Institute of Electrical and Electronics Engineers (IEEE) 802.16e Mobile WiMAX specifies a cell reselection procedure in which a Mobile Station (MS) measures signal strengths of neighbor base stations (BSs) using network topology information acquired from the MOB_NBR-ADV message broadcast by the serving BS and selects a target BS based on the measurement result.
FIG. 1 is a diagram illustrating a general cell selection procedure of a MS in a wireless communication system.
In FIG. 1, reference numeral 110 denotes a serving BS, 120 denotes a neighbor BS, 130 denotes a MS, 141 denotes a Carrier to Interference plus Noise Ratio (CINR) of the serving BS 110 measured by the MS 130, 145 denotes a scanning threshold triggering the MS 130 to start scanning for cell selection, and 147 denotes a movement direction of the MS 130.
If the scanning threshold is detected, the MS 130 transmits a MOB_SCN-Request message to the serving BS 110 for scanning as denoted by reference numeral 171, and the serving BS 110 replies by transmitting a MOB_SCN-RSP message containing information required for selecting the neighbor BS 120 as denoted by reference numeral 173.
The cell reselection procedure can be triggered by the MS 130 when a specific condition is fulfilled or by the serving base station 110 without request from the MS 130. In the cell reselection procedure, the serving BS can acquire the physical layer parameters and potential service level and network information through an association ranging with the neighbor BSs, and three association levels are proposed in the standard: association without coordination, association with coordination, and network assisted association report.
In the association without coordination, the neighbor BS has no information about the MS and thus provides the MS with only contention-based ranging allocations. After the receiving a ranging code, the BS sends an RNG-RSP message with ranging status ‘success’ and allocates resource for the MS to transmit RNG-REQ message with TLV parameters (including serving BS ID and MS MAC address) related to the association ranging.
In the association with coordination, the serving BS provides association parameters to the MS and coordinates association between the MS and neighboring BSs. The serving BS provides the parameters such as ranging code to be used for the association ranging with the neighbor BSs and ranging start time. After ranging code, the same procedure is performed as in the association without coordination.
In the network assisted association report, the MS transmits the RNG-REQ message in similar way to the contention based association or non-contention based association. However, the MS receives the RNG_RSP message sent by the neighbor BS from the serving BS. At this time, the serving BS sends a MOB_ASC_REPORT message containing the information about the neighbor BSs to the MS.
FIG. 2 is a sequence diagram illustrating signaling among the MS and serving and neighbor BS during the scanning procedure in a conventional wireless communication.
Referring to FIG. 2, the MS first sends a MOB_SCN-REQ message with scanning parameters to the serving BS. Upon receipt of the MOB_SCN-REQ message, the serving BS sends the MS a MOB_SCN-RSP message with scanning duration and start time, scanning result report type, neighbor BSs information, etc. After a number of frames (M frames) designated by the serving BS elapse, the MS performs synchronization with the neighbor BSs, B1 and B2, in a predetermined number of frames (N frames). In FIG. 2, it is assumed that the scanning iteration is set to 2.
In the meantime, IEEE 802.16j standard specifies about Mobile Relay Station (MRS) which is installed in a bus or train for providing wireless access service to the MSs on it. The MRS supports the handover between neighbor BSs to avoid service loss as the MS does. The MRS can acquire the scanning information of the MS served by the MRS from the MOB_SCN-INFO message transmitted by the serving BS for scheduling the MSs.
FIG. 3 is a diagram illustrating an exemplary moving wireless network including an MRS.
The moving wireless network is a network in which the MRS and the MSs, MS1 and MS2, connected to the MRS moves in the same pattern, and the MSs are seen as fixed from the viewpoint of the MRS but as moving from the viewpoint of the BS, thereby requiring handover. Since the MRS and MSs are moving in the same pattern, they perform handovers approximately at the same time.
The MSs within the moving wireless network maintain their connections to the MRS regardless of the connection status between the MRS and the serving BS. However, since the MRS has to perform handover as an MS, unlike a fixed relay station, it performs cell reselection procedure periodically.
FIG. 4 is a timing diagram illustrating a principle of the conventional cell selection procedure of the MRS and MS in a wireless communication system, and FIG. 5 is a sequence diagram illustrating operations of MS, MRS, and serving and neighbor base stations for the conventional cell reselection procedure in a wireless communication system.
Referring to FIG. 4, the BS communicates with the MRS in Time Division Duplex (TDD) scheme. The MRS exchanges the MOB_SCN-REQ and MOB_SCN-RSP messages with the BS for cell reselection and relays data between the MS and the BS. When searching for the neighbor BSs, the MRS switches to a Radio Frequency (RF) of a neighbor BS and performs scanning for the neighbor BSs. During the scanning procedure of the MRS, the communication with the MS is broken.
While the MRS performs the cell reselection as shown in FIG. 4, the MS cannot receive the packets transmitted by the BS, and the BS cannot receive the packets transmitted by the MS too. In order to solve this problem, the MRS can perform the cell reselection using the neighbor BS information provided by the BS without the cell reselection procedure. In this case, however, the information accuracy decreases, resulting in deterioration of service quality and increase of system complexity. Furthermore, in case that the MS and MRS perform the cell reselection procedures independently, the scanning delay increases, thereby making worse the service quality and reducing the network resource efficiency.
FIG. 5 shows the service loss time increase problem caused by the continuous cell reselection procedures of the MRS and MS in the moving wireless network when the MRS performs scanning for the neighbor BSs. In order to solve this problem, it can be considered to remove the cell reselection procedure of the MS belongs to the moving wireless network. In this case, however, when the MS detaches from the moving wireless network, the MS has no information about neighbor cell, resulting in increase of handover failure rate.
As aforementioned, in the conventional cell selection/reselection method of the MRS in a moving wireless network, the MRS exchanges the scanning information with the serving BS and performs scanning at a predetermined interval in a predetermined duration for handover. However, the conventional cell selection/reselection method has a drawback in that the MS connected to the MRS experiences the service loss while the MRS performs scanning neighbor BSs. The scanning delay of the MRS is prolonged as a number of the neighbor BSs to scan, resulting in deterioration of the service loss problem and unnecessary waste of radio resource. Furthermore, since the MRS performs scanning of the neighbor BS by its lone, it is difficult to expect of diverse scanning results about the neighbor BSs.