Abstract:
In a wireless communication system, a method includes monitoring an error condition on an active communication channel, establishing a simultaneous communication channel, and searching the simultaneous communications channel while continuing to monitor the active communication channel.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to call recovery in a mobile communications system.  
         [0002]     Handoff is a general term describing the function of transferring a communication signal for a mobile subscriber from one base station transceiver to another base station transceiver as a subscriber negotiates the wireless territory. There are two general methods of conducting handoff in a wireless communication system, soft and hard handoff. Quality may suffer using either handoff method as the mobile subscriber switches between base stations for individual cells.  
         [0003]     Hard handoff typically occurs near the boundaries between cells. Through continuous measurements of received signal power from a mobile subscriber, the base station where the subscriber has established communication determines if the power is reduced below a nominal value near the cell boundary. The hard handoff to a candidate cell&#39;s base station occurs instantaneously without disrupting the call in progress. At handoff, the received power at the candidate cell&#39;s base station is much greater than required to ensure an ideal handoff. The difference between the current cell&#39;s power and the candidate cell&#39;s power; however, significantly reduces system capacity by interfering with other users.  
         [0004]     A soft handoff occurs throughout a given range of distances from the current and candidate cells&#39; base stations. In the soft handoff method, a mobile subscriber is connected to both base stations as the subscriber travels near the common cell boundary. The decision to switch is made depending upon the reception of the mobile subscriber&#39;s pilot signal. A central switching center, often referred to as a mobile switching center, decides at what point one of the base stations should be dropped. Shared communication is performed for a finite period of time, during which time transmission from the current and candidate cells is required.  
         [0005]     In a typical wireless communication system, the plurality of signals are transmitted within the same frequency band. This not only applies to mobile subscribers in the same cell, but also to those in all other cells. Since the same frequencies are used, the transmitted power levels from the mobile subscriber and the base station must be monitored closely. If power control is not strictly adhered to, the overall transmission interference and the total number of usable channels is adversely affected. Therefore, the number of signals which can be successfully transmitted and received is associated with the total power of all users.  
         [0006]     Both soft and hard handoff methods have shortcomings. Hard handoff can potentially suffer from high drop-out rates. The soft handoff method requires a duplication of transmission resources from the current base station and a base station from at least one candidate cell. The mobile subscriber must establish two concurrent communication links, thereby requiring twice the transmission power which would otherwise be required. The increase in transmission power wastes power, capacity of the air interface to carry transmissions, and contributes to total system interference while the handoff is taking place. The power or energy transmitted in a wireless system by each mobile subscriber should be kept at the minimum necessary to convey information and to minimize interference with the other users. Careful control of transmission power also contributes to extended use of portable devices relying on battery power. Furthermore, diversity combining of the duplicate signals at the central switching center is cumbersome unless the delay from both cell base stations is nearly identical.  
       SUMMARY OF THE INVENTION  
       [0007]     According to an exemplary embodiment of the present invention, a method may include performing operation for establishing a new communication channel if a current communication channel is judged to potentially drop.  
         [0008]     According to yet another exemplary embodiment of the present invention, a method may include monitoring an error condition on an active communication channel; establishing a simultaneous communication channel; and searching the simultaneous communications channel while continuing to monitor the active communication channel.  
         [0009]     According to yet another exemplary embodiment of the present invention may include initiating call recovery by supplying specific session information to a new channel. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     Exemplary embodiments of the present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limiting on the exemplary embodiments of the present invention and wherein:  
         [0011]      FIG. 1  illustrates a illustrates a general wireless communication system;  
         [0012]      FIG. 2  illustrates a block diagram of general components found in the wireless communication system of  FIG. 1 ;  
         [0013]      FIG. 3  illustrates a call recovery method, depicted in flowchart form, according to an exemplary embodiment of the present invention; and  
         [0014]      FIG. 4  illustrates a continuation of the call recovery method illustrated in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0015]     The exemplary embodiments of the present invention described herein are merely illustrative of the present invention. Therefore, the exemplary embodiments should not be considered as limiting of the present invention.  
         [0016]     First, a general wireless communication system will be described. Second, a method of providing call recovery according to exemplary embodiments of the present invention will be described.  
         [heading-0017]     Wireless Communication System  
         [0018]      FIG. 1  illustrates a wireless communication system  100  that uses multiple access techniques, such as code division multiple access (CDMA), wideband code division multiple access (WCDMA/cdma2000) for communicating between users of user stations (e.g., mobile telephones) and cell sites or base stations.  
         [0019]     In  FIG. 1 , a mobile station  102  communicates with a base station controller  104  by way of one or more base stations  106 A,  106 B, etc. Similarly, a fixed user station  108  communicates with the base station controller  104 , but by way of one or more predetermined and proximate base stations, such as the base stations  106 A and  106 B.  
         [0020]     The base station controller  104  is coupled to and typically includes interface and processing circuitry for providing system control to the base stations  106 A and  106 B. The base station controller  104  may also be coupled to and communicate with other base stations, and possibly even other base station controllers. The base station controller  104  is coupled to a mobile switching center  110  that in turn is coupled to a home location register  112 . During registration of each user station at the beginning of each call, the base station controller  104  and the mobile switching center  110  compare registration signals received from the user stations to data contained in the home location register  112 , as is known in the art. Handoffs may occur between the base station controller  104  and other base controllers, and even between the mobile switching center  110  and other mobile switching centers, as is known by those skilled in this technology.  
         [0021]     When the system  100  processes voice or data traffic calls, the base station controller  104  establishes, maintains, and terminates the wireless link with the mobile station  102  and the fixed station  108 , while the mobile switching center  110  establishes, maintains, and terminates communications with a public switched telephone network (PSTN). While the discussion below focuses on signals transmitted between the base station  106 A and the mobile station  102 , those skilled in this technology will recognize that the discussion equally applies to other base stations and to the fixed station  108 .  
         [0022]     Referring to  FIG. 2 , the mobile station  102  includes an antenna  202  that transmits signals to, and receives signals from the base station  106 A. A duplexer  203  provides a forward link channel or signal from the base station  106 A to a mobile receiver system  204 . The receiver system  204  down-converts, demodulates, and decodes the received signal. The receiver system  204  then provides a predetermined parameter or set of parameters to a quality measurement circuit  206 . Examples of parameters might include measured signal-to-noise ratio (SNR), measured received power, or decoder parameters such as symbol error rate, Yamamoto metric, or parity bit check indication. A processor  207 , may be included for use with exemplary embodiments of the present invention described herein. The processor  207  may be physically connected to one or more components of the mobile station  102 .  
         [0023]     The quality measurement circuit  206  receives the parameters from the receiver system  204  and determines a quality measurement signal or power level of the received signal. Based on these measurements, the quality measurement circuit  206  produces a power level signal. This process is generally understood by those skilled in the art.  
         [0024]     A power control processor  208  receives the power level signal from the quality measurement circuit  206 , compares the signal to a threshold, and produces a power control message based on the comparison. Each power control message can indicate a change in power for the forward link signal. Alternatively, the power control processor  208  produces power control messages representing the absolute power of the received forward link signal, as is known in the art. The power control processor  208  may produce several power control messages in response to several power level signals per frame. While the quality measurement circuit  206  and power control processor  208  are generally described herein as separate components, such components can be monolithically integrated, or the operations performed by such components can be performed by a single microprocessor.  
         [0025]     A mobile transmission system  210  encodes, modulates, amplifies, and up converts the power control messages, via the duplexer  203  and the antenna  202 . In the illustrated exemplary embodiment, the mobile transmission system  210  provides the power control message in a predetermined location of an outgoing reverse link frame.  
         [0026]     The mobile transmission system  210  also receives reverse link traffic data, such as voice or data, from the user of the mobile station  102 . The mobile transmission system  210  requests a particular service (including power/rate) from the base station  106 A based on the traffic data to be transmitted. In particular, the mobile transmission system  210  requests bandwidth allocation appropriate for the particular service. The base station  106 A then schedules or allocates bandwidth (power/rate) resources based on requests from the mobile station  102  and other users to optimize such resource allocation, given power constraints of the system. Thus, effectively managing transmission power in the system will permit more effective bandwidth use.  
         [0027]     The base station  106 A includes a receiving antenna  230  that receives the reverse link frames from the mobile station  102 . A receiver system  232  of the base station  106 A down converts, amplifies, demodulates, and decodes the reverse link traffic. A backhaul transceiver  233  receives and forwards to the base station controller  104  reverse link traffic. The receiver system  232  also separates the power control messages from each reverse link traffic frame and provides the power control messages to a power control processor  234 .  
         [0028]     The power control processor  234  monitors the power control messages and produces a forward link transmitter power signal to a forward link transmitter system  236 . The forward link transmitter system  236 , in response thereto, either increases, maintains, or decreases the power of the forward link signal. The forward link signal is then transmitted via a transmitting antenna  238 . Additionally, the power control processor  234  analyzes the quality of the reverse link signal from the mobile station  102  and provides appropriate feedback control messages to the forward link transmitter system  236 . The forward link transmitter system  236 , in response thereto, transmits the feedback control messages via the transmitting antenna  238  over the forward link channel to the mobile station  102 . The transmitter system  236  also receives forward link traffic data from the base station controller  104  via the backhaul transceiver  233 . The forward link transmitter system  236  encodes, modulates, and transmits via the antenna  238  the forward link traffic data.  
         [0029]     Both the forward link signal and the reverse link signal include various channels. Generally, the forward link signal may include the following: a pilot channel, a paging channel, a sync channel, a fundamental channel, a supplementary channel, a forward dedicated common control channel and a quick paging channel. The reverse link signal may generally include the following: a pilot channel, an access channel, a dedicated control channel, a fundamental channel, a reverse dedicated control channel and a supplementary channel. Other channels on the forward and reverse signals may also be present depending on the operation of the wireless communication system  100 . The definitions and operational characteristics of the stated channels have not been discussed in detail herein, as such definitions and characteristics are understood by those having ordinary skill in the relevant arts.  
         [0030]     Unless described otherwise herein, the construction and operation of the various blocks and elements shown in  FIGS. 1 and 2  and the other figures are of conventional design and operation. Thus, such blocks or elements need not be described in further detail because they will be understood by those skilled in the relevant art. Any additional description is omitted for brevity and to avoid obscuring the detailed description of exemplary embodiments of the present invention. Any modifications necessary to the blocks of the wireless communication system  100  illustrated in  FIGS. 1 and 2 , or the other systems shown therein, can be readily made by one skilled in the relevant arts.  
         [heading-0031]     Call Recovery Method  
         [0032]     The call recovery method according to an exemplary embodiment of the present invention may be implemented in the wireless communication system  100  illustrated in  FIGS. 1 and 2 . For example, the call recovery method may be software coded or hardware imbedded into the processor  207  of mobile station  102 . However, it should be understood that this is only one example of an implementation of an exemplary embodiment of the present invention.  
         [0033]      FIGS. 3-4  illustrates a call recovery method, in flowchart form, according to an exemplary embodiment of the present invention. Initially, a transmit call recovery timer is sent to the mobile station  102  by way of the base station controller  104  and/or the mobile switching center  110  (S 400 ). The transmit call recovery timer may also originate from any one of the base stations  106 A and  106 B.  
         [0034]     In one exemplary embodiment of the present invention, the call recovery timer is a 3-bit value that is representative of a predetermined number of seconds. For example, if the 3-bit number is ‘000’, the call recovery timer would have a value of 1.5 seconds. Generally, the call recovery timer is one of: ‘000’=1.5 sec., ‘001’=2 sec., . . . ‘111’=5 sec. As those of ordinary skill in the art recognize, if the call recovery timer is set to ‘111’, then the call recovery timer will have no affect on the operation of the wireless system  100 . In particular, most mobile communication devices in operation at the time of the present invention have a fade timer that functions to drop a call if particular signal weakness is detected after the expiration of a 5 second period.  
         [0035]     It is assumed that the mobile station  102 , in receipt of the call recovery timer, is currently on a voice call (S 302 ). As it appreciated by those of ordinary skill in the relevant arts, the mobile station  102  monitors a received signal for errors, such as bad frames (S 304 ). Once a bad frame is detected, the mobile station waits for 11 more frames for a total of 240 ms to determine if the signal has been recovered. If not the mobile station fade timer, normally having a duration of 5 seconds, is then initiated (S 306 ).  
         [0036]     After approximately 240 ms, or 12 bad frames received by the mobile station  102 , the mobile station  102  initiates the call recovery timer (S 308 -S 310 ). When the call recovery timer is active, the mobile station  102  will monitor two channels simultaneously. In particular, the mobile station  102  will monitor its current channel for possible receipt of good frames (S 312 ). If good frames are received on the current channel, the mobile station  102  will maintain the current channel as active, and reset the fade and call recovery timers (S 314 ).  
         [0037]     The process of monitoring a second channel may be generally considered the preliminary processing that occurs to establish call recovery. The process and processing of call recovery will be further described in the following paragraphs.  
         [0038]      FIG. 4  illustrates a continuation of the flowchart illustrated in  FIG. 3 . As described, the mobile station  102  monitors two channels simultaneously while the call recovery timer is active. During repetitive reception of bad frames on the current channel and while the call recovery timer is active, the mobile station  102  initiates a search of a pilot channel on the active carrier (S 400 ). Once a pilot channel is found, the mobile station  102  decodes the sync channel (S 402 ). The mobile station  102  then decodes the broadcast common channel and/or the paging channel (S 404 ). During this process, the call recovery timer is still active.  
         [0039]     After completing the decoding process described in S 402 -S 404 , the mobile station  102  is in possession of sufficient information to establish a voice session over the reverse access channel. In particular, the mobile station  102  is ready to receive and send voice information over a new channel.  
         [0040]     The mobile station  102  continues to monitor its current channel for good frames until the call recovery timer expires (S 408 ). If two or more consecutive good frames are received before the call recovery timer expires, the current channel is maintained. However, if the call recover timer expires before two or more consecutive good frames are received, call recovery is initiated over the subsequently decoded reverse access channel.  
         [0041]     Namely, to complete call recovery, the mobile station  102  sends a origination message containing information relating to the call being recovered via the subsequently decoded reverse access channel. The origination message may include the system ID, network ID and packet zone ID of the of the session lost on the original channel. Moreover, the origination message may include an indication that the call recovery is being used. In response to the origination message, call recovery is established once the mobile station  102  receives a channel assignment message from one of the base stations  106 A and  106 B, the base station controller  104  and the mobile switching center  110 . In other words, one of the base stations  106 A and  106 B, the base station controller  104  and the mobile switching center  110  retrieves the relevant call data associated with the origination message so that the information of the session lost on the original channel may be used on the new channel. At this time, traffic over the new channel may commence, where the traffic is that which was on the original channel that experienced bad frames. Therefore, according to an exemplary embodiment of the present invention, a call that would normally be dropped is recovered and continues to commence over the new channel.  
         [0042]     The exemplary embodiments of the present invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the exemplary embodiments of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.