Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-03-31
2001-07-10
Chang, Vivian (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000, C455S442000
Reexamination Certificate
active
06259918
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a cellular telephone network implementing a smart antenna technology in addition to sector antenna technology at its base stations and, in particular, to a hand-off procedure for use in connection with such a network, wherein the handoff procedure preserves the cell borders defined for each cell by sector antenna operation.
2. Description of Related Art
It is well known in the art to utilize directive antennas in cellular communications networks. The most commonly recognized example of directive antenna use in cellular communications networks is based on the principle of sectorization, as is illustrated in
FIG. 1. A
cell site
10
may comprise either one omnidirectional cell or a plurality, for example, three (or more), sector cells
12
. Directive antennas
14
, each with an appropriately selected beamwidth for the sector cell
12
, are then utilized at each base station
16
to form a plurality of wide beams
18
, one per sector cell, with the totality of the beams formed thereby providing substantially omni-directional radio frequency coverage throughout the cell site area. In operation, each of the formed wide beams
18
is in continuous use to provide service within each corresponding sector cell
12
.
Another example of directive antenna use in cellular communications networks is based on the use of smart antenna technology, as is illustrated in FIG.
2
A. Directive antennas
20
are utilized at each base station
16
of a cell site
10
to form a plurality of separate, perhaps slightly overlapping, narrow beams
22
within each sector cell
12
, with the totality of the beams formed thereby providing substantially omni-directional radio frequency coverage throughout the cell site area. In operation, and in contrast to the operation of the sectorized beams
18
of
FIG. 1
, the narrow beams
22
are intermittently used only when necessary to provide service to one or more mobile stations
24
, as is illustrated in FIG.
2
B. Put another way, in smart antenna technology, the base station
16
controls its directive antenna
20
to activate at any given time only those individual ones of the plurality of separate, perhaps slightly overlapping, narrow beams
22
as are needed to serve active mobile stations
24
within the cell site
10
.
Reference is now made to
FIG. 3
wherein there is shown a diagram of directive antenna beam coverage from adjacent cells
12
. Suppose a first cell
12
(
1
) includes a directive (sector) antenna
14
, having an appropriately selected beamwidth for the sector, at its base station
16
(
1
) that is operable to provide a sector coverage beam
18
to serve a mobile station
24
. Suppose further that a second, adjacent, cell
12
(
2
) includes a directive (smart) antenna
20
at its base station
16
(
2
) that is operable to form, at any one time, a narrow beam
22
to serve a mobile station
24
. The theoretical (or ideal) hand-off border
26
between the first cell
12
(
1
) and the second cell
12
(
2
) would lie approximately half-way between the base stations
16
for the respective cells
12
and would be defined by approximate interaction of the sector coverage beam
18
in the first cell and a theoretical (or perhaps physical, if present) sector coverage beam
18
′ in the second cell.
It is noted here that the theoretical border
26
is illustrated in the manner of a zone between lines
26
(
1
) and
26
(
2
) to account for the fact that hysteresis values, as is well known in the art, affect the relative location between the base stations where hand-off would actually occur. More specifically, because of the introduced hysteresis value, which must be met by the signal strength measurements made with respect to, and compared between, the two cells
12
, a mobile station
24
moving from the first cell
12
(
1
) into the second cell
12
(
2
) would not actually achieve a hand-off until at least reaching line
26
(
2
). Conversely, a mobile station
24
moving from the second cell
12
(
2
) into the first cell
12
(
1
) would not actually achieve a hand-off until at least reaching line
26
(
1
).
One advantage of the use of smart antennas
20
is the extended range of coverage, as generally indicated at
28
, obtained when compared to the range of coverage provided by sector antennas
14
. One consequence of this extended coverage range
28
is a disturbance in location of the cell border
26
, as generally shown at
30
. A number of drawbacks arise from such a disturbance
30
in the cell border
26
. First, the base station
16
(
2
) tends to provide service to mobile stations
24
which are not located within or near its cell
12
(
2
), and thus service load between the cells is not properly shared and system management issues become too complex. Second, the base station
16
(
2
) downlink broadcasts made from the second cell
12
(
2
) in order to provide service to a distant mobile station may be made at such a high power level as to inject downlink co-channel interference into other cells within the network which reuse its same frequency. Third, distant mobile station
24
uplink broadcasts may be made at such a high power level as to more quickly and unnecessarily drain battery life. Further, because the mobile station is in the vicinity of a cell other than cell
12
(
2
), uplink co-channel interference will also increase in the reuse cells.
There is accordingly a need for a hand-off technique that accounts for the extended coverage range
28
of smart antennas
20
in comparison to sector antennas
14
and thus corrects for any introduced disturbance
30
in location of the cell border
26
in making the hand-off determination.
SUMMARY OF THE INVENTION
In connection with making a determination to hand-off a mobile station, the mobile station makes downlink signal strength measurements on its own serving traffic channel as well as the measurement (control) channels of at neighboring cells. Hand-off is appropriate if the signal strength measurement on a neighboring cell's measurement channel exceeds the signal strength measurement for the own serving cell's traffic channel (as off-set by any imposed hysteresis value). In connection with measurements made on measurement and traffic channels with respect to cells possessing both sector antenna and smart antenna capabilities, however, these downlink signal strength measurements have to be adjusted to take into account the operational and physical differences between sector antennae and smart antennae.
Turning first to the downlink signal strength measurement on the own serving cell's traffic channel: in accordance with one aspect of the present invention, the downlink signal strength measurement on the own serving cell's traffic channel is adjusted by a first factor accounting for any noted difference in gain between the sector antenna and smart antenna of that cell as a function of azimuth angle of orientation of the mobile station to the serving cell's base station. In accordance with another aspect of the present invention, the downlink signal strength measurement on the own serving cell's traffic channel is adjusted by a second factor accounting for any difference between the output power backoff value for the smart antenna traffic channels (with respect to the digital control channel) and the output power backoff value for the sector antenna traffic channels (again with respect to the digital control channel). In accordance with yet another aspect of the present invention, the downlink signal strength measurement on the own serving cell's traffic channel is adjusted by a third factor accounting for power control attenuation provided by the serving base station on the serving traffic channel.
Turning next to the signal strength measurement on a neighboring cell's measurement channel: in accordance with one aspect of the present invention, the signal strength measurement on a neighboring cell's measurement channel is adjusted by a f
Desgagne Michel
Kongara Gopichand
Labonte Sylvain
Minichiello Pat
Chang Vivian
Jenkens & Gilchrist
Nguyen Duc
Telefonaktiebolaget LM (publ)
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