Abstract:
An electronic device capable of performing automatic frequency control (AFC) to maintain frequency and timing without good received bursts, in which an oscillation unit and a baseband processing unit are provided. Wherein, the baseband processing unit computes a compensation adjustment according to a prediction model and stored information regarding a previous digital value adjustment when detecting that the baseband processing unit is incapable of controlling the oscillation unit according to received bursts from the remote communication unit, and adjusts the oscillation unit according to the determined compensation adjustment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to communication systems, and in particular to electronic devices and methods for improvement of radio transmitter and receiver frequency accuracy of a local radio communication unit that communicates with a remote communication unit. 
     2. Description of the Related Art 
     Communication systems often comprise a plurality of local units such as radiotelephone handsets that communicate digital data by radio transmissions with a remote unit such as a cellular phone base station. The radio frequencies of the communication channels and frequency error tolerances for transmissions on the channels are typically specified by regulatory rules. The frequency tolerances ensure that the level of radio interference between channels is tolerable and that accurate data demodulation is possible at the local unit and the remote unit. In the base stations, the transmitter and receiver radio frequencies are typically phase locked to very stable reference oscillator signals available, in order to meet regulated radio frequency tolerances. However, the cost of the stable reference oscillators is typically very high for radiotelephone handsets. As such, provision for accurate transmitter and receiver frequencies in the local unit at the lowest possible cost is important. 
     For local communication units, the conventional solution for accurate radio frequencies is to use a relatively low cost voltage controlled crystal oscillator (VCXO) to serve as a reference oscillator, wherein the oscillator frequency is approximately linearly related to the magnitude of a VCXO control voltage. Sometimes, the VCXO control voltage is adjusted based on estimated radio frequency error of the receiver in accordance with well known feedback control principals, such that the radio frequency errors of the receiver and transmitter are sufficiently reduced by feedback control principals. The methodology for radio frequency control in the local unit is a conventional automatic frequency control (AFC) loop. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of an electronic device capable of communicating with remote communication units are provided, in which an oscillation unit and a baseband processing unit is provided. Wherein, the baseband processing unit computes a compensation adjustment according to a prediction model and stored information regarding a previous adjustment of a digital value when detecting that the baseband processing unit is incapable of controlling the oscillation unit according to received bursts from the remote communication unit, and adjusts the oscillation unit according to the determined compensation adjustment. 
     The invention provides an embodiment of a method for controlling an oscillation unit in an electronic device communicating digital data with a remote communication unit. A compensation adjustment is determined according to a prediction model and stored information regarding a previous adjustment of a digital value when detecting incapable of controlling the oscillation unit according to received bursts from the remote communication unit. The oscillation unit is adjusted according to the determined compensation adjustment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram illustrating a hardware environment according to an embodiment of the invention; 
         FIG. 2A  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention; 
         FIG. 2B  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention; 
         FIG. 2C  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention; 
         FIG. 2D  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention; 
         FIG. 3  is a block diagram illustrating a hardware environment according to an embodiment of the invention; and 
         FIG. 4  is a flowchart illustrating an embodiment of a method for maintaining the frequency of the VCXO. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     In some conditions, for example, passing through a tunnel, entering a cellar, and the similar, local units (i.e., radiotelephone handsets) cannot receive a good burst from remote units (i.e., base station) and thus, the automatic frequency control (AFC) digital control logic in the local units cannot accordingly adjust frequency of VCXO based on estimated radio frequency error of the receiver. Further, VCXOs have many error contributors, but the most dominant source is VCXO temperature characteristic. Since the local units cannot receive a good burst from the remote units, the transmission power thereof causes the variation of temperature and frequency of VCXO therein will seriously drift which may cause call drop. Embodiments of the invention can maintain frequency of VCXOs according to one or more calculated frequency errors in a normal state. Otherwise, in a reception gap state (i.e. when receiving no good bursts), embodiments of the invention can also periodically generate a new digital value to maintain the frequency of VCXOs according to a prediction model with the last adjustment of the digital value in a fixed period. It is to be understood that the last adjustment of digital value corresponds to the calculated frequency errors in response to the last received bursts, which may be stored in a memory or storage device. 
       FIG. 1  is a block diagram illustrating a hardware environment according to an embodiment of the invention. A local communication unit  10 , such as a radiotelephone handset, a cellular phone, a smart phone and the like, communicates with a remote communication unit  99 , such as a cellular phone base station. In the embodiment, the local communication unit  10  and the remote communication unit  99  adhere to the Global System for Mobile communications (GSM) standard as an illustrative example. However, the embodiments are not restricted to the GSM standard and apply more generally to any system in which a local communication unit derives a transmitter and receiver frequency by frequency tracking of radio transmissions from the remote station (i.e., cellular phone base station). That is, those skilled in the art may also implement the local communication unit  10  and the remote communication unit adhere to enhanced data rates for global evolution (EDGE), general packet radio service (GPRS), code division multiple access (CDMA), wideband code division multiple access (WCDMA), Wi-Fi, worldwide interoperability for microwave access (WiMAX) standard, and the similar. 
     As shown in  FIG. 1 , the remote communication unit  99  transmits bursts (i.e., RF signals) containing voice or data to the local communication unit  10  on an assigned radio channel. The local communication unit  10  comprises a radio frequency (RF) unit  12  having a synthesizer  20  and a voltage-controlled crystal oscillator (VCXO)  14 , and a baseband processing unit  16 . 
     The synthesizer unit  20  resident on the RF unit  12  receives the bursts (RF signals) from the remote communication unit  99  and transmits bursts (RF signals) to the remote communication unit  99 . For example, the RF unit  12  comprises not only the synthesizer  20  but also an RF front end (i.e., a combination of an antenna, a diplexer, a power amplifier, a low noise amplifier and RF filters), a quadrature down converter, a quadrature up converter, and programmable gain amplifiers (PGAs), not shown in  FIG. 1 . 
     For downlink operations, the RF front end receives the burst from the remote communication unit  99 , performs detection and amplification by a low noise amplifier (not shown) and performs RF filtering. The processed received burst is downconverted in frequency to baseband by the quadrature down converter which provides complex (inphase and quadrature) baseband components. Then, the quadrature downconverter mixes the local oscillator signal with the received burst to generate the complex analog baseband signal. The PGAs amplify the respective analog signals and then the amplified analog signals are processed by the baseband processing unit  16 . 
     For uplink operations, DACs (not shown) convert complex digital signals from the baseband processing unit  16  to analog complex signals and then the transmitter mixes the analog baseband complex signal with the local oscillator to the RF frequency of the assigned transmission channel. Thus, the transmitter mixes the local oscillator signal with the analog baseband complex signals to obtain a (RF) signal at the assigned transmission channel frequency. Then, the RF signal is amplified by power amplifier, filtered, and radiated out by the antenna in the RF front end. 
     The VCXO  14  controlled (or adjusted) by the baseband processing unit  16  provides oscillation signal to the RF unit  12 , and the local oscillation signals of the synthesizer  20  is phase locked to the oscillation signal generated by the VCXO  14 , such that the frequency and timing of the RF unit  12  can synchronize with the assigned channel. For example, the VCXO  14  comprises a resistor-capacitor (RC) circuit and a crystal oscillator (not shown), and the frequency of the oscillator signals generated by the VCXO  14  can be controlled (or adjusted) by adjusting the capacitance of the RC circuit. The RC circuit can be controlled by a digital value or an analog signal. 
     The baseband processing unit  16  controls the VCXO  14  which provide oscillation signal and processes voice and data in the received bursts (RF signals). For example, the baseband processing unit  16  comprises analog-to-digital converters (ADCs)  21 , a digital signal processor (DSP)  22 , a microcontroller unit (MCU)  24 , a digital-to-analog converter (DAC)  25  and a storage unit  26 . The ADCs  21  convert the processed analog complex signals to respective digital inphase I and quadrature Q samples. The complex (I, Q) sample pairs of the burst are read by DSP  22  and then the DSP  22  can decode voices or data. 
     In an embodiment, during a normal state, i.e., the local communication unit  10  can receive good bursts (RF signals) from the remote communication unit  99 , the DSP  22  calculates a frequency error according to the received burst and outputs the calculated frequency error to the MCU  24 . Then, the MCU  24  generates a digital value Dafc according to the calculated frequency error and stores the digital value Dafc in the storage unit  26 . The MCU  24  outputs the generated digital value Dafc to control the frequency of the VCXO  14  through the DAC  25 . Namely, the MCU  24  continuously compensates the digital values Dafc according to frequency errors of the received bursts calculated by the DSP  22  to control frequency of VCXO  14  and store the historical digital values with timing information, in the storage unit  26 . In addition, the MCU  24  further computes a compensation adjustment (i.e., increment or decrement) of the digital value Dafc in a predetermined time period T and stores the generated adjustment in the storage unit  26 . 
     During a reception gap state, i.e., the local communication unit  10  cannot receive good bursts (RF signals) from the remote communication unit  99 , the MCU  24  cannot update the digital value Dafc according to the received bursts. In the reception gap state, according to at least one prediction model and the last compensation adjustment of the digital value Dafc in a predetermined time period T, the MCU  24  periodically updates the digital value Dafc to maintain the frequency of VCXO  14  until the local communication unit  10  reenters the normal state. It is to be understood that the last compensation adjustment of the digital value Dafc is generated lastly according to received bursts (i.e. generated during the normal state). Detailed description of generating the digital value Dafc to adjust frequency of the VCXO  14  will be illustrated later. 
     The digital value Dafc is used for frequency compensation, such that the frequency of VCXO  14  can synchronize with the frequency of the assigned channel. The DACs  25  convert the digital value Dafc to the analog voltage Vafc, for adjusting the frequency of the VCXO  14 . The storage unit  26  stores programs and/or the selected processing parameters, and can be a flash memory, a DRAM or registers, but is not limited thereto. It should be noted that, once the local communication unit  10  reenters to the normal state (i.e., capable for receiving bursts from the remote communication unit  99  again), the baseband processing unit  16  adjusts the frequency of VCXO  14  according to the received bursts. 
       FIG. 2A  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention. As shown in  FIG. 2A , the MCU  24  continuously updates the digital values Dafc according to frequency errors of the receive bursts calculated by the DSP  22 , thereby maintaining frequency of VCXO  14 , and store the digital values Dafc in the storage unit  26 , in a normal state (i.e., Non-reception gap state). In addition, the MCU  24  compute a compensation adjustment X of the digital value Dafc in predetermined time period T and stores the generated adjustment X in the storage unit  26 . The compensation adjustment X of the digital value Dafc represents the difference from a previous digital value of a predetermined period T. In the example, the compensation adjustment of the digital value Dafc is an increment. 
     When the reception gap state occurs at time t 0 , the MCU  24  periodically compensates the digital value Dafc to maintain the frequency of VCXO  14  according to a prediction model and the compensation adjustment X of the digital value Dafc of the last period T. For example, the prediction model is X N+1 =α×X N , i.e., the next compensation adjustment X N+1  can be obtained by the previous compensation adjustment X N  multiplying by a decaying factor α, in which the decaying factor α&lt;1 and α&gt;0. The MCU  24  adds the currently obtained compensation adjustment to the last digital value to obtain a new digital value to update the digital value Dafc. 
     Namely, the MCU  24  calculates the compensation adjustment at time t 1  as α×X, according to the prediction model and the compensation adjustment X of the digital value Dafc for the last period T during the normal state, and then adds the calculated compensation adjustment (i.e., α×X) to the digital value D 0  (provided at time t 0 ) to obtain a digital value D 1  as the digital value Dafc at time t 1 . At time t 2 , the MCU  24  calculates the compensation adjustment at time t 2  as α 2 ×X, according to the prediction model and the previous adjustment at time t 1  (i.e., α×X), and then adds the obtained compensation adjustment (i.e., α 2 ×X) to the digital value D 1  (provided at time t 1 ), to obtain a digital value D 2  as the digital value Dafc at time t 2 . 
     Similarly, the MCU  24  obtains the compensation adjustments a α 3 ×X, α 4 ×X, α 5 ×X, and α 6 ×X at times t 3 ˜t 6 , respectively, according to the prediction model, and obtains corresponding digital values to accordingly update digital value Dafc. Namely, during the reception gap state, the MCU  24  outputs the digital values D 1 ˜D 6  at time t 1 ˜t 6  as the digital value Dafc, thereby adjusting the frequency of VCXO  14  stepwise to synchronize with the frequency of the assigned channel according to the prediction model and the stored compensation adjustment, rather than receiving bursts from the remote communication unit  99 . It is to be understood that the digital value adjustment may avoid call drop resulting from frequency drift caused by temperature variation before reentering to normal state, for example, leaving a tunnel, a cellar, and the similar. 
       FIG. 2B  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention. As shown in  FIG. 2B , the MCU  24  continuously decreases the digital values Dafc according to frequency errors of the received bursts calculated by the DSP  22 , thereby maintaining frequency of VCXO  14 , and stores the digital values Dafc in the storage unit  26 , in a normal state (i.e., Non-reception gap state). The MCU  24  computes a compensation adjustment X of the digital value Dafc of time period T and stores the generated adjustment X in the storage unit  26 . In the example, the compensation adjustment X of the digital value Dafc is a decrement. 
     When the reception gap state occurs at time t 0 , the MCU  24  periodically updates the digital value Dafc to maintain the frequency of VCXO  14 , according to a prediction model and the compensation adjustment X of the digital value Dafc for the last period T. According to the prediction model (i.e., X N+1 =α×X N ,) and the compensation adjustment X of the digital value Dafc for the last period T, the MCU  24  calculates the compensation adjustment at time t 1  as α×X, in which the decaying factor α&lt;1 and α&gt;0, and then subtracts the calculated compensation adjustment (i.e., α×X) from the digital value D 0  (provided at time t 0 ) to obtain a digital value D 11  serving as the digital value Dafc at time t 1 . 
     At time t 2 , the MCU  24  calculates the compensation adjustment as α 2 ×X according to the prediction model and the previous adjustment at time t 1  (i.e., α×X), and then subtracts the obtained compensation adjustment (i.e., α 2 X) from the digital value D 11  (provided at time t 1 ) and to obtain a digital value D 12  as the digital value Dafc at time t 2 . Similarly, the MCU  24  respectively obtains the compensation adjustments α 3 ×X, α 4 ×X, α 5 ×X, and α 6 ×X at times t 3 ˜t 6  according to the prediction model, and obtains corresponding digital values D 13 ˜D 16  to accordingly update digital value Dafc. Namely, during the reception gap state, the MCU  24  respectively outputs the digital values D 11 ˜D 16  at time t 1 ˜t 6  as the digital value Dafc, thereby adjusting the frequency of VCXO  14  stepwise. 
       FIG. 2C  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention. As shown in  FIG. 2C , according to the received burst from the remote communication unit  99 , the MCU  24  continuously adjusts the digital values Dafc according to frequency errors of the received bursts calculated by the DSP  22 , thereby maintaining frequency of VCXO  14 , and stores the digital values Dafc in the storage unit  26 , in a normal state (i.e., Non-reception gap state). Similarly, the MCU  24  computes a compensation adjustment X of the digital value Dafc of time period T and stores the generated adjustment X in the storage unit  26 . In the example, the MCU  24  selectively utilizes one of a plurality of predication models to update the digital value Dafc to maintain the frequency of VCXO  14 . For example, when a reverse condition is not satisfied, the MCU  24  updates the digital value Dafc according to a first prediction model. Conversely, when the reverse condition is being satisfied, the MCU  24  updates the digital value Dafc according to a second prediction model. After the reverse condition has been satisfied, the MCU  24  updates the digital value Dafc according to a third prediction model. 
     When the reception gap state occurs at time t 0 , the MCU  24  updates the digital value Dafc to maintain the frequency of VCXO  14  according to a first prediction model and the compensation adjustment X of the digital value Dafc for the last period T. The first prediction model is X N+1 =αX N , in which the decaying factor α&lt;1 and α&gt;0. Hence, the MCU  24  computes the compensation adjustment at time t 1  as α×X, and then adds the calculated compensation adjustment (i.e., α×X) to the digital value D 0  (provided at time t 0 ) to obtain a digital value D 21  serving as the digital value Dafc at time t 1 . In addition, the MCU further detects whether the obtained digital value reaches (or exceeds) a predetermined maximum to determine if the reverse condition is satisfied. Because the digital value D 21  is less than the predetermined maximum, the reverse condition is not satisfied at time t 1 . 
     At time t 2 , the MCU  24  computes the compensation adjustment as α 2 ×X, according to the first prediction model and the previous adjustment at time t 1  (i.e., α×X), and then adds the obtained compensation adjustment (i.e., α×X) to the digital value D 21  (provided at time t 1 ) to obtain a digital value D 22  serving as the digital value Dafc at time t 2 . Because the digital value D 22  is less than the predetermined maximum, the reverse condition is not satisfied at time t 2 . The generation of digital Dafc values D 23  and D 24  at times t 3  and t 4  can be deduced by analogy, and only briefly described herein. Because the digital value D 24  reaches (or exceeds) the predetermined maximum, the reverse condition is being satisfied at time t 4  and thus, the MCU  24  utilizes a second prediction model to update the digital value Dafc for the next time period. 
     At time t 5 , as the second prediction model is X N+1 =−X N  and the previous adjustment is a α 4 ×X, the MCU  24  calculates the compensation adjustment at time t 5  as −α 4 ×X, and then adds the obtained compensation adjustment (i.e., −α 4 ×X) to the digital value D 24  (provided at time t 4 ) to obtain the digital value D 23  serving as the digital value Dafc at time t 5 . Because the reverse condition has been satisfied after time t 5 , the MCU  24  then utilizes a third prediction model to update the digital value Dafc for the next time period. 
     At time t 6 , as the third prediction model is 
               X     N   +   1       =       X   N     α           
and the previous adjustment is −α 4 ×X, the MCU  24  computes the compensation adjustment at time t 6  as −α 3 ×X, and then adds the obtained compensation adjustment (i.e., −α 3 ×X) to the digital value D 23  (provided at time t 5 ) to obtain a digital value D 22  as the digital value Dafc at time t 6 . Similarly, the MCU  24  obtains the compensation adjustments −α 2 ×X and −α×X at times t 7  and t 8 , respectively, according to the third prediction model and obtains corresponding digital values D 21  and D 0  to accordingly update the digital value Dafc.
 
     Namely, the compensation adjustment of the digital value Dafc follows an increment pattern before time t 5 , and the compensation adjustment of the digital value Dafc follows a decrement pattern after time t 5 . 
       FIG. 2D  shows a diagram illustrating adjustment of the VCXO according to an embodiment of the invention. As shown in  FIG. 2D , the MCU  24  continuously adjusts the digital values Dafc according to frequency errors of the received bursts calculated by the DSP  22 , thereby maintaining the frequency of VCXO  14 , and stores the digital values Dafc in the storage unit  26 , in a normal state (i.e., Non-reception gap state). Similarly, the MCU  24  computes a compensation adjustment X of the digital value Dafc of time period T and stores the generated adjustment X in the storage unit  26 . In the example, the MCU  24  selectively utilizes one of a plurality of predication models to update the digital value Dafc to maintain the frequency of VCXO  14 . For example, when a reverse condition is not satisfied, the MCU  24  updates the digital value Dafc according to the first prediction model as described above. When the reverse condition is being satisfied, the MCU  24  updates the digital value Dafc according to a fourth prediction model. After the reverse condition has been satisfied, the MCU  24  updates the digital value Dafc according to a fifth prediction model. 
     At times t 1 ˜t 4 , the operations of the MCU  24  is similar to that shown in  FIG. 2C  and thus, detailed descriptions are omitted for brevity. Because the digital value D 24  at time t 4  reaches (or exceeds) the predetermined maximum, the reverse condition is being satisfied at time t 4  and thus, the MCU  24  then utilizes a fourth prediction model to update the digital value Dafc for the next time period. 
     At time t 5 , as the fourth prediction model is X N+1 =−β×X N , and the previous adjustment is α 4 ×X, the MCU  24  calculates the compensation adjustment at time t 5  as −β×α 4 ×X, and then adds the obtained compensation adjustment (i.e., −β×α 4 ×X) to the digital value (provided at time t 4 ) to obtain a digital value as the digital value Dafc at time t 5 . For example, the decaying factors are 0&lt;β&lt;α&lt;1. At time t 6 , as the fifth prediction model is X N+1 =β×X N , and the previous adjustment is −β×α 4 ×X, the MCU  24  calculates the compensation adjustment at time t 6  as −β 2 ×α 4 ×X according to the fifth prediction model and the previous adjustment, and then adds the obtained compensation adjustment (i.e., −β 2 ×α 4 ×X) to the previous digital value (provided at time t 5 ) to obtain a digital value D 33  as the digital value Dafc at time t 6 . 
     Similarly, the MCU  24  obtains the compensation adjustments −β 3 ×α 4 ×X and −β 4 ×α 4 ×X at times t 7  and t 8 , respectively, and obtains corresponding digital values to accordingly update the digital value Dafc. The compensation adjustment of the digital value Dafc follows an increment pattern before time t 5 , and the compensation adjustment of the digital value Dafc follows a decrement pattern after time t 5 , where the increment pattern with a greater slope than that of the decrement pattern. 
       FIG. 3  is a block diagram illustrating a hardware environment according to an embodiment of the invention. As shown, a local communication unit  10 ″ is similar to the local communication unit  10  shown in  FIG. 1 , the VCXO  14  can be controlled by the digital value Dafc and/or the analog voltage Vafc from a baseband processing unit  16 . The VCXO  14 ″ comprises a RC circuit  15  and a crystal oscillator (not show), and the frequency of the oscillator signals generated by the VCXO  14 ″ can be adjusted by adjusting the capacitance of the RC circuit  15 . 
     The RC circuit  15  can be controlled by a digital value Dafc or an analog voltage Vafc. For example, the capacitive element (not shown) in the RC circuit  15  is a voltage controlled element, and the capacitance of the RC circuit  15  can be adjusted by the analog voltage Vafc (i.e., automatic frequency control voltage) from the baseband processing unit  16 . In some embodiments, the capacitive element in the RC circuit is a capacitor. matrix controlled by a digital signal. Additionally, the capacitance of the RC circuit can be adjusted by a digital value Dafc directly. Namely, when the capacitance of the RC circuit can be adjusted by a digital value Dafc directly, the DAC  25  can be omitted. In some embodiments, the DAC  25  can be integrated into the RF unit  12  for converting the digital value Dafc to the analog voltage Vafc. Structures and operations of the local communication unit  10 ″ similar to that of the local communication unit  10  are omitted for brevity. 
       FIG. 4  is a flowchart illustrating an embodiment of a method for maintaining the frequency of VCXO according to an embodiment of the invention. Referring to  FIG. 4 , in step S 410 , the RF unit  20  in the local communication unit  10  receives bursts (RF signals) from the remote communication unit  99 . Then, the RF unit  12  mixes the local oscillator signal with the received burst to generate the complex analog baseband signals which are output to the baseband processing unit  16 . 
     In step S 420 , the MCU  24  determines whether a reception gap state occurs according to the burst received by the RF unit  20 . For example, the ADCs  21  converts the processed complex baseband signals to respective digital inphase I and quadrature Q samples which are processed by DSP  22 . MCU  24  determines the occurrence of the reception gap state according to whether a series of bad bursts are received by the local communication unit  10 . When a series of bad bursts are received by the local communication unit  10 , the MCU  24  determines that the gap state has occurred. The MCU  24  may detect the reception power of the local communication unit  10 , and signal to noise rate (SNR) of the received burst and decide whether a reception gap state has occurred. When the detected reception power is lower than a threshold value and/or the SNR of the received burst is lower than a threshold value, the reception gap has occurred. Otherwise, it means that the local communication unit  10  can receive good bursts (RF signals) from the remote communication unit  99 . Namely, the local communication unit  10  operates in a normal state, then step S 430  is executed. Conversely, if the reception gap state occurs, it means that the local communication unit  10  cannot receive good bursts (RF signals) from the remote communication unit  99 , and then step S 455  is executed. 
     In step S 430 , normal operations are executed. For example, the DSP  22  obtains voices or data according to the burst sample sequences from the ADCs  21 . In step S 440 , the MCU  24  obtains a digital value Dafc to adjust the frequency of VCXO  14  according to the received bursts, such that the frequency of VCXO  14  can synchronize with the remote communication unit  99 . Namely, an automatic frequency control (AFC) method is executed. For example, the DSP  22  computes the frequency error between the remote communication unit  99  and the local communication unit  10  according to the received burst (sample sequences) from the RF unit  12  and outputs the determined frequency error to the MCU  24 . 
     The MCU  24  then generates a digital value Dafc according to the determined frequency error, and the MCU  24  computes the digital value Dafc to control the frequency of VCXO  14 . As shown in  FIG. 1 , the digital value Dafc can be converted to the analog voltage to control the VCXO  14  by the DAC  25 , such that the frequency of VCXO  14  can synchronize with the remote communication unit  99 . As shown in  FIG. 3 , the digital value Dafc can also be output to the VCXO  14 ″ to adjust (or control) the capacitance of the RC circuit (known as DCXO). 
     In step S 450 , the MCU  24  stores the digital value Dafc in the storage unit  26  and generates a compensation adjustment (i.e., increment or decrement) of digital value Dafc and stores the generated adjustment in the storage unit  26 . Then, the method returns to step S 410 , i.e., the RF unit  12  receives another burst from the remote communication unit  99  again, thereby dynamically adjusting digital value Dafc or an analog signal Vafc to maintain the frequency accuracy of VCXO  14  or  14 ″. 
     In step S 455 , a predetermined time period has expired. When the time period of the reception gap state has expired, step S 460  is executed. On the contrary, when the time period of the reception gap state has not expired, the step S 410  is executed to receive another burst from the remote communication unit  99 . For example, burst reception and reception gap state determination (i.e. steps S 410  and S 420 ) are periodically performed about every 4 ms, and frequency compensation (i.e. steps S 460  and S 470 ), during the reception gap state, is periodically performed about every 2 seconds or after receiving 200 bursts. 
     In step S 460 , the MCU  24  determines a new digital value Dafc according to a prediction model, and the adjustment of the digital value Dafc for the last period, and then the MCU updates the new digital value Dafc in the storage unit  26 . Detailed description of generating the digital value Dafc can be referenced to that shown in  FIG. 2A˜2D  and thus, are omitted for brevity. 
     In step S 470 , the MCU  24  obtains the new digital value Dafc and outputs the new digital value Dafc to the VCXO  14  or  14 ″ for frequency compensation. For example, the digital value Dafc can be converted to the analog voltage to control the VCXO  14  by the DAC  25  as shown in  FIG. 1  Alternatively, the digital value Dafc can also be output to the VCXO to control the frequency. 
     In view of the above, the embodiments of the invention not only generates a new digital value to maintain frequency of VCXO according to the calculated frequency error in a normal state, but also periodically generates a new digital value to maintain the frequency of VCXO according to a prediction model and the adjustment of the digital value for the last pre-determined period, until the local communication unit reenters to a normal state. Hence, the invention can significantly improve the frequency and timing error at reception gap. Thus, call drops resulting from frequency and timing drift caused by temperature variation can be prevented. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.