Save energy with reflections
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 10/1/2003
A prototype system to prove that Inficomm, a technology that reflects modulated RF energy, could transmit encoded digital data without consuming battery power in wireless devices.
THE CHALLENGEDevelop a demonstration system to test the "modulated reflectance" concept.
THE TOOLS- Ajeco ANDI-FG PC/104 frame grabber. www.ajeco.fi.
- Alligator Technologies AFF-3/2G2/2F filter card. www.alligatortech.com.
- Analog Devices EVAL-AD9830EB RS-232-to-TTL Direct-Digital-Synthesizer. www.analog.com.
- Datalight ROM-DOS operating system. www.datalight.com.
- Nikon Coolpix 885 digital camera. www.nikonusa.com.
- Mathworks Matlab technical computing software. www.mathworks.com.
- Maxim MAX2701 direct digital downconverter receiver. www.maxim-ic.com.
- Measurement Computing PCI-DAS4020/12 data-acquisition card and PCI-DIO24H digital I/O card. www.mccdaq.com.
- Nova Engineering NOVASource G6 RF source. www.nova-eng.com.
- Sierra Microwave SFS0512-001 pin diode. www.sierramicrowave.com.
- TRI-M Engineering MZ104-EV PC/104 computer. www.tri-m.com.
- Wisp Gear 2.4-GHz Quad Patch antenna and WG-19-PARC antennas. www.wispshop.com.
Engineers at Los Alamos National Labs (Los Alamos, NM) developed a reflective wireless communications system that promises to drastically improve battery life in cell phones and other battery-powered devices (www.lanl.gov/partnerships/pdf/license/inficomm_3_021.pdf).
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Reflective wireless communications use an antenna whose impedance changes. The changes represent encoded bits. |
Instead of using a battery-powered transmitter, the technology uses a passive antenna and modulates an incoming carrier wave from a base station. An RF carrier wave from the base station's transmitting antenna reflects off the passive antenna and is received by another antenna on the base station. The reflecting antenna's impedance changes, causing changes in the reflected signal's amplitude. The impedance changes can convey encoded digital data.
Digital-signal-processing engineer Scott Briles developed a system to test the technology and demonstrate that it is feasible. Briles mounted two antennas on the top of a radar box's case. One antenna transmits a 2-GHz carrier wave to the passive antenna and the other receives the modulated, reflected signals from the reflecting antenna.
The radar box's receiving antenna feeds an RF downconverter that demodulates the carrier to its baseband frequencies, creating a signal containing modulations at 172 kHz and 115 kHz, which represent logic levels. A low-pass filter card removes any high frequencies and adjusts the signal gain, thus eliminating digitizer errors caused by aliasing. A data-acquisition card digitizes the demodulated signal, and a Matlab script converts the signal into its original format. The Mathworks Data Acquisition Toolbox controls the data-acquisition card, the filter card, and a digital I/O card that controls an RF attenuator, which adjusts the signal amplitude.
Briles used a consumer-grade digital camera to generate digital images. A PC/104-based computer located in a case uses a frame-grabber to produce a video stream at approximately 3 frames/s from the images. The computer sends the video stream over an RS-232 link to a direct-digital synthesizer evaluation board, which converts the data stream into a frequency-shift-keyed (FSK) TTL-level stream. A pin diode modulates the 172 kHz and 115 kHz FSK signals, changing the reflecting antenna's impedance.
RESULTSBriles demonstrated the reflective transmission system at Los Alamos and proved that it works over distances from 20 m to 500 m. Bandwidth, center frequency, transmit power, and antenna gain play critical factors in range. Los Alamos engineers are working to improve the range and lower the power radiated by using error-correction coding, different modulation techniques, and adaptive-beam forming.
Besides having a potential use in cell phones, the reflective wireless technology may someday find use in wireless LANs. A laptop computer could control a reflecting antenna, and the powered signal could come from a line-powered transmitter. Thus, the laptop PC needn't use battery power to transmit over the LAN. Other potential modulation techniques include bipolar phase-shift keying (BPSK) and code-division multiple access (CDMA). Los Alamos National Laboratory is now looking for a US company to transfer this technology to the commercial sector.
Martin Rowe, Senior Technical Editor, m.rowe@tmworld.com.
