1.  Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to an apparatus and method for determining soft or softer handoff on cell switching in a mobile communication system. 
2.  Discussion of the Related Art
In the world of cellular telecommunications, those skilled in the art often use the terms 1G, 2G, and 3G. The terms refer to the generation of the cellular technology used.  1G refers to the first generation, 2G to the second generation, and 3G to the third generation. 
1G is used to refer to the analog phone system, known as an AMPS (Advanced Mobile Phone Service) phone systems.  2G is commonly used to refer to the digital cellular systems that are prevalent throughout the world, and include CDMAOne, Global System for Mobile communications (GSM), and Time Division Multiple Access (TDMA).  2G systems can support a greater number of users in a dense area than can 1G systems. 
3G is commonly used to refer to the digital cellular systems currently being developed.  Recently, third-generation (3G) CDMA communication systems have been proposed including proposals, such as cdma2000 and W-CDMA.  These 3G communication systems are conceptually similar to each other with some significant differences. 
A cdma2000 system is a third-generation (3G) wideband, spread spectrum radio interface system which uses the enhanced service potential of CDMA technology to facilitate data capabilities, such as Internet and intranet access, multimedia applications, high-speed business transactions, and telemetry.  The focus of cdma2000, as is that of other third-generation systems, is on network economy and radio transmission design to overcome the limitations of a finite amount of radio spectrum availability. 
FIG.  1 illustrates a wireless communication network architecture. 
Referring to FIG.  1, a subscriber uses a Mobile Station to access network services.  The Mobile Station may be a portable communications unit, such as a hand-held cellular phone, a communication unit installed in a vehicle, or even a fixed-location communications unit. 
The electromagnetic waves from the Mobile Station are transmitted by the Base Transceiver System (BTS) also known as node B. The BTS consists of radio devices such as antennas and equipment for transmitting radio waves.  The Base Station Controller (BSC) receives the transmissions from one or more BTS's.  The BSC provides control and management of the radio transmissions from each BTS by exchanging messages with the BTS and the Mobile Switching Center (MSC) or Internal IP Network.  The BTS's and BSC are part of the Base Station (BS). 
The BS exchanges messages with and transmits data to a Circuit Switched Core Network (CSCN) and Packet Switched Core Network (PSCN).  The CSCN Provides traditional voice communications and the PSCN provides Internet applications and multimedia services. 
The Mobile Switching Center (MSC) portion of the CSCN provides switching for traditional voice communications to and from an Mobile Station and may store information to support these capabilities.  The MSC may be connected to one of more BS's as well as other public networks, for example a Public Switched Telephone Network (PSTN) or Integrated Services Digital Network (ISDN).  A Visitor Location Register (VLR) is used to retrieve information for handling voice communications to or from a visiting subscriber.  The VLR may be within the MSC and may serve more than one MSC. 
A user identity is assigned to the Home Location Register (HLR) of the CSCN for record purposes such as subscriber information, for example Electronic Serial Number (ESN), Mobile Directory Number (MDR), Profile Information, Current Location, and Authentication Period.  The Authentication Center (AC) manages authentication information related to the Mobile Station.  The AC may be within the HLR and may serve more than one HLR.  The interface between the SC and the HLR/AC is an IS-41 standard interface. 
The Packet Data Serving Node (PDSN) portion of the PSCN provides routing for packet data traffic to an from Mobile Station.  The PDSN establishes, maintains, and terminates link layer sessions to the Mobile Station's and may interface with one of more BS and one of more PSCN. 
The Authentication, Authorization and Accounting (AAA) Server provides Internet Protocol authentication, authorization and accounting functions related to packet data traffic.  The Home Agent (HA) provides authentication of MS IP registrations, redirects packet data to an from the Foreign Agent (FA) component of the PDSN, and receives provisioning information for users from the AAA.  The HA may also establish, maintain, and terminate secure communications to the PDSN and assign a dynamic IP address.  The PDSN communicates with the AAA, HA and the Internet via an Internal IP Network. 
FIG.  2 illustrates a data link protocol architecture layer for a wireless network. 
Referring to FIG.  2, the upper layer contains three basis services; voice services 62, data services 61 and signaling 70.  Voice services 62 include PSTN access, mobile-to-mobile voice services, and Internet telephony.  Data services 61 are services that deliver any form of data on behalf of a mobile end user and include packet data applications (e.g., IP service), circuit data applications (e.g., asynchronous fax and B-ISDN emulation services), and SMS.  Signaling 70 controls all aspects of mobile operation. 
The Link Layer 30, is subdivided into the Link Access Control (LAC) sublayer 32 and the Medium Access Control (MAC) sublayer 31.  The link layer provides protocol support and control mechanisms for data transport services and performs the functions necessary to map the data transport needs of the upper levels 60 into specific capabilities and characteristics of the physical layer 20.  The Link Layer 30 may be viewed as an interface between the upper layers and the Physical Layer 20. 
The separation of MAC 31 and LAC 32 sublayers is motivated by the need to support a wide range of upper layer services, and the requirement to provide for high efficiency and low latency data services over a wide performance range (from 1.2 Kbps to greater than 2 Mbps).  Other motivators are the need for supporting high QoS delivery of circuit and packet data services, such as limitations on acceptable delays and/or data BER (bit error rate), and the growing demand for advanced multimedia services each service having a different QoS requirements. 
The LAC sublayer 32 is required to provide a reliable, in-sequence delivery transmission control function over a point-to-point radio transmission link 42.  The LAC sublayer manages point-to point communication channels between upper layer entities and provides framework to support a wide range of different end-to-end reliable link layer protocols. 
The MAC sublayer 31 facilitates complex multimedia, multi-services capabilities of 3G wireless systems with Quality of Service (QoS) management capabilities for each active service.  The MAC sublayer 31 provides procedures for controlling the access of data services (packet and circuit) to the physical layer 20, including the contention control between multiple services from a single user, as well as between competing users in the wireless system.  The MAC sublayer 31 also provides for reasonably reliable transmission over the radio link layer using a Radio Link Protocol (RLP) 33 for a best-effort level of reliability.  Signaling Radio Burst Protocol (SRBP) 35 is an entity that provides connectionless protocol for signaling messages.  Multiplexing and Quality of Service (QoS) Control 34 is responsible for enforcement of negotiated QoS levels by mediating conflicting requests from competing services and the appropriate prioritization of access requests. 
The Physical Layer 20, is responsible for coding and modulation of data transmitted over the air.  The Physical Layer 20 conditions digital data from the higher layers so that the data may be transmitted over a mobile radio channel reliably. 
The Physical Layer 20 maps user data and signaling, which are delivered by the MAC sublayer 31 over multiple transport channels, into a physical channels and transmits the information over the radio interface.  In the transmit direction, the functions performed by the Physical Layer 20 include channel coding, interleaving, scrambling, spreading and modulation.  In the receive direction, the functions are reversed in order to recover the transmitted data at the receiver. 
Generally, in the 1×EV-DV (1×Evolution-Data and Voice) system, when a mobile station is in a soft or softer handoff area, a base station transmits packet data on forward-packet data channel (hereinafter abbreviated F-PDCH) in forward link from (or through) a serving sector only among various sectors in an active set.  The active set is the list of pilots that are being used for the current communication.  In other words, the active set is the list of sectors that are in communication with the mobile station. 
If it is requested to switch the serving sector as the channel status or signal power is changed, the serving sector is replaced by a sector having the best status (or target sector), for example the strongest signal power level.  The data is then communicated between the mobile station and the base station of the replaced target sector as a new serving sector.  Such a procedure is called ‘cell switching’. 
FIG.  3 is a diagram of determining and managing one serving sector in an active set.  Referring to FIG.  3, a mobile station measures strength of a pilot signal from each sector in a neighbor set and tests whether the measured signal meets certain requirements, such as the signal being greater than a certain threshold value (for example, T_ADD).  If a result of the test is acceptable, the terminal informs a base station of information about the strength of the pilot signal of each of the corresponding sectors through an extended pilot strength measurement message (EPSMM).  The neighbor set is a set of sectors or pilots in the vicinity of the sectors currently transmitting to the mobile station.  The contents of the neighbor set are normally configured by the base station. 
The base station uses the strength information of the received pilot signal to determine the active set and then informs the mobile station of the determined active set through, for example, a universal handoff direction message.  Conversely, the mobile station measures strength of a pilot signal from each sector in the active set and tests whether the measured strength is smaller than a predetermined threshold value (for example, T_DROP).  If the measured strength is smaller than the predetermined threshold value T_DROP, the mobile station informs the base station of information concerning the strength of the pilot signal of each corresponding sector in the active set through an extended pilot strength measurement message (EPSMM).  The base station uses the strength information of the received signal to determine the corresponding sectors as an active set and then sends the active set through, for example, the universal handoff direction message (UHDM).  Generally, the sectors that are not included in the UHDM as the active set will be included in the neighbor set. 
Moreover, the mobile station measures strength of the pilot signal of each sector in the active set for cell switching to select an optimal sector as a serving sector and then informs the base station of the determined serving sector through a channel quality indicator (CQI) cover of a reverse channel quality indicator channel (R-CQICH). 
If the CQI cover received through the R-CQICH indicates a sector of its own, the corresponding sector recognizes to be selected as the serving sector and starts transmission of the packet data accordingly. 
FIG.  3 illustrates an example of sector 1 being selected as a serving sector.  FIG.  4 illustrates an example of cell switching. 
Referring to FIG.  4, a mobile station is receiving packet data 1, packet data 2, and packet data 3 through sector 2, which is a current serving sector.  If sector 3 is designated as a new serving sector since status of channel R-CQICH is changed, the mobile station should direct the sector 3 with the CQI cover.  The sector 3 used as the serving sector then receives packet data 4 from a base station controller (BSC) and transmits the received packet data to the mobile station. 
As mentioned in the foregoing explanation, in order to perform the cell switching correctly in the mobile communication network, an efficient method is needed to judge that the corresponding cells/sectors for the cell switching lie in what kind of handoff areas, respectively.  However, the related art fails to propose any method of having the base station or mobile station judge the areas efficiently.