[Product Innovation]
One-Chip Baseband Processor Lifts CDMA To Integration Nirvana
Having integrated all of the mixed-signal and digital processing, the processor finally makes a two-chip CDMA solution possible.
As CDMA technology continues to wind its own way through the cellular diaspora, it is as susceptible to the pressures of increased integration as any of its counterparts. The reasons are myriad, but they mostly revolve around the almost universal needs to conserve board space, decrease power consumption, improve signal integrity, reduce cost, and speed time-to-market.
Well aware of the advantages of high integration, LSI Logic Inc. has developed the first completely integrated baseband processor for CDMA applications. The chip, dubbed the CBP 3.0, is fully IS-95b compliant. It also includes all the mixed-signal, modem, digital processing, and voice-codec functions on a single slab of silicon.
Based on the company's G11 CMOS process, which features 0.18-µm (L-effective) line widths and an operating voltage of 1.8 V, the CBP 3.0 incorporates two OakDSPCores—one for modem assistance, and one for vocoder operations. Meanwhile, an ARM7TDMI processor acts as a central control processor. The device is supported by the company's CoreWare building blocks, which allow the quick addition of a wide range of telephone/computing capabilities.
The end result is a turnkey baseband solution with standard RF/IF interfacing that simplifies the selection of the remaining components required for a complete device. Depending on those components and the mode of operation, the CBP 3.0 permits development of CDMA cellular telephones with up to 300 hours of standby time and two hours of talk time. The device comes complete on a 280-pin CSP that measures 16 by 16 mm.
Originally developed by Qualcomm for military applications, code-division multiple-access (CDMA) technology has only flourished in the last three or four years. Popular in Asia (particularly in Japan and Korea, as well as China), Latin America, and now North America, CDMA tripled its subscriber base between 1997 and 1998. And, it's expected to surpass the 50 million mark this year. While Global Systeme Mobile (GSM) technology has a much higher numeric penetration, CDMA continues to hold the advantage in terms of current and predicted percentage growth— and for good reason.
According to the Telecommunications Research and Action Center (TRAC), Washington, D.C., CDMA outperforms other digital and analog technologies on every front, including signal quality, security, power consumption, and reliability. Analog came out ahead overall for availability.
CDMA's key attributes are its flexibility, system capacity, quality of service, and implementation economies. In terms of flexibility, the IS-95 standard allows service providers to allocate data in 8-kbit/s increments within the CDMA channel's 1.25-MHz bandwidth, based on how providers configure software download to already installed network controllers.
This enables operators to implement return data speeds at rates much lower than 64 kbits/s, extending the handset's battery life. An improved coding-gain/modulation scheme, voice activity, three-part sectorization, and spectrum reuse in every cell and all sectors give CDMA the advantage in capacity, as well.
The quality-of-service advantage is significant. It results almost entirely from CDMA's spread-spectrum modulation scheme. Where other narrowband systems can suffer severe degradation at the hands of multipath fading, CDMA actually benefits.
Using RAKE receivers and other signal-processing techniques, each mobile station selects the three strongest multipath signals and coherently combines them to produce an enhanced signal. Also, soft handoff eliminates the ping-pong effect mobile users suffer when passing over cell borders. As for economics, CDMA requires fewer cell sites and no costly frequency reuse pattern. Additionally, the average transmitted power is lower, thereby extending the average battery life.
One of CDMA's major advantages, though, is actually a result of its technical merits. CDMA has been selected as the scheme of choice for the greatly anticipated third generation (3G) of mobile telephones. The 3G cdma2000 uses a CDMA air interface, based on the existing IS-95B standard, to provide wireline-quality voice service and high-speed data services at rates ranging from 144 kbits/s for mobile users to 2 Mbits/s for stationary users.
Despite the activity surrounding CDMA, the fact that GSM has by far the larger portion of the subscriber pie cannot be ignored. Many RF and digital IC companies still devote the bulk of their efforts toward cementing their standing in that area, though they are now beginning to set their sights on the CDMA alternative.
Regardless of the modulation scheme, the ideal situation from a purely integration point of view (disregarding the loss in flexibility it would entail) would be to have the complete RF and baseband processing sections on a single substrate. Unfortunately, differences between the processes required to implement the two make a single-chip solution more of a goal than a realistic alternative right now.
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