FOURTH-GENERATION (4G) CELLULAR SERVICE has begun to roll out in earnest. In contrast to characterizations by the mainstream media and some advertisements by leading service providers, however, it is by no means ubiquitousnor is it likely to be for some time. In fact, the 4G infrastructure rollout is going much the way of its third-generation (3G) predecessor. The buildup is consistent but slow, with the largest markets gaining the most attention. For manufacturers supplying infrastructure products, meeting new performance, cost, and size demands has created the need for new design approaches and techniques.
"It is probably going to take a decade for 4G to completely roll out," says Lance Wilson, Research Director, RF Components and Systems, ABI Research. Wilson points out that the major difference between 4G and 3G is frequency and, to some degree, a trend toward slightly higher power levels with 4G. In addition, 4G designs need to address the data requirements for orthogonal-frequency-division-multiplexing (OFDM) -type modulations.
Overall, though, Wilson doesn't see much in the way of technical challenges for this transition. "This is one case where the technology is on par with the development of the market," he states. "The only issue is getting the rollout speeded up. The rate of deployment is based on the ability of the market (subscriber base) to absorb 4G. It has become a chicken or the egg' problem for operators: Do you roll out the network first or get subscribers?"
To meet the needs of next-generation base stations, many manufacturers are working to streamline the RF signal chain. Scott Kulchycki, Marketing Manager for National Semiconductor's High-Speed Signal Path products, believes that the main focus for 4G base stations is on lower power consumption, size, and cost. "From the perspective of a signal-path silicon provider," Kulchycki notes, "this means that National Semiconductor needs to integrate the functionality of its high-performance amplifiers, analog-to-digital converters (ADCs), and clock solutions with filters and other RF components into as few chips as possible" (Fig. 1).
The RF integrated circuits (RFICs) used in 4G base stations also must be multi-bandoperating from 700 MHz to 2.7 GHz, notes James Wong, Product Marketing Manager High Frequency Products, Linear Technology Corp. In addition, they need to be wideband with very flat frequency response over a 300+- MHz bandwidth. To compensate for the increased noise floor of wider bandwidths, components for 4G infrastructure also need higher performance.
Paul Hart, Freescale Semiconductor's RF Systems Manager, RF Division, notes that 4G base stations are being designed to meet LTE's high data rates and complex 64-state quadrature-amplitude-modulation (64QAM) format. But they also must be backward compatible to GSM/EDGE, WCDMA, and CDMA networks while supporting existing hardware. As Hart sees it, the challenge is that "operators are requesting multi-standard, wide-bandwidth BTS equipment in increasingly smaller form factors."
When designers decrease power-amplifier (PA) form factors, Hart notes that they need to significantly increase PA efficiency to meet the thermal requirements required for high reliability (Fig. 2). Increased efficiency is even more important for newly emerging micro/picocell applications and active antenna systems, where board space and cooling are at an even greater premium. Marcus Wise, Vice President of Product Management at ANADIGICS, adds that the stringent linearity requirements of 4G base stations require optimal adjacent-channel-power-ratio (ACPR) specifications.
Despite the industry build-up to 4G, there are still some things that component vendors would like base-station designers to know. Kulchycki gets right to it: "At National, we believe the most cost-effective, flexible, and power-efficient solution to the challenge of 4G integration is direct RF sampling using ultra-high-speed, Gigasample-persecond ADCs in combination with high-performance, highly integrated clocking solutions." He believes that RF sampling is necessary to enable a software-defined radio (SDR), which will allow base-station designers to develop radio platforms that can be reconfigured in the field.
Cost seems to be another area that is subject to misconception. "We would like customers to think of cost reduction in terms of total ownership cost, which isn't always well understood," says Wong. He points out the cost savings that can be achieved when selecting a vendor that offers technical and design support. There also are cost advantages in selecting components that are known for high quality and reliability.
At ANADIGICS, Wise explains that the benefits of improved PA efficiency are not always obvious in base-station applications. "The reduction in current consumption can help reduce the size of heatsinks and power supplies, while proper thermal management of the power amplifier helps to eliminate the need for a fan to cool the device. Such benefits allow designers to reduce base-station size and cost as well as achieve more aesthetic designs."
Although the technology base may be in good shape to support the needs of 4G, some technical challenges still remain. "The greatest challenge is supporting the multi-carrier transceiver requirements over wider signal bandwidths," reports Jon Hall, Strategic Marketing Manager, High Speed ADCs, Analog Devices, Inc. "This aspect requires state-of-the-art noise-spectral-density levels and spurious-free dynamic range while reducing power and size to address the smaller-form-factor base stations."
Wong sums up the conundrum of lower power, smaller size, and higher performance: "The physical reality is that all three are diametrically opposing. No matter which of the three attributes one pushes, something else will give ... Design innovation, however, can make a huge difference in the way we design certain chip functions."
Technical challenges also remain for the PA in particular. Hart notes that wide OFDM signals with extremely fast rise and fall times stress the ruggedness of the PA components more so than previous-generation systems do. Wise points out that complex modulations drive a high peak-to-average power ratio, making it challenging for the PA to achieve the necessary linearity and efficiency.
The manufacturers of components for 4G base stations are looking for creative ways to address the age-old challenges of size, power, and cost, as well as the new ones that require different levels of performance from RF components. National has concentrated on developing new design approaches that enhance analog circuit performance at higher frequencies for 4G deployment. "This heightened design focus has led to the release of a new technology that enables RF sampling," says Kulchycki.
"To properly address the bandwidth challenge, we've needed to overhaul the manner in which RF power transistors are designed," reveals Freescale's Hart. His company has looked to enhanced computer-aided-design (CAD) flows for both active and passive components in the RF signal chain. "Additionally, novel low-frequency decoupling techniques have been embedded into the design of high-power RF products to greatly increase the video bandwidth to support full-band and multiband PA applications."
The firm's Airfast family of products will deliver significant efficiency improvements at all power levels over other silicon-based RF power transistorsachieving nearly 70% efficiency with over 200 W output power at 2.1 GHz. Additionally, Freescale continues to work on lower-power LDMOS and gallium-arsenide (GaAs) monolithic-microwave-integrated-circuit (MMIC) products for the femto, pico, and microcell markets. Recently, the company also introduced several products with enhanced video-bandwidth technology.
As mentioned, National recently announced the ADC12Dxx00RF family of RF-sampling analog-to-digital converters, which enable RF-sampling implementations of 4G base-station systems. To support the needs of smaller-sized legacy base stations, the company also released a quad-channel digital variable gain amplifier (DVGA), the model LMH6522.
ADI offers the AD6649 intermediate-frequency (IF) diversity receiver, which works with its ADL5202 digital variable gain amplifier (DVGA; see "Novel Topology Supports Wideband Passive Mixers," p. 90). ANADIGICS offers PAs for 4G small-cell base stations and high-power base-station-driver applications.
Linear Technology offers its LTC5569 dual mixer, which integrates on-chip balun transformers in a 4-x-4-mm footprint (Fig. 3). The mixer offers a +26.8-dBm input third-order intercept point and 2-dB conversion gain from 400 MHz to 4 GHz. The firm also targets 4G base stations with the LTC6946 integer-N frequency synthesizer with integrated voltage-controlled oscillator (VCO).
Going forward, National will continue to reduce component power and size while maintaining wideband performance for 4G systems. Linear is committed to "push the envelope of the available processes, design know-how, and innovative architecture to our products," says Wong. As for ANADIGICS, Wise reports that the company will focus on expanding its product portfolio: "Specifically, we intend to introduce power amplifiers that support additional frequency bands including band 5, band 7, and 700 MHz."
At Freescale, the focus remains firmly fixed on improved bandwidth, efficiency, and integration for 4G base-station products. Hart summarizes: "Ongoing technology improvements are needed to drive higher efficiency over the coming years. Additionally, increasing levels of integration, multiple PA stages, and increasingly wider bandwidth matching will be needed to keep pace with ever-expanding data rates and the push for smaller footprints and system-level cost reductions."