Original URL: https://www.theregister.co.uk/2012/11/07/wtf_is_rf_mems/
WTF is... RF-MEMS?
Apparently, a way to make smartphones much, much better phones
Feature Smartphones nowadays come with big screens, megapixel-packed cameras and, thanks to apps, many, many more features than anyone could have dreamed of in the early days of mobile telephony. It has even reached the stage where making telephone calls is just one small part of a modern phone. And yet the need to support all the radio technologies punters expect to be able to use, for voice and for data, ensures that wireless communications is still the hardest part of a phone’s design to get right.
Just ask the guys who worked on the iPhone 4...
Steve Jobs explains Apple's grip-of-death iPhone 4 antenna design
That was in 2010. Today, more than two years later, your typical smartphone is even more complex, wirelessly speaking. A 2012 handset might be expected to feature Wi-Fi in two different bands: 2.4GHz and 5GHz. Bluetooth too, in the 2.4GHz band. Then there’s 4G LTE for fast data communications and 3G for voice - because 4G can’t yet do voice properly - and for data in places where 4G hasn’t been rolled out yet. Just in case the user roams into a region without 3G either, phones still have to support 2G. All this cellular goodness has to work across a range of frequency bands to support different carriers in different countries.
Oh, and don’t forget there’s more wireless goodness coming. Devices are soon going to have to start supporting 60GHz short-range, high-speed data transfer communications if they’re to continue offering the full 802.11 standard. WiGig - aka Wireless Gigabit - builds on the agreed 802.11ad 60GHz specification and it’s coming in 2013-2014, trailing new pick-up specifications in its wake.
To get the promised ever higher data transfer rates, devices need to stick to these newer radio specifications very closely. Old, broad tolerances which might have been acceptable in the GSM and GPRS days will no longer do. That means more effort needs to be put in to get each antenna turned correctly from the off.
To make it all work, today’s smartphones need multiple, pre-tuned antennae and a host of different chips to manage the signals for each of these technologies. And they all have to fit within the phone’s casing. No one, after all, wants to go back to extendible external aerials.
Were punters happy with ever-fatter phones, that would be much less of an engineering problem than it is, but they’re not - they want thin, pocket-friendly devices.
The solution might seem obvious: build in a single, universal radio able to hop across all those radio technologies and frequencies at will. It’s an answer that’s easy to state, rather harder to realise.
Back in September 2011, Samsung released a Windows Phone smartphone, the Focus Flash, which featured a little-known first: it contained a radio frequency micro-electromechanical system (RF-MEMS). This tiny chip, developed by WiSpry, a company based in Irvine, California, was capable of physically changing its impedance under the influence of a software. The upshot: it could be used to dynamically tune the Focus Flash’s antenna to meet the needs of some if not all of the radio technologies the phone uses.
WiSpry has been working on RF-MEMS chips for more than ten years. Indeed, MEMS makers have been shipping these kinds of chips since the middle of the last decade. Getting them to work with mobile phones has long been a goal, but it’s proved hard to attain. While phones were chunky and could be stuffed with all the antennae they needed, there wasn’t much need to implement RF-MEMS in handsets. There were fewer radio technologies to support too.
That changed in 2010 when Apple launched the iPhone 4, at the time the world’s thinnest smartphone. It sported a novel group of antennae fitted around the edge of the phone. It also suffered from the so-called ‘death grip’. More recently, HTC's One X has been the cause of similar grumbles. Hold the handset a certain way, and you impeded its ability to pick up and hold in to a cellular signal. Quite how badly has always been open to question, but enough people were bothered by it for it not to be written off. It was a problem.
A tuneable radio would have minimised it, by dynamically adjusting to the radio conditions, not only from the user’s hand but also the wireless environment around him or her. It’s a noisy world we live in, and no device feels it more than a wireless one. Each year, less and less signal makes it through all the noise to the phone. Some industry experts reckon that, since 2001, radio frequency signal quality has been falling by about 1dB each year. A drop of 3dB is equivalent to a halving of signal strength.
Dynamically adjusting the antenna - to make it resonate on precisely the right frequency - can increase its efficiency and cope better with the weaker signal.
Until 2011, RF-MEMS chips were too big and too unreliable for cellphone usage. WiSpry’s chip for Samsung didn’t necessarily change that, but it did show that the technology has reached the point where phone makers can consider it and start early see-if-it-flies implementations. WiSpry followed up 2011’s Samsung deal by announcing, in August 2012, that it had demo’d an RF-MEMS part able to widen an off-the-shelf 700MHz LTE phone’s reach from 725MHz to 880MHz, covering bands V through XII, without changing the handset’s antenna.
“With the many different LTE bands in use, this enables the creation of a true global LTE phone that consumers will come to demand,” said Jeffrey Hilbert, president and founder of WiSpry, at the time.
“This marks the first time that off‐the‐shelf antennas in Smartphone form factors have achieved such a dramatic LTE bandwidth extension,” the company claimed.
A typical phone RF-MEMS implementation puts the chip right at the front of the handset’s radio frequency sub-system right after the antenna. It provides a tuneable impedance matching circuit that matches and re-matches the power amplifier output to the antenna input on the fly. It’s controlled by software which monitors input and output and, using the RF-MEMS, keeps the two constantly aligned. The one antenna is effectively customised by the software.
Not that RF-MEMS is an ideal solution, at least not yet. They are fragile and that leads to medium- to long-term reliability issues. They have moving parts which, although tiny, wear down. Again, that hinders long-term reliability. The accumulation of electrical charge can reduce performance over time. Movement causes friction, which heats the MEMS unit up, and that can cause problems too.
But the technology is finally coming together. Rival RF-MEMS maker Cavendish Kinetics has been sampling its NanoMech RF-MEMS chips for phones to potential customers since the start of 2012 and hopes to go into volume production early in 2013. One of its major investors is wireless chip maker Qualcomm.
WiSpry's RF-MEMS tuner on its evaluation board
Network operators are keeping an eye on the technology’s evolution too. WiSpry claims that its RF-MEMS products can deliver a 3dB improvement at each frequency band. That’s good news for carriers keen to avoid having to constantly upgrade their equipment to cope with the signal fall-off. That aforementioned 1dB per year drop means, folk in the business say, carriers need to roll out 14 per cent more base-stations to make up the loss. That costs big - and it’s just to stand still. Getting the handset to do more of the work saves carriers hundreds of millions of dollars in equipment costs, it’s estimated.
It may also help prevent the costly loss of customers who go to another network because they can’t get a decent signal strength otherwise.
WiSpry has just announced the first device-integrated Antenna Tuning Developers’ Kit for smartphones and tablets - tiny boards hardware makers can build into their devices to test the tuning capability of WiSpry’s antenna tuning RF-MEMS chips. Persuading them to use kit like this is another matter, and one of the challenges companies like WiSpry and Cavendish face. Radio makers know about RF-MEMS - the concept has been around for long enough - but they don’t yet trust it.
But as users demand thinner phones equipped with more radio technologies, and with more and more of them trying to pull down ever larger volumes of data - and wanting it to arrive more quickly - the current approach appears to be hitting its limits. Phone designers are going to have to go MEMS. ®