Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-09-29
2004-06-08
Maung, Nay (Department: 2684)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S436000, C455S515000, C455S067150
Reexamination Certificate
active
06748215
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to digital wireless communication systems, and more particularly to methods for efficiently performing candidate frequency searches in digital wireless communications systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “mobile stations” and a fixed network infrastructure. Typically, the mobile stations communicate with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Time Division Multiple Access (TDMA) systems, Code Division Multiple Access (CDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital wireless communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to connect the mobile station users with the fixed network infrastructure (usually a wired-line system).
Basic CDMA Communication System
Mobile stations typically communicate with base stations using a duplexing scheme that allows for the exchange of information in both directions of connection. In most existing communication systems, transmissions from a base station to a mobile station are referred to as “forward link” transmissions. Transmissions from a mobile station to a base station are referred to as “reverse link” transmissions. These CDMA systems are well-known in the art. For example, some such system is described in U.S. Pat. No. 4,901,307, issued on Feb. 13, 1990 to Gilhousen et al. and assigned to the owner of the present invention, which is also hereby incorporated by reference for its teachings of CDMA communication systems. Basic radio system parameters and call processing procedures for exemplary prior art CDMA systems is described in the TIA specification, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as “IS-95A”. The update and revision to IS-95A and J-STD-008 (PCS specification analogous to IS-95A) is TIA/EIAJIS-95-B, published in Mar. 1999 by the Telecommunications Industry Association, and referred to hereafter as “IS-95B”. Both the IS-95A and IS-95B specifications are hereby incorporated by reference for their teachings on CDMA communication systems.
As shown in
FIG. 1
, a typical CDMA communication system comprises at least one mobile station and a plurality of fixed base stations geographically distributed over the system's service area and controlled by a mobile telecommunications switching office (MTSO)
20
. The service area is defined as the geographical area within which a mobile station can remain and yet still communicate (i.e., maintain a valid radio link) with the CDMA communication system. Each base station provides communication services to a fixed area within the service area. The service area is known as the base station's “coverage area”. Thus, when a mobile station is within a base station's coverage area the base station is able to provide communication services to the mobile station. A base station that provides service to the mobile is also known as a “serving” base station. The MTSO
20
coordinates all of the switching functions between base stations, mobile stations, and other communications systems (e.g., a Public Service Telephone Network or satellite communication system).
Communication between a base station and a mobile station is established by a negotiation process that is initiated upon call origination. The serving base station begins the negotiation process by assigning a selected one of its available forward traffic channels to the mobile station and thus establishes a forward link with the mobile station. The mobile station then establishes a reverse link with the serving base station. Base stations continuously transmit pilot channels. Mobile stations track base station signals through acquiring and tracking pilot channels. Once communication is established between the serving base station and the mobile station, pilot channels emitted by each base station are used by the mobile station to determine the base station coverage area that the mobile station is within and the quality of the link to the base station. As described in more detail below, these pilot channels aid in a procedure known as a “handoff” procedure. One such method of a handoff procedure is described in U.S. Pat. No. 5,848,063, issued on Dec. 8, 1998 to Weaver et al., assigned to the owner of the present invention, which is also hereby incorporated by reference for its teachings on CDMA communication systems, especially with respect to its teachings on handoff procedures.
A handoff occurs when a mobile station moves across a “boundary line” from a first serving base station's coverage area to a second base station's coverage area. The communication system “hands over” service from the first serving base station to the second base station, also known as the “target” base station. A handoff also occurs when a single base station utilizes multiple frequency channels and switches communication between frequency channels. Each pilot channel is identified by a pseudorandom noise (PN) sequence offset and a frequency assignment. Thus, each pilot channel is uniquely identified with a base station that transmits the pilot channel. Pilot channels aid mobile stations in performing candidate frequency searches.
A candidate frequency (CF) search is a process by which a mobile station searches for pilot channels on a frequency other than the serving frequency. A CF search is performed to assist the base station in determining if handing off to a different frequency is necessary. Thus, CF searches apply to a handoff between (i.e., a hard handoff). The pilot CF search measures received input powers and signal strengths of pilots. These measurements help determine the base station coverage area on CF's that the mobile station is within and the quality of the link to the base station. When the mobile station detects a pilot channel of significant strength (i.e., the mobile station is at or very near the frequency coverage boundary of the serving base station or a different base station), it reports the pilot channel to a base station controller. The base station controller initiates a handoff procedure to switch to a target base station (the base station associated with the pilot channel) where the mobile station active set now contains the new pilot channel. The mobile station active set is defined as a set of base stations in communication with the mobile station. Then, as described below, the MTSO
20
switches the radio link from the serving base station to the target base station.
FIG. 1
depicts a simple CDMA communication system having a service area comprising seven base stations controlled by one MTSO
20
. Each base station services a separate coverage area, represented by a hexagon in
FIG. 1
, and communicates with a specific frequency, frequency one (F
1
) or frequency two (F
2
). Typically, F
1
and F
2
operate either on the Cellular band (800 MHz) or the PCS band (1900 MHz). For example, a first base station
12
, located in the middle of a Service Coverage Area One, communicates on a first frequency F
1
. A mobile station
10
is serviced by the first base station
12
because it is located within the Coverage Area One. When the mobile station
10
moves from the Coverage Area One to a Coverage Area Two, it performs a handoff procedure from the first base station
12
, the serving base station, to a second base station
14
, the target base station. Thus, the mobile station
10
′ (of
FIG. 1
) is now serviced by the second base station
14
. It is critical for the MTSO to determine the appropriate time to initiate a handoff to a different frequency than the serving frequency in order to mai
Achour Baaziz
Chen Qingxin
Brown Charles D.
Maung Nay
Pauley Nicholas J.
Qualcomm Incorporated
Sharma Sujatha
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