HK1063554B - Method and apparatus for call recovery in a wireless communication system - Google Patents

Description

Method and apparatus for call recovery in a wireless communication system

Technical Field

More particularly, the present invention relates to a novel and improved method and apparatus for call recovery in a wireless communication system.

Background

A wireless communication system typically includes a plurality of Base Stations (BSs), each associated with a cell and/or sector, and communicating with a plurality of Mobile Stations (MSs). The base stations are controlled by a Base Station Controller (BSC). As the mobile station moves through the entire system, the quality of the signal received from the base station fluctuates. When a communication link between a base station and a given mobile station deteriorates, it is possible to prevent communication from being lost by establishing a link with at least one other base station. A hand-off procedure provides for the generation of such alternative communication links. In the case of a handover, the infrastructure negotiates with the various base stations and mobile stations. However, the signal quality often degrades too quickly to negotiate.

Therefore, a method and apparatus for call recovery is needed in various situations. There is also a need for a reliable method of call recovery in a wireless communication system.

Disclosure of Invention

The disclosed embodiments provide a novel and improved method of recovering a degraded call in a wireless communication system. In one aspect, in a wireless communication system having a plurality of base stations, each of the plurality of base stations having a neighbor set including neighbor base stations, each neighbor base station having a default channel, a method includes transmitting default channel information to a mobile station, detecting an occurrence of a call recovery trigger; and instructs all base stations in the set of neighbor stations to transmit on their own implied channel.

In one aspect, a wireless device includes an antenna; a processor connected to the antenna; a transmission line connected to the antenna and the processor; a receiving circuit connected to the antenna and the processor; a first set of computer readable instructions executable by a processor for receiving a neighbor list of base stations, the list including a default channel designation for each neighbor; a second set of computer readable instructions executable by the processor for identifying a call recovery trigger and in response disabling the transmission line; and a third set of computer readable instructions executable by the processor for establishing a handover with the at least one neighboring station.

In another aspect, a wireless device includes a transmitter line; a call restoration adjusting unit that operates after the call restoration operation to generate a predetermined power control instruction; and a power adjustment unit coupled to the talk recovery adjustment unit and the transmitter line, the power adjustment unit operating in response to the power control command to adjust the transmitter line.

In yet another aspect, a computer program is embodied on a computer-readable medium containing computer-executable instructions. Wherein the program includes a first set of instructions operative to identify a special event; a second set of instructions operative to disable call recovery during a special event; and a third set of instructions operable to notify the wireless communication system of the special event.

Drawings

The features, objects, and advantages of the presently disclosed methods and apparatus will become apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

fig. 1 illustrates, in block diagram form, a wireless communication system in accordance with one embodiment;

FIG. 2 illustrates, in block diagram form, a portion of the wireless communication system of FIG. 1 in accordance with one embodiment;

fig. 3 shows in a time diagram the signal quality of two base stations in a wireless system as in fig. 2 according to an embodiment;

fig. 4 illustrates, in block diagram form, a portion of the wireless communication system of fig. 1 during recovery in accordance with one embodiment;

FIG. 5 illustrates, in a time diagram, signal quality for two base stations in a wireless communication system in accordance with one embodiment;

FIGS. 6A and 6B illustrate, in flow diagram form, a method of call recovery at a base station in accordance with one embodiment;

FIGS. 7A and 7B illustrate, in flow diagram form, a method of call recovery at a mobile station in accordance with one embodiment;

FIG. 8 illustrates, in block diagram form, a hierarchy of the system of FIG. 1 in accordance with one embodiment;

FIG. 9 illustrates, in a time diagram, a call recovery operation of the system of FIG. 1 in one embodiment;

FIG. 10 illustrates, in a time diagram, the initialization of transmit power levels at a mobile station after call recovery in accordance with one embodiment of the present invention;

fig. 11 illustrates, in flow chart form, operation of a wireless device in the system of fig. 1.

Detailed Description

A method of call recovery in a wireless system according to one embodiment is to provide information about neighboring cell service areas and/or sectors that are available to, and potentially dangerous for, a mobile station with a lost communication link to be able to recover from the call. Each call recovery capable base station has a default forward call recovery channel identified by a predetermined code. In further embodiments, each neighbor cell is assigned more than one default forward call recovery tunnel, and the mobile station uses a hash function with IMSI (international mobile station identity), TIMSI (temporary international mobile station identity), ESN (electronic serial number), system clock, or a combination thereof, to deterministically determine which tunnel to use to receive transmissions from each call recovery capable base station. The mobile station may then use that channel to receive signals from the call recovery base station. When the mobile station accesses the base station, the recovery base station from multiple neighboring calls commands the mobile station to combine the power control subchannels with an additional message. This can occur even when the mobile station is in an idle state without a continuous communication link, by means of a communication channel message on which a call is occurring when the mobile station moves into the coverage area of the base station, or in a handoff situation when the active set changes to the mobile station.

Fig. 1 illustrates a wireless communication system 10 having a plurality of cells 12, 14, 16, 18, 20, 22, 24. The cells 12, 14, 16, 18, 20, 22, 24 communicate with the BSC26 over a radio broadcast interface. Each cell 12, 14, 16, 18, 20, 22, 24 has a corresponding set of neighbor cells, consisting of cells in geographical and/or transmission neighbors. For example, cell 18 has a neighborhood of cells including cells 12, 14, 16, 20, 22, 24. In Spread Spectrum transmission systems, such as the Code Division Multiplexing (CDMA) System specified by TIA/EIA/IS-95Mobile Station-Base Station compatible Standard for Dual-MODE Wideband Spread Spectrum Cellular System, hereinafter referred to as the "IS-95 Standard", or TIA/EIA/IS-2000Standard for CDMA2000 Spread Spectrum System, hereinafter referred to as the "CDMA 2000 Standard", the Spread Spectrum signals occupy the same channel bandwidth, with each signal having its own separate Pseudorandom Noise (PN) sequence. The operation of a CDMA system is described in the following U.S. patents: serial No. 4,901,397 entitled "forward scroll ACCESS COMMUNICATION SYSTEM using scroll title OR TERRESTRIAL REPEATERS" and serial No. 5,103,459 entitled "SYSTEM AND METHOD FOR GENERATING a waveform IN a cdmachine language SYSTEM", both assigned to the assignee of the present patent application and incorporated by reference herein IN their entireties. In this way, multiple users send messages simultaneously on the same channel bandwidth.

Fig. 2 shows a portion of the system 10 of fig. 1 including a base station 32, labeled BS1, in communication with an MS 38. BS132 is in cell 18 of fig. 1. Other base stations 34, 36, labeled BS2 and BS3, respectively, are in the cells 16, 24, respectively. The radio broadcast interface provides a medium for the Forward Link (FL) from BS132 to MS38 and the Reverse Link (RL) from MS38 to BS 132. Note that MS38 may move throughout system 10, degrading the quality of the signal going back and forth from BS 132. To initiate a call, the MS38 sends out a transmission on the access channel. BS132, BS234, and BS 336 send out channel assignment messages on the paging channel. The channel assignment identifies the Walsh code index for each base station.

Signal quality is typically measured in terms of signal-to-noise ratio (SNR) and can be expressed as a ratio of pilot signal energy per chip to total received power density (Ec/Io). Fig. 3 shows a graph of signal quality measured at MS38 for BS132 and BS 234. The signal quality to the BS234 begins to increase at time T0 and continues to increase beyond the threshold of the following T _ ADD marker at T1. The threshold T _ ADD provides a reference signal quality beyond which the MS38 is instructed to inform the base station to ADD a base station to its Active Set (AS). The AS is composed of those base stations. They are actively in transmit and receive communication with the MS 38. The AS is typically selected from the base stations of the Candidate Set (CS). The CS includes base stations that are backed up to be in active communication with the MS 38. The CS is typically selected from base stations in a neighbor set of stations (NS).

With continued reference to fig. 3, the signal quality of BS132 deteriorates while the signal quality of BS234 improves. Because the signal for a given base station is a comparison of the signal energy from that base station to the energy of all other occurring signals, the increase in energy level of the signal received from BS234 adds to the deterioration of the signal from BS 132. At time T1, MS38 measures that the signal energy of BS234 exceeds T _ ADD. This indicates to the MS38 that appropriate action is required, i.e. triggering a handover. At time t2, MS38 sends a Pilot Strength Measurement Message (PSMM) to BS132 and BSC26 containing measurement information for BS132 and BS 234. At time t3, the BSC26 establishes a link for the MS38 from the BSC26 to the BS 234. The BSC26 includes a selector. BSC26 establishes a communication link that forms a "reverse" communication network with respect to MS38 between BS132, BS234, and BSC 26. At time t4, BS132 sends a Handoff Direction Message (HDM) that includes code indices identifying BS132 and BS234 and associated forward link (TL) channels from BS132 and BS 234. This information enables MS38 to receive and demodulate signals from BS132 and BS 234. At time 5MS 38 receives the HDM from BS132 and begins demodulating the signal from BS234 added to the signal from BS 132. Note that in this example, only one new base station is involved in the handover. However, there may be any number of base stations involved in such a handover situation, where those base stations in communication with the MS38 form an AS. When the MS38 receives signals comprising symbols from multiple base stations in the AS, the MS38 can combine the signals to form a stronger signal. This combining process is referred to as "soft combining" of the FL and is done in an optimal proportional combining manner with weighting according to signal quality. At time t6, the MS38 issues an acknowledgement of the HDM received from BS132, or a Handoff Completion Message (HCM) indicating successful completion of the handoff.

Referring again to fig. 3, a situation may arise where the signal quality of the BS234 increases too quickly. In this case, the signal strength of BS234 encourages the deterioration of BS132 signal quality relative to BS 132. MS38 cannot communicate with the infrastructure until it receives information necessary for handoff, such as a pseudo-random noise (PN) offset necessary to identify the BS234 or the channel used by BS234 for MS 38.

During a typical CDMA handoff, when a mobile station moves from the coverage area of one base station to the coverage area of another base station, the handoff avoids the loss of a communication link. In a type of handoff, such as a soft handoff, a mobile station maintains a connection with two or more base stations at the same time. The current location of the mobile station can be considered the source cell and the next cell to which the mobile station moves can be referred to as the target cell. The mobile station demodulates a plurality of signals received on FLs of a plurality of base stations using a search type receiver. The two signals are combined to form a composite signal with improved quality. Although each of the plurality of base stations involved in the soft handover demodulates the respectively received signal, each transmits the demodulated and decoded information to the BSC. The BSC includes a selector that selects a best frame from the plurality of received frames. Additional types of handover may be used in various situations and system requirements.

In mobile-Assisted handover (mobit-Assisted Hand-Off-MAHO), the mobile station makes signal quality measurements of FL pilot signals from multiple mobile stations. This information is reported to the source base station. The signal quality is compared to a plurality of thresholds to make an add base station to AS decision. If the signal quality of a given pilot is greater than the pilot detection threshold, T _ ADD, then this pilot is added to the AS. In another embodiment, the pilot is added to the CS first, and then to the AS. In fact, the threshold facilitates the transfer of the state of the base station from one group to another.

In the case that handoff negotiation is not possible, call recovery provides information to the mobile station in advance. Call resumption is initiated in various situations. In normal operation, the mobile station and the base station use triggers to determine their proper operation. For example, a mobile station operating in system 10 uses multiple thresholds to make decisions regarding what information to report back to a base station. A threshold T ADD, AS discussed above, indicates the signal quality level to be used to ADD the base station to the AS. When the mobile station receives a signal with a measurement value higher than T _ ADD, the mobile station moves the base station into CS, searches for that base station more frequently, and reports this status to the system through the existing AS. The further threshold T DROP provides a signal quality class below which the base station will DROP from the AS. When the mobile receives a signal whose measurement value is below tddrop for a period longer than TDROP, the mobile reports this to the system through the existing AS. In each case, the base stations in the AS relay this information to the base station controller.

For call recovery, the base station in the AS looks for any of various possible triggers. A first type of call recovery trigger occurs when the FL signal quality is below a threshold for longer than another threshold time. Such triggers include increasing the transmission energy level at the base station when the base station receives a continuous Power Control (PC) request from a mobile station. Typically, the base station always transmits to the mobile station at the maximum capped energy level. For example, FL traffic transmission is maintained at a high level for a predetermined period of time. The mobile station can issue a number of requests to increase power, i.e., UP commands. In addition, the mobile station may report many erasure cases. An erasure occurs when more than one bit of threshold level is received without an expected confidence. In another case, the mobile station sends a message indicating to the base station that its outer loop set point is high or at the highest allowed level, or too long time has elapsed at that level.

The second type of trigger occurs when a certain response is expected from the mobile station but no or a different response is received. Such triggers include the lack of acknowledgement from the mobile station for the message sent by the base station requiring acknowledgement. The message may be retransmitted a predetermined number of times before the triggering condition is met. This predetermined number can be fixed or variable and can be changed by broadcast. Also, the base station may receive a repeated RL message from the mobile station that requires an acknowledgement, wherein the message is received after the base station sends the acknowledgement.

The third type of trigger involves a low quality reverse link, such as when the Frame Error Rate (FER) of the RL exceeds a threshold level. In addition, RL can remain at a high energy level for a predetermined period of time. There is a case where there is a high RL set point. The base station to be added to the AS also has a call recovery trigger that initiates the recovery action. The most important trigger is a notification from the BSC that there is a potential problem at a given mobile station. In that case, the base station starts searching for a signal from the mobile station.

The mobile station can also enter into call recovery using various call recovery triggers. The first type of trigger occurs when there is an abnormal number of errors in the received signal. For example, FL erasures in a moving window may exceed a predetermined threshold. In one embodiment, the threshold limit is 12 consecutive frames subject to erasures. In this case, the mobile station will turn off the transmitter portion of the mobile station and turn on the transmitter again when at least two FL consecutive frames are clear of erasures.

A second type of call recovery trigger to the mobile station occurs when the mobile station receives a PC command from the base station commanding an increase in power. The base station may have difficulty receiving the RL signal due to a large path loss from the mobile station.

A third type of recovery trigger occurs when one or more RL messages requiring acknowledgement from the base station are not acknowledged. This is called a retransmission retry trigger. There may similarly be an inappropriate or no response from the base station to the message from the mobile station. A similar type of trigger occurs if a repeated FL message is received that requires an acknowledgement after the mobile station actually sends the acknowledgement.

A fourth type of call recovery trigger occurs when the mobile station transmits a predetermined time interval at a high level. In this case, the RL is considered to not pass through to the base station with sufficient energy.

In one embodiment, flexible thresholds are utilized for one or more of the various call recovery triggers. The call recovery trigger may be based on various transmission attempts in system 10. These attempts are often made in the link layer between the signaling and the physical link. The link layer is referred to as layer 2. Discussed below with reference to fig. 8. In a system capable of call recovery, such as system 10 of fig. 1, the MS38 completes the call recovery process to maintain the call when the communication link, such as the FL, becomes bad. A trigger always initiates a recovery operation, where the trigger indicates when a parameter or measurement passes a threshold. These thresholds can be dynamic, adapting to the conditions and environment of the system 10. Similarly, the threshold can be adjusted based on historical or statistical records of the operation of the system 10.

In one embodiment, the number of repeated transmissions on the RL, the time between successive erasures, or the disablement of the MS38 transmitter may be in response to a command transmitted from the infrastructure of the system 10, such as the BS132 and/or the BSC 26. In yet another embodiment, the state and/or location of the mobile station provides a trigger. The current transmit energy level of the MS38 approaching the predetermined maximum value can trigger call resumption. Additional triggers include the quality of the FL AS measured by erasures transmitted in the current AS, a deficiency in inner loop power control where the desired SNR for the MS38 is different than that provided by the inner loop, etc. Further embodiments may combine certain parameters and mobile station conditions as triggers.

The infrastructure of system 10 may provide MS38 with operational class information that facilitates determining a call recovery threshold and use that information in selecting a fixed parameter for providing MS38 with a trigger threshold. In one embodiment, the usual number of call retries that are encountered in trouble or are cut off. Further embodiments use the input of the RL to set and adjust the threshold. Yet another embodiment may use the location of the MS38 in the system 10, such as the region of a given cell. Yet another embodiment considers the day of the week and/or time of day in conjunction with known patterns of mobile station traffic distribution. Combinations of any of these mechanisms are also achievable where applicable and desired.

In the system 10 of fig. 1 and 2, the additional information for each base station 32, 34, 36 includes its corresponding neighbor list. The neighboring station table identifies the corresponding pseudo-random noise (PN) code offset of the neighboring station.

Referring to fig. 4, BSC26 responds to any of a variety of triggers by establishing a reverse link with BS132 and BS 234. According to one embodiment, the call recovery method 100 is initiated as shown in FIG. 6. A specific signal quality diagram of an example is shown in fig. 5. The MS38 is sometimes considered to have a potential problem in this example.

In one embodiment of the call resumption method 100 shown in fig. 6A and 6B, BS132 issues a default tunnel assignment to the group of neighboring base stations of MS38 at step 102. The base stations in the neighbor set are call recovery capable units with the necessary software and/or hardware to implement call recovery and have coverage areas that cover the area of the base stations sent out of the neighbor set. The default channel assignment identifies a default channel code index, including a code for BS234, used by base stations in the neighbor set. Each base station in the set of neighbor stations capable of call recovery has a default spreading code for identifying the mobile station when call recovery is required. The spreading code of one embodiment is a special Walsh code. In step 104, the BS234 sends a retransmission retry trigger to the MS 38. The resend retry trigger defines the number of retries of the MS38 allowed before initiating the call recovery operation. BS132 then determines whether a call recovery trigger has occurred at decision diamond step 106. If no recovery trigger occurs, the process waits until a trigger occurs. Upon the occurrence of the trigger, the process continues to step 108, commanding all base stations in the NS of BS132 to transmit on the corresponding default tunnel of the corresponding MS 38. Note that some base stations in the NS may not be able to establish a communication link due to the weakness of the FL or RL, but each base station in the NS begins transmitting to the MS 38. Multiple transmissions provide a stronger FL signal at the MS38 and a more reliable RL to the BSC 26.

Note that in this embodiment, the number of retries of the RL message, or the amount of time allowed between successive erasures, is determined by the BSC26 and provided to the MS38 over the dedicated message and broadcast radio link. Further embodiments use fixed parameters that differ from other parameters. One embodiment adds a function of the condition of the mobile station. The mobile station condition may be considered how close the actual transmission energy level of the MS38 is compared to the maximum transmission energy level. Similarly, additional mobile station conditions take into account the quality of the FL, such AS erasures on the current AS. Yet another mobile station condition accounts for lack of inner loop. The lack of an inner loop is the difference between the target ANR and the SNR delivered by the inner loop PC. Another embodiment combines mobile station status with the type of transmission.

The number of retries allowed can be adjusted based on statistics regarding a fault cutting call or a problematic call. For example, there may be an average number of retries beyond which most problematic calls cannot be recovered. Additional considerations include RL input, the location of the MS38, and/or the time of day, or date. In the latter case, the traffic distribution of certain mobile stations affects the number of mobile stations that need fast call recovery.

Continuing with fig. 6A, the BSC26 determines the current AS of the MS at step 110. The BSC26 then starts the HDM timer at step 112 and sends the HDM at step 114. At this point, the system 10 wishes to care that you move away from the default path with the communication link can be used by any mobile station in the system 10 and so use can be optimized. While the MS38 utilizes a given default channel, that channel cannot be used by another mobile station. Each base station in the NS is instructed to start transmission on an additional or new channel in parallel with the transmission on the default channel. This is the initiation of the switching situation.

If the BSC26 receives a message from the MS38 indicating that the handoff is complete at decision diamond 118, the process continues to step 120 where the communication link between the MS38 and the members of the NS on the default tunnel is stopped. The process then continues to step 124. If, instead, the handover complete message is not received, the BSC26 checks whether the HDM timer has expired at decision diamond 122. If the HDM timer has expired, the appropriate default channel is terminated for transmission to the MS38, call resumption is cancelled at step 124, and use of the default channel and the new channel is terminated at step 125. Normal operation resumes at step 126. If the timer has not expired at decision diamond 122, the process returns to waiting for a handoff completion message from the MS38 at decision diamond 118.

Fig. 6B refines a portion of the method 100 where step 110 is shown as initializing a timer at step 130. The BSC26 checks the PSMM from the MS38 at decision diamond 132. If the PSMM has been received, the process continues to step 134 to set the AS for the neighbor stations included in the PSMM. If a PSMM has not been received, the process continues to decision diamond 138 to determine whether the timer (initialized at step 130) has expired. If the timer has expired, the process continues to decision diamond 144. If the timer has not expired, the process continues to decision diamond 144. If the timer has not expired, the process returns to decision diamond 132.

After setting AS in step 134, if RL is enhanced AS determined at decision diamond 140, the BSC26 determines whether any neighbors not included in the PSMM have acquired the MS38 signal at decision diamond 140. These neighbors are called heard neighbors (Hearing neighbor-HN) and are added to the AS at step 142. The process then returns to step 112 of fig. 6A.

If the timer runs out and the PSMM is not received, the BSC26 determines whether any neighbor cells have acquired an RLMS 38 signal, HN, at decision diamond 144. In this case, at step 146, the AS is set to include these HNs. If at decision diamond 144, the HN is not found, then call recovery is terminated at step 148 and the call is terminated.

At decision diamond 110, the method determines whether the transmitter of the MS38 is off. If the transmitter is off, the BSC26 commands the MS38 to turn the transmitter on at step 110.

A mobile station call recovery method 200 of one embodiment is shown in fig. 7. At step 202, the MS38 communicates with a base station in the AS (0). This identifies the current AS. If a call recovery trigger occurs at decision diamond 204, the process continues to decision diamond 208. The call recovery trigger may be the trigger discussed above or another indication that the MS38 needs a rescue operation, i.e., the MS38 may have lost the FL communication link. If no trigger is present, normal operation begins at step 206. Decision diamond 208 determines whether the transmitter of the MS38 is enabled. If the transmitter is enabled, the process continues to step 214, whereas if not, the MS38 checks for a trigger condition at decision diamond 210. If a trigger condition exists indicating that the MS38 has disabled the transmitter, appropriate action is taken at step 212 and the process continues to step 214. If no trigger indicates that the transmitter is disabled, the process continues to step 214. A wait timer is set at step 214. The wait timer is checked at decision diamond 216 and upon expiration a recovery timer is started at step 218. If the wait timer has not expired, the process continues by determining whether the MS38 is back in normal operation at decision diamond 222. Normal operation continues from step 206, otherwise the process returns to the wait state, waiting for the timer to run out.

Continuing with FIG. 7, from step 218, if the transmitter of the MS38 is disabled, the transmitter is enabled at step 220. The MS38 transmits a predetermined preamble duration period Y. The preamble provides information about the MS38 transmissions rather than the actual data or symbols. The MS38 transmits the PSMM information in step 228. At decision diamond 228, the MS38 waits for a predetermined time period Y, after which the AS is updated, if an HDM is received or if an acknowledgment is received acknowledging the PSMM. If no HDM or PSMM acknowledgement is received at decision diamond 230, the process continues to decision diamond 232 to check if the PSMM is not being sent more than the maximum allowed number of times. If the PSMM can be retransmitted, i.e., the maximum value is not reached, the process returns to step 228 and the PSMM is retransmitted. If, however, the maximum value is received, the process continues to step 236 and call recovery terminates.

In an alternative method of call recovery, the BSC26 notifies the potentially problematic BS132 of all call recovery capable neighbors. The BSC instructs the MS38 to turn on the transmitter portion of the MS38 and instructs the base stations in the set of neighbor stations to listen to the MS 38. After detecting or obtaining the signal from the MS38, each base station in the set of neighbor stations sends a report. The report is received from a subset of base stations, wherein the subset can include all or a portion of the base stations in the set of neighbor stations. The BSC26 informs the MS38 of the default tunnel for each base station in the subset. The base stations of the subset then initiate communication with the MS38 using the appropriate default channel.

In yet another embodiment, the subset of the set of neighbor stations is determined based on the most recently transmitted PSMM. There is a problem in that the most recently transmitted PSMM may not be correctly received, in which case the PSMM used to identify the subset is incorrect. For example, when the most recently received PSMM identifies BS132 and BS 336, MS38 transmits a PSMM identifying BS132 and BS234, which is not received and call recovery is hindered. The BSC26 establishes a reverse network connection with the BS 336 and the BS 336 begins transmitting to the MS38 on the implied path. Unfortunately, the MS38 considers the call to resume as establishing communication with the BS234 and preparing to receive on a different default channel. More than the transmission from BS 336 is wasted and actually causes more noise in system 10.

When call resumption is initiated by the MS38, a timer delay such as initiation may be used until after all call resumption has occurred. The time period of the timer can be set by the BSC 26. After the timer has expired, the MS38 sends a preamble on the RL pilot channel. The preamble includes a call recovery message. In one embodiment, the preamble is a constant that can be set by the BSC 26. In a further embodiment, the preamble is a variable length signal determined by the system operator. After transmitting the preamble, the MS38 sends a message regarding the FL change. The message can be a PSMM. The message may be sent several times to ensure receipt by BS 234.

The combination of the above methods provides various advantages of call recovery. In one embodiment, the call recovery method is based on the radio transmission environment of the source cell base station. When the number of neighbors that can recover from a call is small (e.g., 2), the BSC26 will instruct all neighbors to transmit on their respective default tunnels. The AS is updated and the transmitter of the MS38 is enabled without delay. For a large set of neighbor stations capable of call recovery, the BSC26 will instruct the neighbor stations to listen for signals from the MS 38. Those overhearing neighbors are instructed to use the default channel after a delay caused by waiting for neighbors to report signals they receive from the MS 38. Similarly, if a PSMM is received from the MS38 within a predetermined time period, those base stations identified by the PSMM are instructed to use the default channel. Note that when the FL is functioning correctly, (which may be determined by a fixed number of consecutive good frames) a PC command sent over the PC subchannel is considered valid.

Fig. 8 illustrates the construction of the wireless communication system 10 of fig. 1 in a hierarchical structure. Construction 700 includes three layers: a signal layer 702; a link layer 704; and a physical layer 706. The signaling layer 702 provides upper layer signaling 708, data services 710; and voice services 712. The signal layer 702 provides voice, packet data, simple line data, and synchronous voice and packet data services. On this layer protocols and services are provided corresponding to the two layers below. The link layer 704 is subdivided into a Link Access Control (LAC) sublayer 714 and a Medium Access Control (MAC) sublayer 716. The protocols of the application and signaling layer 712 use the services provided by the LAC sublayer 714. The link layer 704 serves as an interface between the upper layer protocols of the signaling layer 702 and the application and physical layers 706. The MAC sublayer 716 also includes a multiplexing and quality of service (QOS) submit block 722. Link layer 704 connects signal layer 702 to physical layer 706. The physical layer 706 constitutes a physical channel 724 for transmission.

Fig. 9 provides a timing diagram of the operation of system 10 of fig. 1 in accordance with one embodiment. Reference is made to the methods of fig. 6A, 6B and 7. The horizontal axis represents time, while the vertical axis represents the individual channels used for transmission. A source cell base station BS132 is provided in the middle, where signals are transmitted to the MS38 via a communication channel. Two channels are shown for the MS 38: a transmission channel Tx; and a receive channel Rx. For the binding channel: rx1 and Rx2 illustrate two cases. Also shown is a neighbor base station, which is the target base station BS 234. The default channel and the new channel are shown. The new channel is the channel used for communication with the MS38 after the handover. The process begins with the MS38 receiving a transmission from a first AS identified AS (0). The MS38 simultaneously transmits on the communication channel to the source cell BS 132. At time t1, a call recovery trigger occurs. Both the MS38 and the BS132 recognize the trigger. Note that the trigger may be a common event, such as a persistent PC request from MS38 to BS132 to increase the transmit power of the FL, or may be separate events for MS38 and BS 132. Also, the MS38 and BS132 may not recognize the trigger at the same time. MS38 discerns the trigger before BS132 during the FL failure.

When a trigger is identified at time t1, the BSC26 initiates a default channel transmission from the neighbor BS 234. At time t2, the BS234 begins transmitting to the MS38 on the default tunnel. This transmission is in parallel with the same transmission from BS 132. In the presence of the trigger, the MS38 disables the transmitter for a predetermined wait time. At time t3, the wait time expires and the MS38 transmits a preamble for a period Y. At the same time, the AS of the MS38 changes from AS (0) to AS (1). The base stations identified in AS (1) are all the base stations listed in the most recent PSMM. In another embodiment, AS (1) can be all neighbors of BS132 and BS132 itself.

At time t4, the preamble ends and the MS 28 begins transmitting the current PSMM. In response to the receipt of the PSMM at time t5, BS132 and BS234 transmit HDMs at time t 6. At time t8, the HDM signal the AS to change to AS (2). Note that the next PSMM is sent out at time t7, where PSMMs are sent out periodically or continuously to identify signals received at MS 38.

At time t8, the BS234 begins transmitting to the MS38 on the new tunnel. The MS38 sends the HCM, which triggers the end of the transmission to the MS38 on the default tunnel at time t 9. In one embodiment, the HCM is transmitted periodically or continuously until its correct reception is acknowledged by the base station. In the scenario shown in fig. 9, call resumption starts at time t2 and ends at time t 9. At time t9, the handover is complete and the BS234 is the current source cell base station to the MS 38.

Additional cases are shown for the receive path Rx 2. Here AS (0) remains active until time t 5. After time t5, the MS38 continues to receive information from AS (0) for a predetermined time period X, and thereafter changes to AS (1). This allows for additional time on the part of the base station to determine the transmission to MS38 for call recovery on a subset of the call recovery capable neighbors of BS 132. At time t8, there is a subsequent change in response HDM from AS (1) to AS (2). This case corresponds to a method in which only those neighbors that can obtain signals from the MS38 are commanded to transmit over the respective default channels.

Once call recovery is complete and handoff has been achieved, the MS38 must determine the initial transmit power level. According to one embodiment. The system 10 of fig. 1 uses closed loop power control to adjust the transmit power level. Alternative embodiments may use additional open loop power control methods. Open loop involves transmitter (whether mobile or base station) -controlled operation, where the receiver is not directly involved. For example, certain reverse link open loop power controls require the mobile station to adjust the reverse link transmit power based on the power level of the signal received from the base station over the forward link. Closed loop power control extends open loop operation so that the receiver actively participates in making power adjustment decisions. For RL closed loop power control, for example, the base station compares the power level of a signal received from a given mobile station to a threshold. The base station then commands the mobile station to increase or decrease the reverse link transmit power based on the comparison. Instead, the mobile station monitors the power level of the signal received on the FL and provides feedback to the base station regarding the quality of the FL. Closed loop operation is used to compensate for power fluctuations associated with the attenuation of a given link, such as Raleigh attenuation.

The MS38 begins transmitting at the initial power level just prior to running the wait time and establishing power control. The RL transmit power level may be restored from just before disabling the transmitter of MS 38. The power level may remain at this initial level until closed loop power control resumes.

In an alternative embodiment, the power level is initialized to a final level before disabling the transmitter and then gradually increased at a predetermined rate until functional control is restored. The rate of increase is typically set by BS132 and/or BS234 and may be a fixed value or variable. This increase continues until RL closed loop power control resumes.

Further embodiments initiate call recovery with open loop control based on the total received power in the frequency band. This procedure IS similar to the access procedure defined in IS-95 and IS-2000. This enables corrections to be made for a plurality of forward link base stations visible to the MS 38. Open-loop control continues until closed-loop control resumes. Fig. 10 shows the power regulation according to this embodiment. The horizontal axis represents time and the vertical axis represents transmission power level. At a first time t1, the transmit power is an initial power level. At time t2 after the first time period, the transmit power is increased by a predetermined incremental value. The increment value may be a fixed value, may be a variable, and may increase or decrease over time. In one embodiment, the delta value is adaptive and responsive to conditions of system 10, wherein the delta value may increase or decrease from one time period to the next. Finally, a predetermined maximum transmission power level may be reached after a predetermined number of time periods. The transmit power then reaches the cap and waits for the closed loop power control to resume.

In yet another embodiment, the initial transmit power is based on the received pilot signal quality. The desired AS signal quality is measured with pilot Ec/Io or pilot Ec. In open loop control, the transmission power generally has the following relationship:

T_x＝(-R_x)+k (1)

where k is a constant, Tx is the RL transmit energy, and Rx is the FL receive energy. For closed loop power control methods, the transmit power typically has the following relationship:

T_x＝(-R_x)+k+y(t) (2)

and y (t) is a correction variable accumulated based on all active power control commands received until time t. The term (k + y (t)) is referred to as β. In another way, there is the following relationship:

T_x+R_x＝k+y(t) (3)

the previously transmitted beta is applied to the new transmission when the initial transmission power is determined. The new transmit power level is then calculated as follows:

T_x(t)＝(-R_x(t)+T_x(0)+R_x(0)， (4)

wherein T is_x(0) Is the transmit energy, R, before call recovery_x(0) Is the received energy before call recovery. In this way, the transmission power is adjusted by the ratio of the transmission power level to the reception power level.

In fig. 11 there is shown a wireless device MS38 operating in the system 10 of fig. 1 as a mobile telephone or Personal Digital Assistant (PDA). The MS38 includes an antenna 300 for transmitting and receiving. The antenna 300 is connected to a duplexer 302 for isolating the receiver path from the transmitter path. The duplexer is connected to a receiver line 308 forming a receiver path and to an amplifier 304 and a transmit line 306 forming a transmit path. The amplifier 304 is also connected to a power conditioning unit 310 which provides control of the amplifier 304. Amplifier 304 receives a transmit signal from transmit line 306.

The signal received via the antenna 300 is supplied to a power control unit 314 which implements a closed loop power control scheme. The power control unit 314 is connected to a communication bus 318. The communication bus 318 provides a common connection between the various modules in the MS 38. The communication bus 318 is also connected to a memory 322 and a talk recovery adjustment unit 316. Memory 322 stores computer readable instructions that may be used for various operations and functions of MS 38. Processor 320 executes instructions stored in memory 322. For normal operating conditions, the power control unit generates a PC signal that is sent to power regulator 310 via multiplexer 312. The power conditioning 310 then passes the PC signal as an amplification stage to the amplifier 304.

The MS38 may disable the transmitter when call recovery occurs. When the transmitter is re-enabled, a handoff completion signal is provided to the talk recovery adjustment unit 316. The switch completion signal instructs the call resumption adjustment unit 316 to generate a predetermined PC signal. The PC signal so generated can implement any of the schemes of initial RL transmit power generation described above. Or alternative methods may be implemented. A switch complete signal is also provided to control the multiplexer 312. After the call is restored, the PC signal generated by the restoration adjustment unit 316 goes to the power adjustment unit. At the same time, closed loop power control is started. Once closed loop power control is complete and resumes, the switching signal is removed and the multiplexer 312 selects the PC signal generated by the power control unit 314 to provide the power adjustment 310. The operation of the call recovery adjustment unit 316 can be performed by the microprocessor 320 operating on software instructions or implemented in hardware for efficient and reliable operation.

In one embodiment, the specific operation of the MS38 or BS132 is considered a special event. Special events include various conditions and processes that cause false triggers to occur. In other words, a special event may create a situation where a call recovery trigger occurs, but the call is not degraded. One special event is a mobile station locator search. The MS38 is instructed to search on an additional frequency for Global Positioning System (GPS) signals. The GPS provides part of the location of the MS38 or the location of the MS 38. The mobile position locator search is performed periodically or non-periodically. Typically, the MS38 has a priori information about the timing of such a search. Additional events may include candidate frequency searches in preparation for inter-frequency hard handoffs, where the mobile station tunes to additional frequencies to search for signals from the base station on different frequencies.

Further events may include actions taken by the MS38 during which the trigger is ignored. Among these types of events, the MS38 notifies the BS132 of the source cell of the special event. In one embodiment, the special event is a candidate frequency search in which the MS38 tunes to a different frequency to look for signals from neighboring base stations on that frequency. This allows for transitions between different frequency coverage areas, such as switching between personal communications system PCS) frequencies and cellular (cellular) frequencies. Upon the occurrence of such a special event of mobile station startup, the MS38 notifies the source cell BS132 to ignore the trigger for the MS38 for a specific period of time or until the next notification.

In one embodiment, to avoid such false triggering during a special event, the source cell base station, such as BS132, allows the event and informs the MS38 of the timing of the event, including at least when the event begins and the length of time allocated for that time. The MS38 and base stations in its AS prohibit call recovery triggers from initiating call recovery during special events.

In an alternative embodiment, the MS38 notifies the BS132 of an upcoming special event or group of special events. In response to the notification, BS132 may acknowledge the special event, disable the event, or reschedule the event. And this provides the MS38 and the base stations in its AS with sufficient information to disable call recovery triggers during special events.

Accordingly, presented herein are novel and improved methods of maintaining communications in a wireless communication system. When the link between the mobile station and the corresponding source cell base station experiences trouble, the mobile station and infrastructure rearrange the potential rescue base stations. The source cell base station contacts all call recovery capable neighbor stations as potential rescuers. The call recovery capable neighbor station has a predetermined default channel adapted to soft handoff of the mobile station. During the initial part of the handover, the default channel is only used temporarily. Each rescue base station is instructed to use the acquiescent channel for rescue transmissions. The rescue transmission is considered a call recovery operation. The mobile station establishes a soft handoff with the rescue base station, wherein the FL uses a default channel. The rescue base station then initiates transmission on the additional channel. Once the handoff is complete, the rescue base station ceases using the default channel in transmitting with the mobile station. In one embodiment, the source cell base station provides the mobile station with a list of neighbor stations that can resume a call during transmission and before a communication link problem occurs as additional information. In this way, the mobile station has sufficient information to perform a handoff for the case where the FL fails before receiving the handoff information.

In yet another embodiment, multiple default channels are assigned to the neighbor BS 234. Using multiple default or rescue channels increases the call recovery energy of the system 10. Each of the neighboring stations can then participate in call recovery for a plurality of mobile stations, such as MS 38. In operation, prior to call resumption, source cell BS132 provides MS38 with identifiers corresponding to the plurality of tunnels associated with BS 234. Both the MS38 and the BS234 store deterministic functions, such as hash functions, that map identifiers to specific channels. A hash function, in particular a pseudo-random process, is used. Further, an electronic serial number is assigned to the MS 38. The electronic serial number may be stored in the MS38 or provided to the MS38 upon call resumption. Upon call resumption, source cell BS132 provides the electronic serial number of MS38 to BS 234. Both the BS234 and the MS38 apply a predetermined function to calculate the appropriate default channel.

The hash function on the data structure enables a keyword to be identified in a group of words using only one probe message to the database. The hash function maps its arguments to a predetermined type of result. The hash function is deterministic and stateless. That is, the return value depends only on the arguments, and equal arguments yield equal results. It is important that the hash function minimize collisions, where a collision is defined as two different arguments that hash to the same value. It is also important that the distribution of hash values is uniform; that is, the probability that the hash function returns any particular value of a predetermined type is roughly the same as the probability that it returns any other value. In alternative embodiments, other forms of cryptographic functions can be utilized for identifying multiple default channels upon call resumption.

For example, the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components such as registers and FIFO, a processor executing a set of firmware instructions, any conventional programmable software module and a processor, and any combination thereof designed to perform the functions described herein. The processor is preferably a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A software module may reside in RAM memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. The processor can be contained in an ASIC (not shown). The ASIC can be placed in a telephone (not shown). Alternatively, the processor can be included in the phone. A processor can be implemented as a combination of a DSP and a microprocessor, or as a combination of two microprocessors in conjunction with a DSP core.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (13)

1. In a wireless communication system having a plurality of base stations, wherein each of the plurality of base stations has a neighbor set including neighbor base stations, each neighbor base station having a default channel, a method comprising:

transmitting default channel information to the mobile station;

detecting the occurrence of a call recovery trigger; and

when the occurrence of the call recovery trigger is detected, all base stations in the neighbor set are instructed to transmit in respective default channels.

2. The method of claim 1, wherein the call recovery trigger occurs when a forward link signal quality is below a predetermined threshold.

3. The method of claim 1, wherein the call recovery trigger occurs when:

acknowledging the first message from the mobile station; and

the first message is received from the mobile station a predetermined number of times after acknowledging the first message.

4. The method of claim 1, further comprising:

a soft handoff is established with the set of neighbor stations, wherein each set of neighbor stations transmits on an additional channel.

5. The method of claim 4, further comprising:

a handover complete message is received from the mobile station.

6. The method of claim 1, further comprising:

an active set of the mobile station is determined.

7. The method of claim 6, further comprising:

receiving a pilot signal measurement message from a mobile station, the pilot signal measurement message comprising signal strength measurements for a set of base stations; and

the set of base stations is assigned to the active set.

8. The method of claim 6, wherein at least one base station in the set of neighbor stations obtains signals from the mobile station, the method further comprising:

at least one base station is assigned to the active set.

9. The method of claim 1, wherein the first neighbor station in the set of neighbor stations has a plurality of default channels.

10. The method of claim 9, wherein the mobile station determines one of the plurality of default channels for call recovery using a deterministic function.

11. The method of claim 10, wherein the deterministic function is a hash function.

12. The method of claim 10, wherein the first identifier is associated with the mobile station, the method further comprising:

transmitting the first identifier to the first neighboring station; and

the first neighboring station is instructed to determine one of a plurality of default channels for call resumption using the determining function and the first identifier.

13. A wireless apparatus, comprising:

an antenna;

a processor connected to the antenna;

a transmission line connected to the antenna and the processor;

a receiving circuit connected to the antenna and the processor;

the processor is configured to:

receiving a neighbor list of base stations, the list including a default channel designation for each neighbor;

identifying a call recovery trigger and in response disabling the transmission line; and

a handover is established with at least one neighboring station.