Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2002-08-27
2004-12-28
Vo, Don N (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S365000
Reexamination Certificate
active
06836506
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention generally relates to wireless communications networks. More particularly, the present invention relates to a method and apparatus for synchronizing timing in circuits associated with wireless communication terminals that support multiple airlinks or multi-mode phones.
II. Related Art
There are presently many different types of radiotelephone or wireless communication systems, including different terrestrial based wireless communication systems and different satellite based wireless communication systems. The different terrestrial based wireless systems can include Personal Communications Service (PCS) and cellular systems. Examples of known cellular systems include the cellular Analog Advanced Mobile Phone System (AMPS), and the following digital cellular systems: Code Division Multiple Access (CDMA) systems; Time Division Multiple Access (TDMA) systems; and newer hybrid digital communication systems using both TDMA and CDMA technologies.
The use of CDMA techniques in a 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
” and U.S. Pat. No. 5,103,459, entitled “
System And Method For Generating Signal Waveforms In A CDMA Cellular Telephone System
,” both of which are assigned to the assignee of the present invention and are incorporated herein by reference.
The method for providing CDMA mobile communications was standardized in the United States by the Telecommunications Industry Association in TIA/EIA/IS-95-A entitled “
Mobile Station
-
Base Station Compatibility Standard for Dual
-
Mode Wideband Spread Spectrum Cellular System
,” referred to herein as IS-95. Combined AMPS & CDMA systems are described in TIA/EIA Standard IS-98. Other communications systems are described in the IMT-2000/UM, or International Mobile Telecommunications System 2000/Universal Mobile Telecommunications System, standards covering what are referred to as wideband CDMA (WCDMA), cdma2000 (such as cdma2000 1× or 3× standards, for example) or TD-SCDMA.
In the above patents, CDMA techniques are disclosed in which a large number of mobile station users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations. The satellite repeaters are known as gateways and the terrestrial base stations are known as cell base stations or cell-sites. The gateways provide communication links for connecting a user terminal to other user terminals or users of other communications systems, such as a public telephone switching network. By using CDMA communications, the frequency spectrum can be used by multiple terminals thus permitting an increase in system user capacity. The use of CDMA techniques result in much higher spectral efficiency than can be achieved using other multiple access techniques.
In a typical CDMA communications systems, both the remote units and the base stations discriminate the simultaneously received signals from one another using modulation and demodulation of the transmitted data with high frequency pseudo-noise (PN) codes, orthogonal Walsh codes, or both. For example, in the forward link, i.e., base station to mobile station direction, IS-95 separates transmissions for different users from the same base station into different channels by the use of different Walsh codes for each transmission, while the transmissions from different base stations are distinguished by the use of a uniquely offset PN code. In the reverse link, i.e., mobile station to base station direction, different PN sequences are used to distinguish different channels or user terminals.
The forward CDMA link includes a pilot channel, a synchronization (sync)-channel, one or more paging channels, and a larger number of traffic channels. The reverse link includes an access channel and a number of traffic channels. The pilot channel transmits a beacon signal, known as a pilot signal, and is used to alert mobile stations of the presence of a CDMA compliant base station. After a mobile station has successfully acquired the pilot signal, it can then receive and demodulate the sync-channel in order to achieve frame level synchronization and system time, etc. The synch channel carries a repeating message that specifically identifies the base station, provides system level timing, and provides the absolute phase of the pilot signal. This feature will be discussed in greater detail below. The paging channel is used by the base station to assign communication channels and to communicate with the mobile station when it has not been assigned to a traffic channel. Finally, the traffic channels assigned to individual mobile stations are used to carry user communications traffic, such as speech and data.
To communicate properly in a CDMA system, the state of the particular codes selected must be synchronized at the base station and mobile station. Code level synchronization is achieved when the state of the codes at the mobile station system are the same as those in the base station, less some offset to account for processing and transmission delay. In IS-95, such synchronization is facilitated by the transmission of the pilot signal, which comprises the repeated transmission of the uniquely offset PN code (pilot PN code), from each base station. In addition to facilitating synchronization at the Pilot PN code level, the pilot channel allows identification of each base station relative to the other base stations located around it using the pilot channel phase offset. The pilot channel, therefore, provides the mobile station with access to a first level of detailed PN sequence timing information.
Mobile stations initially acquire an IS-95 based communications system by searching for a valid pilot signal within a definable search window. Pilot signals associated with different base stations are distinguished from one another on the basis of the phase (time offset) of the pilot signal. Thus, although each base station transmits an identical pilot signal, pilot signals from different base stations have different phases. A 9-bit number can be used to identify the pilot phase and is called the pilot offset.
After a mobile phone has acquired a valid pilot signal and has associated that pilot signal with a particular base station, the mobile station can receive and demodulate the sync channel. In addition to providing the mobile station with the phase of the pilot signal and identification of its associated base station, the synchronization message also includes CDMA system level timing information. Although system time can be provided through a number of different timing sources, traditional wireless communication systems derive system timing information through the global positioning system (GPS) satellite system located at each base station.
Due in part to convenience and availability of mobile phones, in the United States the Federal Communications Commission (FCC) now requires that wireless communication system (WCS) providers implement a mechanism to automatically route 911 calls to the nearest emergency services processing center. This is referred to as the E911 requirement. In order to accommodate this requirement, the WCS must be able to quickly and accurately determine the geographic position of a mobile phone or wireless device. Conventional WCSs typically determine a user's or mobile station position using what is referred to as either a handset based solution or a network based solution.
The conventionally used handset-based method typically relies on GPS capabilities to provide user position information. This GPS solution, however, exhibits degraded performance and availability in areas where satellite coverage is limited or obscured, such as indoors or in major urban areas. GPS solutions are also relatively slow to provide a position determination and can be costly. Network-based solutions rely on a signal transmitted from the mobile station to multiple fixed base stations. The limit
Ogrod Gregory D.
Qualcomm Incorporated
Vo Don N
Wadsworth Philip R.
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