Abstract:
Systems and methods for handover of a mobile station to a base station are provided. Handover criteria for determining which base station to handover to is different for different candidate base stations. This difference can account for base station loading and/or environmental factors. The handover criteria can include a threshold and hysteresis value. A wireless communication system can employ a fixed threshold and hysteresis value for all base stations, and these fixed values are adjusted differently for different candidate base stations.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 60/696,801, filed Jul. 7, 2005, the entire disclosure of which is herein expressly incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     In wireless communication networks, such as cellular networks, mobile stations can move through large geographic areas, while maintaining a call, due to the concept of mobility. In wireless networks mobility refers to the ability of a mobile station to move between wireless access points, e.g., base stations, while maintaining communications with another communication station. Known handover techniques use mobile station measurements, base station measurements, or a combination of these measurements. 
     Because handovers may result in a disruption or dropping of a call, and because handovers consume network resources, handovers are typically controlled in some manner. For example, handover determinations are typically based on a comparison of measured signal strengths (either of a signals transmitted from the mobile station being measured by the base station or a signals transmitted from a base station being measured by the mobile station) with a threshold and hysteresis value. When the signal strength of signals between a mobile station and a base station currently serving the mobile station (herein referred to as a serving base station) falls below a threshold value and the strength of signals between the mobile station and another base station (herein referred to as the target base station) are above the threshold value by a predetermined hysteresis value, then the mobile station hands-off from the serving base station to the target base station. Various other ways of using hysteresis and threshold values are known in the art. 
     SUMMARY OF THE INVENTION 
     Different types of wireless networks use different types of handover techniques. One type of wireless network is the iDEN network, such as the one owned and operated by Sprint Nextel Corporation. Currently, handover threshold and hysteresis values in iDEN are set a on per cell site basis, such that these values are the same regardless of which particular neighbor base station is the target base station a mobile station is attempting to handover to. 
     Exemplary embodiments of the present invention provide different handover criteria for different base stations. The different handover criteria can be adjusted to control base station loading and/or compensate for environmental factors. The handover criteria can include thresholds and hysteresis values, and these values can be adjusted by a different adjustment value for at least two base stations. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a block diagram of an exemplary wireless communication system in accordance with exemplary embodiments of the present invention; 
         FIG. 2  is a flow diagram of an exemplary method for handover in accordance with exemplary embodiments of the present invention; and 
         FIGS. 3   a - 3   c  are exemplary logic diagrams of handover criteria in accordance with exemplary embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a block diagram of an exemplary wireless communication system in accordance with exemplary embodiments of the present invention. The system includes a mobile station  105 , serving base station  120  and handover candidate base stations  110  and  115 . As illustrated in  FIG. 1 , mobile station  105  is in two-way communication with base station  120 , and receives signals from base stations  110  and  115 . The signals received from base stations  110  and  115  can be signals directed to mobile station  105  as part of a handover procedure, signals transmitted for any mobile station to use as part of a handover procedure, or signals directed to other mobile stations as part of a normal communication. Mobile station  105  measures the signals from base stations  110  and  115 , and these measurements are used to determine whether to handover to one of these base stations, as will be described in more detail below. 
     Base station  120  includes an antenna  125 , processor  130  and memory  135 . Processor  130  includes logic  140 - 150 , which will be described in more detail below in connection with  FIG. 2 . Processor  130  can be a microprocessor, field programmable gate array (FPGA), application specific integrated circuit (ASIC) and/or the like. Memory  135  can be any type of memory including volatile memory (such as random access memory) or non-volatile memory (such as read-only memory). When processor  130  is a microprocessor, logic  140 - 150  can be processor-executable code loaded from memory  135 . 
     For ease of explanation, and not limitation, base stations  110  and  115  are illustrated with only the base station tower and not the details of the base station. It should be recognized, however, that these base stations can include the same components as base station  120 . 
       FIG. 2  is a flow diagram of an exemplary method for handover in accordance with exemplary embodiments of the present invention. Initially, base station  120  receives measurements from mobile station  105  (step  205 ). These measurements can include received signal strength indications (RSSIs) and signal quality estimates (SQE) for each handover candidate base station  110  and  115 , as well as measurements for serving base station  120 . Logic  140  adjusts the handover criteria for each base station for which it has received measurements (step  210 ). In accordance with exemplary embodiments of the present invention, the handover criteria can be different for each base station. The handover criteria can be adjusted to balance the loads placed on the different base stations, i.e., to “encourage” handover to base stations with lower loads. The handover criteria can also be adjusted to account for radio frequency environmental factors. For example, when a mobile station is supported by base station  120  it may be that mobile station measurements of base station  110  indicate an acceptable signal quality. However, due to environmental factors, such as the presence of a building or other obstruction, it is known that this acceptable signal strength is temporary and that when the mobile station moves only a small distance towards the base station the signal quality will drop below an acceptable level. 
     The handover criteria adjustment can be performed in a number of different ways. Specifically, an adjustment value can be added or subtracted from a handover threshold and/or hysteresis value. In some systems mobile station&#39;s measurements may account for fixed threshold and hysteresis values for all base stations. In these systems the base station can adjust the threshold and hysteresis values while accounting for the mobile station&#39;s adjustment of the measurements for the fixed threshold and hysteresis value. 
     Logic  145  then compares the received base station measurements with the adjusted handover criteria (step  215 ) and logic  150  determines whether any base stations meet the handover criteria (step  220 ). When no base stations meet the handover criteria (“No” path out of decision step  220 ), then handover processing using these measurements ends (step  225 ). When at least one base station meets the handover criteria (“Yes” path out of decision step  220 ), then the base station determines whether more than one base station meets the handover criteria (step  230 ). When only one base station meets the handover criteria (“No” path out of decision step  230 ), then the base station instructs the mobile station and/or the base station meeting the handover criteria to initiate a handover (step  235 ). If, however, more than one base station meets the handover criteria (“Yes” path out of decision step  230 ), then the base station that provides the highest signal quality is selected (step  240 ) and the base station instructs the mobile station and/or the selected base station to initiate a handover (step  245 ). 
     It should be recognized that the method of  FIG. 2  is merely exemplary and can be subject to a variety of modifications. For example, instead of the adjustment of handover criteria (step  210 ) being performed after receiving base station measurements from the mobile station, this can be performed at a different time and/or can be performed continuously, adjusting for variations of the wireless communication system. 
       FIGS. 3   a - 3   c  are exemplary logic diagrams of handover criteria in accordance with exemplary embodiments of the present invention. Any one of these handover criteria can be employed by the present invention. Moreover, when a wireless communication system divides mobile stations into different classes (e.g., providing differing quality of service (QoS) levels), then the handover criteria illustrated in  FIG. 3   a  can be employed for class 1 mobile stations, the handover criteria illustrated in  FIG. 3   b  can be employed for class 2 mobile stations and the handover criteria illustrated in  FIG. 3   c  can be employed for class three mobile stations. 
     Although exemplary embodiments of the present invention describes the base station as performing the handover criteria adjustments, this can be performed by other network elements, such as an access control gateway (ACG). The method described above in connection with  FIG. 2  uses measurements received from a mobile station in handover-related messages. However, the present invention can also be employed to use measurement reports from a mobile station that are sent while the mobile station is engaged in a call, such as a Measurement Coverage Response message. 
     As used in the description above, the term mobile station includes wireless telephones, personal digital assistants (PDAs), pagers, computers and/or the like. Although exemplary embodiments of the present invention are described in connection with an iDEN network, the present invention can be implemented in other networks. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.