Figure 1 iPhone 4 antennas (courtesy of intomobile.
Most people have heard about the iPhone 4 antenna problems. Apple released the new phone with a novel set of external antennas that immediately had reception problems. The company cleverly used the stainless steel band around the phone as the antenna for GSM, UMTS, WiFi, GPS and Bluetooth with spaces separating the multiple antennas around the phone to electrically isolate them (see Figure 1).
Placing the antennas on the outside of the phone seemed like a nice approach that freed up space on the inside and maximized the size of the antennas, which should have helped increase the efficiency at the cellular frequencies. A recent clandestine poll revealed that Apple was not alone in deriving plans for such "edge antenna" designs. The ever-increasing feature sets of smartphones require more space for components and batteries translating into less volume for the antenna. This forces antenna designers to seek novel ways for developing radiating elements.
Two problems occurred that Apple did not seem to account for in the design. First, touching between the antennas will short them together causing them to perform poorly. Second, covering them up while holding the phone can attenuate the signal to the point where the call is dropped (which can happen with any mobile phone if the area over the antenna is covered). The first problem was solved by giving away free skins or cases that protected the perimeter of the phone so a person's hand would not come into contact with the antennas, but the second one is more universal and difficult to solve. But how was this not discovered in testing? According to an insider, it was found later that the dummy hands used to simulate human use situations in testing are not malleable enough to cover the area between the antennas; human hands, however, are malleable enough to do this.
It is common to design and test for phone reception issues using simulated hand and head positions with special dummies that simulate actual use, but the combinations are infinite so it is not possible to test all of them. Many current mobile phones do suffer from over-the-air (OTA) performance degradation due to hand and head effects. However, future phones will have to deal with an even greater range of frequencies and modes as 4G technologies come online, which can lead to an even more difficult situation of the overall OTA performance degradation. Therefore, antenna designers need a solution to this problem.
The answer could be adaptive antenna tuners that help solve the performance degradation, while reducing antenna size and current consumption of the mobile phone. If the antenna impedance can be dynamically changed so the antenna is always tuned to the appropriate frequency for maximum efficiency, these problems will be minimized and dropped calls and poor reception will be reduced significantly.
Several companies are working on tunable antenna technologies to solve this problem. Many are using tunable MEMS devices that switch in various values of capacitance to tune the antenna to the desired frequency. There are many devices like varactors that are tunable, but the tuning range is typically not wide enough to cover all the cellular frequencies. Varactors typically tune over a 2:1 range where as MEMS devices are now achieving 10:1 or more, which is needed for the wide range in frequencies that mobile phones utilize. Also, MEMS devices have much higher Q factors than varactors, which are relatively low Q devices. Other key issues for varactors are power handling and linearity as there are 10:1 range varactors, although they have strong nonlinearities.
Figure 2 TDK-EPC prototype 5 × 5 mm antenna tuner module.
Some of the MEMS companies involved in this market are TDK-EPC and WiSpry among others. TDK-EPC offers a high performance antenna tuner that employs a closed-loop algorithm that instantly optimizes the matching to the conditions of use. Unlike open-loop systems, the antenna tuner only requires a synchronization signal, making it easy to design into advanced multiband/multimode mobile devices. The adaptive antenna tuner is now in the advanced sampling stage. Pilot production was expected to begin at the end of 2010. It supports all common frequency bands from 824 to 2170 MHz in a module size of 5.0 × 5.0 × 1.0 mm (see Figure 2).
Figure 3 WiSpry fully integrated Tunable Impedance Matching (TIM) solution (3.5 × 4.0 mm).
WiSpry recently announced a partnership with IBM to develop single-chip tunable RF front-ends for mobile handsets, which WiSpry will market to tier-one original equipment manufacturers (OEM). The first of these customers was due for initial production before the end of 2010, with others planning production throughout 2011. WiSpry's tunable MEMS technology uses arrays of capacitive devices that can be quickly tuned in and out to provide over 3 dB of link resilience by adapting to changes in frequency, antenna conditions (such as being touched by the user) and other ongoing operational conditions (see Figure 3). WiSpry's standard RF CMOS manufacturing process through IBM allows for increased integration by building RF MEMS devices on active CMOS silicon structures. This enables a roadmap from discrete RF capacitors all the way to a fully integrated and monolithic single-chip transceiver.
Figure 4 TDK-EPC comparison of RF output power.
Figure 4 shows a comparison of maximum RF output power of a commercial phone without an antenna tuner (a) and with a tuner (b). Evaluation of RF MEMS technology in antenna tuners by TDK-EPC revealed significantly improved mobile phone efficiency:
- 50 percent average improvement was observed in both low and high band for a typical commercial phone
- >200 percent average improvement (>800 percent in low band) was also observed for a commercial phone
WiSpry reports similar results with its tuner solution, including a broadband tuning range of 10:1, +3 to +6 dB of transducer gain and overall efficiency gains of 30 percent or more, depending on the implementation within the handset.
iSuppli recently reported that they anticipate RF MEMS revenue to rise to $8.1 M this year, $27.9 M in 2011 and then $223.2 M in 2014. Much of this is projected to be from cell phone front-end adoption of tuning using RF MEMS switches and varactors. There are other technologies competing for this solution, but RF MEMS seem to be leading the way and could see adoption in the very near future. Maybe we can finally get rid of the phrase "Can you hear me now?"
