Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1999-12-29
2002-08-06
To, Doris H. (Department: 2682)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S432300
Reexamination Certificate
active
06430414
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The, present invention relates to cellular telephone systems. More specifically, the present invention relates to a novel and improved system for providing more efficient soft handoffs in a code division multiple access (“CDMA”) cellular telephone system to accommodate uninterrupted voice and high data rate transmission.
2. Description of Related Art
The next generation of wireless networks will provide multiple services requiring high data rate transmission and uninterrupted connections. This next generation is often referred to as the “third generation” of CDMA wireless systems. The range of services include text paging, two-way radio connections, internet connectivity using microbrowsers, two-way wireless e-mail capability and wireless modem functionality. The CDMA cellular telephone system offers the capability to provide reliable radio links between a wireless communications device such as a mobile station (“MS”) and a base station (“BS”) with a much higher data capacity than conventional networks that only support voice service. As an example, in the third generation CDMA wireless systems, radio links supporting high rate (up to 2 Mbps) data transmissions will be established between the MS and the BS to provide multimedia services such as Internet access.
One particularly important feature of CDMA systems for effective third generation wireless communication is the soft handoff, which allows the MS to move smoothly from the coverage of one cell to another without interruption. The soft handoff is accomplished by establishing simultaneous communications between the MS and multiple base stations. A soft handoff is illustrated in
FIG. 1. A
MS
10
passes to the edge of the coverage area
12
a
of a serving BS
12
.
While the MS
10
is within a serving BS coverage area
12
a
and a receiving BS
14
coverage area
14
a,
both base stations
12
,
14
simultaneously communicate with the MS
10
. As the MS
10
passes further into the coverage area
14
a
of the receiving BS
14
, the server BS
12
stops communicating with the MS
10
. In this manner, there is uninterrupted communication for the user of the MS
10
as he or she passes from the serving cell to the receiving cell. An efficient soft handoff algorithm plays an important role in maintaining the link quality as well as conserving the capacity-related network resources. As the demand to support high rate data services increases, the need to improve the efficiency of the handoff algorithm becomes more critical.
For a third generation system based on CDMA technologies, a highly efficient handoff algorithm is essential to successfully provide the infrastructure to support the new range of services. A conventional protocol for soft handoffs in a CDMA system has been adopted by the Telecommunications Industry Association in the industry standards IS-95, IS-95A or IS-95B (collectively “IS-95 A/B”) for implementing a CDMA cellular system. Under the IS-95 A/B standard, a MS communicates with one or more base stations dispersed in a geographic region. Each BS continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a BS will be simply referred to as a pilot. The MS monitors the pilots and measures the received energy of the pilots.
The IS-95 A/B standards define a number of states and channels for communication between the MS and the BS. For example, in the Mobile Station Control on the Traffic State, the BS communicates with the MS over a Forward Traffic Channel, and the MS communicates with the BS over a Reverse Traffic Channel. During a call, the MS must constantly monitor and maintain four sets of pilots Collectively referred to as the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set comprises pilots associated with the Forward Traffic Channel assigned to the MS. The Candidate Set comprises pilots that are not currently in the Active Set but have been received by a particular MS with sufficient strength to indicate that the associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set comprises pilots that not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set comprises 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.
The MS constantly searches the Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a threshold value. As the MS moves from the region covered by one BS to another, the MS promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the BS or base stations of the promotion via a Pilot Strength Measurement Message (“PSMM”). The BS determines an Active Set according to the PSMM, and notifies the MS of the new Active Set via a Handoff Direction Message. When the MS commences communication with a new BS in the new Active Set before terminating communications with the old BS, a “soft handoff” has occurred.
In IS-95 A/B compliant CDMA systems, each BS is identified by the pseudo-random (“PN”) offset of its pilot channel signal. The details of the PN offset identification procedures in IS-95 A/B are well known to those of ordinary skill in the art and are therefore not discussed further herein. The MS categorizes all pilots into different sets based on the pilot's likelihood to be used as a candidate for handoff.
The value T_ADD consists of the pilot strength threshold specified by the BS (IS-95 A) or dynamically determined at the MS (IS-95 B), above which the pilot is considered sufficiently strong to be added to the Active Set. The value T_DROP reflects the pilot strength threshold below which the pilot is considered sufficiently weak to be removed from the Active Set. The PSMM is sent from the MS to the BS to report the strength of all pilots in the Active and Candidate Sets. In response to the PSMM, an Extended Handoff Direction Message (“EHDM”) which includes an updated Active Set is sent from the BS to the MS.
The IS-95 A/B compliant MS typically has a searcher unit that continuously measures the pilots in various sets and reports to the BS the pilots that are sufficiently strong for an addition to the Active Set, and the pilots that are relatively weak to be removed from the Active Set. Pilots in the Neighbor Set are of particular importance, and normally they are more frequently measured than pilots in the Remaining Set.
The procedure of adding a pilot from the Neighbor Set to the Active Set in the IS-95 A/B soft handoff algorithm is briefly described as follows:
1. Each BS has a stored Neighbor List (“NL”) in terms of the PN offsets and configuration information of the neighboring cells. The MS receives a Neighbor List Update Message (“NLUM”) containing the NL from the BS and places the corresponding pilots into the Neighbor Set.
2. The MS is required to perform continuous measurement of the pilot channel strength of every pilot in the Neighbor Set using its searcher unit.
3. The MS compares the measured pilot strength with the T_ADD. Those neighbor pilots whose strengths are above T_ADD are placed in the Candidate Set and the PSMM is sent to the BS.
4. Based on the content of the PSMM and the availability of the network resources, the BS sends an EHDM to the MS indicating a new Active Set.
A similar reporting procedure is followed when the MS needs to delete a pilot from its Active Set. In this case, the strength of a pilot in the Active Set is compared with the threshold T_DROP and a timer T_TDROP is activated whenever the pilot strength decreases below T_DROP. Upon the expiration of T_TDROP, a PSMM is sent to the BS and the BS usually responds with an EHDM indicating a reduced Active Set.
According to the IS-95 A/B standard, when the MS
Chen Qingxin
Sorokine Vladislav
Brown Charles
Cheatham Kevin
Qualcomm Incorporated
Ramos-Feliciano Eliseo
To Doris H.
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