Once in a while, a technology has the numbers to back up the buzz it’s generating, and that’s the case with LTE. By the end of this year, ABI Research predicts that LTE subscriptions will hit 183 million worldwide. In the first two years it was commercially available – Q4 2010 to Q3 2012 – LTE racked up far more customers than W-CDMA did in that technology’s first two years.

For OEMs of smartphones, tablets, laptops and other devices, the message is clear: an LTE strategy including innovative devices is critical to stay ahead of the competition and remain relevant. But less obvious are the challenges, costs and competitive risks of choosing the wrong LTE RF solution. Mistakes are easy to make because LTE has fundamentally different requirements and considerations than those of 3G and 2G:

• Highly fragmented spectrum. LTE is designed for use in more than 40 bands between 450 MHz and 2.7 GHz. Roughly half of those are already in commercial use. To enable regional or global roaming on par with what 3G provides, or single-SKU products, OEMs currently must build support for a dozen bands into their devices. That increases cost, complexity and development time, all of which increases further when those bands are widely spaced. LTE-Advanced introduces carrier aggregation, which makes fragmentation even more challenging when the aggregated frequencies are far apart.
• Operators prefer lower bands.Many mobile operators prefer to use low frequency bands, 700 MHz, for LTE because lower frequencies require fewer base stations, thus reducing their CapEx and OpEx. But lower frequencies require electrically larger antennas, which are literally a bad fit for the trend toward thinner devices. In smartphones, for example, the amount of space available for antennas and other RF components is shrinking 25 percent annually to make room for bigger batteries while enabling increasingly thin, sleek form factors. Although M2M devices such as vending machines, smart meters, and telemedicine monitors appear to have ample room for large antennas, they are actually often as space-constrained as smartphones and tablets.
• MIMO (Multiple Input, Multiple Output) is required. MIMO increases the number of cellular antennas. The two antennas must have enough separation between them to benefit from the differences in signal conditions. That amount and spacing increase the challenge of finding enough room as devices become thinner.
• Multi-technology support is a must-have. Although 193 LTE networks have launched and another 123 will debut over the next two years (according to ABI Research), the technology will not have ubiquitous coverage in most countries until late this decade. So in that interim, many LTE devices still need the ability to use other wireless technologies, such as 2G/3G fallback in places where LTE is not yet available, or WiFi when it’s more cost-effective. GPS is another common requirement. Each additional technology increases the challenge of finding enough room for antennas and other RF components.

Device OEMs must overcome all of these challenges. If they do not, their products will not deliver the high performance and fast speeds that consumers and business users expect from 4G. Sub-standard performance would directly affect their competitive position and ultimately revenue. OEMs that rely on mobile operators for distribution also risk losing their sales channels if customers inundate operators with complaints about poor performance, or if those devices create problems that sap network capacity. To overcome these challenges, device OEMs need to focus on two things: the trend toward active antenna systems and where each RF vendor’s products fit into that trend.

Active Antenna Systems

Active antenna systems are not just the future of LTE – they are also the present. An active antenna system went into production in August of 2011 in a medical device monitoring critical medication dispensing and inventories. Several months later, the Galaxy S II LTE SC-03D phone, using an active antenna system with active impedance matching techniques, launched on the DOCOMO network. Most recently, an ultrabook from a tier one OEM utilized band aperture techniques to provide global 3G and 4G coverage. Active antenna systems are gaining traction in the marketplace to help OEMs solve LTE’s toughest challenges.

Unlike passive antennas, active systems can be dynamically tuned to cover significantly wider bandwidths, achieve smaller physical volumes, and provide more degrees of freedom in the design process. This flexibility reduces the cost, complexity and lead time of developing antenna systems capable of meeting unique device or application requirements, such as an LTE M2M module mounted inside a metal box or in an underground vault, or a smartphone that needs to be ultra-thin and capable of global roaming on LTE. This flexibility also increases the likelihood that a device will pass operator certification on the first try, which means faster time-to-market and faster time-to-revenue for the OEM.

Active antenna systems also enable single-SKU products, reducing the OEM’s development and support costs while expanding those products’ addressable market to the world rather than just a single region or country. For example, a single active antenna system can support multiple LTE bands, plus the bands for 3G and 2.5G fallback, ISM, WiFi, Bluetooth and ZigBee. Covering all of those bands with multiple passive antennas is somewhere between difficult and impossible, depending on both the amount of space available in the device and the requirements for cost and performance.

Unlike passive antennas, active antenna systems can seamlessly adjust the antenna’s characteristics to compensate – all in real time – for frequency shifts due to environmental changes such as the position of the user’s head and hand, or a large truck parked over an underground utility vault. Active antenna systems also can make those adjustments to overcome challenging installations, such as when an M2M module is mounted on a metal surface that would wreak havoc with a passive antenna.

The ability to mitigate detuning effects directly affects an LTE device’s market potential and support costs, as well as its OEM’s brand reputation. Active antenna systems enable those devices to provide data speeds, video performance and call quality that are noticeably superior to what’s available from LTE products that try to make do with passive antennas.

Quality of Service (QoS) and reliability also affect mobile operators’ cost of delivering service. For example, when M2M modules use active antenna systems to maintain connectivity even under difficult, changing environmental conditions, the mobile operator is under less pressure to increase its cell site density. The
CapEx and OpEx of dozens, hundreds or thousands of additional cell sites would make it difficult for the operator to price its M2M services competitively, yet profitably.

What Makes an Ideal Active Antenna System?

Not all active antennas, or tunable antennas, provide equal performance and integration benefits. That is a key point for OEMs, systems designers and others to keep in mind because many vendors are now talking about tunable antenna products. It is important to understand the key differences between them.

For starters, the ideal active antenna system for many applications is an all-in-one module that device OEMs can quickly and cost-effectively integrate into their products instead of spending weeks or months on custom designs with components from multiple vendors. The plug-and-play approach has obvious benefits, such as reduced development costs and faster time-to-market. A less obvious benefit is that OEMs now do not need to hire staff to create an RF engineering team to handle integration in house. That benefit is particularly valuable for M2M and consumer electronics OEMs, which typically have limited or no RF experience.

The turnkey plug-and-play module approach includes being able to dynamically sense and optimize the antenna system without external control signals from the device. That is possible by using advanced antenna architecture, tunable capacitors and adaptive algorithms designed and integrated in conjunction with a microprocessor.

To achieve the highest performance and ease integration, an active antenna system needs to perform dynamic impedance matching at the feedpoint rather than farther back in the system, such as in the transceiver chipset. The feedpoint approach maximizes performance because the tuning is focused entirely on the antenna. When tuning is done farther back in the signal chain, the process can be undermined by the transmission line’s electrical delay and losses.

Band switching is another important feature to look for. Also known as active aperture, this technique dynamically changes the electrical length of the antenna element to shift its frequency response. An alternative method is active matching, where a tuning circuit at the feedpoint changes the antenna’s impedance. The main difference between the two methods is that active aperture/band switching is a coarse tuning of the antenna element followed by active impedance matching for a fine tuning of the frequency response at the feedpoint. This allows the device to quickly tune across the required frequency bands in the smallest antenna volume.

OEMs used to have to choose between the two techniques because of the cost of implementing multiple components. Impedance matching requires tunable capacitors, while band switching requires a switch. OEMs are always looking for ways to reduce costs given the competitive nature of the wireless industry.

The latest active antenna solutions minimize the cost factor by combining a four-port switch and multiple tunable capacitors in a single RFIC. These products combine the active aperture/band switching technique to quickly tune across frequencies and then use the active impedance matching function to fine tune the impedance to the desired frequency for the best performance.

Many of LTE’s biggest RF challenges occur below 1 GHz because above that, antennas do not have to get larger to the point that they’re difficult to integrate inside space-constrained devices. That is why these next-gen active antenna system solutions focus on impedance matching at the lower frequencies.

Device OEMs have been asking RF vendors to provide single-RFIC active antenna systems to maximize performance benefits without significant cost increases, so it is a milestone that they are now commercially available. They also do not come with many tradeoffs. For example, multiport switches and tunable capacitors typically have low current consumption – well under 200 micro amps – so the performance and reliability that they enable do not come at the expense of battery life.

Even so, not all RFIC active components are equally effective. The reason is the heritage of the companies that produce them. For example, some RFIC vendors know chips but not antennas, so they are developing products that are unnecessarily expensive because they conduct tuning at higher frequencies.

By comparison, a chipset vendor with extensive antenna experience knows that all the tunable components in the world will not compensate for a poorly designed, inefficient antenna. An experienced antenna vendor also knows that a systems approach is a key to performance: the antenna, chips and algorithms need to be designed together in order to deliver the easiest integration, best performance and the lowest cost. By comparison, a chip vendor will design an RFIC that will attempt to work with as many bands, antenna types and impedances, which inevitably translates into compromises in performance and higher cost.

Single-RFIC chips and active antenna systems are an idea whose time has come because they solve a variety of unique challenges created by the arrival of LTE. They’re also the first in a wave of next-gen active antenna solutions, which will feature even more complexity to enable higher performance and lower costs.