Editor’s Note: Welcome to our weekly Reality Check column. We’ve gathered a group of visionaries and veterans in the mobile industry to give their insights into the marketplace.
Anytime there’s talk about the promises of LTE, look around the room. You’re bound to find someone wondering how we’re really going to deliver on these promises. From a marketing perspective, LTE is impressive as mobile users will be able to take advantage of faster data speeds and cool, new devices. From a technical perspective, LTE presents several challenges. The good news is that there are some innovative solutions available to solve these challenges.
For starters, LTE has a myriad of spectrum issues. It’s no secret that there’s limited spectrum available worldwide, nor is it a stretch to say that radio spectrum is one of the most valuable assets in the world right now given the growing demand for wireless – from the most developed to third-world countries. In some markets where carriers are using whatever spectrum they can get their hands on, the need for band harvesting is creating a situation where LTE is now required to operate across a wide range of bands, with many of these bands being in the lower frequency range, especially here in the United States. Operating at these lower frequencies isn’t always conducive to delivering the high data rates and connectivity speeds as promised by LTE.
While the carriers have their fair share of issues to address for the deployment and continued roll out of LTE, so too, do device OEMs. The desire for global roaming is impacting how device OEMs approach the design of LTE devices – from handsets to notebooks. In some instances, there are more than 13 bands to cover, ranging from the highest to lowest frequency levels. To handle 13-plus bands, OEMs need to significantly increase the number of antennas that are integrated into devices, while at the same time trying to shrink form factors given consumer pressure for lighter, sleeker mobile devices. For example, the number of antennas required for LTE, especially since MIMO is required at this point, coupled with the multiple antennas required for 3G, GPS, Wi-Fi and more, is resulting in antenna-packed devices. In some cases five antennas are required in a given device. LTE’s requirement for two antennas makes achieving isolation difficult since both antennas operate at the same frequencies and are located fairly close together given shrinking device size. In addition, having multiple antennas located within a device leads to significant interference and ultimately detuning. As a result, in order for two antennas to work well in an LTE MIMO application, low correlation is needed – the two antenna radiation patterns need to be dissimilar.
So what can be done?
The current process of integrating multiple passive antennas into a device, then tuning them through several design iterations to ensure that the antennas maintain adequate isolation and do not interfere with one another won’t provide good connectivity with LTE given the various challenges the technology already presents. Instead, device OEMs can differentiate their product offerings for LTE by utilizing antenna and RF systems that are active, rather than passive, especially since current passive antennas must be able to cover the entire bandwidth simultaneously, whereas active antenna systems do not need to cover all the bands at the same time. Instead, active antenna system solutions are dynamically tuned, allowing for the design of a narrower bandwidth antenna, which can be tuned across a wide frequency range, resulting in high isolation between pairs of antennas in the handset. In addition, proper placement of two antennas in a single device is critical to reduce or minimize correlation co-efficient. If antennas are placed in the wrong locations, high correlation will result in poor MIMO performance or low data rates, dropped calls, etc. – all outcomes that carriers and device OEMs work hard to avoid.
Active antenna system solutions utilize different technologies and techniques that can be used as standalones or as complements to one another. One such solution is band switching. Band switching meets LTE’s multiple band challenge in that it is able to cover more than 13 bands for LTE global roaming Since active band switching technology can handle significant spreads in bands, from the low 700 MHz U.S. bands to 2.7 GHz, this ability simultaneously allows both voice and data usage at the same time, all without compromising performance for the user. This is pertinent for carriers needing to harvest bands or make the most of any spectrum they can get their hands on. Band switching is also capable of bringing the smallest physical volume antenna systems to fit in the thinnest devices for consumers.
Active impedance matching is another viable solution, especially given the current LTE requirement for a second MIMO antenna. With this technique, you can impedance match an antenna covering a wider range without increasing the antenna’s physical volume, and still provide the same performance to meet carrier, OEM and regulatory specs. With active impedance matching, initial “open loop” designs keep the antenna impedance matched in a band-switched scenario, optimizing the antenna for the frequency band in use to provide the best performance. Future iterations will implement “closed loop” solutions, which will be able to dynamically tune the antenna for the given frequency and to offset head and hand effects. Both band switching and active impedance matching can be used independently or in tandem with one another to achieve even better results.
Bottom line, given the innovative new active antenna system technologies available to device manufacturers, consumers will soon reap the benefits promised by LTE.
Jeff Shamblin is the CTO at Ethertronics, and is responsible for overseeing all research and development projects for the corporation. Shamblin brings 29 years experience in antenna engineering to this position.