Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1999-03-24
2002-09-24
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C455S436000
Reexamination Certificate
active
06456606
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The invention relates generally to wireless communications. More particularly, the invention relates to handoff control in a wireless communication system.
II. Description of the Relaxed Art
FIG. 1
is an exemplifying embodiment of a terrestrial wireless communication system
10
.
FIG. 1
shows the three remote units
12
A,
12
B and
12
C and two base stations
14
. In reality, typical wireless communication systems may have many more remote units and base stations. In
FIG. 1
, the remote unit
12
A is shown as a mobile telephone unit installed in a car.
FIG. 1
also shows a portable computer remote unit
12
B and the fixed location remote unit
12
C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, remote units may be any type of communication units. For example, the remote units can be hand-held personal communication system units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment.
FIG. 1
shows a forward link signal
18
from the base stations
14
to the remote units
12
and a reverse link signal
20
from the remote units
12
to the base stations
14
.
An industry standard for a wireless system using code division multiple access (CDMA) is set forth in the TAU/EIA Interim Standard entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/ELA/IS-95, and its progeny (collectively referred to herein as IS-95), the contents of which are also incorporated herein by reference. More information concerning a code division multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated in its entirety herein by this reference.
In an IS-95 system, each base station synchronizes its operation with other base stations in the system. For example, in one embodiment, the IS-95 base stations synchronize operation to a universal time reference such as Global Positioning Satellites (GPS) signaling. Based upon the synchronizing time reference, each base station in a given geographical area is assigned a sequence offset of a common pseudo noise (PN) pilot sequence. For example, according to IS-95, a PN sequence having 2
15
chips and repeating every 26.66 milliseconds (ms) is transmitted by each base station in the system at one of 5
12
PN sequence offsets as a pilot signal. According to IS-95 operation, the base stations continually transmit the pilot signal which can be used by the remote units to identify the base stations as well as for other functions.
Various methods exist for transferring communication with the remote unit from one base station to another through a process known as handoff. Handoff may be necessary if a remote unit operating in the coverage area of an original base station moves into the coverage area of a target base station. One method of handoff used in CDMA systems is termed a “soft” handoff. Through the use of soft handoff, communication with the target base station is established before termination of communication with the original base station. When the remote unit is communicating with two base stations, both the remote unit and base stations create a single signal from the multiple received signals. Through the use of soft handoff, communication between the remote unit and the end user is uninterrupted by the eventual handoff from the original base station to the target base station. U.S. Pat. No. 5,267,261 entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM,” which is assigned to the assignee of the present invention and which is incorporated herein, discloses a method and system for providing communication with the remote unit through more than one base station during the handoff process. Further information concerning handoff is disclosed in U.S. Pat. No. 5,101,501, entitled “METHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM”, U.S. Pat. No. 5,640,414, entitled “MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR COMMUNICATIONS SYSTEM”, and U.S. Pat. No. 5,625,876 entitled “METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION,” each of which is assigned to the assignee of the present invention and incorporated in its entirety herein by this reference. The subject matter of U.S. Pat. No. 5,625,876 concerns so-called “softer handoff.” For the purposes of this document, the term “soft handoff” is intended to include both “soft handoff” and “softer handoff.”
As described in the above mentioned patents, remote unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the remote unit: the active set, the neighbor set, the candidate set and remaining set. The active set is the set of base stations through which active communication is established. The neighbor set is a set of base stations surrounding the active base stations and comprising base stations that have a high probability of having a pilot signal strength of sufficient level to establish communication. The candidate set is a set of base stations having a pilot signal strength of sufficient level to establish communication but through which active communication is not yet established. The remaining set is a set of base stations which are not in any of the other three sets.
The remote unit uses these sets to control the handoff process. In this example, we shall assume that when communications are initially established, a remote unit communicates through a first base station, and the active set contains only the first base station; although in many cases, the active set contains more than one base station before the handoff process is begun with respect to yet another base station. The remote unit monitors the pilot signal strength of the base stations in the active set, the candidate set, the neighbor set and the remaining set. When a pilot signal strength of a base station in the neighbor or remaining set exceeds a predetermined threshold level, the base station is added to the candidate set and removed from the neighbor or remaining set at the remote unit. The remote unit communicates a pilot strength measurement overhead message through the first base station identifying the new base station. A system controller receives the pilot strength measurement overhead message from the first base station and decides whether to establish communication between the new base station and the remote unit. Should the system controller decide to do so, the system controller sends a message to the new base station with identifying information about the remote unit and a command to establish communications with the remote unit.
A handoff message is also transmitted to the remote unit through the first base station. The handoff message is an overhead message which identifies a new active set that includes the first and the new base stations. The handoff message also identifies which channel has been allocated for use by the remote unit with the new base station. The remote unit searches for the new base station's transmitted signal, and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations such that two or more base stations are in the active set.
When the remote unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the active set, the candidate set, the neighbor set and the remaining set. Should the signal strength corresponding to a base station of the active set drop below a predetermined threshold for a predetermined period of time, the remote unit generates and transmits an overhead message to report the event. T
Baker Kent D.
English Sean
Kizou Hassan
Qualcomm Incorporated
Wadsworth Philip R.
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