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Energy-Efficient Architectures for 4G and LTE Design

Mobile devices are evolving rapidly. To keep pace, processor architecture architectures need to, too.

By Dan Holden, Low-Power Design

ARM techcon

ARM Techcon November 9, 2010 [Updated] -- "Ninety five percent of mobile phones are ARM-powered," said Chris Turner, product marketing manager at ARM. "But that doesn't tell the whole story. On average, every cell phone contains two and a half ARM proceessor cores."

Why so many?

Because mobile devices have grown up.  No longer just for talking and texting, mobile devices are now more powerful than a desktop PC was just one generation ago.  That pushes the capabilities of every device in the data pipeline as well, from servers to transmitters and even modems.

In fact, said Turner, the average cell phone modem must today be capable of churning out millions of instructions per second at the lowest possible clock speed due to the expanded data throughput requirements.

Still, at every point in the data transmission, there is a crying need for power efficiency. After all, the last thing a consumer wants to see is a dead battery signal while watching a movie on a mobile device, and server companies don’t want to keep throwing money away in electricity bills.

It's this market-savvy understanding of the real drivers of power efficiency that have made ARM the company to watch in the mobile computing space. In fact, ARM has identified a wide range of power efficiency requirements for mobile handset and future (“long-term evolution”) devices.

So while the rest of the industry was busy defining the methodology for SoC design, ARM was already moving beyond that.

"You can't satisfy all these terminal requirements with a single processor," said Turner.

That’s why many terminals include one ARM Cortex processor for baseband processing and another ARM Cortex processor for system-level processing, such as playing video, transmitting calls and sending data to the display screen.

Turner said ARM’s R&D investment “far exceeds other processor IP vendors” in the development of fabric  and processor IP and development software and tools. In addition, the company has over 700 ecosystem partners who provide broad support and compatibility with terminal devices.

Many smartphones use the ARM Cortex 4 today for user-level applications, real-time data communications and storage, and microcontrol.

High-performance processor IP such as the ARM  7, ARM 9 and ARM 11 provide baseband processing. In the future, said Turner, that's likely to evolve.

"If you put an ARM11 or ARM9 or an ARM Cortex-R4 into a device at, say, 40 nanometers today, you will see different performance and energy consumption results. As the industry moves to 22-nm technology, which whould be mainstream about 2015, most designs will be using the Cortex-R4."

“We’ve evolved from a single chip to multiple chip solutions because in the past two years," continued Turner. "Handsets have gone from 2 GHz chips to 3G and 4G smartphones requiring multiple ARM cores and/or chips. Of course, low-cost handset still use single-chip solutions, but smartphones today require dedicated baseband chips, as do tablets, laptops, datacards and other LTE connected devices.”

Turner said advanced LTE requirements for emerging 4G devices include wireless baseband performance features up to 1Gbps, round-trip latency of 5ms as well as support of multiple radio access formats. All this power will be necessary to process the datastream that is hitting it, he said.

“If you look at the data rate of these LTE devices, 20MHz of spectrum contains about 100 ODFM resource blocks. That’s the equivalent of about 7200 bits at the symbol rate.  An LTE frame must be captured and processed in about 10ms, for a symbol rate of about 71.43 microseconds. Of that,” he said, “About 66.7 microseconds is payload.”

These aggressive specs require the state of the art in processing power, said Turner.

“Forty nanometer process technologies and high performance processors and DSPs are the enabling technologies for these LTE devices,” he said.

Turner said an illustrative LTE baseband architecture includes a Cortex R for baseband protocol processing and a Cortex A for system-level tasks such as multimedia applications processing and support for multiple radio formats.

And while traditionally some of these tasks were given to VLIW (very long instruction word) DSPs, today’s requirements have moved well beyond the capabilities of that architecture, he said.

“DSPs are good for repetitive signal processing, but we’re now in a realm where its about flexible instruction processing,” he said. “DSPs are very efficient when you only need them to do one thing. But power-efficient processing must be optimized for different tasks, and VLIW DSP engines are not very compiler friendly.”

Taylor said that a typical modem must now contain millions of logic gates to handle new software defined radio technology. If this task were given to DSPs, it would represent a significant power and area increase. Instead, he said, the Cortex R4 real-time embedded processor on a 40nm G process  is far more power and area efficient.

“When the processor is tightly coupled with memories, you get highly deterministic performance,” said Turner, noting that the Cortex R4 has been in production since 2006 and boasts 18 licenses from most of the major semiconductor companies.  It’s also used in volume applications for automotive electronic systems, such as steering and braking for vehicle stability.

Even compared to other ARM processors, Turner said, the Cortex is a superior choice due to its lower power consumption.

“Today’s smart phones are used for texting, voice calling, MP3 players, web browsing and mobile TVs, said Turner. “In every case, Cortex R uses less power.”

“So get ready,” he said. “The LTE market is coming fast. It’s all about data connectivity, and it will be in every laptop, tablet, and smartphone for anytime, anywhere, always on Internet access with voice, video and Web 3.0.

“It’s why we like to say that although we’ve already enabled 1 billion smartphones, we’re delivering the super phone.”

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