Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2000-02-25
2004-04-20
Yao, Kwang Bin (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
Reexamination Certificate
active
06724739
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method for enabling a mobile station to execute a handoff between an asynchronous base station and a synchronous base station in a CDMA wireless communication system.
II. Description of the Related 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 unit. For example, the remote units can be hand-held personal communication system units, portable data units such as personal data assistants, 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 TIA/EIA Interim Standard entitled “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, TIA/EIA/IS-95, and its progeny (collectively referred to here in 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.
Third-generation CDMA wireless communications systems have also been proposed. The cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission proposal forwarded by the Telecommunications Industry Association (TIA) to the International Telecommunication Union (ITU) for consideration for the IMT-2000 CDMA standard is an example of such a third-generation wireless communication system. The standard for cdma2000 is given in draft versions of IS-2000 being generated by TR45 of the TIA. The cdma2000 proposal is compatible with IS-95 systems in many ways. For example, in both the cdma2000 and IS-95 systems, each base station time-synchronizes its operation with other base stations in the system. Typically, the base stations synchronize operation to a universal time reference such as Global Positioning Satellites (GPS) signaling; however, other mechanisms can be used. 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 512 PN sequence offsets as a pilot signal. 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.
Base station time-synchronization as provided in the cdma2000 and IS-95 systems has many advantages with respect to system acquisition and handoff completion time. Synchronized base stations and time-shifted common pilot signals as discussed above permit a fast one-step correlation for system acquisition and detection of neighboring base stations. Once the mobile station has acquired one base station, it can determine system time which is the same for all neighboring synchronous base stations. In this case, there is no need to adjust the timing of each individual mobile station during a handoff between synchronous base stations. Additionally, the mobile station does not need to decode any signal from the new base station in order to obtain rough timing information prior to handing off.
Another recently-proposed 3G communication system is referred to as W-CDMA. One example of a W-CDMA system is described in the ETSI Terrestrial Radio Access (UTRA) International Telecommunications Union (ITU) Radio Transmission Technology (RTT) Candidate Submission forwarded by ETSI to the ITU for consideration for the IMT-2000 CDMA standard. The base stations in a W-CDMA system operate asynchronously. That is, the W-CDMA base stations do not all share a common universal time reference. Different base stations are not time-aligned. As a result, W-CDMA base stations employ a 3-step acquisition procedure with multiple parallel correlations in each step. In the W-CDMA system, each base station transmits a “synchronization” channel that comprises two sub-channels. The first of the two sub-channels, the primary synchronization channel, uses a primary synchronization code, c
p
, that is common to all base stations. The second of the two sub-channels, the secondary synchronization channel, uses a cyclic set of secondary synchronization codes, c
s
, that are not shared by other base stations that are not in the same code group. The mobile station in a W-CDMA system can acquire the synchronization channel of one or more base stations by searching for the primary synchronization code, c
p
of the primary synchronization channel, and then using the timing information derived from the primary synchronization channel to process the secondary synchronization channel.
Recently, a combined CDMA IMT-2000 standard has been proposed in which cdma2000 -compliant equipment and W-CDMA-compliant equipment may be optionally supported by any manufacturer. Thus, it is expected that synchronous base stations of a cdma2000 -compliant system will be geographically located near asynchronous base stations of a W-CDMA-compliant system. This creates a need to be able to handoff a mobile station that supports both cdma2000 and W-CDMA operation between the asynchronous base stations of a W-CDMA system and the synchronous base stations of a cdma2000 system, and vice versa.
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.” U.S. Pat. No. 6,456,606, entitled “HANDOFF CONTROL IN AN ASYNCHRONOUS CDMA SYSTEM”, assigned to the assignee of the present invention and incorporated in its entirety herein by reference, also provides additional information on handoff procedures involving both synchronous and asynchronous base stations.
Each base station is associated with a set of neighboring base stations surrounding the base station. Due to the physical proximity of the coverage areas of the neighboring base stations to the coverage area of the active base station, the remote units which are communicating with the
Sarkar Sandip
Terasawa Daisuke
Tiedemann Jr. Edward G.
Baker Kent D.
Jones Prenell
Martin Roger W.
QUALCOMM Incorporated
Wadsworth Philip R.
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