Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-06-10
2003-05-06
Olms, Douglas (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C370S337000, C455S436000
Reexamination Certificate
active
06560210
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to telecommunications in general, and, more particularly, to a technique for handing off a wireless terminal from one base station to another as the wireless terminal moves within a wireless telecommunications system.
BACKGROUND OF THE INVENTION
FIG. 1
depicts a schematic diagram of a portion of a wireless telecommunications system in the prior art, which system provides wireless telecommunications service to a number of wireless terminals (e.g., wireless terminals
101
-
1
through
101
-
3
) that are situated within a geographic region. The heart of a wireless telecommunications system is a wireless switching center (“WSC”), which also may be known as a mobile switching center or mobile telephone switching office. Typically, a wireless switching center (e.g., WSC
120
) is connected to a plurality of base stations (e.g., base stations
103
-
1
through
103
-
5
) that are dispersed throughout the geographic region serviced by the system and to the local and long-distance telephone and data networks (e.g., local-office
130
, local-office
138
and toll-office
140
). A wireless switching center is responsible for, among other things, establishing and maintaining a call between a first wireless terminal and a second wireless terminal or, alternatively, between a wireless terminal and a wireline terminal (e.g., wireline terminal
150
), which is connected to the system via the local and/or long-distance networks.
The geographic region serviced by a wireless telecommunications system is partitioned into a number of spatially distinct areas called “cells.” As depicted in
FIG. 1
, each cell is schematically represented by a hexagon. In practice, however, each cell has an irregular shape that depends on the topography of the terrain surrounding the cell. Typically, each cell contains a base station, which comprises the radios and antennas that the base station uses to communicate with the wireless terminals in that cell and also comprises the transmission equipment that the base station uses to communicate with the wireless switching center.
For example, when a user of wireless terminal
101
-
1
desires to transmit information to a user of wireless terminal
101
-
2
, wireless terminal
101
-
1
transmits a data message bearing the user's information to base station
103
-
1
. The data message is then relayed by base station
103
-
1
to wireless switching center
120
via wireline
102
-
1
. Because wireless terminal
101
-
2
is in the cell serviced by base station
103
-
1
, wireless switching center
120
returns the data message back to base station
103
-
1
, which relays it to wireless terminal
101
-
2
.
Typically, a wireless terminal communicates with the base station that is nearest to it. But because a wireless terminal can move from cell to cell, the base station that the wireless terminal is nearest to can change. Therefore, a wireless terminal usually communicates with different base stations as it moves from cell to cell. For example, if wireless terminal
101
-
1
exits from the cell serviced by base station
103
-
1
and moves into the cell serviced by base station
103
-
2
, wireless terminal
101
-
1
ends communication with base station
103
-
1
and begins communication with base station
103
-
2
. The process in which a wireless terminal ends communication with one base station and begins communication with another is, for the purposes of this specification, defined as a “hand off.”
Although there are several techniques in the prior art for performing a hand off, all of the techniques are similar in that they attempt to orchestrate the process so deftly that there is no perceptible interruption in service.
One technique in the prior art for performing a hand is known as “hard hand off.” Before a hard hand off, a wireless terminal uses one pair of communications channels (one for transmitting, the other for receiving) for communicating with a first base station. At the moment when the wireless terminal exits the cell serviced by the first base station and enters the cell serviced by a second base station, the wireless terminal re-tunes its radio from the first pair of communications channels to a second pair of communications channels for communicating with a second base station. Furthermore, at the same moment that the wireless terminal re-tunes its radio, the wireless switching center stops using the first base station for communication with the wireless terminal and begins using the second base station.
For example,
FIG. 2
depicts a block of spectrum that has been partitioned into four frequency bands, channel #
1
through channel #
4
. If wireless switching center
120
assigns channels #
1
and #
2
to base station
103
-
1
and channels #
3
and #
4
to base station
103
-
2
, then base station
103
-
1
could transmit in channel #
1
and receive in channel #
2
, and base station
103
-
2
could transmit in channel #
3
and receive in channel #
4
. Because neither base station
103
-
1
nor
103
-
2
use the same channels for communicating, there is no co-channel interference between the base stations.
To continue with the example, assume that when wireless terminal
101
-
1
is in the cell serviced by base station
103
-
1
, wireless terminal
101
-
1
uses channels #
1
and #
2
to communicate with base station
103
-
1
. Thereafter, when wireless terminal
101
-
1
exits the cell serviced by base station
103
-
1
and moves into the cell serviced by base station
103
-
2
, wireless terminal
101
-
1
ends communications in channels #
1
and #
2
and begins communication in channels #
3
and #
4
with base station
103
-
4
. Furthermore, at the moment that wireless terminal
101
-
1
re-tunes from channels #
1
and #
2
to channels #
3
and #
4
, wireless switching center
120
stops routing the signals intended for wireless terminal
101
-
1
to base station
103
-
1
and begins routing the signals to base station
103
-
2
. And still furthermore, at the moment that wireless terminal
101
-
1
re-tunes, wireless switching center
120
expects the signals from wireless terminal
101
-
1
to be received from base station
103
-
2
, rather than from base station
103
-
1
.
Hard hand off is advantageous in that it can be used with all multiplexing technologies (e.g., frequency-division multiplexing (hereinafter “FDM”), time-division multiplexing (hereinafter “TDM”), code-division multiplexing (hereinafter “CDM”), etc.) and all access technologies (e.g., frequency-division multiple access (hereinafter “FDMA”), time-division multiplex access (hereinafter “TDMA”), code-division multiple access (hereinafter “CDMA”), etc.). Hard hand off is further advantageous in that it is a rather simple process to implement.
Hard hand off is, however, disadvantageous in that it is a fragile process. If the re-tuning of wireless terminal
101
-
1
is not precisely synchronized with the switching from base station
103
-
1
to base station
103
-
2
, then there could be an interruption in service. Such a lapse is not only an inconvenience, but also creates the perception that the quality of service offered by the wireless telecommunications system is inferior. This, in turn, causes customers to use competing services and/or to use less of the service than they might otherwise. The net result is that the owner of the wireless telecommunications system loses revenue because hand off in the prior art is such a fragile process.
A second technique in the prior art for handing off a wireless terminal is known as “soft hand off.” Before a soft hand off, a wireless terminal uses one pair of communications channels (one for transmitting, the other for receiving) for communicating with a first base station. Before the wireless terminal exits the cell serviced by the first base station and enters the cell serviced by the second base station, the wireless terminal begins communications with the second base station on a second pair of channels. T
DeMont & Breyer LLC
Lucent Technologies - Inc.
Phunkulh Bob A.
LandOfFree
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