Abstract:
A high integrity process is provided to apply pilot phase rates to call processing decisions. An estimate or measurement of the rate of change of a pilot&#39;s PN phase offset is used to assist in making CDMA call processing decisions. The phase rate is a measure of the rate of change of a pilot&#39;s PN phase offset relative to a mobile time reference and is directly related to the multipath environment and movement of the mobile. Both an integrity algorithm to compute an integrity indicator and a technique to apply the integrity indicator to predict future pilot strengths are included.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of U.S. Provisional application No. 60/245,363, filed Nov. 2, 2000, the content of which is herein incorporated by reference in its entirety. 

   TECHNICAL FIELD 
   This invention relates to wireless communication systems, and more particularly to predicting the pilot strength signal condition in a communication system. 
   BACKGROUND 
   The use of wireless communication systems is growing with users now numbering well into the millions. One of the most popular wireless communications systems is the cellular telephone, consisting of a mobile station (or handset) and a base station. Cellular telephones allow a user to talk over the telephone without having to remain in a fixed location. This allows users to, for example, move freely about the community while talking on the phone. 
   The wireless communication systems may communicate using the Code Division Multiple Access (CDMA) standard. CDMA is a communication standard permitting mobile users of wireless communication devices to exchange data over a telephone system wherein radio signals carry data to and from the wireless devices. A set of standards that define a version of CDMA that is particularly suitable for use with the invention include IS95, IS-95A, and IS-95B, Mobile Station-Base Station Compatibility Standard for Dual-Mode Spread Spectrum Systems; TIA/EIA/IS-2000-2, Physical Layer Standard for cdma2000 Spread Spectrum Systems; and TIA/EIA/IS-2000-5 Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems, all of which are herein incorporated by reference in their entirety. 
   CDMA call processing decisions are largely dependent on forward and reverse link signal conditions. For example, a soft-handoff requires that the mobile select pilots that, as a set, will be usable for maintaining a communications link with the base station in the short term. There is, therefore, significant value in being able to predict signal conditions in the short-term in order to make better call processing decisions. 
   Pilot strengths are often, if not generally, predictable in the short-term. If reliable pilot strength predictions were available, mobile station or base station call processing could use the predictions to make informed decisions such as which pilot(s) to handoff to so that the frame error rate is maintained low and system capacity is minimally impacted. 
   While an estimate of a pilot&#39;s phase rate gives an indication of the velocity component of the mobile in the direction of the signal path from the base station, it may not consistently give a good indication of the expected change in pilot strength from that base station. In order to judge the usefulness of such estimates of phase rates in real-time, the mobile can use an integrity indicator. 
   SUMMARY 
   A high integrity process is provided to apply pilot phase rates to call processing decisions. An estimate or measurement of the rate of change of a pilot&#39;s PN phase offset, here-after called a pilot&#39;s phase rate, is used to assist in making CDMA call processing decisions. The phase rate is a measure of the rate of change of a pilot&#39;s PN phase offset relative to a mobile time reference and is directly related to the multipath environment and movement of the mobile. Examples of decisions that could benefit from this information include, but are not limited to, soft, softer, hard, and idle handoffs, or establishing or adapting pilot search priorities or search sequencing. The present invention includes both an integrity algorithm to compute an integrity indicator and a technique to apply the integrity indicator to predict future pilot strengths. 

   
     DESCRIPTION OF DRAWINGS 
     These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
       FIG. 1  illustrates the components of an exemplary wireless communication system used by one embodiment of the present invention. 
       FIG. 2  is a block diagram showing features of a mobile station according to one embodiment of the invention. 
       FIG. 3  illustrates a process for determining a pilot phase rate integrity for use in call processing decisions according to one embodiment of the invention. 
       FIG. 4  illustrates a process for determining the integrity algorithm according to one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates components of an exemplary wireless communication system  100 . A mobile switching center  102  communicates with base stations  104   a–   104   k  (only one connection shown). The base stations  104   a–   104   k  (generally  104 ) broadcasts data to and receives data from mobile stations  106  within cells  108   a–   108   k  (generally  108 ). The cell  108 , corresponding to a geographic region, is served by a base station. Practically, said geographic regions often overlap to a limited extent. 
   A mobile station  106  is capable of receiving data from and transmitting data to a base station  104 . In one embodiment, the mobile station  106  receives and transmits data according to the CDMA standards. Under the CDMA standards, additional cells  108   a ,  108   c ,  108   d , and  108   e  adjacent to the cell  108   b  permit mobile stations  106  to cross cell boundaries without interrupting communications. This is so because base stations  104   a ,  104   c ,  104   d , and  104   e  in adjacent cells assume the task of transmitting and receiving data for the mobile stations  106 . The mobile switching center  102  coordinates all communication to and from mobile stations  106  in a multi-cell region. Thus, the mobile switching center  102  may communicate with many base stations  104 . 
   Mobile stations  106  may move about freely within the cell  108  while communicating either voice or data. Mobile stations  106  not in active communication with other telephone system users may, nevertheless, scan base station  104  transmissions in the cell  108  to detect any telephone calls or paging messages directed to the mobile station  106 . One example of such a mobile station  106  is a cellular telephone used by a pedestrian who, expecting a telephone call, powers on the cellular telephone while walking in the cell  108 . The cellular telephone scans certain frequencies (frequencies known to be used by CDMA) to synchronize communication with the base station  104 . The cellular telephone then registers with the mobile switching center  102  to make itself known as an active user within the CDMA network. 
   At times it is desirable for a different base station  104  to communicate with the mobile station  106 . This may be due to the original base station  104  losing signal strength, the mobile station  106  traveling out of range of the original base station  104 , or other factors. When the mobile station  106  changes base stations  104 , it is referred to as a handoff. Currently, one technique for determining if a handoff is to occur is to monitor the energy level of a pilot signal from a base station. If the energy level of the pilot signal falls below a predetermined threshold for a specific period of time, the mobile station  106  initiates a handoff. 
     FIG. 2  shows a block diagram of the mobile station  106 , including a processor  200  and memory  205 . The processor  200  may be driven by a program stored in the memory  205 . A portion of memory  210  may be used to store search parameters. 
     FIG. 3  illustrates a process  300  for determining a pilot phase rate integrity measure for use in call processing decisions according to one embodiment of the invention. The present invention does not include the mechanism by which phase rate estimates or measurements are determined or the mechanism used for pilot strength measurements. The present invention describes a mechanism that is independent of the means by which pilot PN phase offset and energy measurement inputs are determined. 
   The integrity mechanism is shown in the context of call processing in  FIG. 3 . The process  300  begins at block  305 , where a Search Element of a mobile station  106  receives a pilot signal from one or more base stations  104 . The pilot strength is denoted by PS, pilot PN phase offset is denoted by ΔΦ, and derivative/rate-of-change is denoted by d(ΔΦ)/dt. The rake receiver in block  305  takes the pilot signal and processes the signal to obtain the pilot strength PS and the pilot PN phase offset ΔΦ. The pilot strength PS is then forwarded to a pilot energy filter in block  315 , while the pilot PN phase offset ΔΦ is sent to a rate of phase change estimator in block  310 . The pilot energy filter in block  315  provides filtered pilot energy to both an integrity algorithm in block  325  and the call processing decision logic in block  350 . The rate of phase change estimator in block  310  provides the derivative of the pilot PN phase offset, known as the phase rate d(ΔΦ)/dt to the integrity algorithm in block  325 . The integrity algorithm is part of a pilot energy predictor  320 , which also includes a decision time-frame module  330  as well as multipliers  335 ,  340 , and adder  345 . 
   As indicated above, the integrity algorithm in block  325  takes the pilot energy and phase rate as inputs. The operation of the integrity algorithm  325  is shown in  FIG. 4 . The algorithm  325  receives the pilot strength PS and may send the PS through an optional filter  405 . The algorithm  325  then computes the derivative of the filtered pilot strength (or unfiltered, if the optional filter is not used) in block  410  to obtain a derivative d(PS(t))/dt. The algorithm  325  may the optionally perform filtering of the derivative d(PS(t))/dt in block  415 . The derivative (or filtered derivative) d(PS(t))/dt is then provided as an input to a divider  430 . 
   The algorithm  325  also receives the phase rate d(ΔΦ)/dt as an input. The algorithm  325  may filter the phase rate dΔΦ(t)/dt measurements through an optional phase rate filter  420 . The phase rate d(ΔΦ)/dt (or filtered phase rate) is then optionally delayed in time in block  425 . The time delay T predictive  is a parameter controlling the timing nature of the rate of change. The T predictive  parameter optionally adjusts the time window applicability of the phase rate filter output. This can be envisioned, in one embodiment, as a delay applied to the output of the phase rate filter. The algorithm  325  then computes the ratio of the derivative of the pilot&#39;s strength over the pilot&#39;s phase rate, i.e. d(PS(t))/dt/dΔΦ(t−T predictive )/dt (in this example no filtering is used) at the divider  430 . The ratio is taken such that the rate of phase offset change (denominator) is optionally delayed in time (by T predictive ) relative to the derivative of the pilot energy (numerator). This ratio may additionally be filtered. The integrity algorithm  325  computes this ratio continuously. The ratio is essentially dPS″/dΔΦ which, over time, represents an indication of whether the phase rate and energy change rate are correlated and can be used for predictive means, i.e. to predict each other&#39;s future change. For example, in one embodiment, a negative ratio may be considered as indicating an unreliable predictability of either the phase rate or energy change rate based on the other and thus may be ignored or changed to zero (0) to represent this condition mathematically. Optionally, a sustained negative value may be considered as an indication of a reliable value such as might occur when moving toward a cell but closer to a significant signal obstruction such as an underground parking garage. 
   The calculated ratio is used in block  435  to obtain an integrity decision, outputting a value T integrity . The output value T integrity  may be based directly on the ratio. The algorithm  325  may either output the ratio directly or choose to output a zero value (0) if the data is a poor indication of future pilot signal strength. For example, if the measurements and processing results indicate that movement in the direction of the signal corresponds to a significant decrease in energy this may indicate that the data may not be currently applicable for prediction. Alternatively, if the ratio is filtered, then it may be directly output. 
   The output T integrity  of the integrity algorithm  325  is multiplied at the multiplier  335  by the current pilot phase rate dΔΦ(t)/dt to determine the predicted rate of change in pilot strength in the short term d(PS(t))/dt. This result is then multiplied at multiplier  340  by the time T decision  that the mobile station  106  associates with decision making. This will be discussed further below. The result of the multiplication is the predicted change in pilot strength (ΔPS) in the short term. This change is added to the current pilot strength at adder  345  to determine the predicted short-term pilot energy. This prediction is communicated to the call processing decision logic in block  350  for interpretation and decision-making. In the case where the integrity algorithm  325  determines that the ratio is not usable by setting the output to 0, the integrity mechanism will yield a predicted future pilot strength equal to the current pilot strength. The filters and output choice are design parameters of the integrity algorithm  325 . 
   The decision time frame block  330  in  FIG. 3  shows that the integrity mechanism may use the mobile call processing state and signal conditions as real-time input for determining the time-frames for both pilot strength prediction and integrity monitoring. The T predictive  time is the offset in time that is used for computing the ratio between future change in pilot strength associated with current phase offset change. The T decision  time determines how far into the future the pilot strength is predicted. The reason for supporting variability in these parameters is the varying requirements on pilot strength prediction time-frames from a call processing perspective. In cdmaOne or cdma2000 slotted-mode operation, for example, the mobile station  106  may be unable to make pilot strength measurements for a considerable amount of time during the sleeping period. For this reason, predictions may be required far into the future. T decision  should be extended so that the prediction is made further into the future and T predictive  should be extended so that the integrity algorithm generates an integrity indicator using a longer time-frame. Signal conditions may also have an impact on this time-frame since handoffs during poor signal conditions may take considerable time to perform due to retransmissions by mobile or base station. The time parameters may be adjusted similarly. 
   A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.