Abstract:
Methods and apparatus are disclosed for handing over of a high-speed mobile terminal or user equipment from a source network node to a target network node. Reliable methods for measuring the speed of a moving user equipment are disclosed. Parameters used in a handover procedure are also adjusted in accordance with the speed of the user equipment. For handing over a high-speed user equipment, the present application discloses that the source network node can coordinate with the target network node and other candidate network nodes in transmitting a handover command to prevent handover failures.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to handover of a UE (user equipment) in an LTE network and, more specifically, to methods and apparatus for improving handover for high speed UEs in an LTE network. 
       BACKGROUND 
       [0002]    In a wireless communications network, when a UE moves away from a first cell and into a second cell, handover of the UE from the serving node of the first cell to the serving node of the second cell is necessary. A handover decision is generally made by the serving node of a UE based on measurement results reported by the UE. Parameters controlling the measurement reports from a UE, such as how often the UE reports and what types of measurements to report, are also configured by the serving node. 
         [0003]    When a UE moves at a high speed, the regular handover procedure often does not work well, due to incorrect estimate of the speed of the UE, degraded channel conditions experienced by the UE, and insufficient time for the UE to perform and report measurement results. 
         [0004]    One existing method for measuring the speed of a UE is to count the number of handovers the UE has experienced in a given time period. Such method is based on the assumption that base stations or eNBs (evolved NodeBs) are distributed evenly, an assumption that may not always hold true. Another problem with such method is that the UE may experience a sudden change of speed, for example, when entering a highway. In such case, the speed of the UE estimated based on its handover history would not accurately reflect the new speed of the UE. As a result, the regular handover procedure designed for a UE moving at a normal speed may fail to work properly for a UE moving at a high speed. Calls may drop as a result. 
         [0005]    A high speed UE also experiences rapidly degrading channel conditions because of the strong interference from neighboring cells, especially the cell the UE is moving towards (i.e., the target cell). Because of the high speed, the UE may have insufficient time to report measurement results, send a handover request, and decode a received handover command before the UE moves out of the current serving cell. Because of the degraded channel conditions, the high-speed UE may fail to receive the handover command from the serving cell. 
         [0006]    There is a need for improving regular handover procedures to accommodate high-speed UEs. The present application discloses robust handover methods and apparatus that reduce call drops and handover failures during handover of high speed UEs. 
       SUMMARY 
       [0007]    The present invention relates to improvements in handover procedures designed for high speed UEs or wireless devices. 
         [0008]    In some embodiments, a method of handing over a high-speed wireless device is disclosed. The wireless device is moving at a high speed from a source base station to a target base station. The method is implemented on the source base station. The method comprises measuring a speed of the wireless device and estimating a channel quality based on a channel quality index report from the wireless device. The method further comprises signaling one or more candidate base stations to coordinate with the source base station for transmitting a handover command to the wireless device depending on the measured speed and the estimated channel quality. In the embodiments, the one or more candidate base stations are included in a recent measurement report from the wireless device. The one or more candidate base stations include the target base station. 
         [0009]    In some embodiments, a source network node is configured to perform a method of handing over a high-speed wireless device from the source network node to a target network node. The source network node comprises a transceiver, a network interface, and a processing circuit. The transceiver is configured for transmitting and receiving signals to and from a wireless device. The network interface is configured for communicating with one or more candidate network nodes. The one or more network nodes are included in a recent measurement report. The processing circuit is configured to measure the speed of the wireless device and estimate a channel quality based on a channel quality report from the wireless device. The processing circuit is further configured to signal the one or more candidate network nodes to coordinate with the source network node for transmitting a handover command to the wireless device based on the measured speed and the estimated channel quality. 
         [0010]    In some embodiments, a method implemented on a wireless device located in a wireless communications network is disclosed. The method is implemented for the wireless device to perform a handover from a source network node to a target network node. The method comprises measuring a channel quality for a channel between the source network node and the wireless device and reporting the measured channel quality to the source network node. The method further comprises receiving a handover command from the source network node, or the target network node, or a candidate network node other than the target network node. The handover command directs the wireless device to hand over from the source network node to the target network node. Upon receiving the handover command, the wireless device sends an acknowledgement for the received handover command to the source network node and starts a handover procedure. 
         [0011]    In some embodiments, a wireless device configured to perform a handover procedure from a source network node to a target network node comprises a transceiver and a processing circuit. The transceiver is for transmitting and receiving signals to and from the source network node and the target network node. The processing circuit is configured to measure a channel quality for a channel between the source network node and the wireless device and report the measured channel quality to the source network node. The processing circuit is further configured to receive a handover command from the source network node, or the target network node, or a candidate network node other than the target network node. The handover command directs the wireless device to move from the source network node to the target network node. The process circuit is further configured to send an acknowledgement for the received handover command to the source network node and start a handover procedure upon receiving the handover command. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0012]      FIG. 1  illustrates an exemplary wireless network. 
           [0013]      FIG. 2  is a flow chart illustrating an exemplary handover procedure. 
           [0014]      FIG. 3  is a flow chart illustrating an exemplary method of configuring a handover procedure based on the speed of a moving UE estimated by an eNB. 
           [0015]      FIG. 4  illustrates an exemplary method of configuring a handover procedure based on the speed of a moving UE estimated by the UE. 
           [0016]      FIG. 5  is a flow chart illustrating an improved handover procedure. 
           [0017]      FIG. 6  illustrates a first embodiment of an improved handover procedure. 
           [0018]      FIG. 7  illustrates a second embodiment of an improved handover procedure. 
           [0019]      FIG. 8  illustrates an exemplary network node configured to support the improved handover procedures disclosed herein. 
           [0020]      FIG. 9  illustrates an exemplary wireless device configured to support the improved handover procedures disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In  FIG. 1 , a wireless network  100  comprising several base stations or eNBs,  104 ,  108 , and  110 , is shown. In the present disclosure, terms such as eNB, base station, and cell are treated as synonyms and are used interchangeably. In the wireless network  100 , a UE (user equipment)  106  is moving away from the eNB  104  and moving towards the eNB  108 . As the UE  106  moves closer to the eNB  108 , the signals from the eNB  108  become stronger and the signals from the eNB  104  become weaker. Eventually, the UE  106  is handed over from the eNB  104  to the eNB  108 . In describing the handover procedure, the eNB  104  is referred to as the source eNB and the eNB  108  is referred to as the target eNB. The third eNB  110 , along with other eNBs in the vicinity (not shown), is referred to as neighbor eNBs. 
         [0022]      FIG. 2  illustrates an exemplary procedure of handing the UE  106  over from the source eNB  104  to the target eNB  108 . When the UE  106  is served by the source eNB  104 , the source eNB  104  configures the UE  106  for reporting measurement results (step  210 ). For example, the source eNB  104  may set the time when the UE  106  needs to report measurement results, which may be defined by parameter TimeToTrigger value (TTTV). The source eNB  104  may set the criterion that triggers the reporting of measurement results by the UE  106 . The criterion may be defined by parameter a3offset. Other parameters include filterCoefficient value (FCV) and hysteresis. FCV specifies the measurement filtering coefficient, and hysteresis is a parameter used to ensure that the target neighbor cell is indeed a better cell for the UE to be handed over from the current cell. These parameters and others used by the source eNB  104  to configure measurement reports by the UE  106  are referred herein as handover parameters. 
         [0023]    Once the UE  106  has been configured by the source eNB  104 , the UE  106  reports its measurement results (step  212 ). Based on the measurement reports from the UE  106 , the eNB  104  makes the handover decision, i.e., whether to hand over the UE  106  to the target eNB  108  (step  214 ). The source eNB  104  sends a handover request to the target eNB  108  (step  216 ). In response, the target eNB  108  sends a handover request acknowledgement (ACK) to the source eNB to acknowledge the handover request ( 218 ). The source eNB  104  then sends an RRC_Connection_Reconfiguration message to the UE  106  (step  220 ). The UE  106  interprets the received RRC_Connection_Reconfiguration message as a handover command and starts accessing the target eNB  108  using a random access channel (RACH). The UE  106  sends a RACH Access message to the target eNB  108  (step  222 ) and receives a RACH Response from the target eNB  108  (step  224 ). In turn, the UE  106  sends an RRC_Connection_Reconfiguration_Complete message to the target eNB ( 108 ) to confirm that the handover is complete and the target eNB  108  can start transmitting data to the UE  106 . 
         [0024]    As discussed above, handover parameters are configured by the source eNB  104 . When the handover parameters are not set properly, it can cause handover failures, dropped calls, or the so-called ping-pong handover scenario. For example, if TTTV is too short, the UE  106  may report measurement results too frequently, and fluctuations in channel conditions could cause a UE  106  located in the overlapping region of two neighboring cells to be handed over back and forth, i.e., a ping-pong handover. If TTTV is too long, the UE  106  may wait too long to report its measurement results. By the time a handover decision is made by the source eNB  104 , the signal from the source cell may have become too weak and the call may be dropped as a result. 
         [0025]    In most cases, the handover parameters can be set to some proper values to ensure that the handover procedure works well. However, handover parameters properly configured under normal conditions may cause problems when a UE  106  is moving at a high speed. For example, the TTTV may be too long compared to how long it takes a UE  106  to move across a cell. One solution is to adjust the handover parameters based on the speed of the UE  106 , as shown in  FIG. 3 . 
         [0026]      FIG. 3  illustrates a flow chart of an exemplary method for configuring a handover procedure. The speed of the UE  106  is first estimated by the source eNB  104  (step  202 ). The estimated speed of the UE  106  is then used to retrieve a TTTV parameter from a speed-value mapping table (step  304 ). The speed-value mapping table stores a mapping of different TTTV parameters to different speed values of the UE  106 . The eNB  104  compares the retrieved TTTV parameter to the current TTTV parameter (step  306 ). If the two parameters are not equal, the eNB  104  updates the current TTTV parameter with the retrieved TTTV parameter (step  310 ). The eNB  104  also sends a RRC_Connection_Reconfiguration message to the UE  106  to re-configure the handover parameters (step  312 ). Upon receiving the message, the UE  106  updates its handover parameters (step  314 ). In the above description, parameter TTTV is used as an example of handover parameters. Other handover parameters, such as a3offset, etc., can be adjusted according to the speed of the UE in the same way as described above. 
         [0027]      FIG. 4  illustrates a different embodiment for updating handover parameters based on the estimated speed of the UE  106 . In  FIG. 4 , the speed of the UE  106  is first estimated by the UE  106  itself (step  402 ). Based on the estimated speed, the UE  106  retrieves a scaling factor from a mapping table (step  404 ). The mapping table may be a pre-stored file that maintains a mapping between different scaling factors and different speed values. The retrieved scaling factor is used to scale the handover parameters, e.g., the TTTV parameter. After retrieving the scaling factor, the UE  106  compares the scaling factor to the current scaling factor it maintains (step  406 ). If the retrieved scaling factor is not the same as the current scaling factor, the UE  106  updates the TTTV parameter by applying the retrieved scaling factor (Step  408 ). 
         [0028]    In some embodiments, the speed of the UE  106  may be measured using a GPS device. In some embodiments, the speed of the UE  106  may be measured based on Doppler shift or time adjustment value. A time adjustment value is used to keep a UE  106  time-aligned with the serving eNB. When a UE  106  initially accesses a radio network served by an eNB, the UE  106  adjusts its transmission timing to be time-aligned with the eNB. After the radio connection between the UE  106  and the eNB has been established, the transmission timing of the UE  106  needs to be adjusted frequently to keep the UE  106  and the eNB in sync. For example, the eNB may send a time adjustment value in a time adjustment command to the UE  106  to adjust the UE&#39;s timing. The time adjustment value reflects the varying propagation delay experienced by signals transmitted from the UE  106  to the eNB. One reason that causes the propagation delay to vary is that the UE  106  is either moving towards or away from the eNB. Therefore, a time adjustment value can be used to estimate the speed of a UE in certain scenarios. 
         [0029]    In both  FIG. 3  and  FIG. 4 , the estimated speed value is used to update or reconfigure the handover parameters. In some embodiments, the speed value of a UE  106  may be classified as LOW, MEDIUM, and HIGH. A set of handover parameters may be pre-determined for each of the three classes. Depending on the classification of the speed value of the UE  106 , the corresponding set of handover parameters is selected by either the UE  106  or the eNB  104  for updating the UE&#39;s handover parameters. Because there are only three classes, the selected handover parameters are not necessarily the best choice for the speed of the UE  106 . Using a mapping table allows selected handover parameters to better match to the speed value of the UE  106 . 
         [0030]    In the above described embodiments, the source eNB  104  adjusts handover parameters based on the speed of the UE  106  to improve handover performance and reduce handover failures. Handover performance may be improved in other ways. In some embodiments, the source eNB  104  may signal the target eNB  108  to request the target eNB  108  to coordinate with the source eNB  104  for transmission of handover commands to the UE  106 . In some embodiments, the source eNB  104  may signal all candidate eNBs, for example, the target eNB  108  and neighbor eNB  110 , to coordinate with the source eNB  104  for transmission of handover commands to the UE  106 . 
         [0031]      FIG. 5  describes an exemplary handover process during which the source eNB  104  coordinates with the target eNB  108  or all candidate eNBs to ensure that a handover command is received by the UE  106 . 
         [0032]    In  FIG. 5 , the source eNB  104  receives one or more measurement reports from the UE  106  on the pilot strengths of neighboring eNBs. The target eNB  108  is selected based on the most recent measurement reports from the UE  106 . The source eNB  104  also measures the speed of the UE  106  (step  502 ) and estimates a channel quality based on one or more channel quality index (CQI) reports from the UE  106  (step  504 ). Based on the measured speed of the UE  106  and the estimated channel quality, the source eNB  104  signals the target eNB  108  or all candidate eNBs to coordinate with the source eNB  104  for transmitting a handover command to the UE (step  506 ). In some embodiments, to coordinate with the source eNB  104 , the target eNB  108  turns off its transmission on certain resource elements during a particular transmission time interval (TTI). In other embodiments, all candidate eNBs may turn off their transmissions on certain resource elements. Alternatively, in some embodiments, both the target eNB  108  and the source eNB  104  transmit a handover command to the UE  106  to ensure that the UE  106  indeed receives the handover command. In other embodiments, all candidate eNBs and the source eNB  104  transmit a handover command to the UE  106 . 
         [0033]    As examples,  FIGS. 6 and 7  illustrate two processes of the source eNB  104  coordinating with the target eNB  108  in transmitting a handover command to the UE  106 .  FIG. 6  illustrates a process in which the target eNB  108  turns off its transmission to assist the source eNB  104  in transmitting the handover command. The source eNB  104  receives one or more measurement reports from the UE  106  (step  602 ). The source eNB  104  also estimates the speed of the UE  106  (step  604 ) and estimates a channel quality based on the channel quality index (CQI) report from the UE  106  (step  606 ). Based on the speed of the UE  106  and the channel quality, the source eNB  106  determines whether to signal the target eNB  108  to coordinate in the handover command transmission (step  608 ). 
         [0034]    If it is determined that (1) the speed of the UE  106  is larger than a speed threshold and that the channel quality is smaller than a first channel quality threshold, or (2) the channel quality threshold is smaller than a second channel quality threshold, the source eNB  106  signals the target eNB  108  and requests the target eNB  108  that it stop transmission on the resource elements (REs) that are used to transmit a handover command by the source eNB  106 . For example, control channel elements (CCEs) on a physical downlink control channel (PDCCH) may be used for transmission of control information of a handover command during a particular transmission time interval (TTI) (step  612 ). In the meantime, the source eNB  104  transmits a handover command using a radio link control (RLC) message on a physical downlink shared channel (PDSCH) to the UE  106  during the same TTI (step  610 ). If it is determined otherwise, the source eNB  104  simply transmits a handover command to the UE  106  without involvement of the target eNB  108  (step  610 ). After transmitting the handover command to the UE  106 , the source eNB  104  waits for an acknowledgement (step  614 ). 
         [0035]    If the source eNB  104  does not receive an acknowledgement from the UE  106 , the source eNB  104  informs the target eNB  108  to turn off its transmission on the resource elements used by the PDCCH CCEs and PDSCH for retransmission of the control information of a handover commands during a particular TTI. The source eNB  108  then attempts retransmission of a handover command (step  616 ). The source eNB  108  then again waits for an acknowledgement ( 618 ). The source eNB  108  attempts a pre-determined maximum number of retransmissions if no acknowledgement is received from the UE  106 . The procedure ends when the source eNB  104  receives an acknowledgement from the UE  106  or the maximum number of retransmissions has reached (step  620 ). The procedure may also end when the target eNB  108  receives a RRC_Connection_Reconfiguration_Complete message from the UE. 
         [0036]    In  FIG. 6 , the target eNB  108  turns off its transmission on certain resource elements when the source eNB  104  is transmitting a handover command to the UE  106 . Because the interference caused by the transmissions from the target eNB  108  is eliminated or mitigated, there is a better chance for the UE  106  to receive and decode the handover command. As described above, because a high-speed UE often experiences degraded channel conditions due to interference from neighboring cells, especially the target cell, turning off transmissions by the target eNB  108  on certain resource elements during a particular TTI improves the channel condition during the time period the handover command is transmitted by the source eNB  104 . 
         [0037]    Alternatively, the target eNB  108  may be asked by the source eNB  104  to transmit a handover command to the UE  106 , in addition to the handover command sent by the source eNB  104 , as shown in  FIG. 7 . 
         [0038]      FIG. 7  illustrates another exemplary handover procedure. In the method shown in  FIG. 7 , the source eNB  104  receives one or more measurement reports from the UE  106  (step  702 ), estimates the speed of the UE  106  (step  704 ), and estimates a channel quality based on the channel quality index report (step  706 ). The source eNB  106  determines whether the speed of the UE  106  is larger than a speed value threshold and the channel quality is smaller than a first channel quality threshold, or whether the channel quality is smaller than a second channel quality threshold (step  708 ). If yes, the source eNB  104  signals the target eNB  106  to send a handover command to the UE  106  using a radio link control (RLC) message (step  710 ). 
         [0039]    In some embodiments, after receiving the request from the source eNB for sending a handover command, the target eNB  108  will transmit the handover command to the UE  106  using a special cell radio network temporary identifier (C-RNTI) reserved for handover coordination only. The special C-RNTI plus the control information is transmitted on a physical downlink control channel (PDCCH) and the handover command plus the identifier of the UE  106  is transmitted on a physical downlink shared channel (PDSCH). On the side of the UE  106 , the UE  106  first tries to decode the control information transmitted on the PDCCH of the target eNB. If successfully, the UE  106  will check if the special temporary identifier used for the handover is included in the PDCCH or not. If yes, the UE  106  will use the decoded control information to decode the handover command plus the UE identifier on the PDSCH. If the UE  106  successfully decodes the handover command plus its UE identifier, it will use the target eNB information contained in the handover command to start accessing the target eNB  108  using a random access channel (RACH). 
         [0040]    The source eNB  104  also sends a handover command to the UE  106  (step  712 ) and waits for an acknowledgement from the UE  106 . On the side of the UE  106 , the UE  106  first tries to decode the control information transmitted on the PDCCH. If successfully, the UE  106  will check if its temporary identifier (which is assigned by the source eNB during the call set-up) is included in the PDCCH or not. If yes, the UE uses the decoded control information to decode the handover command on the PDSCH and use the information about the target eNB  108  contained in the handover command to start accessing the target eNB  108  using a random access channel (RACH). The acknowledgment from the UE  106  may be in response to the handover command sent by either the source eNB  104  or the target eNB  106 . If the source eNB  104  does not receive an acknowledgement from the UE  106 , the source eNB  104  will signal the target eNB  108  to retransmit the handover command. Both the source eNB  104  and the target eNB  108  retransmit the radio link control message to resend the handover command, until an acknowledgement is received from the UE  106  or the maximum number of retransmissions has been reached (step  718 ). In either case, the transmission of handover commands terminates (step  720 ). The transmission of handover commands may also terminate when the target eNB  108  receives a RRC_Connect_Reconfiguration_Complete message from the UE  106 . 
         [0041]    In some embodiments, the target eNB  108  may be configured to re-transmit the handover command without any explicit command from the source eNB  104 . The target eNB  108  will continue to retransmit until the source eNB  104  signals it to stop, which may take place when the source eNB  104  receives an acknowledgement from the UE  106 . In some embodiments, the target eNB  108  may be configured to attempt retransmission of the handover command for a pre-determined number of times. 
         [0042]    In  FIGS. 6 and 7 , the source eNB  104  is coordinating with the target eNB  108  to ensure that the UE  106  receives a handover command. In some embodiments, the source eNB  104  may be configured to coordinate with all candidate eNBs, e.g., eNB  108  and eNB  110 , included in the most recent measurement report received by the source eNB  104  from the UE  106 . 
         [0043]    For example, the source eNB  104  may request all candidate eNBs to transmit a handover command to the UE  106 . After receiving the request from the source eNB  104  for sending a handover command, all candidate eNBs transmit the handover command to the UE using a special cell radio network temporary identifier (C-RNTI) reserved for handover coordination. The special C-RNTI plus the control information is transmitted on their PDCCHs, respectively. The handover command plus the identifier of the UE  106  is transmitted on their PDSCHs, respectively. This process is repeated until one of the following conditions are met: (1) the source eNB receives an acknowledgement from the UE; (2) one of the candidate eNBs receives the RRC_Connection_Reconfiguration_Complete message from the UE; and (3) the number of the retries reaches the pre-defined maximum number of retransmission of the handover command. 
         [0044]    On the side of the UE  106 , it decodes the handover commands from the serving eNB as well as from all the candidate eNBs. The procedure for decoding the handover commands from all the candidate eNBs is similar to from the target eNB described before. The UE  106  first tries to decode the control information transmitted on the PDCCH of each candidate eNB. If successfully, the UE  106  will check if the special temporary identifier used for the handover is included in the PDCCH or not. If yes, the UE  106  will use the decoded control information to decode the handover command plus the UE identifier on the PDSCH. If the UE  106  successfully decodes the handover command plus its UE identifier, it will use the target eNB information contained in the handover command to start accessing the target eNB  108  using a random access channel (RACH). 
         [0045]    In the above descriptions, eNBs are used as an example of a network node that handles the handover procedure of a UE  106 . The same methods and techniques can be implemented on other types of network nodes that serve a cell or sector of a cell. 
         [0046]      FIG. 8  is a block diagram illustrates an exemplary network node configured to perform the methods and techniques described above. The network node in  FIG. 8  comprises a transceiver  802 , a network interface  804  and a processing circuit  806 . The transceiver  802  is configured to transmit signals to and receive signals from one or more UEs. The network interface  804  is configured to communicate with other network nodes. The processing circuit  806  is configured to measure the speed of a UE, e.g., the UE  106 , using, for example, Doppler shift or time adjustment value, and estimate a channel quality based on one or more channel quality reports sent by the UE. The processing circuit  806  is also configured to signal another network node to coordinate with the network node  800  for transmitting a handover command to the UE, based on the measured speed and the estimated channel quality. 
         [0047]    In some embodiments, the network node  800  may be configured as the source eNB  104  or the target eNB  108 . When configured as the target eNB  108 , the processing circuit  806  may receive and process a signal from another network node that requests the network node  800  to coordinate in transmitting a handover command to a designated UE. 
         [0048]      FIG. 9  illustrates an exemplary UE  900  configured to perform the above described handover procedure. The UE  900  comprises a transceiver  910  and a processing circuit  920 . The transceiver  910  is configured for transmitting and receiving signals to and from multiple network nodes, e.g., eNB  104 , eNB  108 , and eNB 110 . The processing circuit  920  comprises a measurement processor  922  which is configured to measure a channel quality for a channel between the UE  900  and the source network node, e.g., eNB  104 , and to report the measured channel quality to the source network node. The measurement processor  922  is also configured to measure the pilot strengths of the serving and all the candidate cells and to report them in a measurement report message when certain condition is met. The processing circuit  920  also comprises a handover processor  924  configured to receive a handover command from multiple network nodes. Upon receiving a handover command, the handover processor  924  sends an acknowledgement to the source network node for the received handover command and commences a handover procedure. If the UE  900  does not receive a handover command or can&#39;t decode the handover command successfully for a certain time period, it will start a random access procedure to establish a radio connection with the target eNB  108 . 
         [0049]    The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. One or more of the specific processes discussed above may be carried out in a cellular phone or other communications transceiver comprising one or more appropriately configured processing circuits, which may in some embodiments be embodied in one or more application-specific integrated circuits (ASICs). In some embodiments, these processing circuits may comprise one or more microprocessors, microcontrollers, and/or digital signal processors programmed with appropriate software and/or firmware to carry out one or more of the operations described above, or variants thereof. In some embodiments, these processing circuits may comprise customized hardware to carry out one or more of the functions described above. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.