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Externally Synchronized, Coherent Data Transmission Optimization, 10-9047

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Principal Investigators
Gary L. Ragsdale
Thomas J. Warnagiris

Inclusive Dates: 10/01/97 - 10/01/99

Background - The demand for untethered data communication over wireless modems is growing as evidenced by new mobile data applications in the transportation, utility, and service industries. Greater demand translates into the need for wireless modems that convey information with greater accuracy and at lower transmit power.

The most effective modems (both for speed and power) operate in mutual synchronization to a common clock reference. State-of-the-art technology in wireless synchronization relies on clocks embedded within the modems. These modems exhibit varying degrees of immunity to noise, depending, in part, upon the accuracy of their clocks. Cost, size, environmentally induced noise, and other technical factors limit the accuracy of practical internal clocks, thereby reducing the modem's accuracy and speed. To advance wireless communication, better means for synchronizing modems must be developed.

Approach - This project sought to improve modem synchronization by exploiting highly accurate clocks broadcast from a double-sideband residual carrier (DSB-RC) broadcast, such as amplitude-modulated (AM) radio stations, television stations, or time reference broadcasts. The study investigated improvements in wireless information transfer by using an existing DSB-RC signal as a modem clock. Using an approach called triad modem synchronization, researchers compared the current synchronization methods against an externally synchronized modem. The research team used computer models and laboratory prototypes to test the behavior of triad modem designs when exposed to noise.

Accomplishments - The investigations performed under this research effort included developing models for a radio-frequency link using an external reference signal, determining the effects of noise on the simulated reference signal, and comparing the simulated link performance with conventional links that recover a carrier from the received signal. The model consisted of three parts: 1) a bandpass limiter-squaring loop referenced to a conventional reference signal; 2) a bandpass limiter-phase locked loop referenced to an external DSB-RC signal (the triad configuration); and 3) a design model that predicts triad bit error rate performance. A prototype binary phase shift keying modulated link using a local AM broadcast station as an external reference signal was designed, built, and tested to verify the feasibility of such links.

10-9047-2.jpg (23717 bytes)
An AM broadcast station provides synchronization for a Triad system.


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A Triad prototype receives data over a low-power, unlicensed channel.

Electronic Systems and Instrumentation Program
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