proved to be a poor fit. Hardware-based solutions have been used effectively here because of their inherently lower cost and lower power consumption. Unfortunately, there are demands from the market that complicate the application and reduce the utility of these devices in 3G wireless systems.

First, it is almost an oxymoron to refer to 3G standards - the air-interface options are numerous and they will continue to evolve for years. It is a challenge to deliver a system that can accommodate the evolution of even one of these standards with such inflexible approaches as provided by the hardware-based solutions. Should an OEM choose to deliver products for multiple air interfaces, it must decide whether to serve those markets by building multiple market-specific modems or by building a single modem with air-interface-specific blocks.

One problem with building market-specific modems is that the approach does not permit the OEM to take advantage of economies of scale and it multiplies the development costs almost linearly by the number of different modem designs delivered. On the other hand, designing a single modem with the potential to serve multiple air interfaces has the disadvantage of increasing the end-unit cost.

To address these deficiencies in the traditional approaches and permit a compelling business case for the OEM, reconfigurable solutions for these portions of the modem will be required. Moving closer to the antenna, signal-processing demands are also staggering. It is clear that commercially viable solutions require a different technology than the traditional DSP. This is where the vendor's toolbox has to have more than just the hammer described in that famous adage.

Silicon is the tool; software provides the solution. Tomorrow's vendors are being required to provide an ever larger portion of the basic system software. Customers can no longer afford to invest in the development of the base functions and must focus their limited resources in areas where product differentiation is possible.

The 3G market presents the solutions vendors with a large challenge. These air interfaces are more complex than the existing air interfaces, yet the market demands a cost per channel of less than one-tenth the cost per equivalent voice channel. Programmable DSP is the technology solution to meet these challenges.

Figure 2: A single ASIC/FPGA platform can provide an alternative to ASIC-oriented baseband for existing 2.5G/3G wireless networks.

- Roman Robles

Digital Technologies Operation Manager

RF and DSP Infrastructure Division

Semiconductor Products Sector

Motorola Inc.

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