1. Field of the Invention
The present invention relates to an apparatus and method for supporting the mobility of a Mobile Station (MS) in a wireless communication system. More particularly, the present invention relates to a handover apparatus and method for maintaining the maximum Quality of Service (QoS) of an MS in a wireless communication system.
2. Description of the Related Art
Wireless communication systems use a handover technology such that Mobile Stations (MSs) can move between cells while maintaining communication. According to the Institute of Electrical and Electronics Engineers (IEEE) 802.16-2004 COR2_D3 standard, there are three basic modes of handover: 1) hard HandOver (HO), 2) Macro Diversity HandOver (MDHO), and 3) Fast Base Station Switching (FBSS). Wireless communication systems adopt a hard HO mode as an Inter Operability Test (IOT) between an MS and BS manufacturer and a service provider due to the stability of technology application and the ease of technology realization.
A hard HO technology is divided into an MS-initiated HO technology and a BS-initiated HO technology. Comparing the MS-initiated HO technology and the BS-initiated HO technology, an HO process performed between an MS and a serving BS has few differences, and an HO entry process performed between an MS and a target BS is substantially identical.
When performing handover, an MS selects a target BS for handover through an HO process with a serving BS. After that, the MS acquires synchronization with the target BS and performs an HO entry process.
The MS may fail to complete the HO entry with the target BS process because of a channel environment, thus causing the occurrence of an HO drop. A wireless communication system can process the HO drop as described below with reference to FIG. 1.
FIG. 1 illustrates a method of processing an HO drop in a wireless communication system according to the conventional art. The following description is made assuming that the wireless communication system uses an MS-initiated HO technology.
Referring to FIG. 1, an MS 100 receives a service from a serving BS 110. That is, the MS 100 transmits/receives traffic with the serving BS 110 in step 131.
The MS 100 measures signal strength of the serving BS 110 and neighboring BSs, determining whether to perform handover in step 133. If the MS 100 determines to perform handover, the MS 100 transmits an HO request signal (MOB_MSHO-REQ) to the serving BS 110 in step 135. The HO request signal includes information on the neighboring BSs to which the MS 100 can perform handover.
The serving BS 110 identifies the information on the neighboring BSs included in the HO request signal, thus identifying if the neighboring BSs can support handover of the MS 100. Then, the serving BS 110 transmits an HO response signal (MOB_BSHO-RSP) to the MS 100 in step 137. The HO response signal includes information on neighboring BSs constructed such that the neighboring BS that can best support the handover of the MS 100 is listed first.
The MS 100 selects a target BS 120 for handover using the HO response signal that includes information on whether neighboring BSs support the handover of the MS 100. Then, the MS 100 transmits an HO indication signal (MOB_MS-IND) to the serving BS 110 so as to inform of HO initiation in step 139. At this time, the MS 100 drives a resource maintenance timer for releasing information on the serving BS 110. If the resource maintenance timer expires, the MS 100 releases the information on the serving BS 110. The information on the serving BS 110 includes a frequency of the serving BS 110, a preamble index, a serving BS 110 IDentifier (ID) and service related context information.
After transmitting the HO indication signal, the MS 100 acquires downlink synchronization with the target BS 120 in step 141.
If the HO indication signal is received, the serving BS 110 drives a resource maintenance timer for releasing information on the MS 100 in step 143. If the resource maintenance timer expires, the serving BS 110 releases the information on the MS 100.
Then, the MS 100 exchanges a signal for HO entry with the target BS 120 in step 145. If the MS 100 acquires Connection ID (CID) information and authentication information through a ranging response signal (RNG-RSP) received from the target BS 120, the MS 100 completes HO entry with the target BS 120.
However, if an HO ranging attempt is repeatedly made more than a threshold number of times due to channel degradation during the HO entry, the MS 100 recognizes that an HO drop occurs in step 147.
When the HO drop occurs, the MS 100 reselects a BS for handover in step 149. Although not shown, when the reselected BS is not the serving BS 110, the MS 100 performs HO entry to the reselected BS.
When the reselected BS is the serving BS 110, the MS 100 identifies if the resource maintenance timer driven to release the information on the serving BS 110 has expired in step 151.
If the resource maintenance timer has not expired, the MS 100 can be aware of information for communication with the serving BS 110 and thus, transmit an HO indication signal to the serving BS 110 to return to service with the serving BS 110 in step 153. That is, the MS 100 transmits an HO indication signal of a type different from the previous HO indication signal, which has been transmitted to the serving BS 110 in step 139, to the serving BS 110. Then, the MS 100 recognizes that it returns to the serving BS 110, thus operating in a mode for transmitting/receiving traffic with the serving BS 110 in step 157.
However, the serving BS 110 may fail to receive the HO indication signal from the MS 100 that was transmitted in step 153 due to channel degradation between the MS 100 and the serving BS 110 in step 155.
At this time, the serving BS 110 fails to recognize that the MS 100 desires to enter the serving BS 110. Thus, the serving BS 110 identifies if a resource maintenance timer driven to release information on the MS 100 expires in step 159.
If the resource maintenance timer expires, the serving BS 110 releases the information on the MS 100 in step 161. That is, because the serving BS 110 fails to receive the HO indication signal, it erroneously assumes that the MS 100 performs handover to a different BS. However, the MS 100 transmits the HO indication signal, thus erroneously indicating that it performs communication with the serving BS 110. Thus, a problem of inconsistency of communication states of the MS 100 and the serving BS 110 takes place.
As described above, when the communication states of the MS 100 and the serving BS 110 are inconsistent with each other, the MS 100 transmits a signal but the serving BS 110 fails to receive the signal from the MS 100, thus causing a problem of communication interruption.
Also, when the serving BS 110 deletes information on the MS 100, the serving BS 110 can allocate a CID, which had previously been allocated to the MS 100, to a different MS. At this time, the MS 100 can receive a downlink signal including the allocated CID from the serving BS 110 and send a signal to the serving BS 110 in response to the downlink signal. In this case, a problem may occur in that the response signal from the MS 100 interrupts a communication between the different MS, to which the serving BS 110 now allocates the CID, and the serving BS 110.