Patent Publication Number: US-7711367-B2

Title: Fast handover with reduced service interruption for high speed data channels in a wireless system

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to telecommunications, and, more particularly, to wireless communications. 
   2. Description of the Related Art 
   In the field of wireless telecommunications, such as cellular telephony, a system typically includes a plurality of base stations distributed within an area to be serviced by the system. Various users within the area, fixed or mobile, may then access the system and, thus, other interconnected telecommunications systems, via one or more of the base stations. Typically, a mobile device maintains communications with the system as the mobile device passes through an area by communicating with one and then another base station, as the user moves. The mobile device may communicate with the closest base station, the base station with the strongest signal, the base station with a capacity sufficient to accept communications, etc. 
   Historically, the mobile device has been used for voice communications where the delivery of information is time critical. That is, if even relatively short segments of a conversation are delayed or lost, the meaning and understanding of the parties to the conversation may be substantially impaired. During the period when the mobile device is discontinuing communications with a first base station and beginning communications with a second base station, there is a distinct possibility that communications will be at least temporarily interrupted or delayed. Thus, for voice communications, a process known as soft hand off (SHO) was developed in the CDMA and UMTS systems to have multiple connections in the region of overlapped coverage in order to substantially enhance the likelihood that the conversation will continue unabated even during these transition periods. 
   Recently, the operation of mobile devices has been extended to the field of high speed data, such as might be employed when accessing the Internet or the World Wide Web. The exchange of high speed data, unlike voice communications, has historically not been time critical. That is, the transmission of data may be temporarily interrupted or delayed without affecting a receiver&#39;s ability to “understand” the data. Thus, temporary delays or interruptions during the transition period from one base station to another have been acceptable. 
   However, the use of high speed data connections has expanded to operations that are more time critical. For example, Voice over Internet Protocol (VoIP) is a process that involves digitizing voice signals, organizing the digitized voice signals into packets, and transmitting the packets over a high speed digital connection. A receiving party reassembles the packets and plays the packets to produce an audio communication. Thus, voice communications can be accomplished over a high speed data connection. If this process can be accomplished in real time, then a conversation may occur across the high speed digital connection. Where the high speed digital connection is being used for voice communications, then the transition periods become significant so as to avoid delaying or interrupting the conversation. 
   The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect of the instant invention, a method is provided for controlling a communications system. The method comprises communicating information to a first base station, and selecting a switchover time for beginning communications with a second base station. The switchover time is based on channel conditions associated with the communications to the first and second base stations. 
   In another aspect of the instant invention, a method is provided for controlling a communications system. The method comprises communicating information from a first base station, and selecting a switchover time for beginning communications from a second base station. The switchover time is based on channel conditions associated with the communications from the first and second base stations. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
       FIG. 1A  is a block diagram of a communications system, in accordance with one embodiment of the present invention; 
       FIG. 1B  is a stylistic representation of a region in which the communications system of  FIG. 1A  may be employed; 
       FIG. 2  depicts a block diagram of one embodiment of a Base station and a mobile device used in the communications system of  FIG. 1 ; and 
       FIG. 3  is a flow diagram illustrating the interoperation of the various components of the communications system of  FIGS. 1 and 2 . 
   

   While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
   Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
   Turning now to the drawings, and specifically referring to  FIG. 1A , a communications system  100  is illustrated, in accordance with one embodiment of the present invention. For illustrative purposes, the communications system  100  of  FIG. 1A  is a Universal Mobile Telephone System (UMTS), although it should be understood that the present invention may be applicable to other systems that support data and/or voice communication. The communications system  100  allows one or more mobile devices  120  to communicate with a data network  125 , such as the Internet, and/or a public telephone system (PSTN)  160  through one or more base stations  130 . The mobile device  120  may take the form of any of a variety of devices, including cellular phones, personal digital assistants (PDAs), laptop computers, digital pagers, wireless cards, and any other device capable of accessing the data network  125  and/or the PSTN  160  through the base station  130 . 
   In one embodiment, a plurality of the base stations  130  may be coupled to a Radio Network Controller (RNC)  138  by one or more connections  139 , such as T1/E1 lines or circuits, ATM virtual circuits, cables, optical digital subscriber lines (DSLs), and the like. Although one RNC  138  is illustrated, those skilled in the art will appreciate that a plurality of RNCs  138  may be utilized to interface with a large number of base stations  130 . Generally, the RNC  138  operates to control and coordinate the base stations  130  to which it is connected. The RNC  138  of  FIG. 1  generally provides replication, communications, runtime, and system management services, and, as discussed below in more detail below, may be involved in coordinating the transition of a mobile device  120  during transitions between the base stations  130 . 
   As is illustrated in  FIG. 1B , a region  170  to be serviced by the system  100  is separated into a plurality of regions or cells, each being associated with a separate base station  130 . Typically, each cell has a plurality of adjacent neighboring cells. For example, the cell  175  has six neighboring cells  176 - 181  such that a mobile device  120  entering the cell  175  may travel from one of the neighboring cells  176 - 181 . Thus, as the mobile device  120  enters the cell  175  from any of the neighboring cells  176 - 181 , the mobile device may need to transition from communicating with the cell  175  to communicating with the neighboring cell  176 - 181  that it is entering. 
   Returning to  FIG. 1A , the RNC  138  is also coupled to a Core Network (CN)  165  via a connection  145 , which may take on any of a variety of forms, such as T1/E1 lines or circuits, ATM virtual circuits, cables, optical digital subscriber lines (DSLs), and the like. Generally the CN  165  operates as an interface to the data network  125  and/or to the public telephone system (PSTN)  160 . The CN  165  performs a variety of functions and operations, such as user authentication, however, a detailed description of the structure and operation of the CN  165  is not necessary to an understanding and appreciation of the instant invention. Accordingly, to avoid unnecessarily obfuscating the instant invention, further details of the CN  165  are not presented herein. 
   Thus, those skilled in the art will appreciate that the communications system  100  enables the mobile devices  120  to communicate with the data network  125  and/or the PSTN  160 . It should be understood, however, that the configuration of the communications system  100  of  FIG. 1A  is exemplary in nature, and that fewer or additional components may be employed in other embodiments of the communications system  100  without departing from the spirit and scope of the instant invention. 
   Unless specifically stated otherwise, or as is apparent from the discussion, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system&#39;s memories or registers or other such information storage, transmission or display devices. 
   Referring now to  FIG. 2 , a block diagram of one embodiment of a functional structure associated with an exemplary base station  130  and mobile device  120  is shown. The base station  130  includes an interface unit  200 , a controller  210 , an antenna  215  and a plurality of channels: such as a shared channel  220 , a data channel  230 , and a control channel  240 . The interface unit  200 , in the illustrated embodiment, controls the flow of information between the base station  130  and the RNC  138  (see  FIG. 1A ). The controller  210  generally operates to control both the transmission and reception of data and control signals over the antenna  215  and the plurality of channels  220 ,  230 ,  240  and to communicate at least portions of the received information to the RNC  138  via the interface unit  200 . 
   The mobile device  120  shares certain functional attributes with the base station  130 . For example, the mobile device  120  includes a controller  250 , an antenna  255  and a plurality of channels: such as a shared channel  260 , a data channel  270 , and a control channel  280 . The controller  250  generally operates to control both the transmission and reception of data and control signals over the antenna  255  and the plurality of channels  260 ,  270 ,  280 . 
   Normally, the channels  260 ,  270 ,  280  in the mobile device  120  communicate with the corresponding channels  220 ,  230 ,  240  in the base station  130 . Under the operation of the controllers  210 ,  250 , the channels  220 ,  260 ;  230 ,  270 ;  240 ,  280  are used to effect a controlled scheduling of communications from the mobile device  120  to the base station  130 . 
   Turning now to  FIG. 3 , a flow diagram illustrating the interoperation of the various components of the system  100  is shown. In the flow diagram of  FIG. 3 , it is assumed that a high speed data transmission is underway with respect to the mobile device  120  such that the mobile device  120  is communicating with base station A, but will be transitioning to base station B. Initially, the mobile device  120  is within the cell associated with base station A and is approaching or entering the cell associated with the base station B. 
     FIG. 3  stylistically represents a handover procedure for a high speed data channel, generally depicting a messaging process that may be used to switch over the high speed data channel from a serving cell to a target cell. Generally, the actual switchover begins when the RNC  138  sends a Radio Link Reconfiguration Commit messages to the serving Node B to stop the scheduled transmission at a defined “activation time.” The mobile device  120  begins “listening” to scheduling information from the target cell at the activation time after sending the “Physical Channel Reconfiguration Complete” messages. A significant factor in reducing VoIP service interruption is the setting of the “activation time” for the switchover from the serving cell to the target cell. Historically, the “activation time” is set at the very early stage of Radio Link Reconfiguration Commit messages and is executed at the time when the mobile device  120  sends the “Physical Channel Reconfiguration Complete.” In some prior art systems, the setting time and the execution time may be separated by a significant amount of time (e.g., from hundreds of milliseconds to seconds). The “activation time” is determined based on the process time of the signaling messages (e.g., Iur, Iub and UU). During this interval, a variety of factors could negatively impact the radio channel conditions. For example, the mobile device  120  may move away from the serving cell and the radio link quality could deteriorate significantly such that data could no longer be delivered. It could also happen that mobile device  120  may be unable to decode the scheduling information and missed the data. Thus, VoIP service to the mobile device  120  may be interrupted for a significant amount of time. In one embodiment of the instant invention, the activation time for the switchover process is carefully selected based on current conditions of the radio link between the mobile device  120  and the serving and target base stations. 
   In a first embodiment of the instant invention, as shown in  FIG. 3 , the serving RNC  138 - 1  determines that there is a need to switchover from high speed communications between the mobile device  120  and a source base station  103 - 1  to high speed communications between the mobile device  120  and a target base station  130 - 2 . The serving RNC  138 - 1  prepares an RNSAP (Radio Link Reconfiguration Prepare) message, which is transmitted to a drift RNC  138 - 2  at  300 . 
   In the illustrated embodiment, the source and target cells are controlled by different base stations  130 - 1 ,  130 - 2 . The drift RNC  138 - 2  requests the source base station  130 - 1  to perform a synchronized radio link reconfiguration using an NBAP (Node B Application Part) message “Radio Link Reconfiguration Prepare” at  302 . The source base station  130 - 1  returns an NBAP message “Radio Link Reconfiguration Ready” at  304 . 
   The drift RNC  138 - 2  requests the target base station  130 - 2  to perform a synchronized radio link reconfiguration using the NBAP message “Radio Link Reconfiguration Prepare” at  306 . The target base station  130 - 2  returns the NBAP message “Radio Link Reconfiguration Ready” at  308 . The drift RNC  138 - 2  returns an RNSAP message “Radio Link Reconfiguration Ready” to the serving RNC  138 - 2  at  310 . The drift RNC  138 - 2  initiates set-up of a new Iub Data Transport Bearers using ALCAP (Access Link Control Application Protocol) protocol at  312 . This request contains an AAL2 (ATM Adaptation Layer type 2) Binding Identity to bind the Iub Data Transport Bearer to the high speed data channel. 
   The serving RNC  138 - 1  initiates set-up of a new Iur Data Transport bearer using ALCAP protocol at  314 . This request contains the AAL2 Binding Identity to bind the Iur Data Transport Bearer to the high speed data channel. The high speed data channel transport bearer to the target base station  130 - 2  is established. The serving RNC  138 - 1  proceeds by transmitting the RNSAP message “Radio Link Reconfiguration Commit” to the drift RNC  138 - 2  at  316 . The serving RNC  138 - 1  selected the activation time in the form of a CFN (Connection Frame Number). 
   The drift RNC  138 - 2  transmits the NBAP message “Radio Link Reconfiguration Commit” to the source base station  130 - 1  including the activation time at  318 . Similarly, the drift RNC  138 - 2  also transmits the NBAP message “Radio Link Reconfiguration Commit” to the target base station  130 - 2  including the activation time at  320 . At the indicated activation time, the source base station  130 - 1  stops transmitting and the target base station  130 - 2  starts transmitting on the high speed data channel to the mobile device  120 . 
   The serving RNC  138 - 1  also transmits an RRC (Radio Resource Control) message “Physical Channel Reconfiguration” to the mobile device  120  at  322 . At the indicated activation time the mobile device  120  stops receiving high speed data in the source cell and starts receiving high speed data in the target cell. The mobile device  120  returns an RRC message “Physical Channel Reconfiguration Complete” to the serving RNC  138 - 1  at  324 . The drift RNC  138 - 2  initiates release of the old Iub Data Transport bearer using ALCAP protocol at  326 . Similarly, the serving RNC  138 - 1  initiates release of the old Iur Data Transport bearer using ALCAP protocol at  328 . 
   To reduce or minimize VoIP service interruption during handover, the “activation time” needs to be properly selected. The “activation time” is determined by the processing time of the signaling procedure. If the switchover is set very late and the radio channel condition of the serving cell has deteriorated, the VoIP service will have an open window period of no service. If the activation time is set too early and the radio channel condition of the target cell relative to the specific user is not good, the VoIP service will have an interruption too. The issue of the switchover timing is the unawareness of the radio channel condition when RNC  138 - 1  sets the activation time. It requires L-3 signaling to communicate between the mobile device  120  and RNC in order to identify the best timing of switchover. Nevertheless, the L-3 signaling delay is large to affect the effectiveness of the switchover timing identification. Thus, the criteria of minimizing the VoIP service interruption during the handover are to estimate the required signaling processing time correctly, to reduce the signaling time, and to switch over to the target cell at the right time. 
   Reducing the time required for signaling during switchover can improve the switchover process. The RNC  138  uses feedback of the radio channel conditions, such as SIR (Signal to Interference Ratio) from the base stations  130  in the active set or mobile reported best cell measurement, to trigger the switchover and to estimate a desired switchover time. However, the accuracy of the estimation deteriorates as the prediction interval becomes larger. In particular, radio channel conditions may changes dramatically in hundreds of milliseconds to seconds. By reducing the interval, the switchover process may be substantially improved. 
   The base station  130  periodically reports SIR measurements to the RNC  138 . In one embodiment, the average SIR is calculated over an 80 ms interval and reported to the RNC  138 . The average SIR measurement may be used as a reference of the radio channel condition of each leg during the switchover. The average uplink SIR measurements are a reciprocal of the long-term average of the CQI reports for the downlink radio link. In one embodiment of the instant invention, an algorithm used to calculate “activation time” is as follows, 
           {             SIR   Serving     &lt;     Threshold   Serving                   SIR   Target     &gt;     Threshold   target                       
where SIR Serving  and SIR Target  are the SIR measurements of the serving cell and the target cell, respectively, and Threshold Serving  and Threshold target  are the thresholds of the serving cell and target cell, respectively. The algorithm is designed based on link imbalance during the switchover when the uplink inner loop power control would speed up the migration of the radio link to the leg with good channel condition. Thus, the activation time setting could be improved by setting the threshold properly in accordance with the signaling process delay.
 
   The general signaling procedures of switching the high speed data channel from the serving cell to the target cell are performed in series. Since Iub Radio Link Reconfiguration Commit message requires no acknowledgement, the delay may be reduced by sending Iub Radio Link Reconfiguration messages to the serving and target cells as well as UU RRC Physical Layer Reconfiguration message to the mobile device  120  at about the same time, or at least with some overlap. This would remove significant delays and introduce minimum process delay only. 
   With the concurrent Iub and UU signaling message to commit the switchover of the radio links at the activation time, the processing delay of the UU RRC Physical Channel Reconfiguration message is another factor in the delay. The current performance procedure for the Physical Channel Reconfiguration (see section 13.5.2 of TS25.331) requires the mobile device  120  to execute it within 80 ms after receiving the UU signaling message and update the L1 configuration at the beginning of the next TTI. The DCCH RABs supported in the TS34.108 are 1.7 kbps, 3.4 kbps, and 13.6 kbps with TTI being 80, 40, and 10 ms, respectively. The most common test procedure is the 3.4 kbps DCCH with 40 ms TTI. If the 3.4 kbps DCCH were used to carry the Physical Channel Reconfiguration messages for the high speed data channel switchover, the total delay would exceed 120 ms (80 ms processing+40 ms TTI DCCH reception). Moreover, the 80 ms execution delay is also considered by the low signaling RAB for communication of the primitive between higher layer protocols and physical layers at the mobile device  120 . Using the 13.6 kbps RAB with 10 ms TTI would reduce the execution time and processing time. It will reduce the interruption of the VoIP service during switchover. 
   One significant issue of the high speed data channel switchover is to synchronize the buffers at the base stations  130 - 1  and  130 - 2  since both Iub links for the serving cell and target cell are established before the Radio Link Reconfiguration Commit commands. To reduce the service interruption, the RNC  138  can send the VoIP data to both of the base stations  130 - 1  and  130 - 2  after the Iub data link of the target base station  130 - 2  is established. 
   The target base station  130 - 2  will not schedule any transmission before the “activation time” and has no idea of the scheduled information at the source base station  130 - 1 . Since other channels (e.g., UL DPCH) are operating in the soft-handover mode, the target base station  130 - 2  may decode the Ack/Nack and CQI information before the “activation time” although that they intend to send to the source base station  130 - 1 . Due to the link imbalance during the switchover, the target cell would decode the Ack/Nack and CQI for the serving cell incorrectly with erasure in the beginning and gradually move into correct decoding before the “activation time”. The target cell could decide to drop a VoIP frame every T erasure  interval if the Ack/Nack and CQI are erased. The T erasure  interval is a parameter to be optimized for minimizing the buffer occupancy at the target cell. If the Ack is decoded correctly at the target cell before the activation time, the target cell could determine to drop the next VoIP packet. This procedure is an uncoordinated VoIP counting for the high speed data channel when the VoIP packet has been sent to both the serving and target cells during the switchover before the activation time. This will avoid any data gap for the sequence delivery of the VoIP packet and minimize the buffer occupancy at the target cell. The proposed algorithm is called “Pseudo-Synchronization” because its behavior is similar to the buffer synchronization. 
   Those skilled in the art will appreciate that the various system layers, routines, or modules illustrated in the various embodiments herein may be executable control units (such as the controllers  210 ,  250  (see  FIG. 2 )). The controllers  210 ,  250  may include a microprocessor, a microcontroller, a digital signal processor, a processor card (including one or more microprocessors or controllers), or other control or computing devices. The storage devices referred to in this discussion may include one or more machine-readable storage media for storing data and instructions. The storage media may include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy, removable disks; other magnetic media including tape; and optical media such as compact disks (CDs) or digital video disks (DVDs). Instructions that make up the various software layers, routines, or modules in the various systems may be stored in respective storage devices. The instructions when executed by the controllers  210 ,  250  cause the corresponding system to perform programmed acts. 
   The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. Consequently, the method, system and portions thereof and of the described method and system may be implemented in different locations, such as the wireless unit, the base station, a base station controller and/or mobile switching center. Moreover, processing circuitry required to implement and use the described system may be implemented in application specific integrated circuits, software-driven processing circuitry, firmware, programmable logic devices, hardware, discrete components or arrangements of the above components as would be understood by one of ordinary skill in the art with the benefit of this disclosure. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.