Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-12-15
2003-03-25
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000, C455S438000, C455S440000, C370S331000, C370S332000
Reexamination Certificate
active
06539227
ABSTRACT:
This application claims priority under 35 U.S.C. §§119 and/or 365 to PCT/IB98/02076 filed in WIPO on Dec. 18, 1998; the entire content of which is hereby incorporated by reference.
BACKGROUND
The present invention relates generally to methods and systems for radiocommunications and, more particularly, to such systems in which a connection can be handed over from one channel or base station to another.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
In cellular systems, the capability is typically provided to transfer handling of a connection between, for example, a mobile station and a base station to another base station, as the mobile station changes its position and so moves out of the coverage area of one base station and into the coverage area of another base station. This type of handoff is commonly referred to as an “intercell” handoff as the coverage areas associated with base stations are commonly referred to as “cells”. Depending upon the quality of the current channel, it may also be desirable to transfer a connection from one channel of the base station to another channel supported by the same base station, which handoffs are commonly referred to as “intracell” handoffs.
So-called “hard” handoffs refer to handoffs which are performed wherein there is no overlap in time between transmissions received from an original, serving base station and transmissions received from a new, target base station. As shown in FIG.
1
(
a
), during hard handoff, the mobile station (MS) typically first breaks its connection to its original base station (BTS
1
) and then establishes a connection to its new base station (BTS
2
).
By way of contrast, “soft” handoffs refer to handoffs wherein, for some period of time, a mobile station receives substantially the same information from two (or more) transmission sources. An exemplary soft handoff scenario is illustrated in FIG.
1
(
b
). Therein, before starting soft handoff, the MS is connected to BTS
1
. During the soft handoff, the MS establishes a connection to BTS
2
without dropping the connection to BTS
1
. Each base station which is concurrently communicating with a particular mobile station may be referred to as a member of that mobile station's “active set”. At some time after the connection to BTS
2
is set up, the connection to BTS
1
will be released which is the termination of the soft handover procedure. The overlapping transmissions from BTS
1
and BTS
2
permit the mobile station to smoothly switch from receiving information from its original, serving base station to receiving information from its new, target base station. During soft handoff, the mobile station may also take advantage of the fact that it is receiving substantially the same information from two sources to improve its received signal quality by performing diversity selection/combining of the two received signals.
For the sake of simplicity, the foregoing examples of the hard and soft handoff were described in the context of base stations employing omnidirectional antennas, i.e., wherein each base station transmits signals which propagate in a substantially circular direction, i.e., 360 degrees. However, as will be appreciated by those skilled in the art, other antenna structures and transmission techniques may also be employed in radiocommunication systems. For example, a cell can be subdivided into several sectors, e.g., into three sectors where each sector covers a 120 degree angle as shown in FIG.
2
. Alternatively, the system or cell may employ an array antenna structure as shown in FIG.
3
. Therein, an exemplary radio communication system
200
includes a radio base station
220
employing a fixed-beam phased array (not shown). The phased array generates a plurality of fixed narrow beams (B
1
, B
2
, B
3
, B
4
, etc.) which radially extend from the base station
220
, at least one of which (B
1
) is used to communicate with MS
210
. Preferably, the beams overlap to create a contiguous coverage area to service a radio communication cell. Although not shown, the phased array can actually consist of three phased array sector antennas.
Of course, the principles described above with respect to hard and soft handoff for omnidirectional antennas in FIGS.
1
(
a
) and
1
(
b
) can be directly mapped to other systems which employ sectorized and/or array antennas. In these latter types of systems, hard and soft handoffs can be performed between sectors or beams of the same base station as well as between sectors or beams associated with different base stations.
Both types of handoff have their drawbacks and advantages. On the one hand, soft handoff provides a robust mechanism for changing the connection from one base station to another. However, since the mobile station is connected to more than one base station during soft handoff, soft handoff requires more system resources than hard handoff. An advantage of hard handoff, therefore, is a reduced need for system resources, while its drawback is a higher probability of dropped calls when compared to soft handoff.
Both hard and soft handoffs are employed in some radiocommunication systems. For example,
FIG. 4
illustrates a system described in WO 96/02117, wherein soft and hard handoff are applied sequentially. Therein, a system containing two base station controllers, BSC
1
and BSC
2
, is shown. BSC
1
controls base stations BTS
11
, BTS
12
and BTS
13
, while BSC
2
controls base stations BTS
21
, BTS
22
, and BTS
23
. The area that is served by all of the base stations coupled to a BSC is called a “BSC area”.
Assume for this example, that the mobile station (MS) moves from cell A served by the base station BTS
12
to cell B, which is at the border between two BSC areas. Cell B is served by two overlapping base stations, BTS
11
and BTS
21
. BTS
11
is coupled to controller BSC
1
, and BTS
21
is coupled to base station controller BSC
2
. As the MS moves to cell B, it carries out a soft handoff controlled by BSC
1
to a traffic channel of base station BTS
11
.
Assume further that the MS continues onward toward cell C and finally enters into its area of radiocommunication coverage. The base station BTS
22
, serving cell C, is under the control of BSC
2
. Before it is possible to activate the base station BTS
22
for the handoff, the call control must first be switched to base station controller BSC
2
from the previous controller BSC
1
. This is accomplished by performing a hard handoff. The MS performs a hard handoff from the base station BTS
11
to the base station BTS
21
, and consequently, the base station controller change from BSC
1
to BSC
2
takes place. Finally, a soft handoff from BTS
21
to BTS
22
is performed.
However, these techniques described in WO 96/02117 do not provide a mechanism for controlling the use of either soft or hard handoff. Instead, these techniques are simply provided as an intended mechanism for reducing interference and signaling overhead associated with the handoff of a mobile area from a service area under the control of a first BSC to a service area under the control of a second BSC. Thus, these techniques do not provide any solution for controlling the usage of soft and hard handoff between cells per se.
According to European Patent Application 817 517 A1, as illustrated in
FIG. 5
, a technique is presented for determining an appropriate type of handoff for a mobile station. In the Figure, the received perch channel (i.e., a type of broadcast control channel) level is shown for the cell where the MS resides initially (solid line) as well as for a neighboring cell (dashed line). The received levels are given with respect
Blomberg Petter
Butovitsch Peter
Jetzek Ulrich
Johansson Lars B.
Jonsson Sture
Davis Temica M.
Telefonaktiebolaget LM Ericsson (publ)
Urban Edward F.
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