Abstract:
The present invention a system and method are provided for performing an inter-frequency search with reduced loss of link frames in a CDMA system. The CDMA system includes a base station ( 20 ) and a mobile station ( 50 ). The mobile station ( 50 ) has a searcher ( 164 ), which searches for pilot channels. The signal strengths of these pilot channels are then reported to the base station ( 20 ). This searching results in erased portions of a data frame ( 238 ). After the signal strengths are reported to the base station ( 20 ), the mobile station ( 50 ) informs the base station ( 20 ) of the parameters related to the search. These parameters may include the frame of the search, the start position of the search, and the length of the search. The mobile station ( 50 ) and the base station ( 20 ) then replaces the erased portions of the frame with corrective data such as soft zeros.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     The present application claims priority from Provisional Patent application No. 60/092,957, filed on Jul. 13, 1998, herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to spread spectrum communication systems that utilize an inter-frequency search. More particularly, a technique for performing an inter-frequency search with reduced or eliminated loss of link frames is described. 
     Wireless communication systems have grown dramatically in popularity in recent years. In typical wireless communication systems, mobile stations (e.g., a cellular telephone) communicate with other mobile stations via base stations. To date, a variety of cellular networks have been implemented, and one of the increasingly popular types of networks is referred to as a code division multiple access (CDMA) system. 
       FIG. 1  provides an overview of a CDMA system. In this system, mobile switching center  10  provides simultaneous communications among multiple base stations  20  while simultaneously routing calls from one or more base station  20  to public switched telephone network (PSTN)  30 . PSTN  30  communicates with, for example, telephone  40 . The mobile switching center&#39;s simultaneous routing makes handoffs between base station  20  and other base stations more reliable. CDMA base stations use one or more CDMA radio channels to provide both control and voice functionality. Base station  20  converts the radio channel to a signal that is transferred to and from mobile switching center  10 . Base station  20  can also communicate simultaneously among different sections in a cell, enhancing handoffs. Base station  20  communicates with, for example, mobile stations  50 ,  52  and  54 . Mobile stations  50 ,  52  and  54  can be, for example, mobile telephones and other types devices that provide wireless communication, such as PCSs, laptop computers or PDAs. 
     In a CDMA system, there are two types of handoffs, soft and hard. In a soft handoff, the mobile station is allowed to communicate with two or more cell sites enhancing the signal quality. These cell sites must share the same frequency. The CDMA mobile station measures the pilot channel signal strength from adjacent cells and transmits the measurements to the serving base station. The pilot channel provides a reference for coherent channel demodulation and is used as a reference signal level for handoff decisions. The mobile station must be synchronized to the pilot channel pseudo noise (PN) phase before accessing any other control channel. When an adjacent base station&#39;s pilot channel signal is strong enough, the mobile station moves the base station pilot into a candidate set and sends a pilot strength measurement message indicating the pilot signal energy. Now, both base stations (i.e., the current and the new one) send an extended handoff direction message, which requests the addition of the new base station pilot to the mobile stations active set of pilots. The new base station also starts transmitting a signal to the mobile station, while the mobile station tunes to the arriving signal from the new base station. This tuning occurs when the mobile station assigns a demodulating element (e.g., a finger on a rake receiver) to the arriving signal. Thus, during the soft handoff, the mobile station is communicating with both base stations simultaneously. During soft handoff, the mobile station utilizes time diversity to use signals from both base stations. The mobile station adds the new signal in a maximum ratio combiner before the decoding. 
     In contrast, during a hard handoff, the mobile station terminates the communication link with the current servicing base station before establishing the link with the new base station. This technique is similar to the technique used in time division multiple access (TDMA) and global system for mobile communication (GSM) systems. Hard handoffs occur when the mobile station&#39;s receiver is switching between a base station of one frequency and a base station of a different frequency. Usually, there is only one receiver in a mobile station, and that receiver can only receive data from one frequency at any given time. Therefore, a soft handoff is not possible when switching between base stations with different frequencies.  FIG. 2  illustrates base stations with different frequencies. For example, cells  100 ,  104 ,  108 ,  114 ,  116  and  118  use a first frequency and cells  102 ,  106 ,  110 ,  112  and  120  use a second frequency. Microcells  106  and  110  are used, for example, in shopping malls, office buildings and other indoor facilities. 
     Currently available hard handoff techniques can result in a dropped or lost telephone call. If the searcher in the cellular telephone mobile station uses, for example, a sequential sliding correlator (SSC) algorithm, and the search window size is 192 clips, then the total search time for a typical system with a 1× spreading note is as follows: 
             SF   =       ⁢     192   ×     C   L     ×       0.8     -   6       ⁡     [   S   ]                     =       ⁢     192   ×   768   ×     0.8       -   6     =       ⁢   0.18   ⁢           ⁢   seconds               
 
where: C L  is the average correlation length to achieve 0.99 depiction probability or approximately 20 frames. Because the frequency search message contains more than one base station offset in this example, the loss of service quality can be significant if the mobile station performs all searches in one period. If 20 frames are lost, then the telephone call will likely be so dropped.
 
     The IS-95 standard combines new digital CDMA and advanced mobile phone service (AMPS) functionality. IS-95A CDMA systems do not allow for inter-frequency searches because of the continuous nature of the CDMA waveforms. The inter-frequency search, also called mobile assisted hard handoff, was introduced in the IS-95B CDMA standard. The mobile assisted hard handoff can be performed without any timing restrictions (i.e., there is no restriction on the length of time used for this handoff). As a result, the mobile station is allowed to erase as many data frames (or portions of data frames) of the forward or reverse links as needed to perform the inter-frequency search. 
     The forward link is the data link from the base station to the mobile station, and the reverse link is the data link from the mobile station to the base station. 
     Currently, the CDMA mobile station performing the inter-frequency search will erase one or more of forward link frames and reverse link frames. 
     The IS-95B standard includes a gated-off transmission technique on the reverse link. A gated-off transmission is used when voice activity is low, and this allows voice data to be sent at different rates depending on the voice activity. For example, when voice activity drops to a low rate (e.g., ⅛ of the full rate), the transmission can be gated-off such that ⅛ of the normal amount of date is transmitted. This gated-off transmission on the reverse link allows the mobile station to perform the inter-frequency search during the period when the transmitter is gated-off. This technique minimizes the impact on the reverse link. 
     A mobile station in a third generation CDMA system, such as cdma2000, does not gate-off its transmitter during the transmission of lower rates. Thus, the erasure (i.e., loss) of both forward and reverse link frames is particularly true in this situation. Most of the third generation CDMA systems allow for a mixture of different classes of service, such as speech over data. In the cdma2000 system, this is achieved by allowing a simultaneous transmission on many physical channels. For example, fundamental channels and supplemental channels each carry a different payload. Additionally, since speech is carried on a fundamental channel and uses variable (i.e., speech activity driven) data rates, while a supplemental channel usually uses higher fixed (i.e., assigned) data rates, it is eminent that one or both of these channels will experience erasure during the inter-frequency search. Therefore, the likelihood of erasing links increases with, for example, the cdma2000 standard because the cdma2000 standard allows the mixing of different classes of services (e.g., data and voice services) on the fundamental and supplemental channels. It is desirable to have an inter-frequency search with minimal loss link frames in a CDMA system. 
     SUMMARY OF THE INVENTION 
     The present invention provides a technique for performing an inter-frequency search with reduced or eliminated loss of link frames in a CDMA system. In the preferred embodiment, the CDMA system includes a base station and a mobile station. The mobile station has a searcher, which measures the signal strength of the base station pilot channels. The signal strengths of these pilot channels are then reported to the base station. Usually, there is only one RF receiver in the mobile station. Thus, the mobile station cannot receive data on one frequency while searching for pilot channels on another frequency. As a result, this searching during a hard handoff produces erased portions of at least one data frame. 
     Before the search on the candidate frequency is performed, the mobile station informs the base station of the parameters related to the search. In the preferred embodiment, these parameters include the frame of the search, the start position of the search, and the length of the search. This allows the mobile station and/or the base station to replace the erased portions of the frame(s) with corrective data such as soft zeros. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  provides an overview of a CDMA system; 
         FIG. 2  illustrates cells of base stations with different frequencies; 
         FIG. 3  illustrates a device configuration for implementing the present invention; 
         FIG. 4  provides a process flowchart for the inter-frequency search of the present invention; 
         FIG. 5  provides a diagram illustrating one procedure used by the present invention; 
         FIG. 6  provides a process flowchart for one embodiment of the present invention; 
         FIG. 7  provides a process flowchart for an autonomous inter-frequency search; and 
         FIG. 8  provides a process flowchart for an inter-frequency search with full rate determination. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides several embodiments for providing an inter-frequency search procedure that significantly reduces additional power requirements while achieving comparable or better performance.  FIG. 3  illustrates a device configuration for implementing the present invention. A band limited signal is filtered by low pass filter  130  and then received by analog to digital converter  132  (A/D converter). A pseudo random sequence from base station  1  is received by multiplier  140  along with the signal exiting A/D converter  132 . The signal from multiplier  140  is sent to channel estimator  142  and multiplier  144 . Multiplier  144  also receives a Walsh code from base station  1 . The signals from channel estimator  142  and multiplier  144  are then sent to phase corrector  146 . In this example, phase corrector  146  is also a multiplier. A similar operation is performed for base station  2  with multiplier  150 , channel estimator  152 , multiplier  154  and phase corrector  156 . In this example, phase corrector  156  is not a multiplier. The sum of the signals from phase correctors  146  and  156  are then sent as soft data to viterbi decoder  158  in the preferred embodiment. In  FIG. 3 , the thin signal lines represent real data, and the thick signal lines represent complex data. 
     The signal from AND converter  132  is also sent to multiplier  160 . Multiplier  160  also receives a pseudo random sequence from another base station. The signal from multiplier  160  is sent to searcher  164  and then to controller  166 . Searcher  164  and controller  166  perform the inter-frequency search. 
       FIG. 4  provides a process flowchart for the inter-frequency search of the present invention. At step  200 , the mobile station receives a candidate frequency search control message (CFSCM). At step  201 , the mobile station informs the base station of the position and the length of the search on frequency f2. At step  202 , the mobile station tunes its receiver from frequency f1 to frequency f2. At step  204 , the mobile station performs the search of f2 Pilots. In this embodiment, the mobile station&#39;s receiver can only receive data on one frequency at one given time. Therefore, during the search time, no data is received from frequency f1. At step  206 , the mobile station returns to frequency f1 and continues normal operation. At step  208 , the mobile station reports the frequency f2 Pilots&#39; strength to the base station. During the time the mobile station is searching the frequency f2 (i.e., search time), the frequency f1 forward and reverse links are disrupted. This results in a negative impact on the quality of service of data transferred on frequency f1. The present invention allows the mobile station to perform the inter-frequency search while minimizing frame erasure on the forward and reverse links. This reduces or eliminates the negative impact on the quality of service. 
     At step  212  in  FIG. 4 , the mobile station replaces the erased portion of the frame with, for example, soft zeros. Thus these soft zeros are used to replace any data received in error from frequency f2 and to minimize the negative impact on the quality of service. Alternatives to soft zeros can also be utilized in the present invention. 
     If the inter-frequency search period is short relative to the length of the frame, then the probability of losing the forward channel frame is low because the transmitted symbols are interleaved over the entire frame period. When the inter-frequency search period is short, the inter-frequency search disruption of the forward channel can be seen as a fade (i.e., loss of signal for a short period of time or lowering of the power) of the received signal. If the inter-frequency search position and length are known to the base station, it can also perform the same operation (i.e., replacing the erased portion of the frame with soft zeros), thus minimizing the impact on the reverse link. This is shown in step  214  of FIG.  4 . 
     In the preferred embodiment, a dedicated message is used to provide the inter-frequency search communication and synchronization between the base station and the mobile station. This message is referred to as a candidate frequency search position message (CFSPM) and may be placed, for example, on the reverse dedicated control channel (R-DCCH) or on the reverse common control channel (R-CCCH) to indicate the position and the length of the inter-frequency search. The R-DCCH is dedicated to one mobile station, and the R-CCCH is for all mobile stations. 
     In another embodiment of the present invention, the synchronization is provided by the base station. In this embodiment, the base station pushes search parameters to the mobile station, such that the inter-frequency search is performed at the action time specified in a message from the base station. In either embodiment, the search position can be defined in, for example, units of power control group (PCG) or in milliseconds. In the IS-95B and cdma2000 standards, the length of the PCG is 1.25 ms (800 Hz). 
       FIG. 5  provides a diagram illustrating one procedure used by the present invention. In this diagram,  220  identifies the forward channel and  222  identifies the reverse channel. Frame N+n contains data received by the rake receiver, which is located in the mobile station. Data sections  230  and  232  were received while the receiver was tuned to frequency f1. The receiver was tuned from frequency f1 to frequency f2 at start time position  236 . Date section  238  was received while the receiver was tuned to frequency f2. Thus, time  236  provides the start time or position of the inter-frequency search, and section  238  provides the duration or length of the inter-frequency search. 
     In the preferred embodiment, the following three parameters are used to characterize the inter-frequency search (IFS): (1) the frame in which the search is performed, (2) the start of the search within the frame, and (3) the length of the search. In the preferred embodiment, the message contains the following fields:
         IFS_FRAME_OFFSET 6 bits (describes the frame position from CFSPM message)   IFS_START_PCG 4 bits (defines the PCG in which IFS starts)   IFS_LENGTH_PCG 4 bits (defines the # of PCG used for IFS search)       

       FIG. 6  provides a process flowchart for one embodiment of the present invention. At step  240 , the mobile station (MS) is demodulating frequency f 1  (see also section  230  in FIG.  5 ). At step  242 , the mobile station is directed by the base station (BS) to perform a search of frequency f 2 . At step  244 , the mobile station sends the candidate frequency search position message (CFSPM) on frequency f 1  with the above-described parameters (e.g., searched frame, search start position and search length). At step  246 , in the frame specified by the parameter IFS_FRAME_OFFSET, the base station waits until PCG is specified in parameter IFS_START_PCG. Then, at step  248 , to overcome the loss of the reverse link symbols, the base station replaces the indicated portion of the frame with soft zeros. For example, zeros are inserted by the following:
         For IFS 13  LENGTH_PCG
           Rx_SYMBOLS&lt;=‘soft_zeros’
 
At step  250 , the mobile station performs the same operation for the forward traffic channel frame.
   
               
     In another embodiment, the mobile station performs the inter-frequency search autonomously without network knowledge.  FIG. 7  provides a process flowchart for an autonomous inter-frequency search. At step  260 , the mobile station monitors speech activity when it is present on the reverse channel. At step  262 , the mobile station checks for a natural drop in speech activity. When the speech rate drops to a low rate (e.g., ⅛ of the normal rate), the process moves to step  264 , and the mobile station performs the inter-frequency search for a fraction of the frame period (e.g., for several power control groups). 
     During the inter-frequency search, the mobile station receiver is tuned to frequency f2. Therefore, the mobile station does not receive any signal on the serving frequency f 1 . This will normally cause an erasure of a portion of the forward link frame. However, since the mobile station knows the timing of the inter-frequency search, it can replace the missing channel symbols with the soft zeros. If the inter-frequency search period is relatively short in comparison to the length of the frame, then the probability of losing the forward channel frame is low because the transmitted symbols are interleaved over the entire frame period. In this embodiment, the inter-frequency search disruption of the forward channel can be seen, for example, as a flat (i.e., shadow) fade of the received signal. 
     In yet another embodiment of the present invention, data rates on multiple channels are determined.  FIG. 8  provides a process flowchart for an inter-frequency search with full rate determination. At step  270 , the mobile station again monitors speech activities. When the speech rate drops below a predetermined threshold at step  272 , the process moves to step  274 . At step  274 , the mobile station drops the data rate for the reverse supplemental channel such that its data rate matches the data rate of the reverse fundamental channel. As set forth above, the supplemental channel usually carries data with a higher, fixed rate, and the fundamental channel usually uses a variable data rate that is speech activity driven. If the data rates on both channels are the same, then the position and length of the inter-frequency search will be the same on both channels. This simplifies the correction procedure when both channels are in use (e.g., in voice over IP applications). 
     In normal operation, the base station only checks the reverse fundamental channel for data rate variation because the data rate on the supplemental channel is normally fixed. Therefore, an alteration must be made so that the base station is aware of the rate change on the supplemental channel. In one embodiment, the base station is notified of the change in data rate on the supplemental channel. In this embodiment, the candidate frequency search response message can be used to notify the base station of the change in data rate at step  276 . This notification can be placed, for example, in a new field that is added into the candidate frequency search response message or in the reserved bits of this message. At step  278 , the base station performs a rate determination on the reverse fundamental channel. At step  280 , the base station uses the rate from this rate determination for both the reverse fundamental channel and the reverse supplemental channel. 
     In a another embodiment, the base station performs a rate determination on the reverse supplemental channel without any notification from the mobile station. At step  290 , the process moves forward only if the base station directs the mobile station to perform an inter-frequency search. At step  292 , the process moves forward only if the base station receives a low data rate frame on the reverse fundamental channel (e.g., ⅛ of the full data rate). At step  294 , the base station performs rate determinations on both the fundamental channel and the supplemental channel. Therefore, the base station detects the rate change on the supplemental channel. 
     The present invention can be used with any CDMA system that includes a continuous channel or any other wideband CDMA system such as UMTS.