Abstract:
An equalizer is provided for use in a mobile radio communication system for compensating for changing channel conditions caused by movement of a mobile communication device at varying speeds. The equalizer includes an estimator receiving a digital signal transmitted over a communication channel, the digital signal being adversely modified by fading during transmission caused by movement of the mobile communication device at varying speeds, and a channel estimate signal, the estimator responsively producing a decision signal representing an approximation of the transmitted digital signal with fading eliminated. First and second channel trackers are provided, each tuned to a different speed of the mobile communication device and each receiving the decision signal. A decision circuit receives the decision signal and responsively selects one of the first and second channel trackers to produce the channel estimate signal.

Description:
FIELD OF THE INVENTION 
     The present invention is directed toward digital data demodulation and, more particularly, toward demodulation of a desired digital signal in a mobile communication device where the channel conditions change with movement of the mobile communication device at varying speeds. 
     BACKGROUND OF THE INVENTION 
     Cellular communication systems presently implemented in the U.S. utilize digitized voice and data signals for communication between a mobile communication device, generally a cellular telephone, and a base station. Movement of the mobile communication device at varying speeds (for instance if the user holding the device is walking, running, riding a bike, or in a car or train, etc.) causes channel fading to occur across the communication channels; the change in the signal caused by channel fading varying with time. 
     Prior art equalizers, which are included in the mobile communication device, generally include an estimator which receives the transmitted digitized signal and produces an output representing a distorted version of the digital signal as actually transmitted. A channel tracker is also included within the equalizer which receives both the transmitted digitized signal and the output of the equalizer, and produces a channel estimate signal representing an approximation of the channel fading that has occurred to the transmitted digitized signal. The role of the channel tracker is to use the output of the estimator to estimate the changes in the signal caused by channel fading, and to produce a channel estimate signal representing an approximation of the changes in the signal due to channel fading. The channel estimate signal produced by the channel tracker is utilized by the estimator to produce a more accurate representation of the digital signal as actually transmitted, i.e., get rid of the changes due to channel fading. 
     When the mobile communication device is moving at high speeds, the rate of change of the characteristics of a radio channel used by the mobile communication device typically occurs quickly. The opposite is true when the mobile communication device is moving at low speeds. When a channel tracker is built, an assumption must be made about the speed of the mobile communication device in which it is to be utilized. The channel tracker is tuned to the assumed speed and can accurately track the rate of change of the channel&#39;s characteristics if the mobile communication device is moving at a speed which approximates the speed to which the channel tracker is tuned. As a result, a channel tracker tuned to a high speed (HS) channel is designed such that it tracks fast changes in the channel&#39;s characteristics well. On the other hand, a channel tracker tuned to a low speed (LS) channel is designed such that it tracks slow changes in the channel&#39;s characteristics well. 
     When an HS channel tracker is used in an LS channel, it will tend to track non-existing changes in the channel&#39;s characteristics. This can hurt performance by about 1-2 dB. When an LS channel tracker is used in an HS channel, it may not be able to keep up with the changes in the channel&#39;s characteristics and can lose the channel completely by the end of a frame. For this reason, an HS tracker is typically chosen and a performance hit is taken at lower speeds. 
     However, merely placing an HS channel tracker in a wireless communication device or cellular phone is not quite satisfactory. Many people use their cellular phones essentially as wireless phones, that is, at extremely low speeds, e.g., 1-2 kilometers per hour, where the channel is almost static. The same is generally true for personal communicator phones and phones used as wireless modems for portable computers. In such cases, an HS channel tracker is a mismatch. 
     While there have been successful attempts to actually estimate the speed of the mobile communication device from the received signal, tracking the speed on top of tracking the channel is extremely complex and may result in a computational nightmare. Nevertheless, it is helpful to boost performance by using a channel tracker that is matched to the speed of the mobile communication device. 
     The present invention is directed toward overcoming one or more of the above-mentioned problems. 
     SUMMARY OF THE INVENTION 
     An equalizer is provided for use in a mobile radio communication system for compensating, among other sources of distortion, for changing channel conditions caused by movement of a mobile communication device at varying speeds. The equalizer includes an estimator receiving a digital signal transmitted over a communication channel, the digital signal being adversely modified by fading during transmission caused by movement of the mobile communication device at varying speeds, and a channel estimate signal, the estimator responsively producing a decision signal representing an approximation of the transmitted digital signal with fading eliminated. First and second channel trackers are provided, each tuned to a different speed of the mobile communication device and each receiving the decision signal. A decision circuit receives the decision signal and responsively selects one of the first and second channel trackers to produce the channel estimate signal. 
     In one aspect of the present invention, the estimator includes a maximum likelihood sequence estimator. 
     In another aspect of the present invention, the decision circuit responsively selects one of the first and second channel trackers based on the accuracy of the decision signal to the digital signal actually transmitted. 
     In another aspect of the present invention, the decision circuit includes an error control decoder receiving the decision signal and producing a reliability signal indicative of the accuracy of the decision signal to the digital signal actually transmitted, and a switching circuit receiving the reliability signal and responsively selecting one of the first and second trackers to produce the channel estimate signal. 
     In another aspect of the present invention, the reliability signal has first and second states. The switching circuit selects said one of the first and second channel trackers to produce the channel estimate signal with the reliability signal in the first state, and the switching circuit selects the other of said one of the first and second channel trackers to produce the channel estimate signal with the reliability signal in the second state. 
     In another aspect of the present invention, the digital signal is encoded with a Cyclic Redundancy Check (CRC) code prior to transmission. The error control decoder detects CRC errors in the decision signal and produces the second state reliability signal if a CRC error is detected; otherwise the error control decoder produces the first state reliability signal. 
     In another aspect of the present invention, the estimator produces an estimator metric signal in addition to the decision signal. The error control decoder receives the estimator metric signal and produces the second state reliability signal if the estimator metric signal is above a threshold value; otherwise the error control decoder produces the first state reliability signal. 
     In another aspect of the present invention, the digital signal is encoded with a block or convolutional code prior to being transmitted. The mobile communication device further includes a decoder decoding the decision signal and producing a decoder metric signal in addition to the decoded decision signal. The error control decoder receives the decoder metric signal and produces the second state reliability signal if the decoder metric signal is above a threshold value; otherwise the error control decoder produces the first state reliability signal. 
     In another aspect of the present invention, the decoder includes a Viterbi decoder. 
     In another aspect of the present invention, the digital signal is encoded with a CRC code prior to transmission. The error control decoder detects CRC errors in the decision signal producing a CRC error signal. The estimator produces an estimator metric signal in addition to the decision signal, with the estimator metric signal received by the error control decoder. The error control decoder produces the first or second state reliability signal based on a comparative analysis of the estimator metric and the CRC error signal values. 
     In another aspect of the present invention, the digital signal is encoded with a CRC code and a block or convolutional code prior to transmission. The error control decoder detects CRC errors in the decision signal producing a CRC error signal. The mobile communication device further includes a decoder decoding the decision signal and producing a decoder metric signal in addition to the decoded decision signal, with the decoder metric signal received by the error control decoder. The error control decoder produces the first or second state reliability signal based on a comparative analysis of the decoder metric and the CRC error signal values. 
     In still another aspect of the present invention, the digital signal is encoded with a block or convolutional code prior to being transmitted. The mobile communication device further includes a decoder decoding the decision signal and producing a decoder metric signal in addition to the decoded decision signal, with the decoder metric signal received by the error control decoder. The estimator produces an estimator metric signal in addition to the decision signal, with the estimator metric signal received by the error control decoder. The error control decoder produces the first or second state reliability signal based on a comparative analysis of the decoder metric and estimator metric signal values. 
     In yet another aspect of the present invention, the digital signal is encoded with a CRC code and a block or convolutional code prior to being transmitted. The error control decoder detects CRC errors in the decision signal producing a CRC error signal. The estimator produces an estimator metric signal in addition to the decision signal, with the estimator metric signal received by the error control decoder. The mobile communication device further includes a decoder decoding the decision signal and producing a decoder metric signal in addition to the decoded decision signal, with the decoder metric signal received by the error control decoder. The error control decoder produces the first or second state reliability signal based on a comparative analysis of the estimator metric, decoder metric and CRC error signal values. 
     In a mobile communication device for use in a mobile radio communication system, a method is provided for compensating for changing channel conditions caused by movement of the mobile communication device at varying speeds. The method includes the steps of receiving, at the mobile communication device, a digital signal transmitted over a communication channel, generating a first channel estimate signal tuned to a first speed of the mobile communication device, generating a second channel estimate signal tuned to a second speed of the mobile communication device, different from the first speed, demodulating the received digital signal using one of the first and second channel estimate signals to track the communication channel, determining the accuracy of the demodulated received digital signal, and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received signal. 
     In one aspect of the present invention, the digital signal is encoded with a CRC code prior to transmission. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal includes the steps of detecting CRC errors in the demodulated received digital signal, and selecting the other of the first and second channel estimate signals to track the communication channel if a CRC error is detected. 
     In another aspect of the present invention, the method further includes the step of generating a demodulator metric signal associated with the demodulated received digital signal. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal includes the steps of determining the value of the demodulator metric signal, and selecting the other of the first and second channel estimate signals to track the communication channel if the demodulator metric signal value is above a threshold value. 
     In another aspect of the present invention, the method further includes the steps of decoding the demodulated received digital signal, and generating a decoder metric signal associated with the decoded received digital signal. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal includes the steps of determining the value of the decoder metric signal, and selecting the other of the first and second channel estimate signals if the decoder metric signal value is above a threshold value. 
     In another aspect of the present invention, the digital signal is encoded with a CRC code prior to transmission. The method further includes the step of generating a demodulator metric signal associated with the demodulated received digital signal. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal include the steps of detecting CRC errors in the demodulated received digital signal, determining the value of the demodulator metric signal, and selecting one of the first and second channel estimate signals to track the communication channel based on a comparison of the CRC errors and the demodulator metric signal value. 
     In another aspect of the present invention, the digital signal is encoded with a CRC code prior to transmission. The method further includes the steps of decoding the demodulated received digital signal, and generating a decoder metric signal associated with the decoded received digital signal. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal include the steps of detecting CRC errors in the demodulated received digital signal, determining the value of the decoder metric signal, and selecting one of the first and second channel estimate signals to track the communication channel based on a comparison of the CRC errors and the decoder metric signal value. 
     In still another aspect of the present invention, the method further includes the steps of decoding the demodulated received digital signal, generating a decoder metric signal associated with the decoded received digital signal, and generating a demodulator metric signal associated with the demodulated received digital signal. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal include the steps of determining the value of the decoder metric signal, determining the value of the demodulator metric signal, and selecting one of the first and second channel estimate signals to track the communication channel based on a comparison of the decoder metric and demodulator metric signal values. 
     In yet another aspect of the present invention, the digital signal is encoded with a CRC code prior to transmission. The method further includes the steps of decoding the demodulated received digital signal, generating a decoder metric signal associated the decoded received digital signal, and generating a demodulator metric signal associated with the demodulated received digital signal. The steps of determining the accuracy of the demodulated received digital signal and selecting one of the first and second channel estimate signals to track the communication channel based on the accuracy of the demodulated received digital signal include the steps of detecting CRC errors in the demodulated received digital signal, determining the value of the demodulator metric signal, determining the value of the decoder metric signal, and selecting one of the first and second channel estimate signals to track the communication channel based on a comparison of the CRC errors, the demodulator metric signal value and the decoder metric signal value. 
     It is an object of the present invention to track changing channel conditions in a received digital signal due to movement of a mobile communication device at varying speeds. 
     It is a further object of the present invention to boost equalizer performance by utilizing a channel tracker matched to the speed of a mobile communication device. 
     It is a further object of the present invention to switch among a plurality of channel trackers in an equalizer dependent upon demodulation and decoding information. 
     It is yet a further object of the present invention to accurately demodulate a digitized signal in a mobile radio communication system where the channel conditions change within the duration of a frame. 
     Other aspects, objects and advantages of the present invention can be obtained from a study of the application, the drawings, and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the relevant elements of a prior art communication system; 
     FIG. 2 is a block diagram of the prior art demodulator shown in FIG. 1; 
     FIG. 3 is a block diagram of a mobile communication system for use with the present invention; and 
     FIG. 4 is a block diagram of the equalizer shown in FIG.  3  and according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 generally depicts the relevant elements of a prior art mobile communication system, shown generally at  10 . The system  10  generally includes a base station  12  communicating with a plurality of mobile communication devices, one of which,  14 , is illustrated. For communications from the base station  12  to the mobile communication device  14 , the base station  12  includes a transmitter  16  which transmits a signal over the air that is received at a receiver  18  in the mobile communication device  14 . It should be understood that while FIG. 1 only illustrates “downlink” communication from the base station  12  to the mobile communication device  14 , both the base station  12  and mobile communication device  14  include a transmitter and a receiver for both uplink and downlink communication therebetween. 
     A digital signal  20  to be transmitted at the base station  12  is first conventionally encoded by an error control encoder  22  at the transmitter  16 . The error control encoder  22  may be a CRC (Cyclic Redundancy Check), block or other convolutional encoder. The encoded signal  24  is then conventionally modulated by a modulator  26 , and then the digital signal  28  is transmitted across a communications channel where it is subject to channel fading at  30 . The digital signal  32  subject to channel fading is received by the receiver  14  and conventionally demodulated by demodulator/equalizer  34 . (The terms demodulator and equalizer are used interchangeably herein.) The demodulated signal  36  is then decoded by an error control decoder  38 , which basically performs the reverse function of the error control encoder  22 , producing a decoded signal  40  for further processing. 
     FIG. 2 is a block diagram of the prior art demodulator  34  shown in FIG.  1 . The demodulator  34  includes an estimator  42  and a channel tracker  44 . The estimator  42  may be a Maximum Likelihood Sequence Estimator (“MLSE”) or any other coherent demodulator. The transmitted digital signal  32 , subject to channel fading, is received by both the estimator  42  and the channel tracker  44 . The estimator  42  conventionally demodulates the received signal  32  and produces a plurality of demodulated symbols  36  representative thereof. The demodulated symbols  36  are input to the channel tracker  44 , which analyzes both the received signal  32  and the decisions or demodulated symbols  36  from the estimator  42  and produces a channel estimate signal  46  which is received by the estimator  42 . As previously described, the channel estimate signal  46  represents the change in the received signal  32  due to channel fading and is utilized by the estimator  42  in tracking the channel of the received digital signal  32  in order to reduce the effects of any channel fading that may have occurred on the transmitted signal  32 . The estimator  42  produces an accurate demodulated signal  36  representation of the digital signal  28  as actually transmitted. However, as previously indicated, the channel tracker  44  is tuned to a certain speed and may either be an HS (high speed) tracker or an LS (low speed) tracker. If the mobile communication device  14  is moving at a speed other than the speed to which the channel tracker  44  is tuned, then the channel tracker  44  cannot accurately track the channel and performance is degraded. 
     FIG. 3 depicts a communication system, shown generally at  50 , for use with the present invention. The communication system  50  includes a base station  51  communicating with a mobile communication device  52 , which may include a cellular phone or any other type of wireless communication device. While FIG. 3 depicts downlink communication between the base station  51  and the mobile communication device  52  via a transmitter  53  in the base station  51  and a receiver  54  in the mobile communication device  52 , the present invention is also concerned with uplink communication by a transmitter (not shown) in the mobile communication device  52  with a receiver (not shown) in the base station  51 . For simplicity only downlink communication will be discussed herein. 
     Digital information bits  58  to be transmitted are first conventionally encoded with a parity check code by encoder  60 ; are further conventionally encoded by a convolutional, or block, code by encoder  62 ; are conventionally interleaved by interleaver  64 ; and conventionally modulated by modulator  66 , thus producing the transmitted digital signal  68 . The transmitted digital signal  68  is preferably a TDM (Time Division Multiplexed) signal divided into a plurality of frames. 
     During transmission across the communication channel, the transmitted digital signal  68  may experience what is commonly known as channel fading at block  70 . The digital signal subject to channel fading, indicated at  72 , is received at the receiver  54  and the corresponding operations performed by the transmitter  53  are performed in reverse. More specifically, the channel digital signal  72 , subject to channel fading, is demodulated by the demodulator/equalizer  74 ; de-interleaved by de-interleaver  76 ; decoded by decoder  78 , which may be Viterbi decoder; and further decoded with a parity check code decoder  80 . The resulting received digital signal  82  may be further processed by the receiver  54  or the mobile communication device  52  for providing the signal in audible form to an end user. 
     FIG. 4 depicts a block diagram of the demodulator/equalizer  74  shown in FIG.  3 . The equalizer  74  includes an estimator  84 , a first channel tracker  86 , a second channel tracker  88 , an error control decoder  90  and a tracker choice block  92 . The estimator  84 , which may be an MLSE or any other coherent demodulator, receives both the received digital signal  72 , subject to channel fading, and a channel estimate signal  94 , the channel estimate signal being produced by one of the first  86  and second  88  channel trackers, respectively. The estimator  84  demodulates the received signal  72 , thus producing a demodulated signal  96 . The demodulated signal  96  is received by both the first  86  and second  88  channel trackers, and also by the error control decoder  90 . The error control decoder  90  also receives signals  98  and  100  from decoders  78  and  80 , respectively, which signals will be explained in detail hereafter. 
     In operation, the first  86  and second  88  channel trackers are tuned to different speeds. For instance, assume that channel tracker  86  is tuned a high speed, while channel tracker  88  is tuned to a low speed. At the beginning of the call, i.e., prior to receiving signal  72 , the receiver  54  chooses one of the two trackers, and for simplicity, it is assumed that the receiver  54  will first choose the HS tracker  86 . 
     The received signal  72  is demodulated by the estimator  84 , with the HS channel tracker  86  producing the channel estimate signal  94  which is utilized by the estimator  84  to track the changing channel characteristics during demodulation. The error control decoder  90  receives the demodulated signal  96  and determines whether or not the estimator  84  is accurately demodulating the received signal  72 . For instance, if the mobile communication device  52  was static or moving at a very low speed, the HS channel tracker  86  would be a mismatch and would not accurately track the changing channel characteristics. The error control decoder  90  outputs a reliability information signal  102  which is received by the tracker choice block  92 . In the above example, the reliability information signal  102  instructs the tracker choice block  92  to activate a trigger  104  and switch to the LS tracker  88 . The LS tracker  88  would then produce the channel estimate signal  94 , potentially aiding the performance. There are a number of ways which the error control decoder  90  can determine that the estimator  84  is not accurately demodulating the received signal  72 . These are discussed below for illustrative purposes only. 
     One method of switching between channel trackers can be accomplished by looking at the status of the parity check code decoder  80 . Typically, the parity check code used at the transmitter  53  is a CRC (Cyclic Redundancy Check) code. The parity check code decoder not only decodes the CRC code but is also capable of determining whether there are errors in the CRC code. This error signal  100  is transmitted to the error control decoder  90 . Since the transmitted digital signal  68  is a TDM signal divided into frames, which are further divided into symbols, the parity check is accomplished as follows. 
     At the beginning of the call, it is assumed that the receiver  54  chooses the HS tracker  86 . For frame number i, let c i  denote the parity check flag transmitted by signal  100  (“1” for fail, “0” for pass). For the i-th frame, two cases are considered: 
     (a) If c i =0, it is assumed that the speech frame is good and the tracker choice is good. This frame may be used in speech reconstruction and the error control decoder  90  instructs the tracker choice block  92 , via signal  102 , to continue to use the current tracker (HS tracker  86 ). 
     (b) If c i =1, it is assumed that the speech frame is bad and the tracker choice is bad. This frame is not used in speech reconstruction and the error control decoder  90  instructs the tracker choice block  92 , via signal  102 , to switch trackers (switch to the LS tracker  88 ). If now c i =0, this tracker (the LS tracker  88 ) is kept for the next frame, etc. 
     Every time the CRC check fails, the present invention is capable of using the freed-up computational capability to switch trackers and re-demodulate. Then if the CRC check passes, the new tracker can be utilized for the next frame, potentially improving future performance. 
     Another method of determining whether the estimator  84  is accurately demodulating the received signal  72  is by looking at the final metric of the estimator  84 . The received signal  72  includes a plurality of symbols within each frame, these symbols being denoted by r i . Accordingly, the demodulated signal  96  includes a corresponding number of detected symbols denoted as s i . Likewise, the channel estimate signal  94  includes a plurality of channel estimate symbols denoted as e i . In demodulating the received signal  72 , the estimator  84  finds the sequence of symbols that minimizes the Euclidian metric        m   =       ∑   i                                r   i     -       e   i          s   i              2     .                              
     When a frame has been demodulated, the outcome is a symbol sequence s 1 , s 2 , . . . s n  and its corresponding metric m. The metric m is a measure of the reliability of the frame. A small metric m indicates a high signal-to-noise ratio, whereas as a large metric m indicates a low signal-to-noise ratio. Accordingly, the error control decoder  90  can be implemented to switch trackers if the estimator metric m is above a threshold value indicating a low signal-to-noise ratio. 
     Similar to the estimator  84 , the decoder  78  also produces a final metric. The decoder  78  receives soft values from the equalizer  74 , via de-interleaver  76 , which will be denoted as z i , and the decoded or detected information bits output by decoder  78  will be denoted as x i . In producing the soft value z i , the equalizer, instead of just producing a +1 or a −1 for each bit, produces real numbers, e.g., +10 indicates reliable +1, and −0.1 indicates an unreliable −1. The decoder  78 , which may be a Viterbi decoder, exploits the soft values z i ; it searches the code sequence y 1 , y 2 , . . . y n  that minimizes the metric          m   ′     =       ∑   i                                z   i     -     y   i            2     .                              
     When the search is finished, the outcome of the decoder  78  is the information bit sequence x 1 , x 2 , . . . x k  corresponding to the best code sequence y 1 , y 2 , . . . y n . In addition the metric m′ is produced. It&#39;s interpretation is similar to that of the estimator metric m, namely, a small metric m′ indicates a reliable code sequence (a high signal-to-noise ratio), whereas a large metric m′ indicates an unreliable code sequence (a low signal-to-noise ratio). The error control decoder  90  can be implemented to switch trackers whenever the decoder metric m′ exceeds a threshold value. 
     Still another method of switching trackers is considered below. Let M i  denote the final estimator metric, and for some L (number of frames), let          M   i     =       1   L            ∑     j   =   0       L   -   1                       m     i   -   j                                  
     and also let          C   i     =       1   L            ∑     j   =   0       L   -   1                       c     i   -   j                                  
     Again, it is assumed that the receiver  54  initially chooses the HS tracker  86  at the beginning of the call. A number of cases are considered: 
     (1) If M i  and C i  are below a predetermined value, the call is fine and don&#39;t switch trackers (keep HS tracker  86 ). 
     (2) If M i  is above a predetermined value and C i  is below a predetermined value, the call is okay but switching trackers may help. Switch trackers (switch to LS tracker  88 ) for the next L frames. If M i  decreases and C i  stays below the predetermined value, keep the new tracker choice (LS tracker  88 ). Otherwise, switch back to the original tracker (HS tracker  86 ). 
     (3) If c i =1, the current frame is bad. Switch trackers (switch to LS tracker  88 ) and dernodulate/decode again. If now c i =0, keep the new tracker choice (LS tracker  88 ) for the next frame, otherwise keep the current choice (HS tracker  86 ). 
     It should be noted that instead of using two trackers tuned to high and low speeds, respectively, there can be a number (T&gt;2) of trackers, each tuned to a different speed in the range. When the error control decoder  90  decides to switch from its current tracker, one of several strategies can be used, for instance: 
     (1) Cycle through the remaining T-1 trackers, and terminate the search at the first tracker that gives satisfactory performance. 
     (2) Try the next highest and next lowest trackers (when available) to the current tracker. If neither gives satisfactory performance, try the second next highest and second next lowest trackers (when available), and so on. 
     It should be noted that various other formulas may be utilized to compute M i  and C i , for instance, an exponential average. The information may also be extracted about the change in M i  and C i  to try and detect the change in the speed of the mobile communication device  52 . Finally, the metric m of the estimator  84  may be analyzed not just at the end of the frame, but also throughout the frame. This allows for the detection of the tracking loss phenomenon, which happens when an LS tracker is used in an HS channel. As the tracker begins to lose the channel over the frame, it would be expected that the incremental or branch metric would increase. 
     One skilled in the art will recognize that other methods, along the lines of those previously described above, may be utilized to determine whether to switch trackers. For instance, a comparative analysis of both the estimator metric and the decoder metric, over a frame or a plurality of frames, may be utilized in determining whether or not to switch trackers. Further, a comparative analysis of both the decoder metric and the CRC error signal values may be utilized to determine whether to switch trackers. Still further, a comparative analysis of the estimator metric, decoder metric and the CRC error signal values may be utilized to determine whether to switch trackers. Each such method would use formulas generally similar to the individual formulas outlined above, as is apparent to one skilled in the art. 
     While the invention has been described with particular reference to the drawings, it should be understood that various modifications could be made without departing from the spirit and scope of the present invention.