Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-12-15
2004-06-22
Tran, Sinh (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000, C455S438000, C455S442000, C370S329000, C370S331000, C370S332000
Reexamination Certificate
active
06754493
ABSTRACT:
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 significant overlap 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 may be employed in radiocommunication systems using any type of access methodology, e.g., Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), or any hybrid thereof. For the purposes of illustration, rather than limitation, this description shall primarily describe conventional techniques and techniques according to the present invention in terms of CDMA systems, however those skilled in the art will appreciate that the inventive techniques are equally applicable to systems employing any access methodology. In radiocommunication systems employing a CDMA access technique, a mobile station can be concurrently connected to one or more sectors that belong to one or more base stations. As mentioned above, the sectors which the mobile station uses for communication are called “active set”. Consider, as an example, the system illustrated in
FIG. 4
having cells C
1
and C
2
with the sectors S
1
. . . S
6
each. Let A, B and E denote the sectors C
1
-S
1
, C
1
-S
2
and C
2
-S
5
, respectively, that the mobile station
400
is connected to, i.e., mobile station
400
's active set.
Due to the movement of the mobile station
400
(and possibly other influences), the quality of each connection is time variant. Quality in this context refers to one or more specific types of measurement, e.g., the received signal strength or the downlink signal-to-interference ratio. To adapt to the time variance in quality of a connection, the mobile station
400
is provided with a set of sectors whose transmissions are to be monitored with respect to their received signal quality. This set is called the measurement set. By periodically evaluating the quality of the sectors in the measurement set, the mobile station
400
identifies a set of sectors that are suitable to be members of the active set, referred to as the candidate set. When the membership of the current active set and that of the candidate set differ from each other, the mobile station
400
sends a measurement report to the system, e.g., to a radio network controller (RNC, not shown). The measurement report contains information regarding the quality of transmission received from the sectors of the candidate set. The RNC then decides whether to perform a hard or soft handover, i.e., which sectors shall be added to and/or deleted from the active set. The RNC also requests setup and release of radio and network resources for the connection between the mobile station and the relevant base station(s)in order to perform the handoff. After the handoff has been accomplished, the contents of the active set are updated in the mobile station (and the RNC).
However, there exist a number of different
Davis Temica M.
Telefonaktiebolaget LM Ericsson
Tran Sinh
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