WO2002027966A2

WO2002027966A2 - Method and apparatus for controlling pilot power in a cdma system

Info

Publication number: WO2002027966A2
Application number: PCT/US2001/026129
Authority: WO
Inventors: Robert T. Love; Brent L. Goldstein; Dennis R. Schaeffer; Robert S. Nikides
Original assignee: Motorola, Inc.
Priority date: 2000-09-25
Filing date: 2001-08-21
Publication date: 2002-04-04
Also published as: AU2001285162A1; WO2002027966A3

Abstract

A wireless code division multiple access (CDMA) network control element optimizes pilot signal strength. The element includes at least one pilot signal strength measurement cache (202) containing multiple pilot signal strength measurements from multiple system user devices; and at least one processor (208), operatively coupled to the pilot signal strength measurement cache (202), that generates dynamic pilot power control information (132) for multiple pilot signals in response to a comparison of the multiple pilot signal strength measurements from the multiple system user devices to at least one pilot power threshold associated with each pilot signal. In another embodiment, a method for optimizing pilot signal strength includes receiving multiple pilot signal strength measurements, such as instantaneous pilot signal strength measurements from multiple user devices, and dynamically adjusting a pilot signal power level for at least one pilot signal in response to the multiple pilot signal strength measurements.

Description

METHOD AND APPARATUS FOR CONTROLLING PILOT POWER ΓN A CDMA SYSTEM

Field Of The Invention

The invention relates generally to control of pilot signal power and wireless communication systems, and more particularly to methods and apparatus for use in controlling pilot signal power in code division multiple access wireless communication systems.

Background Of The Invention

Wireless code division multiple access (CDMA) communication systems are known which may use the same frequency for providing pilot signals (e.g., pilot channels) and the same frequency for traffic channels and other channels. As known in the art, pilot signal information is typically used to provide a signal phase reference to demodulate audio or data signals, h addition, pilot signals are used to identify a sector for handing off a system user device, such as a mobile radiotelephone, an Internet appliance or any other suitable portable wireless device. Typically, there is a dedicated pilot channel for each sector of a cell in a CDMA radiotelephone communication system. The dedicated pilot channel is continuously transmitted by each base transceiver station sector. A system user device is informed of the Walsh code, a PN offset, or other suitable channel differentiator for each pilot channel that it receives. This is done through known communication protocols. Typically, a pilot channel in a sector is offset for each mobile unit assigned to the sector.

Wireless CDMA systems are also known that require a system user device, such as a mobile unit, to obtain instantaneous pilot power measurements from a plurality of pilot channels to facilitate, for example, determination of which sector is the best sector to be used as a receiving sector during a handoff. Such CDMA wireless systems use pilot strength measurement messages (PSMM) that are sent by a mobile unit to a plurality of base transceiver stations during a call. The pilot strength measurement message includes pilot strength measurements, such as an instantaneous pilot signal strength measurement (Ec/Io) on a per pilot channel basis. However, such information is used for determining handoff sectors or cells in the communication network and to adjust traffic channel power to enhance traffic channel communication. The pilot signal power level is typically maintained irrespective of the pilot signal strength measurement messages. This results in pilot pollution.

A mobile station in CDMA communication systems typically searches for pilot signals to detect the presence of an available communication channel by making signal strength measurements of the pilot signals. The signal strength and interference levels in combination may be compared against the threshold before accepting presence of an available communication channel. The mobile station monitors pilot signal levels received from neighboring sectors and cells and reports the pilot signal strength measurements to the network.

Various problems arise with the use of pilot channels that are at the same frequency as traffic channels. For example, pilot pollution may occur where too many strong pilot signals occur at a given location. As a result, a mobile station at that location may receive an unnecessary amount of interference resulting in dropped calls. Also too many pilots can delay the handoff process to the best serving cells or sectors for a given area resulting in dropped calls. Overhead channels and traffic channels are also affected by the pilot channel pollution. Moreover, since pilot channels in CDMA systems normally use the same frequency as traffic channels, the number of mobile units that are transmitting at the frequency will affect the signal to noise ratio of the pilots in the area of interest. Accordingly, there is a need to adjust pilot transmission power based on loading in a coverage area. In large systems, hundreds of individual sectors each require a pilot channel power output determination when a system is initially set up and when additional cells or sectors are added to existing systems. A prior determination of pilot power has been used as a mechanism to statically "optimize" pilot power levels. However, as mobile traffic increases, new cells are added, and as other environmental obstructions are added or removed, changes in path loss may occur that was not originally accounted for. One method for initially setting pilot signal power levels to a static setting uses temporary mobile equipment to act as actual users and uses a probability based algorithm for initial system setup. Such a system can require large computer power to find a convergence of preferred pilot power settings and also does not model a real system or the actual user traffic spatial distribution and capacity margin available at each cell . Even with large computer power, convergence to a useful solution is not guaranteed for a simulated annealing approach. Accordingly, it would be desirable to continually optimize pilot signal power levels based on real time system information to reduce unnecessary interference and to provide improved coverage within a system.

Neighbor lists are also statically maintained by a network element which indicate statically set pilot powers of neighboring sectors. These are typically used by wireless network elements, such as base station controllers and/or base transceiver stations to determine which sector has the associated pilot channel and its power level. In addition, where systems are typically statically maintained, too much pilot power can require large power amplifiers in the base transceiver stations. This can result in additional power consumption and the use of unnecessarily large power amplifiers.

Also, one type of system that attempts to control pilot signal strength, sometimes referred to as cell breathing, is based on a number of detected users. For example, a base station controller or other suitable network element determines the number of users in a coverage area and suitably adjusts the pilot signal strength based on the number of detected users. However, such systems do not typically employ pilot signal strength measurements on a real time basis indicative of actual system conditions, such as transmitted by a mobile unit. Accordingly, such cell breathing methods can inadequately adjust pilot signal power levels resulting in interrupted calls and other lost information.

Accordingly, a need exists for a method and apparatus that provides dynamic pilot signal power control on a more real time basis. It would be desirable if such a method and apparatus utilized instantaneous metrics measured from the system itself.

Brief Description Of The Drawings

FIG. 1 is a block diagram illustrating a wireless communication system employing dynamic pilot power control in accordance with one embodiment of the invention.

FIG. 2 is a block diagram illustrating one example of a wireless CDMA network control element in accordance with one embodiment of the invention.

FIG. 3 is a flowchart illustrating one example of a method for dynamically controlling pilot signal power in accordance with one embodiment of the invention.

FIGs.4-5 is a flowchart illustrating one example of a method for dynamically controlling pilot signal power in accordance with one embodiment of the invention. FIG. 6 is a graphical diagram illustrating pilot power pair wise association and capacity limiting criteria in accordance with one embodiment of the invention.

FIG. 7 is a graphical diagram illustrating pilot power pair wise association and capacity limiting criteria in accordance with one embodiment of the invention. Detailed Description Of The Preferred Embodiment

Briefly, a method and apparatus optimizes pilot signal strength in code division multiple access communication systems. In one embodiment, a method includes receiving a plurality of pilot signal strength measurements, such as instantaneous pilot signal strength measurements from a plurality of user devices, and dynamically adjusts a pilot signal power level for at least one pilot signal in response to the plurality of pilot signal strength measurements.

In one embodiment, a wireless CDMA network control element, such as a processing device or other suitable logic in a base transceiver station, a base station controller, or any other suitable network element, dynamically computes optimal pilot signal power values based on the instantaneous system pilot power measurements from multiple system user devices on a system- wide or subsystem- wide basis in an attempt to avoid individual pilot power control by a single system user device.

Also, in one embodiment, the method and apparatus stores (for example, in a cache) the instantaneous pilot power measurements, such as short term average pilot strengths, and applies a time stamp to each measurement. The method and apparatus determines whether to adjust pilot channel powers for a given sector based on the system user device-based pilot signal measurements associated with pilot measurements for those pilot signals other than the best pilot signals. The method and apparatus also updates a neighbor list with the new pilot signal power levels in response to adjusting the pilot signal powers.

In one embodiment, the method and apparatus uses a rule-based approach wherein one of the rules prevents a reduction in pilot power energy below a minimum pilot power threshold. This can help eliminate coverage holes which may result when optimizing an area with little or no traffic. In addition, the rules- based approach may also use a maximum pilot power threshold and a maximum linear power amplifier (LPA) power threshold limit to restrict excessive cell coverage and prevent power amplifier overloading.

FIG. 1 illustrates a portion of a CDMA wireless communication system employing a plurality of network control elements 100 and 102 such as base site controllers each operatively coupled to a plurality of base transceiver stations 104, 106, 108, 110, 112, 114, 116. The network control elements 100 and 102 are in operative communication with a public switch telephone network, the Internet, or any other suitable communication system and are also operatively coupled with one another through a suitable span line 118. The system may be, for example, an IS-2000 system described, for example, in 3GPP2 or a Wideband CDMA system (WCDMA) as described in 3GPP. However, any suitable CDMA system may be used.

Each of the base transceiver stations 104-116 may include a single sector or a plurality of sectors as known in the art. A single sector site is more commonly referred to as an omni-directional cell. Each sector may have independent hardware and software resources in a BTS. The base station controllers 100-102 communicate with the respective BTSs for overall control and operation of the communication system. Such control includes, but is not limited to, management of mobile station 122 and 124 to various BTS resources, as known in the art.

By way of example, each sector has an assigned pilot signal that may be assigned the same carrier frequency. Each pilot signal may be coded according to a unique code, thus distinguishing the pilot signals. All mobile stations, such as system user devices, communicate with a sector and obtain pilot signal power measurements. The set of communication channels, all having a common frequency assignment, is normally associated with a pilot signal. When a mobile station detects a pilot signal from a sector with sufficient strength, the mobile station reports the signal strength to a BTS. Such a BTS is normally associated with a cell that incorporates the sector which originated the pilot signal. The BTS then assigns the forward traffic channel on the same sector to the mobile station and directs the mobile station for other tasks. Other tasks may include soft handoff from a first assigned sector to another sector.

As shown in FIG. 1, mobile station 122 is moving in a direction indicated by arrow 126 and is in a range of BTS's 108, 110 and 112. Accordingly, pilot signal strength measurements from differing sectors from different BTS's are measured by the mobile station 122. The mobile station 122 communicates the pilot signal strength measurements from multiple sectors using an instantaneous pilot signal strength measurement message (known as a PSMM in IS95 or IS2000) or other suitable message. The base station controller 100 receives the pilot signal strength measurement message from one or more system user devices.

Each network control element 100 and 102 includes a suitably programmed processing device that serves as a dynamic pilot power controller 128 and 130. By way of example, the dynamic pilot power controller 128 generates dynamic pilot power control information 132a-132n to each of the base transceiver stations 104-108 to dynamically adjust the pilot signal power level for the pilot signals generated by each of the BTS's 104-108, in response to a plurality of received pilot signal strength measurements from a plurality of system user devices. This may occur, for example, every five minutes or any other suitable interval and is done on a dynamic basis based on instantaneous pilot signal energy measurements performed by the plurality of mobile stations (system user devices).

As known in the art, each of the mobile stations 122 (i.e., system user devices) determines a short term average pilot strength for each detected pilot signal, which serves as the pilot signal strength measurement. Such a measurement may be represented, for example, by a received signal strength indication or a pilot power/total power received ratio referred to as Ec/Io. The mobile stations 122 and 124 communicate pilot strength measurement messages to a BTS without a request from the BTS, or may send responses to pilot measurement request orders from a base transceiver station requesting that the mobile station send pilot measurement information. These messages are shown as messages 140, 142 and 144. Such pilot signal measurement messages contain, as known in the art, Ec/Io values, associated sector identification information, mobile identification information, and other information as known in the art. However, unlike conventional CDMA systems, the network control elements 100 and 102 employ a dynamic pilot power controller to generate dynamic pilot power control information 132a- 134n to suitably vary pilot power transmission strengths in addition to using information to control traffic channel power transmission.

Each network control element 100 and 102 generates the dynamic pilot power control information to control pilot signal power for a respective sector under its control. Accordingly, dynamic pilot power controller 130 generates pilot power control information 134a-134n. It will be recognized that a single dynamic pilot power controller may generate dynamic pilot power control information for all base transceiver stations, if desired.

Each network control element 100 and 102 receives a plurality of pilot signal strength measurements from a plurality of system user devices. Dynamic pilot power control takes into account an entire system of interest or a portion of an entire system so that a single mobile station, for example, does not unduly alter the pilot power for other mobile stations.

FIG. 2 illustrates one example of a CDMA infrastructure element, such as network control element 100. The network control element 100 includes a processor 200, and a pilot signal strength measurement cache 202. The pilot signal strength measurement cache 202 may be a single memory element having suitable locations for storing pilot signal strength measurements from plurality of system user devices on a per pilot basis. For example, each entry in the pilot signal strength measurement cache 202 may include an Ec/Io value for the pilot signal, user device ID and sector identification data. The cache 202 may be organized in any suitable manner and may be distributed among a plurality of network elements, or combined into a single repository, if desired. The processor 200 may be any suitable processing device that executes programming instructions. The programming instructions may be stored in a storage medium that may be, for example, any suitable RAM, ROM, optical storage medium, magnetic storage medium, remote storage medium that may be accessible through a communication link, or any other suitable storage medium.

Operations of the processor 200 are functionally shown as upcounter operations 204, downcounter operations 206, pilot association matrix 205, and dynamic pilot power control signal decision logic 208. Although in one embodiment of the invention, the functions are implemented using software algorithms executed by one or more processing units, it will be recognized that the invention may also be implemented using any suitable combination of hardware, software or firmware. For example, instead of software controlled counters, integrated circuits configured as upcounters and downcounters or any other suitable counter logic may also be used. The network control element 100 generates the dynamic pilot power control information 132a-132n for each pilot signal.

FIG. 3 illustrates one example of a method for optimizing pilot signal strength in accordance with one embodiment of the invention. The method will be described with reference to FIGs. 1 and 2. As shown in block 300, the method includes converting a message received from a system user device, such as mobile station 122 to a suitable format understandable by a base transceiver station.

As shown in block 302, the method includes determining whether a received message from a system user device is a pilot signal measurement JO- message or a pilot measurement request order response or other message. This may be determined, for example, by the network control element 100 or by a suitable controller in the base transceiver station, if desired. If the received message includes pilot signal strength measurements the process proceeds to block 304. As shown in block 304, the method includes extracting the pilot signal strength measurements, such as short term average pilot strength values Ec/Io from messages for all system user devices and sending the appropriate pilot signal strength measurements to their corresponding network control element (e.g. block 128 in block 100). In the embodiment described herein, the extraction of the Ec/Io measurement occurs at the CBSC (base station controller) which is connected to a plurality of BTS's.

The messages are passed from the mobile station to the BTS which best serves that mobile station. From there the message is passed to the CBSC connected to that BTS. As stated earlier, the measurements information will list all BTS's which were in the serving cell set (active set in IS-95 and IS-2000) of the mobile.

There will be cases where the mobile station was measuring a plurality of BTS's which were connected to more than one CBSC. This is known as a CBSC seam. Since the measurements in this case are across a seam, they do not exclusively belong to one CBSC. Therefore, the measurements are discarded because they would not influence the optimization of all the BTS's which were measured.

In another embodiment it is possible to have a network element (e.g combined block 128 and 130) which is connected to all BSCs and processes all of the pilot strength messages and determines all of the dynamic pilot power control information messages 132 and 134 to the BTS's via the CBSC's ( network control elements 100 and 102). As shown in block 306, the method includes applying a time stamp to the pilot signal strength measurements and storing the pilot signal strength measurements, such as short term average pilot strength data, along with sector identification data from the messages as transmitted by each of the plurality of system user devices. In one embodiment, storing the pilot signal measurements includes storing the pilot signal measurements in the appropriate pilot signal strength measurement cache 202 on a per pilot basis such that all the measurements from all system user devices for a given pilot are stored in a common memory block or cache. Storing of the pilot signal measurements may include, for example, storing the measurements in a suitable database. As shown in block 308, the method includes purging undesired cache entries from the pilot signal strength measurement cache 202 for those entries that have a time stamp out of a range of acceptable time stamps. For example, instantaneous pilot signal strength measurements may only be considered valid for a predetermined period of time whereafter new measurements may be required. Accordingly, the pilot signal strength measurement cache 202 maybe suitably purged of those entries that contain measurements outside of the suitable time stamp range. Accordingly, the method includes determining whether a plurality of time stamps associated with the cached pilot signal strength measurements has expired. Those pilot signal strengths that have expired time stamps are subsequently purged from the cache.

As shown in block 310, the method includes determining whether enough pilot signal strength measurements have been stored from the entire system, or a subset of the entire system, such as the sectors of interest to allow a change in pilot signal power. For example, it is desirable to insure that all system user device signal strength measurements for the entire system are considered in controlling the pilot power. It will be recognized that a suitable subsystem level, such as a smaller coverage area such as a predetermined group of sectors may be considered a subsystem or the entire system of interest. In any event, an acceptable number of signal strength measurements from an acceptable number of system user devices must be available before changing pilot power levels. If a suitable number of signal strength measurements are available, the method proceeds to block 312 where the dynamic pilot power controller 128 runs a pilot power control optimization algorithm to determine appropriate pilot power control information per pilot signal (for example, per sector where one pilot is used per sector). Once the dynamic pilot power control information 132a-132n is determined, the dynamic pilot power control information 132a-132n is compared to determine whether the dynamic pilot power control information exceeds several set thresholds before it is sent for controlling pilot signal power levels. In one embodiment, a linear power amplifier (LPA) minimum power threshold and maximum power threshold are used. For example, where dynamic pilot power control information for a particular pilot signal indicates to incrementally increase the pilot power, the method includes comparing that new power level to the linear power amplifier with the maximum pilot power threshold to ensure that the new power level does not exceed the maximum threshold. Also, where the dynamic pilot power control information 132a indicates a reduction in pilot power level, the new power level is compared to the LPA minimum to insure it does not fall below an allowable minimum value.

Accordingly, the method includes determining whether to adjust the pilot signal power for a given pilot signal based on the system user device based pilot signal measurements received from each of the plurality of system user devices. The pilot channel power is not adjusted until a comparison of the determined dynamic pilot power control information is compared to pilot power thresholds such as the linear power amplifier pilot power minimum and maximum thresholds or any other suitable thresholds.

As shown in block 316, if the dynamic pilot power control information is within an acceptable power range, the method includes updating system pilot power levels on a per pilot signal basis based on the acceptable dynamic pilot power control information. This may include, for example, updating a transceiver configuration database accessible by a base transceiver station which includes the appropriate pilot signal power level so that the base transceiver station can suitably increase or decrease a pilot power level as indicated after the update. In addition, the method includes updating a neighbor list in response to adjusting the pilot channel power. For example, if the dynamic pilot power control information 132a indicates an increase or decrease in pilot power, the associated neighbor lists of a given base transceiver station are updated with new pilot power levels. As shown in block 318, once the updates to the database are complete, the method includes purging the appropriate pilot signal strength measurement cache(es) associated with those pilot signals that have been updated since the information has already been used to determine whether a change is required in the pilot signal power.

The dynamic pilot power control information may include, for example, a value indicating an incremental change in power or whether to maintain existing power, or may be an actual final power level, or any other suitable information to control the pilot power dynamically based on the instantaneous pilot signal strength measurement.

As shown in FIG. 3, the method includes evaluating the received pilot signal strength measurements from the plurality of system user devices associated with a defined portion, of the entire system, or all system user devices in the system, if desired, and determining whether to adjust pilot signal strength levels based on the evaluated pilot signal strength measurements. The dynamic pilot power controller receives a plurality of pilot signal strength measurements from the plurality of system user devices from the base transceiver stations under its control. In addition, a dynamic pilot power controller may receive the pilot signal strength measurements from another network control element that controls other base transceiver stations. Accordingly, the processing may be consolidated or suitably distributed among multiple network control elements. FIGs. 4 and 5 illustrate one example of a method for optimizing pilot signal strength such as the pilot power control optimization algorithm referred to in block 312 of FIG. 3. As shown in block 400, the method includes determining whether any additional system user devices, such as mobile stations, have provided instantaneous pilot signal strength measurements that need to be evaluated. If another mobile for a given sector needs to be evaluated, the method includes sorting the received pilot signal strength measurements from that mobile in order by signal strength from the strongest to the weakest pilot signal strength level. For example, where a system employs a three-way soft handoff capability, meaning that a mobile station may simultaneous communicate with three base transceiver stations using three different pilot signals to facilitate a soft handoff from one sector to another. Four or more measurements are preferred. The received pilot signal strength measurements are sorted from each mobile as to which pilot signals are strongest and the associated sector from which the pilot originated. This is shown in block 402. Accordingly, received pilot signal strength measurements are sorted by the dynamic pilot power controller on a per mobile and per sector (pilot signal) basis. As shown in block 404, the method includes determining whether all of the pilot signal measurements have been received for a predetermined number of pilots. For example, if the system user devices are capable of monitoring six pilot channels and of providing six pilot signal strength measurements, the method includes determining whether all six have been sorted. If so, the method includes determining a strongest pilot signal source and a weaker pilot signal source, such as the weakest pilot signal of the sorted pilots, based on the pilot signal strength measurements for a particular mobile. This process is completed for each of the plurality of system user devices under consideration. For each system user device, the method includes comparing the pilot signal strength measurement associated with the weaker pilot signal to a power threshold to determine whether, for example, the weakest pilot signal measured by a mobile station is within lOdB of the strongest pilot signal measured by the mobile station. If so, the method includes updating the associated upcounter for the sector associated with the strongest pilot signal strength and decreasing the downcounter associated with the sector having the weakest pilot signal strength measurement for each mobile station. For example, assuming that four pilot signal strength measurements are evaluated and assuming, for example, that the strongest three are used for purposes of soft handoff, a rule based approach is used to push the fourth strongest pilot down in power when it is with in lOdB of the strongest pilot power level. This can help reduce pilot pollution by suppressing the fourth weakest pilots and increasing the strongest pilots. This updating is shown, for example, in block 408. The dynamic pilot power controller will generate dynamic pilot power control information to reduce the power of the weaker pilot source based on the comparison.

A pilot association matrix function is also a part of the process of block 408. The association matrix function is used to identify pair-wise sets of sectors for capacity limit purposes. For example, if a sector reaches a defined capacity threshold from increasing the pilot power identified above, then that pilot's power is stopped from increasing further. The pilot association is used to identify corresponding sectors that were decreased in conjunction with the capacity limited sector, and prevent these sectors from decreasing further. The combination of these two operations guarantees that no further increase in sector pilot differentials will take place, thus limiting any subsequent increases in sector overload. Accordingly, the method includes generating a pilot association matrix based on at least pair wise associations of pilot powers from cells of interest, and when a capacity threshold is reached, determining whether to adjust pilot signal power based on the pilot association matrix.

To avoid sector overloading, and to allow suitable quality of services for each mobile, the system and method includes performing a pair wise association between pilot powers. For example, the pilot power is changed for paired pilots relative to one another. The frame error rate may be used to determine quality to limit absorption of additional traffic. For example, an association matrix may be -loused that keeps track of all of the cell associations made during mobile voting. Specifically, for each mobile vote, two cells are involved. The strongest, or best serving cell, and the 4^th best serving cell. When a vote is made for each mobile in the system, the association between these two cells is also tracked, i.e., if a vote is made to decrease a mobile's 4^th best pilot that came from cell 5, and a vote to increase the best serving pilot came from cell 17, then the value of corresponding to assoc_matrix[17][5] entry is incremented.

As shown in block 410, other optional steps may be taken to keep a best serving cell with the strongest pilot signal from falling below desired thresholds. For example, the desired threshold may be pilot signal power or pilot signal SNR threshold which are used to trigger the addition of a cell as a serving cell if it's pilot signal strength is detected to be above such a pilot signal threshold (referred to as T_ADD or T_COMP pilot thresholds in IS-95 and IS-2000) or dropping a serving cell when such a cell's pilot signal strength is detected to be below a pilot signal threshold (referred to as the T_D OP pilot threshold in IS-95 or IS-2000) by the mobile station. The serving cells are said to be in the mobile station's serving cell set (called active set in IS-95 or IS-2000) and are also said to be in soft handoff with the mobile station if they simultaneously transmit voice and/or data signals to the mobile station.

For example, as shown in block 410, the method includes, for example, increasing a soft handover factor by raising a second pilot that may be, for example, the second strongest pilot measured by the mobile wherein the second strongest pilot is close to the strongest pilot power. In this case, the method increases the pilot signal power of the second strongest pilot signal if the second best pilot signal power measurement is within a pilot power range, such as within 6dB, of the best (strongest) pilot signal power measurement. Increasing the pilot signal power of the second pilot signal will only occur if the second best pilot signal power measurement is also below a desired threshold (T) such as the threshold (e.g. T_ADD in IS-95 or IS-2000) used to determine whether to add the sector into the serving cell set (called active set in IS-95 or IS-2000). Accordingly, if the second strongest pilot is less than the threshold T for the best server and if the second strongest pilot associated with a particular mobile is within, for example, 6 dB of the strongest pilot, the method includes incrementing the upcounter associated with the second strongest pilot. This is shown in block 412.

As shown in block 414, the method includes determining whether there is a third strongest pilot by determining whether there is a third strongest pilot signal strength measurement for a mobile station. If not, the process includes evaluating the next system user device. However, if a third strongest pilot signal strength measurement exists, the method continues to block 416and the process includes determining if the third best pilot signal power measurement is within another pilot power range, such as 3 dB of the strongest pilot signal strength measurement measured by the mobile station. In addition, the method includes determining whether the third best pilot signal power measurement is below the pilot signal threshold (power or SNR threshold) used to trigger a serving cell add event where the corresponding cell with the third best pilot signal power is added to the mobile station's serving cell set. Accordingly, if both conditions are met, the method includes increasing the pilot signal power of the third best pilot signal. For example, this may include, as shown in block 418, incrementing the upcounter associated with the third best pilot. The process then continues to evaluate the next group of pilot signal strength measurements received from another system user device of interest.

Referring back to block 400 (FIG. 4), if no more system user devices need to be evaluated, the method continues as shown in block 500 (FIG. 5) to determine whether there is another sector to evaluate based on searching for a next sector identifier. If another sector (or cell) includes system user devices that have provided pilot signal strength measurements, the method continues as shown in block 502 to determine whether the cell or sector allows changing of pilot signal strengths. This may be determined for example, by evaluating an entry in a database associated with a particular base transceiver station to see whether a power signal control override bit has been set. If the pilot signal power is allowed to be controlled, the process continues to block 504. The process includes determining whether to increase or decrease pilot signal power. For example, if the upcounter value for a given sector (a particular pilot signal) is higher than the value of the downcounter for that sector, then a check is performed at block 512 to verify if that sector previously exceeded a desired pilot interference capacity threshold via a status indicator. This threshold is typically expressed as a ratio of the pilot power( Ec) to the total cell power(Ior), or Ec/Ior. If the per sector pilot interference capacity threshold was previously set, then processing continues to the next sector in the system. If the sector capacity status was not set, then a check is made in block 514 to verify if the sector has now reached the capacity threshold. If so, the process continues with block 516 where the pilot power for that sector is reduced by a some predefined amount, for example 3dB, then in block 518, the status of the sector is set to identify that maximum capacity has occurred: Processing continues with block 522 where each potential associated pilot is checked for association with the now capacity limited sector. In the preferred embodiment, the association matrix, indexed by the capacity limited sector and each potential sector, is validated for a minimum count value, as set in block 408. If the count exceeds the power capacity threshold then the associated pilot's status is set to associated capacity as shown in blocks 526 and 524. If this was the last associated pilot, then processing continues with the next sector in block 500. Otherwise, the next associated pilot is selected in block 522, and processing continues back with block 526. If the sector capacity was not exceeded in block 514, the pilot signal power for that sector is increased incrementally, for example by 0J Watt. The increase in power is shown, for example, by block 506. Accordingly, if the count value of an upcounter is greater than the count value of the downcounter associated with the same cell identifier, the power for the particular pilot signal is increased. The control of the pilot power is done by issuing dynamic pilot power control information to the appropriate BTS to dynamically adjust the pilot signal power level in response to the pilot signal strength measurements from a plurality of system user devices.

After each pilot update, the overhead channel (such as the paging and sync channels in IS95 and IS2000) power levels are updated and the power limits used for the traffic channels (e.g. fundamental channel, dedicated control channel, and/or supplemental channels as given in IS95 and IS2000) are also updated as given in (530). In the preferred embodiment the channel power levels are set to some fraction of the pilot power level (Ppilot). For example, for IS95 for the fundamental channel power level limits (Pfch_max, Pfch_min) and the paging and sync channel power levels (Ppage, Psync respectively) are, in a preferred embodiment, related to the pilot power level by: Ppage = 0.75*Ppilot Psync = 0J*Ppilot Pfch_max=0.65*Ppilot Pfch_min=0.02*Ppilot.

Hence, on each pilot power update these other channel power levels and thresholds must be updated to maintain the above relationship.

As further noted in block 504, the process also includes determining whether a pilot power is less than the maximum linear power amplifier threshold for the given pilot signal generator. In this way, the pilot power is capped at a maximum level. In addition, the method includes determining whether the pilot signal strength measurement for the pilot of interest is higher than a minimum acceptable pilot signal strength measurement. If the current pilot power is less than the maximum allowable LPA threshold and if the pilot signal strength measurement is greater than the minimum threshold, then the pilot power is increased incrementally through the control of the dynamic power control information. Referring back to block 504, if the downcounter for a given sector (based on sector ID data) is greater than the upcounter value, the method includes generating dynamic pilot power control information to decrease a pilot signal power if any of the following conditions are met. A first condition is that the downcounter value be greater than the upcounter value for a sector. A second condition is if a sector power level is greater than the maximum allowable linear power amplifier threshold. A third condition is if a Walsh code assignment limit has been exceeded. A final condition is if the associated pilot capacity flag has been set. This is shown in blocks 508, 528, and 510. If the pilot is decreased in block 510, then processing proceeds to block 530 again, where overhead and traffic channel gains are updated as noted above.

In summary, the primary rule used is to reduce the fourth strongest pilot (or other pilot that is weaker than the strongest), and weaker pilots that are more than 10 dB down from the strongest pilot. Other rules are optionally used to keep a best serving sector pilot measurement level from falling below pilot signal threshold (such as T ADD or T DROP in IS-95 or IS-2000) threshold to keep at least one good pilot for a given mobile station or for the location that the mobile resides in. For each power control iteration, sector (pilot) pilot power counters are incremented when a mobile has a fourth strongest pilot within 10 dB of its strongest pilot. The increased pilot counter (upcounter) is incremented for the best serving pilot sector and the decrease counter (downcounter) is incremented for the fourth pilot's serving cell for each mobile. After all mobiles have been processed in this manner, the upcount value and downcount value for each sector are analyzed. Ifthe upcounter value is larger for a particular cell than the downcounter value, the pilot signal associated with that cell or sector is increased by an incremental amount, such as 0J Watt. However, ifthe downcounter value is larger for a particular cell than the upcounter counter, then the pilot signal power is reduced for that cell. The increase in pilot signal power is not allowed if the associated power amplifier power level exceeds some threshold. Otherwise the pilot power signal level is decreased ifthe associated power amplifier power level exceeds the threshold.

Stated another way, the pilot signal power levels are dynamically adjusted such that a pilot power is incremented or decremented based on a subset of a total mobile population that meets the measurement criteria, such as those mobiles whose third or fourth strongest pilot signal strength measurement is within a strength range, such as 6 dB, of a strongest pilot signal strength measurement determined from a group of received pilot signal strength measurements for a particular pilot signal. In addition, a subset of total system user devices whose relevant strongest pilot signal strength measurement is above at least a minimum desired pilot strength threshold is also determined. The pilot signal power level of a selected pilot signal is adjusted so that the subset of total system user devices relevant strongest pilot signal strength measurement is within a strength range, and so that the subset of total system user devices relevant strongest pilot signal strength measurement is above at least one minimum desired pilot strength threshold. Accordingly, the apparatus and method determines a best pilot signal power measurement associated with a pilot signal, a second best pilot signal power measurement associated with another pilot signal, and a third best pilot signal power measurement associated with another pilot signal as determined by a system user device. This is done for all mobiles or system user devices prior to dynamically adjusting a pilot signal power level for at least the second or third pilot signals. System pilot powers and neighbor lists are updated upon completion of the determination as to which pilot power levels should be updated.

Pilot signal strength measurement messages sent from a mobile station to its serving base stations contain information indicating the current serving cells (or cells in the active set for IS-2000) and potential serving cells (cells in the candidate set for IS-2000) along with their strength (pilot Ec/Io in IS-2000). As the cells' pilot power are changed their coverage area also changes such that a different set of cells may cover a given area then before the pilot power change. Therefore, as the pilot power is changed the pilot scanning list (or neighbor list which is updated by transmissions on the paging and traffic channel in IS-95 and IS-2000) must be updated. The pilot scanning list is used by each mobile station to efficiently and quickly detect new serving cells so they can be added to the serving cell set. When a mobile stations has multiple serving cells it is said to be in soft handoff with those serving cells. One method of doing this is to keep a candidate pilot scanning list and working pilot scanning list for each cell. A composite pilot scanning list is generated and sent to a mobile based on its current serving cells' working pilot scanning lists and the working pilot scanning list of the cell to be added or dropped from its serving set. The mobile typically receives the composite pilot scanning on add or drop events and merges it with its existing pilot scanning set. Each candidate serving sector list is updated based on information from the pilot signal strength measurement messages. For each cell listed in the pilot signal strength measurement message the corresponding cell's candidate scanning list is updated by increasing the count field for each entry in the list corresponding to the other cells listed in the pilot strength measurement message. After a given collection interval based on time or minimum number of updates to the candidate pilot scanning list, the candidate entries with counts exceeding some threshold are compared to the working pilot scanning list. Ifthe entries do not exist they are added to the working list. Ifthe working list is full then the working entry with the lowest count is replaced if its count is lower than the new candidate pilot entry. In some cases cell entries in the candidate pilot scanning list for a given cell will no longer achieve significant counts during collection intervals. When this happens such cell entries can be removed from the working pilot scanning list if desired. An enhancement to the candidate pilot scanning list update method is to weight the updates based on the pilot signal strength received in the pilot signal strength measurement message.

The dynamic power controller generates the pilot association matrix based on at least pair wise associations of pilot powers from cells of interest, and determines whether to increase pilot signal power based on the pilot association matrix. Table 1 lists psuedocode describing one example of pair wise association to create and maintain the pilot association matrix 205 and provide pilot power control determination .

TABLE 1 Step 0: Initialize the states

FOR each cell in the system (cell_x) DO vote_increase[ cell_x ] = 0 vote_suppress[ cell_x ] = 0 frozen_state[ cell_x ] = NOT_FROZEN

FOR each cell in the system (cell_y) DO assoc_matrix[ cell_x ][ cell_y ] = 0

END IF END FOR

Step 1 : Track the Votes and Pilot Associations

FOR each mobile (mob_i) DO cell_x = cell with strongest pilot for mobj cell_y = cell with 4^th strongest pilot for mobj IF ( 4^th pilot metric for mobj exceeded ) THEN assoc_matrix[ cell_x ][ cell_y ]++ /* Increment association count */ vote_increase[ cell_x ]++ /* Increment the count of the pilot to increase */ vote_suppress[ cell_y ]++ /* Increment the count of the pilot to suppress */ END IF

END FOR

Step 2: Freeze pilot powers when cell capacity is exceeded

FOR each cell in the system (cell_x) DO

IF ( Capacity of ce!l_x exceeds a threshold ) IF ( cell_x not frozen ) THEN

Reduce Pilot Power for cell_x by half Ensure Pilot Power for cell_x does not go below the min. allowed pilot power

END IF frozen_state[ cell_x ] = CAPACITY_FROZEN

FOR ( each cell in the system (cell_y) DO

IF ( assoc_matrix[ ce!l_x ][ cell_y ] > 0 ) AND

( frozen_state[ cell_y ] != CAPACITY_FROZEN ) THEN frozen_state[ cell_y ] = ASSOC_FROZEN END IF

END FOR END IF END FOR

Step 3: Optimize the Pilots (and take into account pilots associated to the frozen pilots)

FOR each cell in the system (cell_x) DO

IF ( vote_increase[ cell_x ] > vote_suppress[ cell_x ] THEN IF (frozen_state[ cell_x ] != CAPACITY_FROZEN ) THEN Increase Pilot Power by 0.1 W

END IF ELSE /* (vote_increase[ cell_x ] < vote_suppress[ cell_x ]) — > try to reduce pilot power 7 flag_decrement = TRUE; /* Initialize the flag 7

IF ( frozen_state[ ceil_x ] = ASSOC_FROZEN ) THEN FOR each cell in the system (ceil_y) DO

IF ( assoc_matrix[ cell_y ][ cell_x ] > 0 ) AND

( frozen_state[ cell_y ] = CAPACITY_FROZEN ) THEN /* lcell_y and cell_x are paired, and cell_x is already associated-frozen to cell_y 7 ratio = assoc_matrix[ cell_y ][ cell_x ] / vote_increase[ cell_y ] IF ( ratio > 20% ) THEN flag_decrement = FALSE; /* Do Not Decrease The Pilot Power 7 END IF

END IF END FOR

END IF

IF( flag_decrement = TRUE ) THEN

Decrease the pilot power of cell_x by 0.1 W END IF

END IF END FOR

The above pseudocode may be briefly described as: Step 0: The variables are initialized.

The votes for increasing and maintaining each cell are set to 0. The frozen state for each cell is set to "NOT_FROZEN" and the association list for every cell pair is set to 0.

Step 1 : Track the mobile votes and pilot associations.

Every mobile in the system now "votes" on how the pilot powers should be altered. The rule is that if a mobile's 4^th best pilot, or other suitable best pilot, is above a particular threshold, then it should be lowered. The act of placing this vote is to increment the value of vote_suppress[ cell_best ] where cell_best transmits the best pilot.

At the same time, the mobile also votes to maintain the power of its best pilot. This means the value of vote_increase[cell_4^th] is increased, where cell_4^th transmits the 4^th best pilot.

For each vote, the association list is also updated. The value of assoc_matrix[cell_best][cell_4 ] is incremented for each mobile vote.

Step 2: Freeze pilot powers when cell capacity is exceeded.

This process loops over all of the cells of the system, or a group of cells of interest. The capacity of each cell is compared to a pre-defined threshold. (This threshold is based on a measurement of "Ec/Ior", a measure of the cell's capacity, rather than the number of mobiles attached to the cell. This way, the threshold is relative to the total traffic that the cell can support based on the surrounding interference.)

Ifthe capacity of a cell is exceeded, AND the cell is in the state "NOT_FROZEN" two things happen:

1. The cell's pilot power is decreased. The result of this will be that the cell will carry less traffic.

2. The cell's pilot power is frozen when it is placed it in the state "CAPACITY_FROZEN". This state means that the cell's pilot power cannot be lowered any further.

Ifthe capacity of a cell is exceeded and that cell has been ALREADY frozen, than the pilot power is not altered at this point. Next, the cell associations are examined. For every cell that has exceeded its capacity threshold, all of its associated cells are also frozen. But, they are placed in the state of ASSOC_FROZEN so they can be identified separately later.

Step 3: Optimize the pilots

The votes are examined for each cell in the system. If vote_increase[cell_x] is equal to vote_suppress[cell_x], than the pilot power for cell_x is not altered.

If vote_increase[cell_x] is greater than vote_suppress[cell_x], then the pilot power for cell_x is increased. (Note that the pilot power is still limited to a predefined maximum value.)

If vote_suppress[cell_x] is greater than vote_increase[cell_x], this indicates that the algorithm wants to decrease the pilot power for cell_x.

However, if this cell is paired (associated) to a capacity limited cell, then it mau not be desirable to reduce its pilot power. Otherwise, both cells might continue to reduce power to the min allowed pilot power. — This is because lowering the power of the paired cell will tend to offload traffic to the capacity limited cell. But, if more traffic is added to the capacity limited cell, its pilot power will also be reduced, resulting in traffic being offloaded back to the associated cell. This ping-pong effect can continue until both pilot powers are reduced to a min value.

To determine ifthe cell pilot power should not be decreased, the algorithm compares the relative importance of:

1. Reducing the cell power based on the mobile votes.

2. Not reducing the cell power because it will adversely affect the capacity of an associated cell. Given that a goal is to reduce the pilot power of cell_x, then the algorithm computes the following metric for every other cell in the system (cell_y) that is in the CAPACITY_FROZEN state:

• ratio = assoc_list[cell_y][cell_x] / vote_increase[cell_y]

If ratio is a large, for example > 20%, then cell_x is strongly associated with cell_y. Lowering the power of cell_x would offload a lot of traffic to cell_y. But, cell_y is in the CAPACITY_FROZEN state. This means that cell_y cannot increase its power to compensate for the additional traffic. As a result, it is desirable not to allow cell_x to lower its power. For example, FIG. 6 graphically illustrates a condition where the capacity limiting may apply. As shown, three cells A, X, Y, 600, 602 and 604, respectively, are in communication with two mobiles 606 and 608. Cell Y is the best serving cell. Cells A and X are the 4^th best serving cells. With a large ratio, most votes to increase Y were also votes to decrease X. Therefore, it is not appropriate to lower X under the capacity limiting rule. Doing so would offload a lot of traffic to Y. But, since Y is CAPACITY_FROZEN, its pilot power cannot increase to compensate for this additional traffic.

Referring to FIG. 7, if ratio is small, then cell_Y is weakly associated to cell_X. That is, there are a lot of other cells that are associated with cell_Y. So, it is appropriate to lower the pilot power of cell_X because the amount of traffic that will be offloaded to cell_Y is relative small. Even though cell_Y is in the state CAPACITY_FROZEN and cannot increase its power to compensate, the affect is small relative to the benefit of decreasing the power of cell_X. Accordingly, most votes to increase Y were votes to decrease cells other than X. Therefore, it is appropriate to lower X under the capacity limiting rule. Lowering X would offload some traffic to Y, but not a large amount relative to the total traffic. The disclosed methods and apparatus provide many advantages. For example, path loss values, traffic location and forward and reverse link powers are not needed as with some conventional pilot power strength determination methods. Only pilot signal strength measurements, total cell power and linear power amplify limits need be used. Instantaneous pilot signal strength measurements are analyzed which takes instantaneous location and traffic loading factors into account. The disclosed invention provides a type of real time pilot optimization technique. Other advantages will be recognized by those of ordinary skill in the art.

It should be understood that the implementation of other variations and modifications of the invention in its various aspects will be apparent to those of ordinary skill in the art, and that the invention is not limited by the specific embodiments described. For example, the steps disclosed herein may occur simultaneously and in any suitable order. Also, the upcounter and downcounters may be single registers for each pilot, that are incremented or decremented so that a final value therein indicates whether to increase or decrease or maintain the associated pilot signal power. It is therefore contemplated to cover by the present invention, any and all modifications, variations, or equivalents that fall within the spirit and scope of the basic underlying principles disclosed and claimed herein.

Claims

ClaimsWhat Is Claimed Is:

1. A method for optimizing pilot signal strength in a code division multiple access communication system comprising steps of: receiving a plurality of pilot signal strength measurements from a plurality of system user devices; and dynamically adjusting a pilot signal power level for at least one pilot signal in response to the plurality of pilot signal strength measurements.

2. The method of claim 1 further comprising steps of: storing received pilot signal strength measurements and sector identification data transmitted by each of the plurality of system user devices; applying a time stamp to each stored pilot signal strength measurement; determining whether to adjust pilot signal power for a given pilot signal based on the pilot signal strength measurements received from each of the plurality of system user devices; and updating a neighbor list in response to adjusting the pilot channel power.

3. The method of claim 2 further comprising a step of comparing a pilot signal strength adjustment level based on dynamic pilot power control information to at least one pilot power threshold prior to adjusting pilot signal power.

4. The method of claim 1 wherein the step of dynamically adjusting a pilot signal power level comprises steps of determining a first subset of total system user devices whose relevant strongest pilot signal strength measurement is within a strength range of a strongest pilot signal strength measurement determined from a group of received pilot signal strength measurements, determining a second subset of total system user devices whose relevant strongest pilot signal strength measurement is above at least one minimum desired pilot strength threshold, and adjusting the pilot signal power level of a selected pilot signal in response to determining the first subset of total system user devices whose relevant strongest pilot signal strength measurement is within a strength range and in response to determining the second subset of total system user devices whose relevant strongest pilot signal strength measurement is above the at least one minimum desired pilot strength threshold, and wherein the method further comprises steps of: sorting the received pilot signal strength measurements by signal strength, on a per system user device basis and on a per pilot channel basis; and prior to the step of determining the subset of total system user devices whose relevant strongest pilot signal strength measurement is within a strength range, determining if a suitable number of pilot signal strength measurements have been received for a selected pilot channel.

5. The method of claim 1 further comprising steps of: determining, on a per system user device basis, a best pilot signal power measurement associated with a first pilot signal; a second best pilot signal power measurement associated with a second pilot signal and a third best pilot signal power measurement associated with a third pilot signal, prior to dynamically adjusting a pilot signal power level for at least one of the second and third pilot signals; increasing a pilot signal power of the second pilot signal ifthe second best pilot signal power measurement is within a first pilot power range of the best pilot signal power measurement and ifthe second best pilot signal power measurement is below a desired threshold; and increasing a pilot signal power of the third pilot signal ifthe third best pilot signal power measurement is within a second pilot power range of the best pilot signal power measurement and ifthe third best pilot signal power measurement is below the desired threshold.

6. The method of claim 1 further comprising steps of: determining a strongest pilot signal source and at least one weaker pilot signal source, for each system user device, based on the plurality of pilot signal strength measurements from the plurality of system user devices; comparing the pilot signal strength measurement associated with the weaker pilot signal to a power threshold; and generating a count value to reduce the power of the weaker pilot source based on the comparison.

7. A wireless CDMA network control element comprising: at least one pilot signal strength measurement cache containing a plurality of pilot signal strength measurements from a plurality of system user devices; and at least one processor, operatively coupled to the pilot signal strength measurement cache, that generates dynamic pilot power control information for a plurality of pilot signals in response to a comparison of the plurality of pilot signal strength measurements from the plurality of system user devices to at least one pilot power threshold associated with each pilot signal.

8. The wireless CDMA network control element of claim 7 further comprising a plurality of per pilot counters, operatively controllable by the at least one processor, that are updated in response to the comparison of the plurality of pilot signal strength measurements from the plurality of system user devices to at least one pilot power threshold associated with each pilot signal.

9. The wireless CDMA network control element of claim 7 wherein the at least one processor determines a strongest pilot signal source and at least one weaker pilot signal source, for each system user device, based on the plurality of pilot signal strength measurements from the plurality of system user devices, compares the pilot signal strength measurement associated with the weaker pilot signal to a power threshold; and generates a count value to reduce the power of the weaker pilot source based on the comparison.

10. The wireless CDMA network control element of claim 7 wherein the at least one processor controls storage of received pilot signal strength measurements and sector identification data transmitted by each of the plurality of system user devices, applies a time stamp to each stored pilot signal strength measurement, determines whether to adjust pilot signal power for a given pilot signal based on the pilot signal strength measurements received from each of the plurality of system user devices; and updates a neighbor list in response to adjusting the pilot channel power.