Patent Publication Number: US-2015085829-A1

Title: Controlling handover of a high-priority user equipment operating near a cell edge

Description:
TECHNICAL FIELD 
     The present invention relates to a method and an apparatus in an eNodeB (eNB) for controlling handover of a User Equipment (UE) operating near a cell edge. 
     BACKGROUND 
     In the Long Term Evolution (LTE) radio access network, UEs send Channel Quality Indication (CQI) reports to their serving eNB to help the eNB determine the physical radio environment, or channel quality, being experienced by each UE and to decide which Modulation and Coding scheme to use. During each Transmission Time Interval (TTI), an eNB scheduler considers the physical radio environment of each UE, prioritizes the Quality of Service (QoS) service requirements among the UEs, allocates radio resources, and informs the UEs of the allocated radio resources. The solution relies on rapid adaptation to channel variations, employing Hybrid Automatic Repeat Request (HARQ) with soft-combining and rate adaptation. 
     The eNB scheduler assigns each radio bearer a certain priority based on an assigned QoS Class Identifier (QCI) of the UE. The operator assigns the QCI to the UE based on the UE type and the type of service requested by the UE. UEs requesting services such as voice calls are assigned a QCI indicating a higher QoS, and the scheduler is required to allocate sufficient resource blocks to the UE to meet the bitrate requirements needed to provide the higher QoS. When prioritizing the QoS service requirements among the UEs, the scheduler supports both delay-sensitive real-time services as well as datacom services requiring high peak data rates. The eNB schedules the UEs both on the downlink (DL) and on the uplink (UL). Since the scheduler schedules DL and UL packets to all UEs in the eNB coverage area, the scheduler has to treat every UE based on its assigned QCI and its reported signal quality. A higher scheduling priority gives the radio bearer serving the UE a higher probability of obtaining the resources necessary to enable the UE to perform transmission or reception at the indicated QoS level. 
     Several trigger events are currently defined in LTE based on the UE measurements of the signal quality from the serving cell and neighbor cells. The trigger events relevant to the present disclosure are referred to as a 2 , a 4 , and a 5 . At a 2 , the signal quality of the serving cell becomes worse than an absolute threshold. At this point, the UE begins to allocate more resources to listening to the serving eNB and to determining which neighboring cells are the best candidates for handover. Thus, in effect, a 2  defines a beginning of the cell edge zone. At a 4 , the Reference Signal Receive Power (RSRP) of a neighbor cell becomes better than or equal to the RSRP of the serving cell. At a 5 , the RSRP of the neighbor cell becomes better than a threshold T 1 , while the RSRP of the serving cell is worse than a threshold T 2 . Thresholds T 1  and T 2  relate to hysteresis values added and subtracted to a defined handover threshold level in order to prevent premature handover and “Ping-Pong” effects. Thus, in effect, a 5  defines the end of the cell edge zone and the boundary of the cell. When the UE&#39;s reported CQI (UEcqi) drops to a 5 , the UE is handed over to the neighbor cell. 
     High-priority subscribers (either with a defined Guaranteed Bit Rate (GBR) or not), who are at the cell edge (i.e., event a 4  has been is triggered but not event a 5 ) suffer because of the low Signal-to-Noise Ratio (SNR) and high interference at the cell edge due to poor RSRP. However, due to the high scheduling priority set for such subscribers, the eNB continues to allocate resources and transmit messages and data to the UE despite the poor RSRP. If the UE does not receive the data or messages at the bitrate required for its assigned QCI, it will signal the serving eNB, which will assign additional resources to the UE. If the UE is unable to decode the messages, it will not respond, and after a timeout period, the serving eNB will retransmit the data that was not acknowledged. This wastes network resources and also deteriorates the user experience in case of data or voice calls. 
     SUMMARY 
     With the existing implementation, even if a high-priority UE has poor signal quality, the eNB scheduler must allocate to the high-priority UE, radio resources consistent with the QoS requirements of the UE&#39;s assigned QCI. This causes several problems. 
     First, the signaling load on the eNB and the UE is increased because of retransmissions. When the UE receives a TB that it is unable to decode, the UE signals this decoding failure to the eNB. In response, the eNB retransmits the TB. Additionally, if the signal quality is so bad that the UE does not receive the TB at all, the UE will not send an acknowledgment. Failure to receive an acknowledgment within a specified time period also causes the eNB to retransmit the TB. The excess use of radio resources caused by these retransmissions causes a second problem, namely, that it may leave insufficient resources to serve other UEs with lower priority. The present disclosure solves this problem of “UE starvation” as well as the increased signaling load on the eNB due to the retransmissions. 
     The present disclosure recognizes that under the existing implementation, high-priority subscribers at cell edge suffer poor service irrespective of their QOS priority because the handover decision is not considering their QCI. Currently, the handover decision only takes into consideration the a 2 , a 4 , and a 5  thresholds. 
     According to different embodiments of the present disclosure, methods and apparatuses are provided for enabling the eNB to instruct the high-QCI/Priority UE, which is experiencing low signal quality and high retransmissions at the cell edge, to move earlier than normal to an adjacent cell with better signal quality in order to save the scheduler bandwidth and also to provide relief to the UE from retransmissions and hence gain a better user experience. 
     According to one embodiment, a method performed in an eNB of a cellular telecommunication system is disclosed. The method controls handover of a UE operating near a cell edge of a serving cell. The method comprises determining whether the UE has a priority level defined for utilizing a signal-quality threshold parameter that is set at a level greater than a signal-quality handover level used to initiate handover of the UE to a neighbor cell; upon determining that the UE has a priority level defined for utilizing the signal-quality threshold parameter, monitoring UE signal-quality reports to determine when the signal-quality of the UE in the serving cell is lower than the signal-quality threshold parameter and greater than the signal-quality handover level; and forcing a handover of the UE from the serving cell to the neighbor cell in response to determining that the signal quality of the UE is lower than the signal-quality threshold parameter and greater than the signal-quality handover level. 
     In another embodiment, an apparatus in an eNB of a cellular telecommunication system is disclosed. The apparatus controls handover of a UE operating near a cell edge of a serving cell. The apparatus includes a processor coupled to a memory that stores computer program instructions. When the processor executes the computer program instructions, the processor causes the apparatus to determine whether the UE has a priority level defined for utilizing a signal-quality threshold parameter that is set at a level greater than a signal-quality handover level used to initiate handover of the UE to a neighbor cell; in response to determining that the UE has a priority level defined for utilizing the signal-quality threshold parameter, monitor UE signal-quality reports to determine when the signal-quality of the UE in the serving cell is lower than the signal-quality threshold parameter and greater than the signal-quality handover level; and force a handover of the UE from the serving cell to the neighbor cell in response to determining that the signal quality of the UE is lower than the signal-quality threshold parameter and greater than the signal-quality handover level. 
     In another embodiment, an apparatus in an eNB of a cellular telecommunication system is disclosed. The apparatus controls handover of a UE operating near a cell edge of a serving cell. The apparatus includes a memory configured to store a CQI threshold table that indicates a CQI handover level used to initiate handover of the UE to a neighbor cell, and a CQI Threshold parameter having a level greater than the CQI handover level; a radio receiver configured to receive from the UE, CQI reports reporting a CQI of the UE (UEcqi) in the serving cell; and a scheduler having an interface to an operator system. The scheduler is configured to receive information from the operator system indicating that the QoS Class Identifier (QCI) level of the UE is defined to utilize the CQI Threshold parameter; instruct a comparison unit to compare the UEcqi with the thresholds stored in the threshold table to determine when the UEcqi is lower than the CQI Threshold parameter and greater than the CQI handover level; and instruct a forced handover unit to force a handover of the UE to the neighbor cell in response to a determination by the comparison unit that the UEcqi is lower than the CQI Threshold parameter and greater than the CQI handover level. 
     Particular embodiments of the described solution provide an enhanced user experience for high-priority UEs operating near the cell edge. The network is able to make handover decisions based on both assigned QCI and reported UEcqi. This saves scheduler bandwidth in retransmissions. It also provides the operator with better control of cell boundary conditions. 
     Further features and benefits of embodiments of the disclosure will become apparent from the detailed description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following section, the disclosure will be described with reference to exemplary embodiments illustrated in the figures, in which: 
         FIG. 1  is an illustrative drawing of two neighboring cells in a cellular radio network in which a high-priority UE is operating near a cell edge; 
         FIG. 2  is a flow chart illustrating the steps of an exemplary embodiment of a method according to the present disclosure; and 
         FIG. 3  is a simplified block diagram of an exemplary embodiment of an apparatus according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments are shown. The disclosed solution may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the solution to those skilled in the art. In the drawings, like reference signs refer to like elements. Additionally, it should be understood that the disclosed solution can be implemented in hardware or a combination of hardware and software such as a general-purpose computer or microprocessor, which executes software stored on a non-transitory memory. 
       FIG. 1  is an illustrative drawing of two neighboring cells, Cell-A and Cell-B, in a cellular radio network in which a high-priority UE  11  is operating near a cell edge of its serving cell, Cell-A. For simplicity, only the a 2  threshold is shown for Cell-B although additional threshold levels may be defined for Cell-B. 
     The UE is illustrated in three different positions (positions  1 - 3 ) with respect to the various thresholds (a 2 -a 5 ) defining an edge zone of Cell-A. In position- 1 , the UE is operating where its reported CQI is lower than the a 2  threshold and greater than the a 4  threshold, where the RSRP of Cell-B becomes better than or equal to the RSRP of Cell-A. In position- 2 , the UE is operating where its reported CQI is lower than the a 4  threshold and greater than a newly defined threshold referred to herein as the CQI Threshold  12 . The CQI Threshold  12  is a configurable threshold set at a level between the a 4  threshold and the a 5  threshold. 
     In position- 3 , the UE is operating where its reported CQI is lower than the CQI Threshold  12  and greater than the a 5  threshold. Thus, in position- 3 , the UE&#39;s signal quality may be quite poor, but it has not yet reached the a 5  threshold where the UE would normally be handed over to Cell-B. 
     The present disclosure enables the operator to define the cell edge based on a new configurable signal-quality threshold defined to force an early handover for UEs having identified QoS or QCI priority levels. In one embodiment, the various a 1 -a 5  thresholds and the new configurable signal-quality threshold may be defined in terms of CQI. Thus, the new configurable signal-quality threshold is a CQI Threshold parameter. In other embodiments, the a 1 -a 5  thresholds and the new configurable signal-quality threshold may be defined in terms of other signal-quality measurements such as RSRP, Reference Signal Receive Quality (RSRQ), SNR, Signal-to-Noise-plus-Interference Ratio (SINR), and the like. The following description uses terminology consistent with thresholds defined in terms of CQI, although the disclosed solution is not limited to this implementation. 
     The disclosed solution provides the operator with the ability to define which QCI range will be subject to the CQI Threshold parameter, and which QCI range will not. In addition, for each QCI level defined as being subject to the CQI Threshold parameter, the operator is able to define a different CQI Threshold parameter, if desired. For example, in LTE, there are nine QCI levels, numbered  1  through  9 , with levels  1 - 4  for GBR UEs and levels  5 - 9  for non-GBR UEs. The operator may determine, for example, that QCI levels  1 - 4  shall be subject to a CQI Threshold parameter for handover that is lower than the a 4  threshold and greater than the standard a 5  handover threshold. Additionally, the operator may define a different CQI Threshold parameter for the GBR UEs (QCI levels  1 - 4 ) than the CQI Threshold parameter defined for the non-GBR UEs (QCI levels  5 - 9 ). 
     By defining the CQI Threshold parameter at a level that is lower than a 4  and greater than a 5 , the operator causes the eNB to force the handover of a high-priority UE at a higher CQI threshold than a lower priority UE (which is handed over at a 5 ). By forcing the handover of the high-priority UEs at a higher CQI threshold, the retransmission load on the eNB is reduced, the high-priority UEs are more quickly moved to adjacent cells with better RSRP thus improving their reception, and resources that were previously used for retransmissions are made available to lower priority UEs, thus reducing the “starvation” effect. 
       FIG. 2  is a flow chart illustrating the steps of an exemplary embodiment of a method according to the present disclosure. The method starts at step  20  and moves to step  21  where the CQI Threshold parameter  12  is set at a level lower than the a 4  threshold and greater than the a 5  threshold for defined QCI priority levels. At step  22 , the eNB determines whether the UE has an assigned QCI priority level that is defined for utilizing the CQI Threshold parameter (i.e., the UE is a high-priority UE). If not, the UE is a lower priority UE, and the method moves to step  23  where the a 5  threshold is used for a normal handover decision. However, if the eNB determines that the UE has an assigned QCI level that is defined for utilizing the CQI Threshold parameter, the method moves to step  24  where the eNB determines whether the UE&#39;s reported CQI (UEcqi) is lower than the CQI Threshold parameter and greater than the a 5  threshold. If not, the UE is still operating with a CQI greater than the CQI Threshold parameter, so no action is taken. The method returns to step  24  where the eNB continues to monitor the UEcqi to determine whether it crosses the CQI Threshold parameter. 
     When the eNB determines that the UEcqi is lower than the CQI Threshold parameter and greater than the a 5  threshold, the method moves to step  25 . At this point, the UE has better signal quality from Cell-B than from Cell-A, but the signal quality from Cell-A has not yet deteriorated to the a 5  threshold. Since the UE is a high-priority UE, the eNB forces a handover of the UE to Cell-B. The method then ends at  26 . 
       FIG. 3  is a simplified block diagram of an exemplary embodiment of an apparatus implemented in an eNB  31  according to the present disclosure. Operation of the eNB may be controlled, for example, by a processor  32  executing computer program instructions stored in a memory  33 . A scheduler  34  controls the allocation of network resources to UEs such as UE  11  using inputs from several sources internal and external to the eNB. A receiver  35  receives CQI reports from the UE including a UE identifier and the reported UEcqi. The receiver provides this information directly or indirectly to the scheduler. 
     The scheduler  34  may also obtain the QCI level of the UE by accessing a QCI table  36 , for example, utilizing the UE identifier. An interface  40  to an operator system provides the scheduler with operator-defined QCI levels  37  that are identified for utilizing the new CQI Threshold parameter. 
     The scheduler  34  may also include or have access to a comparison unit  38 , which compares the reported UEcqi with the different a 2 -a 5  thresholds. The comparison unit may obtain the threshold values, for example, from a table of configurable thresholds  39  stored in memory  33  or another source indicating these operator-defined threshold levels. Upon comparing the UEcqi with the various thresholds, the comparison unit may determine that the UEcqi is lower than the CQI Threshold parameter  12  and greater than the a 5  threshold. When this condition is met, the scheduler  34  may directly, or through the processor  32 , instruct a forced handover unit  41  to initiate handover of the UE  11  to Cell-B. A transmitter  42  may transmit a handover (HO) command to the UE. 
     It is to be understood that the present disclosure addresses only those procedures in the eNB directed toward determining when a handover is to be initiated and then initiating the handover. Details of the handover procedure itself are not disclosed, but are known to those of ordinary skill in the art. For example, it is known that in LTE, the serving (source) eNB also prepares the target eNB for handover concurrently with the handover decision (i.e., the UE&#39;s context is made available and resources are reserved in the target cell when the UE accesses the target eNB). Otherwise, the UE would transition to idle-state and attempt to complete the handover by transitioning back to connected-state. These procedures are outside the scope of the present disclosure. 
     Thus, according to different aspects of the present disclosure, embodiments of methods and apparatuses are provided for enabling the eNB to instruct high-QCI/Priority UEs which are experiencing low quality and high retransmissions at the cell edge to move to an adjacent cell with better RSRP in order to save the scheduler bandwidth and also to provide relief to the UEs from retransmissions and hence gain a better user experience. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the disclosed solution and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation. The scope of the present invention is set forth in the following claims.