AVOID ROAMING CELL PHONE DISCONNECT
     Despite amazing advances in cell phone technology that have propelled the
wireless devices toward being the only kind of phone we have, they can still be
annoying. One of the main irritations is the way you get cut off, sometimes
while driving even short distances. Engineers at Purdue think they have that one
fixed. Not that you will never again be cut off while roaming, but you will be
100 times less likely to have it happen.
     Cut-offs happen when you cross cell boundaries. The wireless phones depend
on a series of base stations that provide service to separate cells, regions
arranged in a honeycomb pattern over the larger service area. Each cell provides
hundreds of channels, each an individual frequency used by one caller. As mobile
phone users cross cell boundaries, they are handed off to a new channel in the
adjacent cell. This is all automatic and seamless--unless all the channels in
the next cell are tied up. Then the caller is abruptly cut off with no warning.
     The Purdue engineers came up with a localized channel-sharing scheme that
increases system capacity without compromising quality of service. Essentially,
it lets channels be shared between adjacent cells, as needed.
     Although frequencies are used over and over again by different cells,
channels used in one cell cannot be used in other cells that are closer than the
minimum reuse distance. The Purdue scheme would allow two adjacent cells to
share many of the same channels, ensuring a continuity of service as you roam
from cell to cell. The channel itself moves with the caller.
     There are two ways channels are allocated now. They are fixed permanently
within a cell and can only be reused in cells the minimum reuse distance away.
This is simple compared to dynamic channel allocation, the other way. But
dynamic channel allocation improves system capacity by exploiting the traffic
fluctuation in different cells. Complexity goes up at least as fast and channel
use must be closely coordinated among cells.
     Purdue's localized channel sharing allows channels to be shared between
adjacent cells. Any user in either cell can use the channel without coordinating
with other cells for the use of the same channels. This way, channel management
can be localized within adjacent cells. To avoid the need for global
optimization, a new fixed-channel assignment scheme is proposed to keep the
co-channel reuse distance as close as possible while allowing channel sharing.
     Two or more adjacent cells are combined in new meta-cells. Each adjacent
cell within a meta-cell can then use the same channels without interference.
Channels are so assigned that each base station is responsible for transmitting
over only a portion of the entire channel set.
     So far the channel-sharing technique has not been field-tested in real-life
applications. Computer simulations show that it could make cell phone users as
much as 100 times less likely to be cut off. The simulations also show that the
technique could increase network efficiency as much as 60%.
     Details: Edwin K. P. Chong, Professor, School of Electrical and Computer
Engineering, Purdue University, 1285 Electrical Engineering Bldg., West
Lafayette, IN 47907-1285. Phone: 765-494-9143. Fax: 765-494-3371. E-mail:
[see Edwin K. P. Chong's web page].
E000070
0400
Copyright 2000, John Wiley & Sons, Inc., New York, NY 10158