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Shrinking Cells Impact Base-Station Design

Aug. 12, 2011
As the telecommunications industry truly begins to explore the use of femto- and picocells, the way we think of cellular base stations is starting to shift.

Cellular Communications continues to experience a worldwide growth trajectory. Consequently, the need for base stations also continues to grow. According to Strategy Analytics, Inc., traditional macro and micro-cell base-station deployment will peak at slightly over 1 million base stations per year in 2012 before falling to slightly less than 1 million in 2014. However, the company sees explosive growth for lower-capacity picocell and femtocell base stations. It estimates that they will exceed 7.8 million units deployed per year by 2014.1 Clearly, the way that cellular networks look and operate is changing. Operators are gaining interest in the use of picocells, remote radio heads, femtocells, and Wi-Fi offload techniques. What will this trend toward smaller cells mean to the wireless-infrastructure market? And how will base stations evolve and adapt to incorporate micro-cells within the network topology?

The year 2011 is already off to a great start in terms of basestation deployments. Neil Shah, Analyst, Global Wireless Practice, Strategy Analytics, anticipates that the total base-station market for CDMA, HSPA, HSPA+, and LTE standards will grow by approximately 70% this year. Markets across the Asia-Pacific region and Africa are witnessing unprecedented cellular growth. And those in China, India, and Nigeria are following closely behind. Of course, all growth is not equal. Shah notes that operators in these markets have been expanding second-generation (2G) cellular networks beyond urban and suburban markets to cover the rural population. In contrast, third-generation (3G) cellular networks and coverage are currently being rolled out to cover the densely populated metropolitan areas and towns.

In developed markets like North America, Western Europe, and Japan, however, 3G coverage has reached saturation. There, the ongoing fourth-generation (4G) deployment will be supported by an extensive cellular network comprising a combination of macro-, micro-, pico-, and femtocell solutions. These smaller cells will not necessarily be Long Term Evolution (LTE), however. In North America and Europe in particular, operators already have more customer demand than available capacity, notes Tom Flanagan, Texas Instruments' Wireless Base Station Director of Technical Strategy. So they want to quickly roll out smaller cell products in Wireless Code Division Multiple Access (WCDMA) that can migrate to LTE when its footprint is finally in place.

"The major challenges that operators and infrastructure vendors are facing is the data deluge brought about by the exponential growth in data-centric devices from smartphones to tablets globally," observes Shah. For infrastructure and chipset vendors, he notes, "This means designing systems and RF transceivers in order to handle this level of traffic as well as tackle coverage, power, heat dissipation, and cost versus performance challenges to ensure higher efficiency, sufficient capacity, data rate, and quality of service (QoS)."

It may sound daunting, but the design challenges for traditional, or macro, base stations are mostly solved. Only ongoing design upgrades and 3G/4G high-volume buildouts remain. Some of the latest design trends include a focus on usability and features that improve QoS. Overall, the latest trends might be summed up in two questions: "How do I run multiple standards?" and "How do I integrate my small cells?" Christophe Cugge, NXP Semiconductors' Marketing Manager Base Station PA, enumerates the latest requirements: more bandwidth, more power (but lower power consumption), lower cost, and smaller size. In the longer term, Cugge sees the goal as multiband/multi-standard base stations and new network architectures, such as those that add low-power base stations at the edge of macrocells to increase capacity and QoS.

TI's Flanagan also sees a multi-standard trend, noting, "In the last year, we are seeing many more requests for a graphical user interface (GUI) to run multiple standards simultaneously." He believes this trend is being driven by operators struggling with how to migrate from 3G to 4G. Because not all customers want to upgrade at once, operators must manage a mix of new and old technologies. Over at AWR Corp., Dr. Gent Paparisto, Wireless Systems Specialist, expects to see the multi-standard trend lead to software-configurable base stations. He says this would "not only extend the life of the base stations, but significantly reduce the network operating costs."

The growing interest in small-cell deployments is driving a change in the way people are thinking about their macrocell designs. This, combined with a more "green" attitude, also is focusing attention on reducing power consumption across all cell sizes, says Charles F. Sturman, Vice President of Marketing at Cognovo. As networks move from a macrocell architecture to a heterogeneous mix of access points of varying sizes, Sturman expects operators to move toward a "single base-station architecture that has to be scalable from hundreds of milliwatts and $500 up to a macro level supporting ~30 km and hundreds of users."

In addition, a rise in "smart" techniquesincluding self-optimizing networks (SONs)is expected by Nick Carter, Tim Beard, and Stamatis Georgoulis, Product Managers for Aeroflex's Test Mobile product range. TI's Flanagan sees multiple-input, multiple-output (MIMO) becoming a differentiator as well as more sophisticated scheduling algorithms and beamsteering, which can maximize the use of the antenna and available spectrum.

Such trends will surely put the squeeze on the components that are used to build the infrastructure. "OEMs must offer scalable, reconfigurable, and cost-/power-efficient solutions to meet current and future operator goals, observes AWR's Paparisto. "Any new base-station RF designs should support a larger number of frequency bands, spanning 2G, 3G, and 4G networks, and even allowing for future expansion."

Even though femtocells support a comparably small number of cellular connections, they still need to handle all of the operating modes and cellular bands offered by the network provider. Components therefore need to be low cost, offer high performance, and operate across multiple cellular bands. Femtocells also will require techniques like output-power flexibility to avoid interference. AWR addresses these concerns in its softwareparticularly with its Visual System Simulator (VSS) RF Architect for system-design-phase exploration and nonlinear co-simulation with Microwave Office for detailed module design and verification.2

David Schnaufer, Senior Manager of Strategic Marketing, Wireless Products Business Unit, RFMD, adds that the design of remote radio heads and picocells dramatically changes expected environmental conditions: "While the base stations are temperature-controlled environments, these smaller cells are exposed to the elements and are required to operate at temperatures to +105C, which is 20 above the usual +85C...Semiconductor suppliers such as RFMD must meet more stringent technical and environmental requirements to compete in this new 3G/4G cellular environment. This results in the development of more highly integrated components with smaller footprints, lower current consumption, and increased temperature compensation."

For instance, the RFDA series of digital variable-gain amplifiers (DVGAs) combines indium-gallium-phosphide (InGaP) heterojunction-bipolar-transistor (HBT) and silicon-on-insulator (SOI) technologies in a laminate-based, multichip module for operation from 50 to 4000 MHz (Fig. 1). The DVGA integration path also supports future market trends toward smaller base-station sizes, microcells, and remote radio heads for new infrastructure deployments. The company also is offering new components using gallium-nitride (GaN) technology. "This new technology offers lower power consumption and high-temperature operation for 3G/4G smaller base-station-architecture designs," observes Schnaufer.

1. This block diagram shows a typical wireless-communications network-infrastructure transceiver. (Courtesy of RFMD; red components are part of the firm's portfolio.)

Cugge lists some of the remaining challenges: designing RF transistors with higher power and wider bandwidths while maintaining or reducing cost, power consumption, and size. He explains that NXP's Gen8 laterally diffused metal-oxide-semiconductor (LDMOS) technology has been designed with these challenges in mind.

TI is answering these challenges with its KeyStone line, which handles applications from the enterprise to high-end macrocells. All of the devices are based on a single architecture. "This enables real scalability," notes Flanagan. "It allows the manufacturer to write one suite of software and have the hardware automatically detect what is being used and scale the software in response." Flanagan reports that this product line allows operators to run multiple standards.

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AWR's most recent software version, AWR 2011, introduces circuit-envelope simulation to help design envelope-tracking power amplifiers (PAs). The software accounts for the PA's memory effects while providing measurements of power-added efficiency (Fig. 2).

With a nod to the future of the base-station air interface, Aeroflex has added support for beamforming, 4x2 MIMO, and carrier aggregation in its TM500 Test Mobile system. Earlier this year, the TM500 Test Mobile was updated to support the 3GPP Dual Cell High-Speed Download Packet Access (DC-HSDPA) Release 9 standard.

2. With a circuit-envelope simulator, this software allows users to simulate circuit-level, time-variant phenomena like memory effects in digital-predistortion systems.

Last month, Cognovo released a software-defined baseband chip, the MCE 160, for LTE-Advanced and 3G/4G femtocell/ picocell applications. The idea behind this chip is to make it possible to create "soft" modems in all wireless standards.

What are the vendors working on for their next-generation offerings? Developers at TI are mapping out their 28-nm process strategy. Planning the same style of device, Flanagan's design teams are looking to integrate more functionality and deliver higher performance.

At Cognovo, the next target is a "higher number of processing resources, which would accommodate the requirements for, say, hundreds of users in a 3G/4G macrocell. In particular, it is the multi-user detection here which drives the architecture," notes Sturman.

NXP is working on enhancing Gen8 for macrocells and integrating a Doherty PA for micro and picocells. For its part, AWR is hoping to take advantage of synergies between its design-validation software and National Instruments' test and measurement products. For example, AWR will be "leveraging NI's wireless virtual- instrument sources and receivers into VSS for standards like LTE and WiMAX, allowing us to bring standard updates to market faster," explains Paparisto.

Aeroflex has its eye firmly fixed on the rise of small cells and smart networks. "Interference management is a key component for the successful deployment of heterogeneous networks, and 3GPP has introduced a number of technologies to address it," states Tim Beard. The company also sees increased interest in LTE-Advanced from its customers. Aeroflex already has demonstrated carrier aggregation running at 300 Mb/s, enabling a Tier 1 infrastructure vendor to demonstrate LTE-Advanced capability. This feature is available now for LTE-A proof-of-concept base-station development work.

RFMD is targeting future base-station designs with multichip modules and designs with integrated active bias circuitry. Those designs will allow digital tuning to handle several frequency bands in one component.



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