As with most engineering endeavors, the design of wireless appliances involves the resolution of specifications with conflicting requirements. For example, the need for higher data rates, increased radiated power and multimode, multiband, multifunction radios is in direct conflict with requirements for higher levels of integration, smaller size and longer battery life.
Many in the industry consider software-defined radio (SDR) the ultimate solution for the low-cost wireless appliances of the future. With SDR, multimode, multifunctional wireless devices can be upgraded via software, thus addressing the most challenging issues confronting the wireless industry.
Successful commercialization of SDRs depends on key technical advances in high-speed digital signal processors (DSPs), data converters with wide dynamic range, efficient and compact software, and broadband high-efficiency radio-frequency technology. To date, RF technology for the transceiver portion of the handset has advanced little toward meeting the goals of SDR.
One critical element of the transceiver has been virtually ignored: namely, the antenna. Engineers at E-tenna Corp. are working to solve the problem by combining proprietary RF and antenna technologies to form the RF2IF line of products.
For optimum performance, the RF2IF line employs key components based on microelectromechanical systems (MEMS) technology. The components must meet stringent performance specifications when implemented as RF switches and variable capacitors (varactors). It is critical that the MEMS community accomplish performance goals in the near term to ensure the success of portable third-generation (3G) wireless devices and SDR handsets.
Gain-bandwidth product is the most relevant parameter for describing the performance of electrically small antennas, such as those used in mobile wireless devices. That figure is a function of the size of the antenna, with larger antennas having larger gain-bandwidth products. The traditional approach has been to accept lower antenna efficiency to achieve smaller size. But given the current trends in wireless handsets and PDAs, that mode of thinking will only degrade efficiency, leading to lower battery life, ever-increasing complexity (and hence ever-increasing cost) of the RF/IF circuitry, and limits on the system data rate. RF2IF seeks to avoid those problems by using reconfigurable technology.
The gain-bandwidth limitation of electrically small antennas is a fundamental law of physics that limits the ability of the wireless-system engineer to reduce the footprint of the antenna while maintaining or increasing its efficiency. A revolutionary approach to circumventing that limitation is to construct a small, highly efficient antenna with a narrow instantaneous operating bandwidth that can be tuned over a much wider operating bandwidth.
The concept was first implemented for small, low-profile antennas that provided adequate gain over the UHF Satcom band (240 to 320 MHz) in high-performance military aircraft, where weight and size are significant design constraints. The antenna employed solid-state switch elements, namely PIN diodes, to dynamically adjust the effective electrical size and, hence, the operating frequency of a microstrip patch antenna over a large frequency band while maintaining an excellent impedance match and efficiency.
Although the approach appears to increase complexity and expense relative to a conventional passive antenna, a more thorough analysis reveals advantages in cost and system architecture design for current 2.5G systems and even more so for planned 3G systems. Indeed, we have found that selectivity of the reconfigurable antenna provides filter-like performance so that duplexing, diplexing and bandpass image rejection may be absorbed into the antenna. That simplifies the transceiver architecture, reducing cost, enhancing quality of service and reducing requirements on prime power.
Furthermore, the increased antenna efficiency leads to higher effective radiated power, which is critical in high-bit-rate, low-bit-error-rate systems for 3G and beyond. Antenna tuning can also compensate for detuning effects caused by the proximity of the user's hand and body, Such effects degrade radiated power levels in conventional wireless devices.
E-tenna's reconfigurable antenna comprises separate transmit and receive antennas and an antenna control unit (ACU). Each antenna has a very narrow instantaneous bandwidth, covering only part of the receive or transmit portion of a given band. The approach reduces the noise and blocker signals entering the receiver while suppressing noise and harmonics generated by the PA, thereby eliminating the need for additional filters. A control algorithm is employed for real-time tuning of the antenna since it can be detuned by random, time-varying proximity effects. The algorithm can be implemented in the device's existing DSP.
In this new paradigm, the antenna not only acts as a simple transformer between the antenna and free space but also provides filter functionality. Because of the elimination of lossy components and the increased radiation efficiency achieved by exploiting the gain-bandwidth product, the transceiver architecture can convert more than 10 times the amount of prime power to radiated power than do more conventional architectures. Further, the use of separate antennas for transmit and receive provides the flexibility to optimize matching separately for the PA and low-noise amplifier (LNA).
The RF2IF architecture employs both coarse-tuning (band switching) and fine-tuning (within each band) capabilities. The coarse-tuning capability requires extremely low-loss switches, and the fine-tuning capability requires high-quality-factor (Q) variable capacitors. In a perfect world, reconfigurable-antenna technology would be implemented using ideal switches and varactors that is, devices that have no loss and perfect isolation. Of course, there are some losses in all practical devices. Full-wave simulations reveal that the series resistance of the switching devices used for band switching in the reconfigurable antenna is an important parameter. If the resistance is too high, it will absorb most of the power fed to the antenna, leading to very low antenna efficiencies as well as loss of selectivity. The effect of the switch's series resistance is more critical at lower frequency because the antenna is so electrically small. E-tenna's proprietary design alleviates that problem in part by using the switches in their off position at the lowest-frequency band. The off-state capacitance of the devices is also important but is not as essential to performance.
The need for high-Q varactors for fine-tuning is even more critical to optimum performance. In examining the effect of Q on maximum efficiency and selectivity, it is clear that the efficiency decreases sharply as the Q decreases. The effect of lower Q can be compensated for somewhat by increasing the thickness of the antenna, but that is generally undesirable for small handsets. The Q of a fine-tuning circuit is even more important than the thickness of the antenna in determining its bandwidth.
A reconfigurable antenna operating over cellular frequencies from 824 to 900 MHz and PCS frequencies from 1850 to 1990 MHz was designed using E-tenna's proprietary technology to determine performance requirements for MEMS devices. Maximum efficiency was sought in an antenna footprint of 20 mm x 20 mm, with instantaneous bandwidth (half-power) of approximately 15 MHz.
The successful realization of SDR requires advancements in nearly all aspects of handset technology. The evolution of DSP and data converter technologies appears to be progressing at a rapid pace and may reach the desired goals in the not-so-distant future.
Even so, wireless appliances that can be completely defined by software will remain elusive unless certain performance milestones are attained in the RF domain. Reconfigurable-antenna technology is one key enabler that empowers wireless-system designers to surmount the challenges. The technology allows more efficient conversion of prime power to radiated signal as well as narrowband selectivity in a very broadband operating regime.
To achieve optimum performance, reconfigurable-antenna technology relies on high-quality switches and variable capacitors. The most promising technology for implementing these components is MEMS.