Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
2000-02-25
2001-06-12
Ngo, Ricky (Department: 2731)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S335000, C455S436000
Reexamination Certificate
active
06246673
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 and apparatus for performing handoff synchronization between synchronous and asynchronous base stations in a CDMA wireless communication system.
II. Description of the Related Art
FIG. 1
a
is an exemplifying embodiment of a wireless communication system having a terrestrial wireless network
140
in communication with a wireless mobile station
110
. Wireless network
140
is shown with two wireless base stations
120
and
130
such that mobile station
110
can communicate with either and can handoff between the two. The wireless communication channel through which information signals, travel from mobile station
110
to a base station
120
or
130
is known as a reverse link. The wireless communication channel through which information signals travel from a base station
120
or
130
to mobile station
110
is known as a forward link.
Though only one serving base station is shown, an MS may communicate with multiple serving base stations in soft handoff and may establish a handoff to multiple target base stations. Mobile station
110
communicates exclusively with one or more serving base stations
120
before handoff is established. After handoff is established, mobile station
110
may communicate in “soft handoff” with both serving base stations
120
and target base stations
130
. Alternatively, following a “hard handoff,” mobile station
110
communicates exclusively with target base stations
130
. In reality, typical wireless communication systems may have many more mobile stations and base stations than shown. Wireless network
140
includes one or more base station controllers or BSC's (not shown) and one or more mobile switching centers or MSC's (not shown). Serving base station
120
may be connected to a different BSC and MSC than target base station
130
, or the two base stations may share the same BSC and MSC.
Mobile station
110
may be any of a number of different types of wireless communication devices such as a portable phone, a wireless communication module incorporated into a portable computer, or a fixed location communication module such as might be found in a wireless local loop or meter reading system. In the most general embodiment, mobile stations may be any type of communication unit. For example, the mobile stations 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.
In an exemplary embodiment, mobile station
110
communicates with serving base station
120
and target base station
130
using code division multiple access (CDMA) techniques. 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 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.
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 and has been approved by 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 26.67 milliseconds (ms) is transmitted as a pilot signal by each base station. The pilot PN sequence is transmitted by each base station at one of 512 possible PN sequence offsets. Each base station transmits the pilot signal continually, which enables mobile stations to identify the base station's transmissions as well as for other functions.
In addition to the pilot PN sequence, each base station in an IS-95 system transmits a “sync channel” signal that is synchronized to the pilot PN signal. The sync channel contains information such as the PN offset of the base station and the CDMA System Time used by the base station. When an IS-95 mobile station first powers on, it must obtain the CDMA System Time from an IS-95 base station before it can communicate with any synchronous base station. To obtain CDMA System Time, an IS-95 mobile station performs a two-step acquisition procedure. First, the mobile station performs a full search for the pilot PN sequence transmitted by any synchronous base station. This search may be performed over the entire 26.67 millisecond pilot PN code space. Upon locating a pilot PN signal, the mobile station then decodes a sync channel message from that base station's sync channel. Once a mobile station successfully reads the sync channel, the mobile station has a reference CDMA System Time that is accurate to within the time it takes for the base station's signal to reach the mobile station. Because this time is dependent on the unknown length of the signal propagation path between the mobile station and the base station, it is called the path delay uncertainty. Because the path delay uncertainty is very small compared to the 512 possible pilot PN offsets used in IS-95, the mobile station can unambiguously distinguish signals transmitted by different base stations by their PN offsets.
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 that 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. Consequently, a W-CDMA base station may not be identified by its pilot signal offset alone. Also, once the system time of one base station is determined, this cannot be used to estimate the system time of a neighboring base
Jou Yu-Cheun
Odenwalder Joseph P.
Sarkar Sandip
Shanbhag Abihijit
Terasawa Daisuke
Ngo Ricky
Qualcomm Inc.
Wadsworth Philip
Yafuso Byron
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