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Digital control fits power amps for 3G

Posted: 15 Apr 2001 ?? ?Print Version ?Bookmark and Share

Keywords:w-cdma? bts? dcsp? edge? acpce?

Third-generation (3G) wireless communications standards, including W-CDMA, cdma2000 and EDGE, are beginning to reshape the mobile communications market. Internet and corporate intranet services, such as real-time data feeds and video conferencing, will soon become standard offerings over wireless networks. Delivering these rich new consumer products will require an overhaul of the existing wireless infrastructure, including the network's base transceiver station (BTS) infrastructure.

Complex to build and sensitive by nature, analog amplifiers are the most expensive components in a wireless basestation. They require lengthy factory calibration before deployment and ongoing field maintenance to ensure transmitter compliance with communications regulations and continued operation without performance degradation due to component and environmental changes. Conversely, digitally controlled power amplifiers are less expensive to build and they do not require time-consuming manual calibration. By design, these digitally controlled power amplifiers automatically adapt to, and compensate for, each amplifier system's unique distortion pattern through firmware. This eliminates tuning and alignment processes during manufacturing and the need for ongoing realignment in the field.

System challenges

In the design of wireless systems, distortion, interference and noise are everyday challenges faced by system developers. Digitally controlled power amplifiers and their control software overcome these impediments to system efficiency by employing a process called digital adaptive pre-distortion. The amplifiers enable the design of linear-response transceivers exhibiting near-perfect fidelity of transmitted power spectral densities for single and multicarrier signals up to 10MHz. Besides offering digital adaptive pre-distortion for wideband amplifier linearization, digitally controlled power amplifiers also provide digital correction for analog modulation distortion and digital soft pre-compression for efficiency management.

These approaches precondition the digital signal to dynamically eliminate analog impairments, such as wideband group delay, thermal and envelope memory dependencies, and bulk AM-AM and AM-PM transistor effects that cause in-channel distortion and adjacent channel interference.

PMC-Sierra's digitally controlled pre-distortion management software allows system operators the flexibility and control to maximize system performance and efficiency. Using a digital correction signal processor (DCSP) chip, an inexpensive and simple single-channel Class AB power amplifier can be transformed into a digitally controlled high-efficiency wideband power amplifier unit that is multicarrier and multi-bearer capable. This architecture replaces the expensive, low-efficiency, feed-forward-based multicarrier power amplifiers commonly used in many 3G BTS designs.

The flexibility offered by the Paladin-10 chip and software solution gives system designers the freedom to adopt new radio resource architectures. This clears a pathway to the design of wireless basestations supporting all emerging 3G wireless standards while extending system performance to include advanced base transceiver station features such as smart antennas and hot-swap redundancy architectures.

The solution

The DCSP chip is the heart of the digitally controlled amplifier solution. System performance is achieved through very high-speed complex operations provided by the proprietary application-specific DSP entities residing within the DCSP device. The rate of the digital signal processing performed by the chip dwarfs the capability of every general-purpose programmable DSP available today. The DCSP chip eliminates distortion in the transmitter chain and power amplifier using fully digital methods, permitting designers to replace many expensive and difficult-to-manufacture analog IF and RF subsystems, which are often required to control distortion and aggregate signals in many existing BTS designs. Paired with an adaptive-control processor and compensation estimator (ACPCE), the DCSP provides a 10MHz wideband distortion elimination and linearization solution for W-CDMA, cdma2000 and GSM/EDGE basestations.

The ACPCE firmware is the brains of the digitally controlled amplifier solution. In order to correct for the nonlinear distortion of the transmitter chain and power amplifier, the signal-plus-distortion output of the amplifier must be sampled and decomposed into its fundamental components. The ACPCE firmware does this by using the sampled observations forwarded to it from the DCSP to compute predictive correction-compensation parameters for the DCSP. The DCSP chip and ACPCE firmware can be controlled from the central control processor radio-resource management entity via an industry-standard SPI serial command interface.

? Kevin Huscroft

Chief Technology Officer

Laurie Wallace

Director of Marketing

PMC-Sierra Inc.

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