Wireless service providers will soon be adding capacity to their networks with small cells powered by 28 nanometer chips that can connect more users than an entire base station once supported. Many of these small cells will be hosted by enterprise customers. While outdoor small cell deployments can be complicated by backhaul challenges and negotiations with municipalities, many carriers are finding that enterprise customers are eager to deploy indoors and use their existing power supplies. For chip designers, this presents a unique challenge as they work to maximize performance and support multiple users with a relatively limited power budget.
“These are typically devices where the entire box consumes less than 10 watts and quite frequently even less than 7 watts,” said Greg Fischer, vice president for carrier access at Broadcom (BRCM). “In order to build a classic enclosure that can thermally deal with the heat dissipation and not be the size of a shoebox sitting on your desk, you need to get the power consumption down significantly.”
Power over Ethernet (PoE) is commonly used to power small cells indoors. Cavium (CAVM), which designs the chipsets that power LTE small cells for Korean operators KT and SK Telecom, says that 14 watts is a very typical PoE budget for a small cell. With PoE plus, the amount of power available rises to 25 watts.
“There’s a lot going on amongst the OEMs and the chip providers to try to understand just how much can you pack into a box in terms of radio access technology and still be within the Power over Ethernet envelope,” said Fischer. The challenge for chip designers is to limit power consumption without limiting performance.
“The handset in your pocket may have more CPU performance than some of the small cell wireless base stations at the other end of the cellular air interface,” said analyst J. Scott Gardner of The Linley Group. “As carriers compress the radio-coverage area to add capacity and fill service gaps, base-station processors often must survive on a miserly power budget that restricts performance – especially in femtocells, because the RF amplifiers use much of the available power.”
“The majority or power consumption occurs at the RF level,” agreed Nick Karter, vice president of business development and product management at Qualcomm Atheros (QCOM). “The power amplifiers consume 50-70% of the power. So if the engineers can figure out a way to reduce [the amplifiers’] power consumption by half, that’s a significant improvement on the overall power budget.” Karter adds that enterprises hosting small cell deployments understand that the amount of power the cell consumes will directly impact their operating expenses.
Focus on power amplifiers
“I think that all of this is … leading to a need and a lot of interest in techniques to make the power amplifiers more efficient … trying to keep the PA very linear in its performance,” said Fischer. “There’s a lot of techniques to look at press factor reduction and digital predistortion techniques that would make the PA operate more efficiently.” Fischer said that Broadcom acquired devices and technology in these areas through its NetLogic acquisition.
Qualcomm’s Karter said that his company’s solution controls power consumption by linearizing the power amplifiers. “We provide a digital front end for RF, and that front end is able to linearize power amplifiers and therefore reduce the system power consumption at the access point level,” he said. Karter added that Qualcomm complements that access point technology with the highly efficient processor technology used in its Snapdragon chips, resulting in a solution that can optimize power consumption for both the access point and the processor.
Texas Instruments (TXN) also incorporates a digital front end in its solution, drawing on its experience with macro base stations. “We’re bringing this to small cells for the first time,” said TI’s Tom Flanagan, director of technical strategy for digital signal processing systems. “Typically it wasn’t viewed as being beneficial to small cells but because the processor is required to do things like crest factor reduction and digital signal predistortion – that processor alone would consume 5 watts. You’re in a device that has a 5 watt or less power amplifier so it really didn’t make sense to try to optimize it with another 5 watts of processing. … So you do that with some digital signal processing that we’re doing now in the SoC, and there’s a feedback loop between the analog front end and the digital radio front end.”
Radio acceleration and multi-mode
Flanagan added that TI’s use of radio acceleration packs is perhaps its biggest power-saving innovation for small cells. “Where you get the big benefit out of this is what we’re doing the radio acceleration packs,” he said. “What that’s doing is pulling the frame rate processing on the wireless network off of the programmable elements- the ARM cores and the DSP cores – handling those functions in hardware so that the software that you need to run on it is free to have the entire access to the ARM cores and the DSP cores. When you do things in hardware like that it’s obviously lower power consumption.”
Texas Instruments and Qualcomm have both recently introduced 3G/LTE, 28 nanometer small cell solutions. They’re targeting the indoor market, although their solutions can also be used outdoors. Broadcom has launched an LTE residential small cell, as well as a 3G/LTE solution and a lower power chipset that can support either 3G or LTE, but not both at the same time. Cavium has launched an LTE indoor chipset that it says can support up to 128 concurrent users. The company demonstrates the solution below.
Next week in Small Cells in Focus: Wi-Fi Connectivity.
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