Antenna breakthrough hints of software radio

Antenna breakthrough hints of software radio
MANHASSET, N.Y. — Using proprietary magnetic-isolation and RF MEMS-based antenna-reconfiguration technologies, e-tenna Corp. has brought the elusive goal of a software-defined radio one step closer to reality. The San Diego company's approach, which eliminates many of the RF front-end obstacles, also promises to improve the performance and lower the cost of antennas for wireless handsets and infrastructure.

Working from the antenna backward, e-tenna's solution could eventually make expensive and bulky bandpass filters, transmit-and-receive switches and multiplexers redundant, while also allowing the antenna to operate in all major communications bands from 800 through 2,100 MHz and higher.

Its antenna also improves power-transfer efficiency by 4 dB over internal antenna technologies, the company said, thereby increasing the range or reducing power consumption, and cutting down on dropped calls.

The company will offer the technology on a license-royalty basis. "We can take out $4 to $6 from a $70 bill of materials for a dual-band phone now," said David McCartney, president of e-tenna. "And that only goes up as you migrate to tri- or quad-band telephones."

"I haven't seen anything else like this before," said Jack Quinn, president of Micrologic Research (Phoenix). "If it works the way they say it does, it should certainly simplify cellular telephones today."

E-tenna's core intellectual property lies in its Artificial Magnetic Conductor (AMC) technology, which it combines with a patented reconfigurable-antenna technology based on microelectromechanical systems (MEMS).

Dave Auckland, director of engineering at e-tenna, described AMCs as high-impedance surfaces that provide significant isolation, a key challenge for small antennas that are mounted inside a phone.

"Think of it as a perfect magnetic conductor, not an electrical conductor," he said. "As a result, the magnetic fields tend to vanish on the boundaries."

The technology relies on its bandgap performance to give it a certain frequency performance at its band edge. The bandgap controls the way the signal is propagated on the surface of the structure by controlling the transverse-electric and transverse-magnetic modes of operation. "You won't find this in the textbooks," said Auckland, "since no one's dissected it this way before. But this dissection allows us to design the surface such that the antennas work the way they're supposed to."

The end result is that e-tenna can put wire antennas directly on top of the AMC surface while allowing them to radiate very efficiently with no impedance ramifications. "Yet," said Auckland, "it gives you very high isolation between the antenna and the circuit board — and also between the handset and the user's head and hand. The specific absorption rate is one-third that of alternative, internal antenna offerings."

The most immediate implication of this high isolation is the ability to place two wire-patch antennas back to back within a very small space without the possibility of interference. According to company president McCartney, "its natural isolation of 30 to 35 dB meets GSM duplex requirements, while 45 to 50 dB is our next step to meet CDMA needs."

This means the handset can eliminate the transmit/receive switch up front, since now both antennas can be designed-in on one device. To date, such a switch has been used to allow alternate operation of the transmit/receive paths to avoid mutual interference.

"The other advantage to having separate Tx and Rx antennas is that designers can better match the antenna to the transmit and receive paths," said McCartney. "These have divergent requirements in terms of impedance matching. We also get rid of the multiplexers and bandpass filters." A dual-band GSM phone typically has three diplexers while a CDMA phone has one 6-mm-high duplexer.

The bandpass filters are eliminated by means of e-tenna's patented autoreconfigurable antenna technology, which also gives the device its wide operating-frequency range and paves the way for software-defined radios (SDRs).

Navy delivery

The autoreconfigurable technology first found its way into e-tenna's delivery of a UHF satellite-communications antenna to the U.S. Navy. It achieved 70 percent efficiency with channel bandwidths of 2 to 3 MHz, the company reports.

That initial version used a wire antenna that was switched between different air interfaces using PIN diodes. "We're now moving to microelectromechanical systems to do the switching," said McCartney.

To date, MEMS have not been usable for this kind of application, since they have required voltages of 20 to 30 V. "We think we have a MEMS design that can go below 3 V through a partnership within Titan Corp.," of which e-tenna is a subsidiary, said McCartney. "That's a separate announcement to come soon. Suffice to say that it will lead to a very low-power, latching-type design for the autoreconfigurable antenna."

In the meantime, McCartney emphasized that the e-tenna solution does not involve just switching among antenna elements, but adds spectral shaping of the antenna's response. "We're really changing the electrical properties of the aperture and the radiating structure — it electrically looks different at each frequency," he said.

"The high-Q antenna ends up being very efficient over the 5 to 10 MHz of the passband," said Auckland. "The combination of the precise electronic tuning and the natural roll-off of the antenna eliminates the need for bandpass filters."

The current version has a 1:3 tuning range, hence the focus on 800 to 2,100 MHz. "The AMC and antenna are currently breadboarded for lower-frequency operation to allow manual adjustment of the components," said McCartney, "though with 2003 design cycles now ramping up, we plan to have intellectual property to offer by the summer."

Design requirements

Exactly how the technology will be incorporated into new designs has yet to be determined. "We're still trying to define that," said McCartney. "What we have is a bit more complicated than a passive antenna. It has some extra components, it needs dc power, it will have an ASIC with the autotuning circuit."

The antenna control unit will need a logic input from a DSP or a digital word from a synthesizer, while the autotuning requires some detection and processing. "That will be done in the antenna," said McCartney. "So when the radio goes on it'll tell the antenna where to go for the transmit-and-receive signal."

Mike Woodward, principal software engineer at soft-radio developer RadioScape (London), termed the antenna "certainly one the three areas of heavy research into SDRs, and any advance there can be considered a key enabler." The two other key areas relate to the baseband processing and RF down conversion.

For baseband processing, the emergence of high-performance, low-power DSPs has been a boon. International Wireless Technologies, for example, incorporates three of VLSI Systems Inc.'s low-power DSPs to handle the processing of the various air-interface standards. International Wireless said it plans to announce its partnership with an RF company in the near future.

The RF segment is being tackled on a multitude of fronts, but has always remained stubbornly power hungry and inflexible. That's due to the myriad stringent requirements of each air-interface standard, whether it's the sensitivity requirements of Global System for Mobile Communications (GSM) or the bandwidth needs of code-division multiple access.

Direct-conversion advances

However, some of the most promising breakthroughs of late have come from advances in direct conversion from companies like Analog Devices, Conexant, Direct2Data and Texas Instruments. These have all opened the door to RF front ends that can better accommodate multiple air interfaces, while at the same time reducing the overall parts count, saving space and reducing cost.

"Direct-conversion radios have eliminated those big expensive parts," said Allen Nogee, analyst at Cahners InStat (Tempe, Ariz.). "But the problem with direct conversion is that the receiver and transmitter become very sensitive to each other if you don't have good isolation between them."

That's where the e-tenna development holds promise.

"We've been able to do SDRs for over 15 years now — but only if you're willing to throw enough money and power at the problem," said Nogee. "As a result, it has not been a viable technology for wireless handsets."