Method and apparatus for providing adjusted handoff parameters based on receiver classification

The present disclosure relates generally to wireless communications systems, and more particularly to mobile station handovers within such wireless communications systems and even more particularly to providing handover parameters to a mobile station prior to handover operations.

As digital signal processor (DSP) speed and power increases over time, wireless communications system digital signal processing may utilize higher complexity approaches that were not feasible previously due to processing constraints.

For example, mobile stations may employ various new approaches to interference cancellation which effectively improves mobile station receiver sensitivity over legacy mobile stations.

In systems such as IS-2000 CDMA, a mobile station having improved receiver sensitivity will be capable of detecting and receiving pilot signals from a much larger subset of its neighbor list than would a legacy mobile. Such improved mobile stations would therefore consume additional Walsh code resources because channels from various neighboring base stations would be assigned to the mobile station for CDMA soft handover implementation.

Therefore, valuable network channel resources would be unnecessarily tied up, and the full capabilities of the improved mobile station receivers would not be effectively utilized.

FIG. 1 is a block diagram of a wireless network having various base transceiver stations and a base station controller.

FIG. 2 is a block diagram of a CDMA wireless network wherein a mobile station has a neighbor list, a candidate list, and an active set.

FIG. 3 is a block diagram of a data packet.

FIG. 4 is a bit map diagram of an information element contained in various messages sent by a mobile station to a base station.

FIG. 5 is a block diagram illustrating the high level components of a base transceiver station in accordance with the various embodiments.

FIG. 6 is a flow chart illustrating the high level operation of the various embodiments.

FIG. 7 is a flow chart illustrating additional detail of the high level operation of the various embodiments.

To address the above-mentioned need, a method and apparatus for providing adjusted handoff parameters is provided herein.

Turning now to the drawings wherein like numerals designate like components, FIG. 1 illustrates a wireless network having various base transceiver stations (BTS) 103 . Each BTS 103 transmits and receives radio signals from a mobile station, such as mobile station 101 , over an air interface 105 . Air interface 105 may be any of a variety of air interfaces such as, but not limited to, IS-95 CDMA, IS-2000 CDMA, UMTS, etc.

Each BTS 103 has a radio coverage area 107 , that is, an area over which its radio transmissions may be received as a strongest signal such that the BTS is the “best server” for any particular mobile station within the coverage area 107 . In general, radio coverage areas may overlap forming an overall network coverage area. It is to be noted that the mobile station 101 may perceive, via its radio receiver, several BTS signals of various signal strengths based upon various conditions such as distance, Rayleigh fading, receiver sensitivity, etc.

Each BTS 103 is connected to a base station controller (BSC) 109 via backhaul connections. Several BTSs may be controlled by a single BSC 109 . Also, the network may comprise many BSCs. As the mobile station travels through the various radio coverage areas 107 , it will handover from BTS to BTS based on criteria referred to as handover parameters. For example, in a CDMA-2000 system various parameters, or thresholds “T,” are used to determine when a mobile station should handover from one BTS 103 coverage area 107 to another. Examples of such threshold parameters in a CDMA system are referred to “T ADD ” and “T DROP .”

Further in a CDMA system in general, soft handover is employed, that is, a make-before-break handover wherein a mobile station such as mobile station 101 may be connected to several BTS channels at any given time. The handover parameters such as T ADD and T DROP may be used by the mobile station 101 to determine when to add and drop a soft handover connection, respectively, to a given BTS. The handover thresholds are constrained by various conditions and considerations one of which may be the receiver sensitivity of the mobile station receiver. Depending on the signal level that may be received and accurately decoded by the mobile station, a number of surrounding BTS signals may be detectable.

Turning now to FIG. 2 , a mobile station 201 will contain a neighbor list 217 which provides BTS identifiers for such surrounding BTS sites that are likely candidates for handover. The mobile station 201 , based on its ability to receive and decode neighbor list BTS signals will promote a BTS, when the BTS received signal meets a criteria set, to a candidate list 219 , and may send a message to the BTS notifying the network of such new candidates. Thus, mobile station 201 may be able to detect only a subset of the BTS sites listed in its neighbor list 217 . For example, in FIG. 2 mobile station 201 can detect BTS 203 , BTS 205 , BTS 207 , BTS 209 and BTS 211 . BTS 213 and BTS 215 cannot be detected even though they are designated in neighbor list 217 . The detected BTS sites are thus promoted to the candidate list 219 .

Depending on traffic resource availability, the network may respond, via BSC 109 for example and a corresponding BTS in communication with the mobile station, by assigning a traffic channel from a BTS on the mobile station 101 BTS candidate list. Thus in FIG. 2 , the mobile station 201 has traffic channels assigned for BTS 203 , BTS 207 and BTS 211 and establishes soft-handover connections 223 to each BTS. BTS 203 , BTS 207 and BTS 211 are therefore part of the mobile station 201 active set 221 . Note that, while BTS 205 and BTS 209 are still detectable via air interface 225 , they are not assigned to the mobile station 201 active set 221 .

However, as the mobile station 201 travels through the network, radio conditions will change and the criteria required for establishing active set connections will likewise change. Therefore, the active set 221 , as well as the neighbor list 217 and candidate list 219 , are dynamic lists that are modified by the mobile station 201 location and various radio conditions occurring at each location.

Further in FIG. 2 , mobile station 201 is unable to detect BTS 213 and BTS 215 . However, if mobile station 201 had a better receiver characteristic, such as an improved receiver sensitivity, it is possible that BTS 213 and BTS 215 might also be detectable and therefore would be part of the candidate list 219 .

New model mobile stations having improved receiver capability would have advantages over older legacy models with respect to the number of detected BTS sites. However, if the criteria for handovers, or for that matter soft-handover connections, are globally set for a network, such new model mobile stations would be assigned to relatively more soft handoff links then mobile stations without an improved receiver sensitivity resulting in decreased system capacity.

Therefore in the various embodiments, a mobile station will provide a capability indication to the network such that criteria may be customized for the particular mobile station. Such capability indications may indicate for example, interference cancellation capability, pilot cancellation capability, receiver sensitivity, or various combinations thereof.

FIG. 3 illustrates a packet of information that may in some embodiments, be used for communication of messages between a BTS and a mobile station. In general, such packets will comprise a header 307 , sub-headers 311 , a data portion 313 and optionally a Cyclic Redundancy Check field 315 . In the embodiments employing packetized communications a message may be sent by a mobile station to the network, wherein a portion of the sub-header 311 bits, or a portion of the data 313 bits may contain a receiver class information of the mobile station. The receiver class information may be contained in messages such as, but not limited to, an origination message, a page response message, a handover message, or a registration message.

In an alternative embodiment, the receiver class information may be contained within unused bit fields of an information element. Various information elements are exchanged between mobile stations and a network during mobile station idle mode, or during call-setup, registrations etc. Therefore, the information element (IE) represented by FIG. 4 is a generic representation of an IE that may be contained in messages such as, but not limited to, an origination message, a page response message, a handover message, or a registration message.

An IE may be of any appropriate length and may therefore be several octets in length. In some embodiments, only a single bit may be used to indicate either binary “0” for legacy mobile stations and binary “1” for new model mobile stations having some enhanced receiver capability. However, a number of bit fields may be used in the various embodiments to contain the receiver class information. For example, a contiguous set of bit fields may be used such as bit 121 , bit 122 , bit 123 and bit 124 , all of which are contained in the second octet 403 . Alternatively, the bit fields may be scattered throughout the IE, or in various configurations, for example bit 002 of the 17 th octet 409 , bit 011 of the 16 th octet 407 , and bits 118 and 120 of the 3 rd octet 405 may be used.

By using four bit fields the binary values “0000” though “1111” may be used to provide sixteen different receiver classes. For example “0000” may be used for a legacy mobile station. Each binary value incremented by one, may correspond to improvements in receiver sensitivity of 3 dB in some embodiments. Therefore, a receiver class “0001” may have a 3 dB better receiver sensitivity than legacy mobile station “0000” in accordance with some embodiments. Also, in some embodiments one or two bits may be used to indicate interference cancellation capability and/or pilot cancellation capability.

FIG. 5 is a block diagram illustrating the primary components of a base transceiver station (BTS) in accordance with some embodiments. BTS 500 comprises one or more transceivers 501 , at least one processor 502 , and at least one memory 507 . Memory 507 has storage sufficient for the BTS operating system 509 , applications 511 and general file storage 513 . Transceivers 501 may be for communicating using various standards such as, but not limited to, UMTS, E-UMTS, IS-95 CDMA, IS-2000 CDMA, etc. The transceivers 501 may be further connected to radio combining equipment 503 and further coupled to antennas 505 .

It is to be understood that FIG. 5 is for illustrative purposes only and is for illustrating the main components of a BTS in accordance with the present disclosure, and is not intended to be a complete schematic diagram of the various components and connections there-between required for a BTS. Therefore, a BTS may comprise various other components not shown in FIG. 5 and still be within the scope of the present disclosure.

Memory 507 is for illustrative purposes only and may be configured in a variety of ways and still remain within the scope of the various embodiments herein disclosed. For example, memory 507 may be comprised of several elements each coupled to the processor 502 . Further, separate processors and memory elements may be dedicated to specific tasks. In any case, the memory 507 will have at least the functions of providing storage for an operating system 307 , applications 309 and general file storage 513 . In some embodiments, applications 511 may comprise a software stack that communicates with a like stack in a mobile station, such that messages may be transmitted there-between, and such messages may contain bit fields corresponding to a mobile station receiver class as disclosed herein.

FIG. 6 illustrates high level operation of the BTS 500 in the various embodiments. In 601 , the BTS receives a message from the mobile station such as, but not limited to, an origination message, a page response message, a handover message, or a registration message. The message will contain bit fields that designate a receiver class of the mobile station such that the mobile station capability may be determined as shown in 603 . In 605 , the BTS (or alternatively the BSC) may adjust or otherwise customize handover parameters for the particular mobile station based upon its previously designated receiver class information. The BTS may then communicate the new handover parameter information to the mobile station in a subsequent message, which may be any type of message having bits available in a packet or IE such as was discussed with respect to packet 300 of FIG. 3 and the IE illustrated in FIG. 4 .

FIG. 7 shows additional operational details, thus in 701 the BTS receives a message from a mobile station such as, but not limited to, an origination message, a page response message, a handover message, or a registration message. The message contains four bits which designate one of sixteen possible receiver classes for the mobile station. In 703 , the BTS determines the radio environment of the mobile station. It is to be understood that 703 may include receiving additional information from the mobile station such as, but not limited to, a signal-to-noise ration (SINR) measurement, or a like indication such as bit-energy-to-noise (E b /N o ) ratio, a frame erasure rate (FER), bit error rate (BER), or any other information appropriate for determining the radio environment of the mobile station at a particular time.

The BTS will subsequently, as shown in 705 , customize the handover parameters such as, but not limited to, T ADD , T DROP , etc., and may further convey the new parameters to the mobile station as mentioned above. For example, a mobile station having a better receiver sensitivity than a legacy mobile may have a much more degraded T DROP value because the mobile station may be able to decode much weaker signals than would a legacy mobile. Therefore, a network employing the method and apparatus of the present disclosure would have additional flexibility in allocating traffic resources and/or redistributing traffic via for example handovers at appropriate times and may therefore improve overall network performance.

In an alternative embodiment, the mobile station may modify the handover parameters. For example, a mobile station with an improved receiver using interference cancellation may have, for example, 3 dB improved performance relative to legacy mobile stations. The mobile station may therefore receive parameters over the air from the base station and modify the parameters before using them to determine which base stations to report to the network as handover candidates.

Further in some embodiments, the mobile station will also send modified signal strength measurements to the network. For example, if the received signal strength is X, and the mobile station has 3 dB improved sensitivity, the mobile station may report a signal strength between X and X-3 dB inclusive. This would result in the mobile station reporting power equivalent to legacy mobiles which would result in similar system loading.

While various embodiments have been illustrated and described, it is to be understood that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.