Abstract:
A pre-determined negative constant is utilized in soft handoff mode to permit a new pilot signal to be added to an wireless phone&#39;s active set. The negative constant is combined with the weakest pilot signal in the active set and then compared to the new pilot signal strength which allows the new pilot to trigger a Pilot Strength Measurement Message (PSMM) even when the new pilot signal is weaker than all active set pilot signals. The negative constant provides a soft handoff while maintaining or reducing drop rate probabilities and frame error rates. After initially triggering a PSMM, the next instance the new pilot may ordinarily cause a PSMM to be triggered is when the new pilot signal exceeds the strongest active set pilot signal. Triggering a PSMM when exceeding the strongest active set signal may increase the probability of dropped signals. To reduce the probability of dropped signals, a negative constant is utilized during soft handoff to add a new pilot to an active set. This step would also decrease the probability that the new pilot will overpower a weaker active set pilot signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to wireless communication systems and in particular to wireless systems utilizing Code Division Multiple Access (“CDMA”). More particularly, the present invention relates to maximizing sector capacity of a given system by improving call drop probability. Still more particularly, the present invention relates to improving soft handoff. 
     2. Description of the Related Art 
     CDMA, as specified by CDMA standard IS95, is a form of digital cellular phone service and generally offers increased capacity over other types of digital cellular phone service. Each phone call is combined with a code that is broadcast across a broad frequency spectrum and another phone, which is aware of the code, receives the signal among all the other signals that may be occupying that frequency band. By coding the signal so that only one phone may receive the signal, more transmissions on the same band are allowable. Each cell may be partitioned, by directional antennas, into a number of sectors. Sectors may have different pilot signals and users and resources are shared by the multiple sectors within a cell. 
     CDMA power requirements represent both an advantage and a disadvantage to the system. In CDMA, strong signals overpower weak signals because the noise level is raised at the base station demodulators to accommodate the strong signal. The noise level problem is somewhat overcome by power control. The base station samples signal strength indicators of each mobile and sends a power change command to the mobiles increasing or decreasing the power requirement as a function of the grade of service requirement. This causes a nearby, strong mobile to decrease its power output and a mobile with a weak signal to increase its power output. 
     CDMA also provides a feature called “soft handoff.” “Hard handoff,” as opposed to soft handoff, is the process a wireless phone (mobile or handset) goes through as it approaches the boundary of a new cell. The network automatically drops resources in the current cell and hands off the connection to the new cell the mobile may be entering. Soft handoff allows a mobile to maintain resource connections with multiple base stations while moving within a system, adding and dropping connections as necessary. 
     IS95 (digital CDMA standard for U.S. cellular radio systems) soft handoff allows an individual handset to maintain a connection with as many as six individual pilot signals. As a mobile demodulates received information and sends modulated information, the mobile is constantly searching for pilot signals. A pilot signal (identifier channel, designated P 1 , P 2 , P 3 , etc.) broadcast from each sector of each base station (fixed station for communication between a network and mobiles within base station cells), is unique to that sector and is identified with a unique code—a PSEUDO NOISE (“PN”) sequence. If a handset (“mobile”) detects a new pilot, not yet in communication with the mobile, whose pilot strength (carrier to total interference ratio) is above an upper signal strength threshold (T_ADD), it will send a Pilot Strength Measurement Message (“PSMM”) to the network via the sector(s) base station with which it is currently communicating. The PSMM is sent to request that this new sector be added to the mobile&#39;s “active set” (a set, on board the mobile and network containing sectors that are currently in communication with the mobile). The network will instruct the mobile to add this new pilot via a Handoff Direction Message (HDM) sent out by all the sectors in the mobile&#39;s current active set. The HDM includes parameter settings based on changes in signal strengths, number of pilots in an active set and new parameter values introduced by the system operator. The mobile, on receiving the message, will add this new sector to the active set utilizing the parameters provided in the HDM and acknowledge via a third message, the Handoff Completion Message (HCM). If the mobile detects that a current, active set pilot signal strength (carrier to total received signal ratio) has dropped below a certain lower signal strength threshold (T_DROP) and has remained below that threshold for a pre-determined period of time (T_TDROP), then a PSMM is sent to the network, requesting that such a sector be dropped from the active set. The HDM and HCM follow in order as explained above. Upon receiving the HCM, the network acknowledges the HCM by sending a BSAO (Base Station Acknowledgment Order). 
     If a PSMM is sent by a mobile requesting addition of a particular pilot into the mobile&#39;s active set, the network may choose not to add such a Pilot. In order to reduce excessive messaging back and forth, the network may not send an HDM because the mobile would just resend the PSMM. However, the network must acknowledge the PSMM to prevent the mobile from continuously sending the PSMM. In such cases, the network acknowledges the PSMM with a Base Station Acknowledgment Order (BSAO), but doesn&#39;t send the HDM. This action prevents the mobile (standard action) from sending a PSMM requesting addition of this particular pilot again, until the pilot exceeds the strength of the current, weakest active set pilot by a factor “T_COMP” (T_COMP is an assigned value for triggering a decision by the network to add a new channel to the active set. New pilot signals must exceed the weakest active set pilot signal plus T_COMP. The IS95 standard requires T_COMP to take on positive values.). As the new pilot exceeds T_COMP (plus weakest signal), a completely new PSMM will be triggered. If the message is again ignored (BSAO sent, not HDM), then another PSMM will be sent only if this particular pilot exceeds the weakest pilot in the active set by T_COMP. Keep in mind that should anything else change, i.e. another pilot needing entry to the active set or a current active set ready to drop a pilot, a subsequent PSMM will be sent. 
     In an active set, as indicated before, there may be as many as six sectors allocating resources for an individual handset. For example, a mobile may have an active set of P 1 , P 2 , P 3 , and a new pilot, i.e., P 4  may be increasing in strength so much so (mobile is in motion within cells causing pilots in an active set to change strength) that P 4  equals or exceeds P 3 . Consequently P 4 &#39;s signal strength generates interference with P 3 . T_COMP should be set so that a PSMM is triggered when the signal to noise ratio is such that the mobile would be dropped from the system because of too much interference. Another way to add a new pilot is that if a current, active set pilot, P 3 , drops below T_Drop (a system specified arbitrary value) for a specified time period (T_TDROP), usually four seconds, a PSMM is triggered. The PSMM requests the removal of the sector from the active set because the pilot C/I (carrier to total interference ratio) dropped below T_Drop for T_TDrop seconds. The HDM and HCM follow in order as explained above 
     Referring now to FIG. 4A, a graph illustrating signal strength of P 3 , as received by a mobile, is illustrated. P 3   400  is a pilot signal increasing in strength as the mobile moves within the system. An active set (not shown), consisting of P 1  and P 2 , will be considering P 3   400  as a candidate for the active set. As the mobile moves closer to P 3 , T_ADD  402  is reached (−6 db from P 2 , the weaker signal in the active set) and a PSMM is sent to a base station. In this instance, P 3  is not added because Delta — 3 (arbitrary value for three pilots in an active set, set by the system) condition is not met, and P 3  continues to increase in strength to −8 db which is the signal strength of P 2 . If the mobile continues moving towards P 3 , the next PSMM to be sent will be PSMM  404 . The signal strength of P 3  when PSMM  404  is sent, meets or exceeds T_COMP  406  (including weakest signal strength) which is the trigger for PSMM  404 . 
     Referring to FIG. 4B, a chart illustrating constants required to add and drop pilot signals to an active set, is depicted. As previously discussed, a signal increasing in strength passes through certain thresholds that permit addition of a pilot signal to an active set and also permit dropping a pilot from the active set. T_ADD  410  is reached by P 3  and P 3  is added to the active set. After a period of decreasing signal strength, the signal strength of P 3  falls through T_DROP  412 . T_DROP  412  threshold is a trigger point that starts measuring the period of time P 3  remains below T_DROP  412 . If the signal remains below T_DROP  412  a predetermined period of time (usually 4 seconds), T_TDROP  414  causes the signal to be dropped from the mobile&#39;s active set. 
     A mobile may demodulate a maximum number of received transmission paths, usually three. Consequently, the mobile will attempt to demodulate the three highest quality paths from any of the links at any instant in time (this changes as radio frequency conditions change). By demodulating three channels simultaneously, probability increases that signal reception is clear and soft handoff would maintain continuity when moving between cells. A problem with high handoff rate is that system capacity is sacrificed, as there are sectors having to transmit (including those in the active set) even though the mobile is demodulating from other sectors. Optimal handoff is required to maintain frame error rates (invalid frame, or packet, identified by the Cyclic Redundancy Check sent with each frame) and dropped call (mobile permanently disconnected from the system during communication) probabilities below selected targets by providing a diversity of signal paths to the mobile. 
     All base stations are expending power transmitting to mobiles that are active within the system. Where a mobile active set contains six pilot channels, there are six sectors expending power maintaining radio connection with the mobile. Therefore, it would be desirable to increase efficiency of the network by providing a method and apparatus for assisting soft handoff between a mobile (or sectors) and base-stations while maintaining low dropped call probabilities and without adversely affecting frame error rates. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide a method and apparatus that will allow soft handoff in a CDMA system to operate more efficiently than prior systems and improve capacity. 
     It is another object of the present invention to provide a method and apparatus that will improve soft handoff while maintaining low call drop rate probabilities and frame error rates. 
     The foregoing objects are achieved as is now described. A pre-determined negative constant is utilized in soft handoff mode to permit a new pilot signal to be added to an wireless phone&#39;s active set. The negative constant is combined with the weakest pilot signal in the active set and then compared to the new pilot signal strength which allows the new pilot to trigger a Pilot Strength Measurement Message (PSMM) even when the new pilot signal is weaker than all active set pilot signals. The negative constant provides a soft handoff while maintaining or reducing drop rate probabilities and frame error rates. After initially triggering a PSMM, the next instance the new pilot may ordinarily cause a PSMM to be triggered is when the new pilot signal exceeds the strongest active set pilot signal. Triggering a PSMM when exceeding the strongest active set signal may increase the probability of dropped signals. To reduce the probability of dropped signals, a negative constant is utilized during soft handoff to add a new pilot to an active set. This step would also decrease the probability that the new pilot will overpower a weaker active set pilot signal. 
     The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a high-level block diagram of a wireless cellular telephone system in which a preferred embodiment of the present invention may be implemented; 
     FIG. 2A illustrates a high-level flow diagram of a method utilizing negative T_COMP to reduce the probability of dropped connections, in accordance with a preferred embodiment of the present invention; and 
     FIG. 2B depicts a graphical representation of a negative T_COMP method for improving handoff reliability in a preferred embodiment of the present invention; and 
     FIGS. 3A-3B illustrates a high-level flow diagram of a method for improving soft handoff reliability, in accordance with a preferred embodiment of the present invention; and 
     FIGS. 4A-4B illustrate constants utilized in determining when to add a pilot signal to an active set. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference now to the figures, and in particular with reference to FIG. 1, a high-level block diagram of a CDMA wireless communication system in which a preferred embodiment of the present invention may be implemented, is depicted. System  100  comprises multiple Base Stations (BS)  102 , each BS servicing and in communication with multiple mobile terminals (MTs)  104 . Each BS  102  has an associated Mobile Service Center (MSC) (not shown). In a typical wireless communication system  100 , there are multiple MSCs and multiple BSs  102  serviced by each MSC. For purposes of clarity and illustration of the present invention, there is shown in FIG. 1, multiple BS cells and a single MT  104 . Each BS  104  generates a pilot signal and the six pilots in the active set are represented by P 1 , P 2 , P 3 , P 4 , P 5 , and P 6 . The pilot signals are received into MT  104  and maintained in the active set on board MT  104  and the network. 
     P 1 , P 2 , P 3 , P 4 , P 5 , and P 6  are all pilot signals that are broadcast from the subject BSs  102  and each one represents one sector of each transmitting BS  102 . MT  104  is receiving six pilot signals (signals (P 1 , P 2 , etc.) of varying strength. As MT  104  moves about the area, each signal changes signal strength and MT  104  may drop and add signals according to pre-defined criteria, usually dependent on pilot signal strength. 
     Referring now to FIG. 2A, a high-level flow diagram of a method utilizing negative T_COMP to reduce the probability of dropped connections, in accordance with a preferred embodiment of the present invention, is depicted. The process begins in step  200 , which depicts two strong pilot signals in the active set with P 1  having a stronger signal strength than P 2 . A new signal, P 3 , is increasing in strength and approaching the strength of signal P 2 . The process then passes to step  202  which depicts a determination of whether P 3  is of greater strength than T_ADD. If P 3  is not stronger than T_ADD, the process proceeds to step  203 , which depicts the network ignoring P 3 . If P 3  is stronger than T_ADD, the process passes instead to step  204 , which illustrates the mobile sending a PSMM. If the new pilot signal, P 3 , exceeds T_ADD, the process then passes to step  206 , which depicts the network receiving the PSMM. 
     Next, the process passes to step  208 , which depicts a determination of whether the absolute value of the difference between the strongest signal in the active set, P 1 , and the new signal, P 3 , is less than or equal to Delta — 3 (an arbitrary value relating the strongest signal to the weakest signal in the active set, set by the system operator). If so, the process continues to step  210 , which illustrates adding P 3  to the active set. If the absolute value of the difference between the strongest signal in the active set, P 1 , and the new signal, P 3 , is not less than or equal to Delta — 3, the process proceeds instead to step  212 , which depicts P 3  not being added to the active set. 
     Further, the process continues to step  214 , which depicts a determination of whether P 3  is greater than P 2  plus T_COMP. If T_COMP is a positive value, as in the soft handoff algorithm depicted in FIGS. 3A-3B, P 3  must exceed P 2  by T_COMP in order to trigger the PSMM. As discussed earlier, the increasing strength of P 3 , without being added to the active set may cause interference and a signal to noise ratio that could cause the mobile to be dropped from the system. However, if T_COMP has a negative value, as in the present invention, P 3  only has to approach the signal strength of P 2  to trigger the PSMM, thereby reducing the probability of a dropped signal. If, in step  214 , P 3  is not of a greater signal strength than P 2  plus T_COMP (T_COMP is a negative value), the process returns to step  214  and repeats until P 3  is added to the set (signal continues to increase in strength) or is dropped from contention (signal grows weaker). If in step  214 , P 3  is greater than P 2  plus T_COMP, the process continues to step  216 , which illustrates the mobile triggering a PSMM for adding P 3  to the active set. 
     Referring to FIG. 2B, a graphical representation of a negative T_COMP method for improving handoff reliability in a preferred embodiment of the present invention, is depicted. Signal strength of P 3 , as received by a mobile, is illustrated as increasing over a period of time. The first instance that PSMM may be sent is T_ADD, at −14 db  252 . P 3  would likely not be added to the active set of the mobile at this strength. As the signal increases in strength, requirement for P 3  to be less than Delta — 3 has been reached. Regularly, the next threshold to reach would be T_COMP  250 . However, utilizing negative T_COMP  254 , P 3  may be added to the active set before becoming too strong and possibly causing the mobile to be dropped from the system. 
     Providing a “buffer” threshold constant, such as negative T_COMP, allows a CDMA wireless system to add signals to a mobile&#39;s active set at a point that reduces the incidence of high signal to noise ratio. Negative T_COMP provides an insertion point that will not threaten stability of the active set as much as positive T_COMP does. 
     With reference now to FIGS. 3A-3B, a high-level flow diagram of a method for improving soft handoff reliability, in accordance with a preferred embodiment of the present invention, is depicted. As a mobile (in a CDMA network) moves about in a region of coverage, the mobile will soft handoff from one sector (divisions within a cell) to another. The mobile demodulates received information, transmits modulated signals and searches for other pilot signals in neighboring sectors. If the mobile detects a new pilot with pilot strength above a threshold (T_ADD), the mobile places the new signal in a candidate set and sends a Pilot Strength Measurement Message (PSMM) to the Base Station (BS). The PSMM requests entry of the detected signal into the mobile&#39;s active set and the network sends a Handoff Direction Message (HDM), transmitted by all the active sectors in the active set, to add the new pilot. 
     The process begins with step  300 , which depicts the network receiving a PSMM signal providing the active set of a mobile and a candidate signal for addition to the active set. The process passes to step  302 , which illustrates a determination by the network whether the mobile is requesting to drop any pilot signal. If so, the process proceeds to step  304 , which depicts the network grouping pilots to be dropped into a set Y. Z represents the active set (X) less pilots to be dropped (Y). The process proceeds on to step  306 . Returning to step  302 , if the determination is made that no pilots are to be dropped, the process passes instead to step  306 . 
     The process continues to step  306 , which illustrates ordering the active set pilot signals in descending order of strength. Each pilot signal in both the active set and a candidate set is ranked from the strongest to the weakest. The process next passes to step  308 , which depicts a determination of the number of pilot signals being demodulated within the process. The process then proceeds to step  310  or step  312 , which illustrate the network creating a HDM with a change in T_ADD, T_DROP, T_COMP and T_TDROP. If the number of pilot channels in the active set is one, the process passes to step  310 , which illustrates the network creating a HDM with changes to the parameter values associated with one pilot signal. If the number of pilot channels in the active set is two, the process proceeds instead to step  312 , which depicts the network creating a HDM with new constant values as in step  310 . 
     From either of steps  310  or  312 , the process next passes to step  344 , in FIG. 3B, which is a determination of whether the new HDM is the same as the most recent previously sent HDM. If so, the process passes to step  346 , which depicts the network sending a BSAO to prevent the mobile from continuously sending a PSMM. If the new HDM is not the same, the process passes instead to step  346 , which illustrates the network sending the new HDM. 
     Returning to step  308  in FIG. 3A, if the number of pilot signals is greater than or equal to three, the process proceeds instead to step  314 , which depicts a determination of whether the absolute value of the difference of signal strengths of P 1  and P 3  is less than or equal to Delta — 3 (arbitrary value set by the system operator based on having three pilots in the active set). If the difference is more, the process passes to step  312 , described above. If the difference is less than or equal to Delta — 3, the process proceeds instead to step  316 , which illustrates a determination of whether there are four or more pilot signals in the active set. If not, the process passes to step  318 , which depicts the network creating an HDM with a change in parameters T_ADD, T_DROP, T_COMP and T_TDROP (described above). The process then continues, as from step  310  and step  312 , to step  344  in FIG.  3 B. 
     Returning to step  316 , if instead there are four or more pilot signals within the active set, the process continues to step  320 , which illustrates a determination of whether or not the absolute value of the difference between the strongest pilot P 1 , and the weakest pilot P 4  is less than or equal to Delta — 4 (an arbitrary value set by the system operator based on having 4 pilots in the active set). If the absolute value is not less than Delta — 4, the process proceeds to step  318 . If the value is less than Delta — 4, the process proceeds instead to step  330  in FIG.  3 B. 
     Referring now to FIG. 3B, a continuation of the process to minimize soft handoff in accordance with a preferred embodiment of the present invention is illustrated. The process continues from step  320  to step  330 , which illustrates a determination of whether there are five or more pilot signals in the active set. If not, the process passes to step  332 , which depicts the network creating an HDM with a change in T_ADD, T_DROP, T_COMP and T_TDROP (the parameters are different based on, among other things, the number of pilots included in the active set). The process then continues to step  344 . 
     Returning to step  330 , if instead there are five or more pilot signals within the active set, the process continues to step  334 , which illustrates a determination of whether or not the absolute value of the difference between the strongest pilot P 1 , and the weakest pilot P 5  is less than or equal to Delta — 5 (an arbitrary value set by the system operator based on having multiple pilots in the active set). If the absolute value is not less than Delta — 5, the process proceeds to step  332 . If the value is less than Delta — 5, the process proceeds instead to step  336 , which illustrates a determination of whether there are six or more pilot signals in the active set. If not, the process passes to step  338 , which depicts the network creating an HDM with a change in T_ADD, T_DROP, T_COMP and T_TDROP. The process then continues to step  344 . 
     Returning to step  336 , if instead there are six or more pilot signals within the active set, the process continues to step  340 , which illustrates a determination of whether or not the absolute value of the difference between the strongest pilot P 1 , and the weakest pilot P 6  is less than or equal to Delta — 6 (an arbitrary value set by the system operator based on having six pilots in the active set). If the absolute value is not less than Delta — 6, the process proceeds to step  338 . If the value is less than or equal to Delta — 6, the process proceeds instead to step  342 , which depicts the network creating an HDM with a change in T_ADD, T_DROP, T_COMP and T_TDROP. The process then continues to step  344 . 
     Delta values are utilized to set upper and lower limits to qualify a new pilot for admittance to an active set. The strongest pilot signal strength is an upper limit and the weakest pilot signal strength is the lower limit. If a new pilot is requesting entry to the active set, it must meet or exceed the strength of the current weakest pilot included in the active set. Delta is a value determined by subtracting the weakest current active pilot from the strongest active pilot. The new pilot signal strength is subtracted from the strongest active set pilot signal to determine whether the new pilot exceeds the signal strength of the weakest active set pilot signal strength. 
     It is important to note that while the present invention has been described in the context of a CDMA wireless network employing a fully functional data processing system, those skilled in the art will appreciate that the mechanism of the present invention is capable of being distributed in the form of a computer readable medium of instructions in a variety of forms, and that the present invention applies equally, regardless of the particular type of signal bearing media utilized to actually carry out the distribution. Examples of computer readable media include: nonvolatile, hard-coded type media such as read only memories (ROMs) or erasable, electrically programmable read only memories (EEPROMs), recordable type media such as floppy disks, hard disk drives and CD-ROMs, and transmission type media such as digital and analog communication links. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.