Abstract:
A frequency offset calibrating method for use in a communication device connected to a communication system is provided. The method includes the following steps: determining a discontinuous reception cycle; awakening the communication device to a working mode from a sleep mode every discontinuous reception cycle and keep the communication device in the working mode for a first time period to receive a paging indication channel message from a communication network periodically; and awakening the communication device at a second time period other than the first time period during a first discontinuous reception cycle, thereby estimating an accumulated timing offset of a clock signal of the communication device and calibrating a frequency offset of the clock signal. In the invention, the accumulated timing offset of the clock signal can be calibrated efficiently to increase the reception performance of the page indication channel message with simple implementation and low hardware cost.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of China Patent Application No. 201310552018.8, filed on Nov. 8, 2013, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless communication device, and in particular to a communication device and method of calibrating frequency offset by awakening the communication device from a sleep mode to a working mode periodically. 
     2. Description of the Related Art 
     In a communication system, for example, a TD-SCDMA communication system, a conventional communication device in sleep mode should be awakened into working mode periodically, such as every discontinuous reception (DRX) cycle, to receive page indication channel (PICH) message. However, the conventional communication device is awakened into working mode at the end of each DRX cycle. Since there may be a frequency offset in the clock signal, the communication device maybe unable to be awakened to receive the PICH message within the time range defined in the communication standard. Conventionally, there are two methods to solve the aforementioned problem. In the first method, the communication device may be awakened at the end of each DRX cycle to estimate the accumulated timing offset, and calibrates the frequency offset of the clock signal (e.g. 32 KHz) according to the estimated accumulated timing offset; while in the second method, a high accuracy clock signal (e.g. 26 MHz) could be used to calibrate the clock signal of 32 KHz. However, the aforementioned methods may have some disadvantages. For example, in the first method, when a longer DRX cycle is used or the frequency offset of the 32 KHz clock signal is too large, the accumulated frequency offset may be too large that it exceeds the estimation capability of the conventional communication device, so that the communication device cannot estimate and calibrate the accumulated timing offset properly, resulting in low PICH reception performance; and regarding the second method, an additional calibrating mechanism is required, resulting in higher system complexity. Accordingly, a communication system is demanded to solve the frequency offset calibration issue when the communication system is in the sleep mode (or the idle mode). 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     A frequency offset calibrating method for use in a communication device connected to a communication system is provided. The method includes the following steps: determining a discontinuous reception cycle; periodically awakening the communication device to a working mode from a sleep mode every discontinuous reception cycle and keep the communication device in the working mode for a first time period to receive a paging indication channel message from a communication network; and awakening the communication device at a second time period other than the first time period during a first discontinuous reception cycle, thereby estimating an accumulated timing offset of a clock signal of the communication device and calibrating a frequency offset of the clock signal. In the invention, the accumulated timing offset of the clock signal can be calibrated efficiently to increase the reception performance of the page indication channel message with simple implementation and low hardware cost. 
     A communication device is provided. The communication device includes a processor, configured to determine a discontinuous reception cycle; and a control unit, configured to awaken the communication device to a working mode from a sleep mode every discontinuous reception cycle and keep the communication device in the working mode for a first time period to receive a paging indication channel message from a communication network periodically, wherein the control unit further awakens the communication device at a second time period other than the first time period during a first discontinuous reception cycle, thereby estimating an accumulated timing offset of a clock signal of the communication device and calibrating a frequency offset of the clock signal. 
     In the invention, the accumulated timing offset of the clock signal can be calibrated efficiently to increase the reception performance of the page indication channel message with simple implementation and low hardware cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present 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 schematic block diagram of a communication device  100  in accordance with an embodiment of the invention; 
         FIG. 2  is a diagram illustrating the communication device  100  being awakened periodically in accordance with an embodiment of the invention; and 
         FIG. 3  is a flow chart illustrating the frequency offset method in accordance with an embodiment of the invention. 
     
    
    
     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. 
       FIG. 1  is a schematic block diagram of a communication device  100  in accordance with an embodiment of the invention. As illustrated in  FIG. 1 , the communication device  100  may include a microprocessor (MCU)  110 , a digital signal processor (DSP)  120 , a baseband buffer  130 , an RF transceiver  140 , a clock generator  150 , and a control unit  160 . In a communication system, the communication device  100  is connected to a communication network to transmit/receive packet data via the RF transceiver  140 . The MCU  110  and the DSP  120  are configured to process the packet data transferred by the communication device  100 , and the MCU  110  may further determine a DRX cycle at the sleep mode. The baseband buffer  130  is configured to store the packet data to be transmitted or received by the RF transceiver  140 . In an embodiment, the communication device  100  can be regarded as user equipment (UE), such as a handheld device (e.g. mobile phone, personal digital assistant, etc.), which supports communication standards such as TD-SCDMA, WCDMA, or CDMA2000. The clock generator  150  may generate clock signals required in each component of the communication device  100  according to the control command from the MCU  110  or the DSP  120 . For example, the clock generator  150  can be implemented by a phase-locked loop (PLL) or an oscillator to generate a stable reference clock signal CLK ref  (e.g. 26 MHz), and generate a clock signal CLK A  (e.g. 32 KHz) to be used in the sleep mode for the communication device  100  based on the reference clock signal CLK ref . The control unit  160  is configured to control activation/deactivation of other components of the communication device  100 . In an embodiment, the control unit  160  may include at least one counter coupled to the clock signal CLK A , configured to count the number of clock cycles required to awake the communication device  100  to the working mode, and the number of clock cycles required in the sleep/idle time, thereby awakening other components of the communication device  100  or controlling other components of the communication device  100  to enter the sleep mode. 
       FIG. 2  is a timing diagram illustrating the communication device  100  being awakened periodically in accordance with an embodiment of the invention. Taking a TD-SCDMA communication system for example, in order to reduce power consumption, the communication device may be in idle mode (i.e. except the clock generator  150 ), and the primary components (e.g. MCU  110 , DSP  220 , baseband buffer  130 , and the RF transceiver  140 ) of the communication device  100  would be awakened into working mode by the control unit  160  to receive the PICH message at each DRX cycle. The sleep time of the communication device  100  is calculated with the clock signal CLK A  having a lower frequency (e.g. 32 KHz) to, and the clock signal CLK A  is usually obtained by dividing frequency of a stable reference clock signal CLK ref  (e.g. 26 MHz) generated by a phase-locked loop device or an oscillator. However, the frequency offset X may happen to the clock signal CLK A  with lower frequency due to the temperature drift. When calculating a fixed time interval, for example, after a time interval of L (L≧0) clock cycles (i.e. L*X/(32K)), the timing offset in each clock cycle will be accumulated, resulting in the components of the communication device  100  being unable to be awakened within the time range defined by the communication standard. Accordingly, the clock signal CLK A  should be calibrated. 
     In one embodiment of the invention, the awakening time of the communication device  100  during each DRX cycle at the sleep mode may be an adjustable number of clock cycles, such as N clock cycles. A conventional communication device may stay in the idle mode during the each DRX cycle, and the timing offset of the clock signal CLK A  will not be estimated and compensated until the communication device  100  is awakened into working mode by the end of each DRX cycle. However, when a longer DRX cycle is used or the frequency offset of the clock signal CLK A  is large, the accumulated timing offset may exceed the tolerance range used in the conventional communication device (e.g. ±0.1 ms), resulting in the conventional communication device being unable to receive the PICH message at the correct time, and the PICH reception performance may reduce due to loss of PICH messages. 
     In one embodiment of the invention, the communication device  100  can be awakened actively in each sleep cycle to estimate accumulated timing offset and calibrate the frequency offset of the clock signal. As illustrated in  FIG. 2 , the frequency offset calibrating method may include the following steps: (1) when the communication device enters the sleep mode, if the DRX cycle being used is long (e.g. 1.28 seconds), after the communication device  100  enters sleep mode at the first cycle DRX  1 , the control unit  160  may actively awaken the communication device  100  to enter the working mode to estimate the accumulated timing offset after a predetermined time period (e.g. approximate ½ DRX cycle). For example, at time t 0 , the communication device  100  enters the sleep mode, and starts counting for the DRX cycle. However, during the period from time t 1  to t 2  (e.g. N clock cycles of the clock signal CLK A ), the communication device is awakened into the working mode by the control unit  160  to receive the PICH message. Afterwards, the communication device may go back to the idle mode at time t 2  from the working mode after N (i.e. N≧0) clock cycles from time t 1 . At time t 3 , components of the communication device  100  except the MCU  110  may be awakened by the control unit  160  to estimate the accumulated timing offset and calibrate the frequency offset after approximately ½ DRX cycle (e.g. 0.64 sec) from time t 2 . It should be noted that the aforementioned ½ DRX cycle is only an example and the invention is not limited thereto. Furthermore, in some embodiments, the MCU  110  and the DSP  120  can be integrated into a processor. 
     (2) At time t 3 , the DSP  120  can calculate the current frequency offset of the clock signal CLK A  (e.g. 32 KHz). When the frequency offset caused by the clock signal CLK A  is obtained, the DSP  120  may calibrate the frequency of the clock signal CLK A  generated by the clock generator  150  according to the calculated frequency offset. Specifically, the frequency of the clock signal CLK A  is obtained by dividing frequency of the reference clock signal CLK ref  by a phase-locked loop or a frequency calibrator, and the DSP  120  may calibrate the ratio for dividing frequency of the reference clock signal, and an accurate clock frequency can be obtained by the calibration. 
     (3) After M clock cycles from time t 3 , the communication device  100  enters the idle mode again at time t 4  to wait for reception of the PICH message. 
     (4) At the subsequent DRX cycles (e.g. DRX  2  or DRX  3 ), the communication device  100  may receive the PICH message regularly. Specifically, since the frequency of the clock signal CLK A  has been calibrated during the period from time t 3  to t 4 , the communication device  100  can be awakened correctly at time t 6  to receive the PICH message. It should be noted that the communication device  100  can be also awakened from the sleep mode actively at other DRX cycles in addition to the first DRX cycle depending on the practical situation, and the details can be referred to  FIG. 3 . 
       FIG. 3  is a flow chart illustrating the frequency offset calibrating method in accordance with an embodiment of the invention. In step S 302 , the communication device  100  enters the idle mode. It should be noted that the communication device  100  has not started to calculate the DRX cycle yet, wherein i denotes the number of DRX cycles passed. In step S 304 , it is determined whether the current DRX cycle is one of the first four DRX cycles. If so, step S 306  is performed. Otherwise, step S 308  is performed. In step S 306 , it is determined whether the DRX cycle is larger than or equal to a predefined DRX cycle (e.g. 1.28 sec). If so, step S 312  is performed. Otherwise, step S 310  is performed. In step S 308 , it is determined whether the DRX cycle is longer than or equal to a second DRX cycle, wherein the second DRX cycle (e.g 2.56 seconds in the embodiment) is larger than the first DRX cycle. If so, step S 310  is performed. Otherwise, step S 314  is performed. 
     In step S 310 , it is determined whether the accumulated timing offset of the previous frequency offset calibration is larger than a threshold value such as 5Tc, wherein Tc denotes the chip period of the TD-SCDMA communication system (e.g. 1/(1.28 MHz)). If so, step S 312  is performed. Otherwise, step S 314  is performed. In step S 312 , the communication device  100  enters the working state at ½ DRX cycle (e.g. 0.64 sec) after entering the idle mode in order to estimate the current accumulated timing offset and calibrate the clock signal CLK A  (e.g. 32 KHz), and then enters the idle mode again to wait for reception of the PICH message. In step S 314 , the communication device  100  enters a regular sleep mode to wait for reception of next PICH message. In step S 316 , when the communication device  100  has received the PICH message, the counter of DRX cycles is increased by 1. In step S 318 , it is determined whether the communication device  100  has to receive the paging channel (PCH) message. If so, step S 320  is performed. Otherwise, step S 304  is performed. In step S 320 , the communication device  100  receives the PCH message. In step S 322 , the communication device  100  enters the idle mode or the working mode according to the PCH message. 
     In view of the above, the communication device and the frequency offset calibrating method of the invention are capable of calibrating periodical sleep timing information by calibrating the accumulated timing offset generated by the clock signal in time, thereby increasing the reception performance of the PICH message with simple implementation and low hardware cost. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On 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.