Abstract:
An intermediate station and method for facilitating handover in a multi-hop communications system is disclosed. The method may include receiving a handover trigger, finding a target station to receive handover, determining whether the handover requires a timing change, wherein if it is determined that a preamble and broadcast timing change for one or more downstream stations is required, estimating when the handover will take place, instructing the one or more downstream stations to resynchronize, and performing the handover to the target station.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to mobile communication systems. 
     2. Introduction 
     A mobile relay station (MRS) is a relay station that is capable of handover between base stations (BS) (or relay stations for above 3-hops). It is usually attached to a public transportation system (e.g., a bus, train, etc.) and may relay information to/from mobile stations (MSs) inside the public transportation. When MRS makes handover from BS 1  to BS 2 , MRS should take care for MSs under communication via the MRS. Due to handover process of MRS, synchronization between MRS and MSs may be lost. It leads to a communication disruption. 
     If MS makes handover as same as conventional cellular system, it would be no problem for MS to prepare handover process and make a decision. When RS makes handover, the RS must be cognizant of MSs under communication via the RS. Synchronization could be an issue between RS and MSs when RS makes handover between BSs. A problem may occur in the change of timing of preamble and broadcast message that is transmitted from RS to MSs. If the MSs lose synchronization, it takes time to recover it. During the recovery process, communication with MSs may be suspended. The synchronization process of MSs consumes radio resources to reestablish the synchronization. If there are many MSs under the RS, the overhead amounts may be large. 
     If the RS can maintain the same preamble and broadcast timing when the RS makes a handover between BSs, this problem does not occur. However, it would be difficult to assign same preamble and broadcast timing after the RS has completed the handover as before the handover because preambles and following broadcasts may interfere. The information exchange between BSs via the backbone network would be too complicated to be performed in a practical manner. 
     SUMMARY OF THE INVENTION 
     An intermediate station and method for facilitating handover in a multi-hop communications system is disclosed. The method may include receiving a handover trigger, finding a target station to receive handover, determining whether the handover requires a timing change of preamble and broadcast, wherein if it is determined that a preamble and broadcast timing change for one or more downstream stations is required, estimating when the handover will take place, instructing the one or more downstream stations to resynchronize, and performing the handover to the target station. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  illustrates an exemplary diagram of a communications network in accordance with a possible embodiment of the invention; 
         FIG. 2  illustrates a block diagram of an exemplary intermediate station in accordance with a possible embodiment of the invention; and 
         FIG. 3  is an exemplary flowchart illustrating one possible mobile communications handover process in accordance with one possible embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth herein. 
     Various embodiments of the invention are discussed in detail below. While specific implementations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the invention. 
     The present invention comprises a variety of embodiments, such as a method, apparatus, and other embodiments that relate to the basic concepts of the invention. 
       FIG. 1  illustrates an exemplary diagram of a communications network  100  in accordance with a possible embodiment of the invention. In particular, the communications network  100  may include intermediate station  110 , anchor station  120 , target station  130 , and downstream station  140 . One of skill in the art may appreciate that intermediate station  110 , anchor station  120 , target station  130 , and downstream station  140  may represent any communications station that may exist in a mobile communications network, including a base station, a relay station, and a mobile relay station. The communications network  100  may represent any communications network or combination of communications networks known to one of skill in the art, including a mobile communications network or cellular communications network. 
     The anchor station  120  communicates through the intermediate station  110  to downstream stations  140  or devices, such as cell phone terminals. The stations send messages in that contain at least in part, preamble and broadcast portions. As shown, anchor station  120  sends preamble  1  and the target station  130  sends preamble  2  to the intermediate station  110 , and the intermediate station  110  sends preamble  3  to the downstream station  140 . The sending of these preamble and broadcast messages must be orthogonal or robust enough in order for the communications to be established and maintained. Otherwise, interference may occur and the communication link may be dropped. 
       FIG. 2  illustrates an exemplary intermediate station  110  which may implement one or more modules or functions of the communications handover process shown below in  FIG. 4 . The exemplary intermediate station  110  may include a bus  210 , a controller  220 , a memory  230 , an antenna  240 , a transceiver  250 , and a communication interface  260 . Bus  210  may permit communication among the components of the intermediate station  110 . 
     Controller  220  may include at least one conventional processor or microprocessor that interprets and executes instructions. Memory  230  may be a random access memory (RAM or another type of dynamic storage device that stores information and instructions for execution by processor  220 . Memory  230  may also store temporary variables or other intermediate information used during execution of instructions by processor  320 . Memory  230  may also include a read-only memory (ROM which may include a conventional ROM device or another type of static storage device that stores static information and instructions for processor  220 . Memory  230  may include a storage device that stores any type of media, such as, for example, magnetic or optical recording media and its corresponding drive. 
     Transceiver  250  may include one or more transmitters and receivers. The transceiver to  250  may include sufficient functionality to interface with any network or communications station and may be defined by hardware or software in any manner known to one of skill in the art. The transceiver  250  may be operable to support communication activities and links within the network  100 . The controller  220  is cooperatively operable with the transceiver  250  to support operations within the communications network  100 . 
     Communication interface  260  may include any mechanism that facilitates the intermediate station  110  to communicate via the communications network  100 . For example, communication interface  260  may include a modem. Alternatively, communication interface  260  may include other mechanisms for assisting the transceiver  250  in communicating with other devices and/or systems via wireless connections. In some implementations of the network environment  100 , communication interface  260  may not be included in the exemplary intermediate station  110  when the communications handover process is implemented completely within the communications network  100 . 
     The intermediate station  100  may perform such functions in response to processor  220  by executing sequences of instructions contained in a computer-readable medium, such as, for example, memory  230 , a magnetic disk, or an optical disk. Such instructions may be read into memory  230  from another computer-readable medium, such as a storage device or from a separate device via communication interface  260 . 
     The communications system  100  and the intermediate station  110  illustrated in  FIGS. 1 and 2  and the related discussion are intended to provide a brief, general description of a suitable computing environment in which the invention may be implemented. Although not required, the invention will be described, at least in part, in the general context of computer-executable instructions, such as program modules, being executed by the intermediate station  110 , such as a communications server, or general purpose computer. Generally, program modules include routine programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that other embodiments of the invention may be practiced in communication network environments with many types of communication equipment and computer system configurations, including cellular devices, mobile communication devices, personal computers, hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, and the like. 
     For illustrative purposes, the communication handover process will be described below in relation to the block diagrams shown in  FIGS. 1 and 2 . 
       FIG. 3  is an exemplary flowchart illustrating some of the steps associated with a communication handover process in accordance with another possible embodiment of the invention. The process begins at step  3100  and continues to step  3150  where the intermediate station  110  determines whether the signal being received from an anchor station  120  upstream is above a threshold or where the anchor station  120  determines whether the signal being received from an intermediate station  110  upstream is above a threshold based on a measurement report from the intermediate station  110  to the anchor station  120 . The threshold may be predetermined or computed using any algorithm known to one of skill in the art. If the intermediate station  110  or the anchor station determines that the signal from the anchor station  120  is at or above the threshold, the process then returns to step  3150 . This step may represent a periodic monitoring process. Although not shown, time delays in the monitoring process may be implemented in this process within the spirit and scope of the invention. 
     If the intermediate station  110  determines that the signal from the anchor station is below the threshold, the intermediate station  110  attempts to find another upstream station in which to hand over communications. If the intermediate station  110  cannot find another station with which to perform the handover, the process returns to step  3200  until a target station  130  is found. If the intermediate station  110  finds a target station  130  in which to hand over communications, then at step  3250 , the intermediate station  110  obtains parameters for synchronization concerning the timing of the preamble and broadcast messages. The preamble and broadcast message timing of all stations involved in the communications chain must be synchronized so that interference does not take place. 
     If the anchor station  120  determines that the signal from the intermediate station is below the threshold, the anchor station forces the intermediate station  110  to attempt to find another upstream station in which to hand over communications. If the intermediate station  110  cannot find another station with which to perform the handover, the process returns to step  3200  until a target station  130  is found. If the intermediate station  110  finds a target station  130  in which to hand over communications, then at step  3250 , the intermediate station  110  obtains parameters for synchronization concerning the timing of the preamble and broadcast messages. The preamble and broadcast message timing of all stations involved in the communications chain must be synchronized so that interference does not take place. 
     The decision for conducting the handover of communications may be conducted in at least two different ways. At step  3300 , the intermediate station  110  may decide to conduct the handover. If the intermediate station  110  decides not to conduct the handover, the process returns to step  3200 . Alternatively, at step  3350 , the intermediate station  110  may receive handover instructions from an anchor station upstream, for example. 
     In either case, once a favorable handover decision has been made, the process goes to step  3400  where the intermediate station  110  determines whether the handover requires any of the downstream stations to perform a preamble and broadcast message timing change. If the intermediate station  110  determines that the handover does not require downstream stations to perform preamble and broadcast message timing changes, then at step  3550 , the intermediate station  110  performs the handover to the target station. The process then goes to step  3600 , and ends. 
     If the intermediate station  110  determines that the handover requires downstream stations to perform preamble and broadcast message timing changes, then at step  3450 , the intermediate station  110  estimates when the handover will take place. At step  3500 , the intermediate station  110  instructs one or more of the downstream stations to resynchronize. At step  3500 , the instruction may include the information to imply when the resynchronization should be started. At step  3550 , the intermediate station  110  performs the handover to the target station. The process then goes to step  3600 , and ends. 
     Embodiments within the scope of the present invention may also include computer-readable media for carrying or having computer-executable instructions or data structures stored thereon. Such computer-readable media can be any available media that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code means in the form of computer-executable instructions or data structures. When information is transferred or provided over a network or another communications connection (either hardwired, wireless, or combination thereof to a computer, the computer properly views the connection as a computer-readable medium. Thus, any such connection is properly termed a computer-readable medium. Combinations of the above should also be included within the scope of the computer-readable media. 
     Computer-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing device to perform a certain function or group of functions. Computer-executable instructions also include program modules that are executed by computers in stand-alone or network environments. Generally, program modules include routines, programs, objects, components, and data structures, etc. that perform particular tasks or implement particular abstract data types. Computer-executable instructions, associated data structures, and program modules represent examples of the program code means for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps. 
     Although the above description may contain specific details, they should not be construed as limiting the claims in any way. Other configurations of the described embodiments of the invention are part of the scope of this invention. For example, the principles of the invention may be applied to each individual user where each user may individually deploy such a system. This enables each user to utilize the benefits of the invention even if any one of the large number of possible applications do not need the functionality described herein. In other words, there may be multiple instances of the intermediate station  110  in  FIGS. 1 and 2  each processing the content in various possible ways. It does not necessarily need to be one system used by all end users. Accordingly, the appended claims and their legal equivalents should only define the invention, rather than any specific examples given.