Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-08-31
2003-04-01
Bost, Dwayne (Department: 2681)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S437000, C455S440000, C455S456500, C370S331000, C370S332000
Reexamination Certificate
active
06542743
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to communications. More particularly, the invention concerns a method and apparatus for reducing search times associated with the handoff of a call from one base station to another base station.
2. Description of the Background Art
Wireless communication systems generally comprise, amongst other elements, a wireless unit, commonly referred to as a mobile telephone (mobile), that communicates with one or more base stations when making a call. The mobile communicates with the base stations on one or more channels that are contained within a frequency band assigned to the mobile by a base station controller. A communication from the mobile to a base station is made on what is called the “reverse link,” and a communication from the base station to the mobile station is made on the “forward link.” During a call, the mobile station is constantly searching for other base stations that the mobile might need to continue the call while the mobile station is moving around.
One important element of a mobile used in such a wireless system is the searcher. The searcher is programmed to search for pilot signals (pilots) transmitted from different bases stations in at least three cases: 1) when a mobile is trying to acquire a base station for communication; 2) in the idle state when the mobile is on the paging or access channels; and 3) in the traffic state where the mobile is in control of the traffic channel. The speed of searching the pilots on the frequency assigned to the mobile and other frequencies determines the search performance of the mobile. In slotted mode, the objective is to search all pilots in the neighbor set before the slot expires. Slotted mode refers to an operation mode of the mobile where the mobile monitors the paging channel, also known as the control channel, only during selected slots of time. While the mobile station is in traffic state, the base station may direct the mobile station to search pilots on a different (candidate) CDMA frequency to detect the presence of CDMA channels and to measure their respective strengths. When searching pilots in a “candidate” frequency, the mobile needs to complete its search of all pilots in the candidate frequency search set as quickly as possible so that it tunes back to the serving frequency and minimizes the voice degradation caused by searching the candidate frequency. As discussed below, the candidate frequency is a potential hard handoff frequency, and these searching techniques are used to coordinate handoffs of communications in the wireless communication system.
A. Handoffs
A mobile used in a code-division-multiple-access (CDMA) wireless system supports three types of handoff procedures when the mobile is in control of the traffic channel. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990 and entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” assigned to the assignee of the present invention and incorporated by reference herein. The three types of handoffs are:
1. Soft Handoff—A handoff in which the mobile commences communications with a new base station without interrupting communications with the old base station. Soft handoff can only be used between CDMA channels having identical frequency assignments.
2. CDMA to CDMA Hard Handoff—A handoff in which the mobile is transitioned between disjoint sets of base stations, different band classes, different frequency assignments, or different frame offsets.
3. CDMA to Analog Handoff—A handoff in which the mobile is directed from a cdma forward traffic channel to an analog voice channel.
To perform soft handoff, the mobile continuously searches for assigned sets of pilots. The term “pilot” refers to a pilot channel identified by a pilot sequence offset and a frequency assignment. A pilot is associated with the forward link traffic channels in the same forward link CDMA channel, or similarly with the reverse link on systems using reverse link pilots. All pilots in a pilot set have the same CDMA frequency assignment. For clarity, pilots are discussed in terms of the forward link only.
The mobile searches for pilots on the current CDMA frequency assignment to detect the presence of CDMA channels and to measure their signal strength. When the mobile detects a pilot of sufficient strength that is not associated with any of the forward link traffic channels already assigned to it, it sends a pilot strength measurement message to the base station with which it is currently communicating. The base station can then assign a forward link traffic channel associated with that pilot to the mobile and direct the mobile to perform a handoff.
The pilot search parameters and the rules for pilot strength measurement message transmission are expressed in terms of the following sets of pilots:
Active Set: The pilots associated with the Forward Link Traffic Channels assigned to the mobile.
Candidate Set: The pilots that are not currently in the Active Set but have been received by the mobile with sufficient strength to indicate that the associated Forward Link Traffic Channels could be successfully demodulated.
Neighbor Set: The pilots that are not currently in the Active Set or the Candidate Set and are likely candidates for handoff.
Remaining Set: The set of all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set. This set of possible pilots consists of pilots whose pilot PN sequence offset indices are integer multiples of some pilot increment.
The base station may direct the mobile to search for pilots on a different CDMA frequency to detect the presence of CDMA channels and to measure their strengths. The mobile leaves the active CDMA channel and searches pilots in the candidate frequency search set. At the end of the search, the mobile reports the results of the search to the base station. Depending upon the pilot strength measurements, the base station can direct the mobile to perform an inter-frequency hard handoff.
The pilot search parameters are expressed in terms of the following sets of pilots:
Candidate Frequency Neighbor Set: A list of pilots on the CDMA Candidate Frequency.
Candidate Frequency Search Set: A subset of the Candidate Frequency Neighbor Set that the base station may direct the mobile to search.
B. Pilot Search
In current systems, the base station sets the search window, that is, the range of PN offsets, in which the mobile is to search for usable multipath components of the active pilots. These multipath components are used by the mobile for demodulation of an associated forward link traffic channel. Search performance criteria, and general wireless system criteria, are sent as part of the In-Traffic System Parameters Message, the Extended Handoff Direction Message, the Neighbor List Update Message, Extended Neighbor List Update Message, and Candidate Frequency Search Request Message. These messages are defined in standards TIA/EIA-95x and TIA/EIA-98-B, each issued by the Telecommunications Industry Association, and ANSI J-STD-018, issued by the American National Standards Institute, all of which are incorporated by reference herein. These searches are generally governed by the following:
Active Set and Candidate Set: The search procedures for pilots in the active and candidate sets are identical. The search window size for each pilot in the active and candidate sets is the number of PN chips specified in Table 1 corresponding to SRCH_WIN_A. For example, SRCH_WIN_A
s
=6 corresponds to a 28 PN chip search window or ±14 PN chips around the search window center. The mobile station centers the search window for each pilot of the active and candidate sets around the earliest arriving usable multipath component of the pilot.
TABLE 1
SRCH_WIN_A
SRCH_WIN_A
SRCH_WIN_N
Window
SRCH_WIN_N
Window
SRCH_WIN_NGHB
Size
SRCH_WIN_NGHB
Size
R SRCH_WIN_
Baker Kent D.
Beladi S. Hossain
Bost Dwayne
Qualcomm Incorporated
Trinh Sonny
LandOfFree
Method and apparatus for reducing pilot search times... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for reducing pilot search times..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for reducing pilot search times... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3005885