Handover method for mobile wireless station

Abstract: A wireless local area network system includes a plurality of base stations (16) connected in a wired LAN (12). A mobile wireless station (20) can roam through communication cells (24) defined by the base stations (16). The base stations (16) transmit beacon messages (70) at regular intervals. The mobile station (20) determines the communications quality of the beacon message (70) for the cell in which the mobile station (20) is currently located and if this quality becomes unacceptable, switches to a search mode wherein beacon messages (70) from any base station (16) are received and their communications quality is determined. The mobile station (20) switches to communicate with a base station (16) providing a beacon message (70) with an acceptable communications quality.

Description: This invention relates to a method of operating a wireless local area network system. With a view to obviating the need for wired cabling connections between stations in local area networks (LANs), wireless local area networks have been developed and are now commercially available. However, the coverage area of wireless LANs is restricted especially in an indoor environment, due to the presence of structural features such as walls and floors in buildings, for example. Also, it may be desirable for stations in a wireless LAN to communicate with remote facilities or resources such as servers. Therefore, it has been proposed to connect the wireless LAN via interface devices to a backbone LAN such as a wired LAN, which can be connected to the remote facilities and/or other wireless LANs. Furthermore, where a mobile wireless station, such as a portable data processing unit provided with a wireless transceiver, for example, moves around it may move in and out of the range of various interface devices connected to the backbone LAN. It is therefore necessary for the mobile station to hand over communication from one interface device to another as it moves around. European Patent Application No. 0 483 544 discloses a wireless communications network including a plurality of mobile data stations in bidirectional communication with header stations which are connected into a wired LAN and which define communication cells. If a mobile data station crosses over from one cell to another, the mobile station determines a new header station for communications access to the wired LAN. The header stations periodically transmit broadcast messages identifying the header station and the loading factors at the header station. Using these messages a mobile station will determine all header stations that are potential owners of the mobile station and maintain in memory a list of potential owner station. If the current owner station remains a potential owner, and the signal strength of the current owner station is acceptable, then the current owner is retained. Otherwise, the mobile station determines a new header station based on the loading factors of the potential owner stations, and, if the loading factors are equal, the signal strengths of the signals received at the mobile station. This procedure has the disadvantages of using memory space in the mobile station to maintain in memory the list of potential owner stations, and the need to constantly monitor the messages periodically transmitted by the base stations. International Patent Application No. WO 92/19059 discloses a system which includes a cable-based network to which are attached controllers which maintain a portable device in communication with the network for data transfer. The portable unit transmits polling packets at regular intervals. Normally, a response packet is received from the current controller. If no response packet is received for a predetermined number of attempts the portable unit initiates a procedure for registering with a new controller. This procedure involves transmitting a packet from the portable unit requesting a response from any controllers, transmitting a response packet from each controller receiving the request, selecting one of the responding controllers at the portable unit according to criteria that enhance data transfer, and transmitting from the portable unit a packet identifying the selected controller to complete registration. This procedure has the disadvantage of the need for the portable unit to transmit a request packet to initiate the handover procedure. U.S. Patent No. 4,829,519 discloses a transfer system based on an assessment of the error rate of messages, which requires error correction code redundancy in all messages. Each base station periodically transmits assessment messages for the mobile to initiate a search for a replacement base station when the error rate detected exceeds a preset level. It is an object of the present invention to provide a method of operating a wireless local area network system wherein mobile station handover may be achieved in a simple manner without the aforementioned disadvantages. Therefore, according to the present invention, there is provided a method according to claim 1. One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings in which:- Fig. 1 shows a block diagram of a wireless LAN system in which the present invention is embodied; Fig. 2 shows a block diagram of a mobile wireless station included in the system of Fig. 1; Fig. 3 shows a block diagram of a base station included in the system of Fig. 1; Fig. 4 shows the format of a beacon message; and Fig. 5A and 5B are a flowchart illustrating the operation of a mobile wireless station during a handover procedure. Referring first to Fig. 1, there is shown a block diagram of a wireless LAN (local area network) system 10, embodying the present invention. The LAN system 10 includes a backbone LAN 12, which is a wired, cable-based LAN, and which includes a cable 14 connecting to a plurality of base stations 16, referenced individually as base stations 16-1, 16-2, ..., 16-N. The base stations 16 have antennas 18, referenced individually as antenna 18-1, 18-2, ..., 18-N. A server 19 is connected to the cable 14 to provide a server function for devices communicating with the LAN 12. Also included in the system 10 is a mobile station 20 having an antenna 22. Of course there may be more than one mobile station operating in the system. The base stations have coverage areas, referred to as cells 24, referenced individually as cell 24-1, 24-2, ..., 24-N. Referring now to Fig. 2, there is shown a block diagram of the mobile station 20. The mobile station 20 includes a wireless transceiver 30 coupled to the antenna 22 and to a bus 32. The mobile station 20 also includes, connected to the bus 32, a processor 34, a memory 36, and (optionally) a keyboard 38 and display 40. Other devices such as a printer (not shown) may be connected to the bus 32. A communications quality measurement circuit 42 is connected to the antenna 22 and to the bus 32, and is adapted to measure the communications quality of signals received on the antenna 22, as will be explained in more detail hereinafter. Referring now to Fig. 3, there is shown a block diagram of a base station 16. The base station 16 includes a wireless transceiver 50 coupled to the antenna 18 and to a bus 52. A further transceiver 54, connected to the bus 52, connects the base station 16 to the cable 14 of the backbone LAN 12 (Fig. 1). Also connected to the bus 52 are a processor 56, a memory 58 and a filtering database 60 which stores data identifying the location of devices in the LAN system 10, in relation to the base station 16. An interference level measurement circuit 62 is connected to the antenna 18 and to the bus 52 and is adapted to measure the interference level experienced by the base station 16. It should be understood that in operation of the LAN system 10, Fig. 1, information messages are transmitted between the mobile station 20 and the backbone LAN 12 via one of the base stations 16. It should be understood that such messages contain a network ID portion which identifies the particular cell 24 in which the message is transmitted. It will be appreciated that as the mobile station 20 moves around it may leave the coverage area of one cell 24 and enter the coverage area of a new cell 24. The present invention is concerned with a handover procedure for handing over communication from one base station 16 to another base station 16 when the mobile station 20 moves in such manner. In this connection, it is provided that the base stations 16 transmit beacon messages at regular intervals, for example at intervals of one second. Referring to Fig. 4, there is shown the format of such a beacon message 70. The beacon message 70 includes a first preamble portion (PR-1) 72, a network ID (NWID) portion 74, a second preamble portion (PR-2) 76, a start delimiter portion 78, a destination address (DA) portion 80, which is a broadcast address ensuring reception by any mobile station in the cell 24 covered by the base station 16 such as the mobile station 20, a source address (SA) portion 82, which is the base station address for the base station's cell 24, a length portion 84, a message identification (MID) portion 86, which identifies the message 70 as a beacon message, a wired address (WA) portion 88, which is the base station address for the wired LAN 12, an interference level (IL) portion 90, which represents the background interference (noise) level as measured by the interference level measurement circuit 62 (Fig. 3), a sequence number (SEQ) portion 92, a pad portion 94 and a frame check (FCS) portion 96. As mentioned hereinabove, the base stations 16 transmit beacon messages 70 at regular intervals. By arranging that the interval between successive beacon messages 70 is slightly different for each base station 16, the incidence of collisions between beacon messages is reduced. Thus station 16-1 may transmit beacon messages 70 at intervals of one second, whereas station 16-2 may transmit beacon messages 70 at intervals of one second plus one microsecond, for example, and station 16-3 may transmit beacon messages at intervals of one second plus two microseconds, for example. With this procedure, the loss of beacon messages due to collisions is rendered very small. Referring now to Figs. 5A and 5B, there is shown a flowchart 100 illustrating the operation of the mobile station 20 during a cell handover procedure. Initially, as shown in block 102, the station is operating in its normal mode, communicating with a current base station, assumed to be base station 16-1 (Fig. 1). When a beacon message is received from the current base station (block 104), the flowchart proceeds to block 106, where it is seen that the communications quality of this beacon message is determined. This measurement involves first determining the maximum of the background noise (interference) level measured at the mobile base station 20 and the background noise (interference) level measured at the base station 16 and included as the IL portion 90 of the beacon message 70 received from base station 16. The measured signal strength of the beacon message 70 at the mobile station 20 is then divided by the determined maximum noise level to provide a communications quality value based on a signal-to-noise ratio. It is then determined whether or not this communications quality value is above or below a first predetermined threshold value TH1. If above, the flowchart returns to block 104 via arrow 110. If below, the flowchart moves to block 112, which shows that the mobile station 20 changes to a search mode of operation. In the search mode of operation, the receiver portion of the transceiver 30 (Fig. 2) switches to a so-called cell-promiscuous mode of operation, in which beacon messages from any of the base stations 16 may be received and processed. This may be achieved by ignoring the network ID portion 74 (Fig. 4) of beacon messages. Assume, as shown in block 116, that the first beacon message 70 received by the mobile station 20 in the search mode is from base station X. As shown in block 118, the communications quality of this beacon message is measured by the communications quality measurement circuit 42 (Fig. 2) as described hereinabove, by first determining the maximum of the background noise (interference) levels at the mobile station 20 and base station X, using the IL portion 90 of the beacon message 70, and then dividing the signal level value of the beacon message 70 by the thus determined maximum noise level. A determination is then made as to whether or not this communications quality value is above a second predetermined threshold value TH 2 (block 120). If below, then the flowchart proceeds as shown via line 122 to block 116, and the next beacon message from a base station 16 is awaited. If above, then the flowchart proceeds to block 123 where it is determined whether the base station X is or is not the current base station. If base station X is not the current base station, then the mobile station 20 switches to the new base station, i.e. switches to a state wherein messages having the network ID of the new base station are processed (block 124), and the receiver is returned from the cell-promiscuous mode to the normal mode (block 126). The flowchart then returns to block 104 as shown by line 128. If base station X is the current base station, the flowchart process directly via line 130 to block 126, and the receiver returns to normal cell mode operation, in communication with the current base station 16. In connection with the above-described handover operation, it should be understood that the value of TH2 is greater than the value of TH1, so that the stop cell search threshold TH2 indicates a better communications quality than the start cell search threshold TH1. This arrangement is effective as a hysteresis feature, arid avoids oscillation between base stations 16 where coverage areas 24 overlap. Modifications to the described embodiment are possible. Thus, in a simplified arrangement measurement of the noise level at the base stations 16 could be omitted and the beacon messages 70 (Fig. 4) transmitted without the IL portion 90. Communications quality at the mobile station 20 would then be determined on the basis of a local signal strength to local noise value ratio only. In another modification when the mobile station is in the cell-promiscuous search mode of operation, searching for a new base station 16, a further comparison is made to determine whether the communications quality value of the current base station 16 is less than a further predetermined threshold value TH3, which is less than the threshold value TH1, and there is no base station 16 with a communications quality value above the stop cell search threshold value TH2. If so, then the mobile station 20 will switch over to any base station which has a communications quality value above the lower threshold value TH3. This fast cell search procedure assists in enabling the mobile station 20 to maintain communication with a base station 16 when the communications quality becomes so low that a new base station 16 has to be identified within as short a time as possible.

Claims: A method of operating a wireless local area network system which includes a plurality of base stations (16) and a mobile station (20), comprising the steps of: transmitting beacon messages (70) from said base station (16) at regular intervals, each beacon message (70) including an identification of the base station (16) transmitting that message ; operating said mobile station (20) in a normal mode wherein beacon messages (70) from a first base station (16) only are monitored; determining a communications quality value of said beacon messages (70) from said first base station (16); determining if said communications quality value becomes unacceptable, and if so, changing the operating mode of said mobile station (20) to a search operating mode wherein beacon messages (70) from any of said base stations (16) may be monitored; selecting a base station (16) providing an acceptable communications quality value for monitored beacon messages (70); and changing the operating mode of said mobile station (20) to said normal operating mode wherein beacon messages (70) from the selected base station (16) only are monitored; characterized by the method determining a background noise level for each base station (16); and including a representation of the background noise level as a portion of the beacon message (70) transmitted by each base station (16). A method according to claim 1, characterized in that said step of selecting a base station (16) providing an acceptable communications quality value includes the steps of: monitoring successively received beacon messages (70) until a beacon message (70) having an acceptable communications quality value is received; and selecting the base station (16) providing the beacon message (70) having the acceptable communications quality value. A method according to claim 2 characterized in that said step of determining if said communications quality value becomes unacceptable includes the steps of: comparing said communications quality value with a first predetermined threshold value; and identifying said communications quality value as unacceptable if said communications quality value is below said first predetermined threshold value. A method according to claim 3, characterized in that said step of monitoring successively received beacon messages (70) includes the steps of comparing the communications quality value of successively received beacon messages (70) with a second predetermined threshold value and identifying such communications quality value as acceptable if such communications quality value is above said second predetermined threshold value. A method according to claim 4, characterized in that said second predetermined threshold value is greater than said first predetermined threshold value. A method according to any one of the preceding claims, characterized in that said step of determining a communications quality value includes the step of determining a signal strength value for a received beacon message (70); determining a background noise value for said mobile station (20); and dividing said signal strength value by said background noise value to obtain said communications quality value. A method according to claim 1, characterized in that said step of determining a communications quality value includes the steps of determining a signal strength value for a received beacon message (70); determining a background noise level for said mobile station (20); determining the maximum value of said background noise level for said mobile station (20) and the background noise level representation of the base station (16) in the received beacon message (70); and dividing said signal strength value by the determined maximum value to provide said communications quality value.

Inventors: BREDERVELD LOEKE, DIEPSTRATEN WILHELMUS J M, HAAGH JOHANNES P N, HOOGENDOORN JAN, MOELARD HENDRIK, BREDERVELD, LOEKE, DIEPSTRATEN, WILHELMUS J. M., HAAGH, JOHANNES P. N., HOOGENDOORN, JAN, MOELARD, HENDRIK
Applicants: LUCENT TECHNOLOGIES INC, LUCENT TECHNOLOGIES INC.
Application year: 1993
Main classification: H04L12
IPC codes: H04L12, H04W36