Abstract:
A method for providing wireless communications system providers to adjust when soft and hard handoffs occur relative to the signal strengths of surrounding antennae. This is achieved by providing two sets of parameters. The first set of parameters is used to adjust either hard or soft handoffs, and the second set of parameters are added as an offset to the first set of parameters to adjust the other handoff, thereby, allowing the wireless communications system providers system to adjust soft and hard handoffs independently.

Description:
TECHNICAL FIELD 
     The invention relates generally to wireless communications systems and, more particularly, to control efficient indication of a suitable location to perform a handoff from a first antenna to a second antenna. 
     BACKGROUND 
     Wireless communications systems provide communication services using geographically dispersed Base Station Transceiver Systems (BTSs) comprising at least one antenna, but typically two, three, or more. Each antenna is capable of transmitting and receiving within a limited geographical service area and may be configured such that the service area is directional or generally concentric. The service area for a BTS is commonly referred to as a cell or a sector. Moreover, a plurality of BTSs are generally coupled to a Base Station Controller (BSC), and, likewise, a plurality of BSCs are generally coupled to a Mobile Switching Center (MSC). The MSC provides interfaces to the Public Switched Telephone Network (PSTN) and the Packet Data Switched Network (PDSN), and, together with the BTSs and BSCs, form a wireless network that provides wireless communications services within a service area. 
     Wireless communication systems may utilize any communication protocol, one of which is the Code Division Multiple Access (CDMA) protocol as defined by the TIA/EIA-95-B and the CDMA 2000 standards. CDMA is a technology employing low-powered radio transmission that utilizes a broad spectrum broadcast to ensure quality of the transmission. 
     The capacity of a CDMA system is dependent upon, among other things, the number of distinct mobile stations (MSs) that may be communicating with a BTS over a single carrier frequency. In an attempt to increase capacity, CDMA systems are typically deployed with multiple carrier frequencies overlaying a single service area. With overlaying frequency coverage, some MSs are serviced on one of the carrier frequencies while other MSs are serviced on other carrier frequencies. Moreover, any particular BTS or antenna typically provides communication services on a subset of the available carrier frequencies, resulting in a situation in which adjacent service areas may utilize different carrier frequencies. 
     As a result of the limited geographic coverage of an antenna and the differing carrier frequencies between service areas, the MS is frequently commanded to handoff from a first antenna to a second antenna when moving from the service area of the first antenna to the service area of the second antenna. If the MS communicates user data, such as voice transmissions, with the first and second antennae, both of which are controlled by a single BSC, on the same carrier frequency, the handoff is commonly referred to as a soft handoff. During soft handoffs, an MS generally communicates user data with the first antenna and the second antenna simultaneously to enhance the transmission before dropping the first antenna. The convention in this application is to refer to soft handoff as both, separately and in combination, the process and state of the second antenna being added to the mobile active set of the MS. The process of adding and removing an antenna from the mobile active set of the MS is well known to one of ordinary skill in the art and will not be discussed in further detail. 
     Furthermore, if the MS is: (1) required to switch to a different carrier frequency; (2) required to switch to an antenna or BTS controlled by a different BSC that is controlled by a same MSC (intra-MSC/inter-BSC); or (3) required to switch to an antenna or BTS controlled by a different BSC and a different MSC (inter-MSC/inter-BSC), the handoff is commonly referred to as a hard handoff. It is well known to one of ordinary skill in the art that during hard handoffs the MS generally only communicates user data with one antenna, i.e., the communication of user data generally switches instantaneously from the first antenna to the second antenna. The process of switching from one antenna to the second antenna is well known to one of ordinary skill in the art and will not be discussed in further detail. 
     When performing a hard handoff, the MS is typically commanded to handoff from a first antenna to a second antenna when the signal strength of the second antenna exceeds a predetermined threshold. One particular method of assisting the MS in a hard handoff from the first antenna to the second antenna is by the use of a pilot signal. Pilot signals are typically transmitted by either the BTS as part of a CDMA forward channel, which comprises a pilot channel, a synch channel, a paging channel, and a traffic channel, or by a pilot beacon unit, whose transmission generally comprises a pilot channel and a synch channel. The second BTS generally transmits the pilot signal if the first antenna and BTS utilize the same carrier frequency as the second antenna and BTS. A pilot beacon unit generally transmits the pilot signal if the first antenna and BTS utilize a different carrier frequency than the second antenna and BTS. The source of the pilot signal will be referred to as being transmitted by an antenna, regardless of whether the pilot signal originates from the BTS or pilot beacon unit. Furthermore, a pilot beacon unit, which is preferably co-located with the second antenna, is generally required if the carrier frequencies of adjacent antennae differ. The use of a pilot signal and a pilot beacon unit are well known to a person of ordinary skill in the art and, therefore, will not be discussed in further detail, except as required to describe the present invention. 
     More particularly, handoffs are generally performed when the pilot signal strength of the second antenna, measured as the ratio of energy per chip to the noise spectral density of the total received forward-link interference, commonly referred to as E c /I 0 , is equal to or exceeds a predetermined system parameter, and may occur in either an idle-state or a traffic-state. Generally, the MS continually monitors the strength of the pilot signal from the second antenna, PS 2 (E c /I 0 ), and compares PS 2 (E c /I 0 ) to the value of two system parameters. 
     The CDMA standards typically provide two system parameters, T_ADD and T_COM, both of which are used to assist in hard and soft handoffs. The MS typically receives and stores T_ADD and T_COM from the BTS and/or BSC. The MS requests handoff from a first antenna to a second antenna when the strength of the pilot signal from the second antenna, PS 2 (E c /I 0 ), is greater than or equal to T_ADD. Furthermore, the MS also requests handoff from a first antenna to a second antenna when the difference between the strengths of the signals from the second and the first antenna, PS 2 (E c /I 0 )−PS 1 (E c /I 0 ), is greater than or equal to T_COM*0.5 dB. 
     For instance, an MS currently communicating with a first antenna having a pilot signal strength of PS 1 (E c /I 0 ), monitors the signal strength of nearby antennae to determine when handoff is appropriate. A second antenna transmits to the MS a pilot signal with a strength of PS 2 (E c /I 0 ). The MS compares PS 2 (E c /I 0 ) with the system parameter T_ADD, and compares the difference of PS 2 (E c /I 0 ) and PS 1 (E c /I 0 ) with the system parameter T_COM*0.5 dB. If either PS 2 (E c /I 0 ) is greater than or equal to T_ADD, or PS 2 (E c /I 0 )−PS 1 (E c /I 0 ) is greater than or equal to T_COM*0.5 dB, the MS requests a handoff from the first antenna to the second antenna. 
     Unfortunately, the system parameters T_ADD and T_COM assist both soft and hard handoffs. As a result, it is difficult for the service provider to adjust the T_ADD and T_COM values to obtain optimal performance for both hard and soft handoffs simultaneously. For instance, if the service provider optimizes T_ADD and T_COM for soft handoffs, hard handoff situations are generally adversely affected. Likewise, if the service provider optimizes T_ADD and T_COM for hard handoffs, soft handoff situations are generally adversely effected. 
     Therefore, there is a need, for one or more reasons discussed or nevertheless existing, for a method of optimizing when handoffs, soft and hard, occur. 
     SUMMARY 
     The present invention provides a method through which wireless communications service providers adjust when hard and soft handoffs occur individually relative to the signal strength of one or more antennae. The method preferably comprises using hysterisis parameters added as an offset to the handoff system parameters in the case of hard handoffs and using the handoff system parameters in the case of soft handoffs, thereby providing a mechanism to adjust hard and soft handoffs individually. Alternatively, the method comprises using hysterisis parameters added as an offset to the handoff system parameters in the case of soft handoffs and using the handoff system parameters in the case of hard handoffs, or a combination thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 schematically depicts the typical network environment that embodies the present invention; 
     FIG. 2 is a message flow diagram illustrating one embodiment in which the mobile station requests handoff and the base station controller determines if handoff is to occur; and 
     FIG. 3 is a flow chart illustrating one embodiment in which handoff from one antenna to a second antenna is determined. 
    
    
     DETAILED DESCRIPTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in FIGS. 1-3. 
     Referring to FIG. 1 of the drawings, the reference numeral  100  generally designates a wireless communication system, which embodies features of the present invention. BSC  102  is coupled to MSC  106  and BTSs  112  and  114 . Similarly, BSC  104  is coupled to MSC  106  and BTS  116 . Although an MSC is typically coupled to more than two BSCs, only two BSCs are shown for purposes of illustration. BTSs  112  and  114  are each configured with one antenna  118  and  120 , respectively. Each of the antennae  118  and  120  is shown, for purposes of illustration, having a generally concentric service area  132  and  134 . Similarly, BTS  116  is configured with one antenna  122 , also for purposes of illustration, having a generally concentric service area  136 . 
     Optionally, BTSs  114  and  116  are co-located with pilot beacons  121  and  123 , respectively, for the purpose of transmitting pilot signals and directing the MS to a supported frequency for continued communications. Generally, pilot beacon  121  transmits via antenna  120  a pilot signal PS 120 , and pilot beacon  123  transmits via antenna  122  a pilot signal PS 122 . Other sources of pilot signals, however, are available, such as the use of a pilot channel of the CDMA forward channel. Furthermore, for the sake of brevity, the antenna will be referred to as transmitting the signals and communicating with the MS. It is well known to a person of ordinary skill in the art that the antenna together with the BTS and/or pilot beacon unit transmits the signal. 
     Other components, such as base transceiver station controllers, additional BSCs and MSCs, connectivity to the PSTN, and the like, necessary for the operation of the present invention are well known to a person of ordinary skill in the art, and are, therefore, not shown or discussed. Additionally, other configurations, such as a BTS with three antennae, and the like, are considered well known to a person of ordinary skill in the art and are to be included in the present invention. Furthermore, configurations other than that illustrated requiring hard handoffs, such as handoffs between sectors of the same BTS utilizing different carrier frequencies, between BTSs controlled by different BSCs utilizing the same or different carrier frequencies, between BTSs controlled by different BSCs and MSCs utilizing the same or different carrier frequencies, and the like, are well known to a person of ordinary skill in the art and are to be included in the present invention. 
     In one embodiment of the present invention, the mobile station (MS)  130  travels from service area  132  through service area  134  and into service area  136 . As the MS  130  travels as described, communications with the MS  130  is handed off from antenna  118  to antenna  120 , and then to antenna  122 . Furthermore, for purposes of illustration, it is assumed that the handoff from antenna  118  to antenna  120  is a soft handoff, e.g., the MS  130  will communicate with antennae  118  and  120  on the same carrier frequency, and that the handoff from antenna  120  to antenna  122  is a hard handoff, e.g., the MS  130  will switch from communicating with antenna  120  on carrier frequency f 1  to communicating with antenna  122  on carrier frequency f 2 . While the hard handoff described above depicts a situation in which the carrier frequencies differ between antenna  120  and antenna  122 , the present invention applies equally to hard handoffs in which antenna  120  and antenna  122  communicates user data with the MS  130  on the same carrier frequency. 
     Whether to initiate a handoff is generally determined by monitoring a pilot signal from a second antenna relative to or received by the MS  130  and handing off when the pilot signal strength from the second antenna relative to or received by the MS  130  is of an acceptable level. For instance, if MS  130  is in communication with antenna  118  but is traveling toward antenna  120  such that the MS  130  will be leaving the service area of antenna  118 , the MS  130  monitors the pilot signal strength of antenna  120 , PS 120 (E c /I 0 ) and will switch to antenna  120  when PS 120 (E c /I 0 ) reaches a predetermined level. Similarly, the MS  130  monitors the pilot signal from antenna  122  and will switch to communicating with antenna  122  when PS 122 (E c /I 0 ) received therefrom is of sufficient strength. 
     Soft handoff from antenna  118  to antenna  120  generally occurs at the desired signal strength of soft handoff  140 . Initiation of soft handoff is dynamically determined or prompted by the MS  130  in reference to T_ADD and T_COM values or system parameters, and the pilot signal strengths of adjacent antennae relative or received by to the MS  130 . If the same parameters were used exclusively for hard handoffs, hard handoffs would occur at a comparable location given similar conditions, such as a distorted signal strength of hard handoff  142 . The desired signal strength of hard handoff  144 , however, may be reached at a different location relative to the pilot signal strengths of the adjacent antennae. As a result, it is preferred that the system operator be provided a method to adjust hard and soft handoffs individually in order to better provide consistent coverage in a geographic area. 
     The preferred embodiment provides the service provider with two hysterisis parameters, T_ADD_HYS and T_COM_HYS, in addition to the standard system parameters T_ADD and T_COM. Preferably, T_ADD and T_COM system parameters are used for soft handoffs and the T_ADD_HYS and T_COM_HYS parameters are added as an offset to the system parameters T_ADD and T_COM, respectively, for hard handoffs. Preferably, the range of values for T_ADD_HYS and T_COM_HYS range from −20 dB to 20 dB. 
     Alternatively, however, the system parameters T_ADD and T_COM are used for hard handoffs and the T_ADD_HYS and T_COM_HYS hysterisis parameters are added as an offset to the system parameters T_ADD and T_COM, respectively, for soft handoffs. Additionally, a combination thereof may also be used, such as using the system parameter T_ADD and the sum of the system parameter T_COM and the hysterisis parameter T_COM_HYS for assisting in hard handoffs and using the system parameter T_COM and the sum of the system parameter T_ADD and the hysterisis parameter T_ADD_HYS for assisting in soft handoffs, and vice versa. 
     Preferably, the parameters T_ADD_HYS and T_COM_HYS are contained in a component other than the MS  130 , and, preferably, contained in the BSC  102 . By locating the parameters in the BSC  102  as opposed to the MS  130 , the communication between the BSC  102  and the MS  130  remains consistent with prevailing CDMA standards. The adjustment and control of hard and soft handoffs are controlled by the BSC  102  and the associated management and control system of the telecommunications network, as desired by the system operator. 
     Using the above-described scenario, the MS  130  requests soft handoff from antenna  118  to antenna  120  when either PS 120 (E c /I 0 ) is greater than or equal to the system parameter T_ADD or the difference between PS 120 (E c /I 0 ) and PS 118 (E c /I 0 ) is greater than or equal to the system parameter T_COM*0.5 dB. Generally, if either of these conditions are met the BSC  102  performs the normal soft handoff procedures, which are well known in the art and will not be discussed in further detail. In this situation, T_ADD and T_COM are adjusted such that soft handoff occurs at the desired signal strength of soft handoff  140 . 
     Likewise, the MS  130  generally requests hard handoff under similar conditions as above for soft handoffs, i.e., when either PS 122 (E c /I 0 ) from antenna  122  is greater than or equal to the system parameter T_ADD or the difference between PS 122 (E c /I 0 ) and PS 120 (E c /I 0 ) is greater than or equal to the system parameter T_COM*0.5 dB. The BSC  102 , however, triggers hard handoff from antenna  120  to antenna  122  when either PS 122 (E c /I 0 ) is greater than or equal to the system parameter T_ADD plus the parameter T_ADD_HYS or the difference between PS 122  (E c /I 0 ) and PS 120 (E c /I 0 ) is greater than or equal to the system parameter T_COM*0.5 dB plus the parameter T_COM_HYS. If neither condition is met, the BSC  102  requests the MS  130  to delay handoff. If either condition is met, however, the BSC  102  triggers the MS  130  to complete handoff. Alternatively, a component other than the BSC  102 , such as an MSC  106 , a BTS  112 , and the like, may determine if a handoff is appropriate and trigger the MS  130  to perform the handoff. 
     In this situation, T_ADD and T_COM retain the values to maintain the desired signal strength of soft handoff  140  as described above. T_ADD_HYS and T_COM_HYS, however, act as an offset to obtain the desired signal strength of hard handoff  144 . The inclusion of the offsets T_ADD_HYS and T_COM_HYS in the case of hard handoffs, therefore, provide system operators the ability to optimize the system for both hard and soft handoffs individually. 
     FIG. 2 illustrates the preferred message flow embodying the present invention during handoffs. A first pilot signal is transmitted by the antenna  120  and a second pilot signal is transmitted by the antenna  122 , both of which are received by the MS  130 . The MS compares the strength of the second pilot signal to the system parameters T_ADD and T_COM. If the strength of the first pilot signal is greater than or equal to T_ADD, or the difference between the strength of the first pilot signal and the strength of the second pilot signal is greater than or equal to T_COM, then the MS  130  transmits a PilotStrengthMeasurementMessage (PSMM) message  210  to the BSC  102  via the BTS  114  (not shown). If the BSC  102  determines that a handoff is necessary, the BSC  102  triggers the handoff by transmitting a group of messages  212 , comprising an ExtendedHandoffDirectionMessage message  216 , to the MS  130 , requesting the MS  130  begin communicating with the antenna  122  corresponding to the second pilot signal. If, however, the BSC  102  determines that a handoff is not necessary  214 , the BSC  102  either transmits a BTS_Acknowledgement message  218  or an ExtendedHandoffDirectionMessage message  220  requesting that the MS  130  continue communicating with the antenna  120 . The content and procedure for transmitting the PSMM message  210 , the ExtendedHandoffDirectionMessage message  216  and  220 , and the BTS_Acknowledgement message  218  are well known to a person of ordinary skill in the art and, therefore, are not described in further detail. 
     FIG. 3 illustrates the procedure the BSC  102  preferably performs to determine whether a handoff from a first antenna to a second antenna is appropriate. Alternatively, either the MSC  106  and/or BTS  114  may perform the illustrated procedure. Initially, in step  310 , the BSC  102  determines whether the cell type from which a pilot signal has been received is a pilot beacon. Generally, the cell type from which a pilot signal is received will be a pilot beacon if the cell is controlled by a different BSC utilizing the same carrier frequency, commonly referred to as a logical pilot beacon, or the cell is controlled by a different BSC utilizing a different carrier frequency. The procedure for determining the cell type is well known to a person of ordinary skill in the art and, therefore, is not described in further detail. 
     If the cell type is not a pilot beacon, the BSC in step  312  performs the typical soft handoff procedures, which are well known to a person of ordinary skill in the art. If the cell type is a pilot beacon, however, in step  314  the BSC  102  determines whether pilot signal strength of: the second antenna, PS 2 (E c /I 0 ), is greater than or equal to the system parameter T_ADD plus the system parameter T_ADD_HYS. If not, in step  316  the BSC  102  determines whether the difference between PS 2 (E c /I 0 ) and pilot signal strength of the first antenna PS 1 (E c /I 0 ) is greater than or equal to the system parameter T_COM*0.5 dB plus the system parameter T_COM_HYS. If either of these conditions are met, in step  318  the BSC  102  transmits via antenna an ExtendedHandoffDirectionMessage message  216  to the MS  130  to direct the MS  130  to handoff to the second antenna from the first antenna. Otherwise, in step  320  the BSC  102  transmits via antenna either a BTS_Acknowledgement message  218  or an ExtendedHandoffDirectionMessage message  220  to the MS to direct the MS to delay handoff. 
     It is understood that the present invention can take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or the scope of the invention. For example, architectures with differing antenna configurations or sources of pilot signals may be used without negating the novelty or usefulness of other aspects of the invention. Moreover, while the above description described the invention in terms, of a pilot signal, other sources of signals may be used, such as the signal strength of a traffic channel. 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.