May 18, 2000 WEST LAFAYETTE, Ind. – Roaming cell phone users are often cut off, sometimes while driving even short distances. Now engineers have devised a method to prevent those annoying disconnections.
The technique could make cell phone users 100 times less likely to be cut off, says one of its creators, Edwin K.P. Chong, an associate professor at the Purdue University School of Electrical and Computer Engineering.
Cell phones depend on a series of "base stations" that provide service to separate regions called "cells," which are arranged in a sort of honeycomb pattern spanning large geographical areas. An individual cell provides hundreds of channels, or frequencies, and each caller occupies a single channel.
As mobile phone users range across cell boundaries, they are "handed off," or transferred, to a new channel in the adjacent cell. However, if all of the channels in the next cell are tied up, the caller is abruptly cut off with no warning.
The technique devised by the Purdue engineers is called a "localized channel-sharing scheme." It would allow two adjacent cells to share many of the same channels, ensuring a continuity of service as a caller traveled from one cell to the next. Essentially, the channel itself would move with the caller into the adjacent cell.
"If I am driving and I start getting too far from the base station, I must at some point be handed off from one base station to the next," Chong says. "In our scheme, you cooperate with your neighboring cell in a way that you can share channels."
Channels of the same frequency cannot be used in cells that are too close to each other, in much the same way that radio stations with the same frequencies must be separated by long distances to prevent them from interfering with each other. To solve this problem, the sharing scheme combines two or more adjacent cells in new "meta-cells." Each of the adjacent cells within a meta-cell can then use the same channels without interference.
"Once you have a system like that, handoff becomes very easy," Chong says.
The channel-sharing technique has not yet been field tested in real-life applications, but computer simulations showed that it would make cell phone users up to 100 times less likely to be cut off, compared with conventional service. Simulations also showed that the technique could increase the network's overall efficiency by up to 60 percent, Chong says.
The technique was detailed in a paper that appeared in December in the journal Wireless Networks. It was written by Junyi Li, a former graduate student, Ness B. Shroff, an assistant professor, and Chong, all from the Purdue School of Electrical and Computer Engineering.
The research was funded by the National Science Foundation.
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