Method and apparatus for performing handoff in a...

Details Method and apparatus for performing handoff in a... Method and apparatus for performing handoff in a...

1996-05-10

2002-09-10

Bocure, Tesfaldet (Department: 2631)

Pulse or digital communications

Spread spectrum

Direct sequence

C455S436000

Reexamination Certificate

active

06449305

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to wireless communication systems and, in particular, to handoff in a spread-spectrum communication system.
BACKGROUND OF THE INVENTION
Wireless communication systems are known to employ handoff methods in which a remote unit (such as a cellular subscriber) is handed off between base stations within the communication system. In particular, when a remote unit moves to the periphery of the serving base station's coverage area, the increase in path losses between the serving base stations and the remote unit may create a situation in which an adjacent base station can better serve the remote unit. As described in the Electronic Industry Association/Telecommunications Industry Association Interim Standard 95A (TIA/EIA/IS-95A), one such communication system employing handoff is a Code-Division, Multiple-Access (CDMA) spread-spectrum communication system. (EIA/TIA can be contacted at 2001 Pennsylvania Ave. NW Washington D.C. 20006). Handoff in a CDMA communication system may be accomplished by simultaneously monitoring several base stations (referred to as an active set) and replacing a base station within the active set if a base station with a stronger signal is encountered. Although TIA/EIA/IS-95A provides for a method for handoff between a CDMA base station and a base station utilizing other communication system protocols (such as the Advanced Mobile Phone Service (AMPS) protocol), TIA/EIA/IS-95A does not provide when such handoff should occur. Because of this, it has been left up to equipment manufacturers to develop methods for handing off from CDMA to other system protocols.
A prior-art method for handoff from a CDMA to an AMPS protocol is illustrated in reference to FIG.
1
. Referring to
FIG. 1
, communication system
100
includes a plurality of CDMA base stations
101
-
107
having respective coverage areas
122
-
128
. Communication system
100
additionally includes a plurality of AMPS base stations
109
-
115
having respective coverage areas
126
-
132
. Remote unit
117
is capable of operating on both an AMPS and a CDMA protocol. At time t=1, remote unit
117
, traveling along path
119
, is in soft handoff (i.e. communicating with more than one base station) with non-border CDMA base stations
101
and
103
. (Non-border base stations are those CDMA base stations with no corresponding AMPS coverage area). At time t=2, remote unit
117
moves into coverage area
126
and continues in a soft-handoff state communicating with non-border CDMA base stations
101
and
103
, and additionally communicating with border CDMA base station
105
. At time t=3, remote unit
117
moves out of coverage area
124
and is placed in soft handoff with non-border CDMA base station
101
and border CDMA base station
105
. Finally, at time t=4, remote unit travels out of coverage area
122
communicating only with border CDMA base station
105
.
The prior-art method of handoff from a CDMA to an AMPS protocol immediately hands off remote unit
117
to AMPS base station
109
when remote unit
117
is no longer communicating with a non-border CDMA base station. In other words, remote unit
117
is handed off to AMPS base station
109
immediately upon losing communication with non-border CDMA base station
101
. This prior-art method of handoff from a CDMA to an AMPS protocol is inefficient in that CDMA capacity may be wasted by prematurely handing off remote unit
117
to AMPS base station
109
.
Thus a need exists for a method and apparatus for handoff in a spread-spectrum communication system that overcomes the deficiencies of the prior art.


REFERENCES:
patent: 5054035 (1991-10-01), Tarallo et al.
patent: 5101501 (1992-03-01), Gilhousen et al.
patent: 5293643 (1994-03-01), Israelsson
patent: 5313489 (1994-05-01), Menich et al.
patent: 5491717 (1996-02-01), Hall
patent: 5548808 (1996-08-01), Bruckert et al.
patent: WO 97/43837 (1997-10-01), None

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