JP4143619B2 - AFC control method, AFC circuit, and mobile communication device - Google Patents

Description

  The present invention relates to a mobile communication device in a mobile communication system, and more particularly to an AFC (Automatic Frequency Control) control method for matching a reception frequency in a mobile communication device with a transmission frequency of a base station.

  In recent years, CDMA (Code Division Multiple Access) communication schemes that are resistant to interference and interference have attracted attention as communication schemes used in mobile communication systems. In this CDMA communication system, a user signal to be transmitted is transmitted by spreading with a spreading code on the transmitting side, and the original user signal is obtained by performing despreading using the same spreading code as the spreading code on the receiving side. It is a communication system.

  Therefore, in a CDMA communication system, despreading cannot be performed on the receiving side unless the phases of the spreading code sequences on the transmitting side and the receiving side are synchronized. For this reason, in a mobile station which is a mobile communication device, an extremely high frequency accuracy VCTCXO (Voltage controlled Temperature Controlled) is used as a reference oscillator for generating a frequency of a local oscillation signal used when demodulating a signal received from a base station. Xtal Oscillator (Voltage Controlled Temperature Compensated Crystal Oscillator) is used, and the frequency of the local oscillation signal is controlled, and AFC (Automatic Frequency Control) control is performed to match the transmission frequency of the base station and the reception frequency of the mobile station. (For example, refer to Patent Documents 1 and 2).

  In this AFC control, a symbol shift is obtained from phase information of I and Q by a Costas loop or the like, and an analog element such as VCTCXO is controlled to correct a reception frequency.

However, when information is transmitted in a narrow band, the transmission speed is inevitably lowered because the band and the transmission speed are in a contradictory relationship. For this reason, the transmission rate is increased as much as possible by multi-level modulation, but the baud rate as a system becomes a low value in proportion to a decrease in the system transmission rate. If the data transmission rate that can be transmitted with a bandwidth of 6.25 kHz is considered, considering the roll-off rate of the root Nyquist filter used for baseband bandwidth limitation, the limit is about 4.8 kBaud at most. . Since the baud rate of 4.8k is equivalent to the angular velocity of 4.8k, when the above-described method for correcting the I and Q phase deviation is applied to such a system, the 90 degree deviation is 1200 Hz in frequency, which is a Gaussian plane. This is the frequency fluctuation for one quadrant in the shape. Therefore, in the case of the correction method as described above, this 1200 Hz (± 600 Hz) is a limit value that can be drawn in as AFC. On the other hand, the stability of crystal parts such as VCTCXO is currently limited to about 0.9 ppm. When a system is designed using such a part, if a radio frequency of 400 MHz is used, The fluctuation is about 360 Hz (400 M × 0.9 ppm). When this frequency variation occurs between the base station and the mobile station, a maximum frequency variation of about 720 Hz occurs between the base station and the mobile station. It cannot be pulled in.
JP 2003-69487 A JP 2003-69488 A

  In the conventional AFC control method described above, if the transmission rate is lowered due to the narrow bandwidth, the AFC pull-in range (pull-in frequency bandwidth) becomes narrow, and it is impossible to pull in the frequency fluctuation between the base station and the mobile station. There was a problem.

  An object of the present invention is to obtain a symbol shift amount from phase information of I and Q by a Costas loop or the like in a digital mobile communication system in a narrow band, and automatically correct a reception frequency by controlling an analog element such as a VCXO. The present invention provides an AFC control method, an AFC circuit, and a mobile communication device capable of expanding the pull-in range (pull-in frequency bandwidth) in a circuit system.

In order to achieve the above object, an AFC control method of the present invention is an AFC control method for adjusting a reception frequency in a mobile communication device to a transmission frequency of a base station, and an amount of deviation of the reception frequency from reception I and Q data. A calculating step;
Based on the calculated deviation amount of the reception frequency, the output frequency of the reference oscillator is controlled, and movement information indicating whether the output frequency of the reference oscillator is controlled to increase or decrease is stored. Steps,
If it is determined that synchronization cannot be detected within a certain period, the stored movement information indicates that the output frequency is controlled to increase, and the median value of the control voltage output to the reference oscillator When the initial value to be set is increased by a certain frequency and the stored movement information indicates that the output frequency is controlled to decrease, set as the median value of the control voltage output to the reference oscillator A step of reducing the initial value to be reduced by a certain frequency.

  According to the present invention, when AFC control is performed, whether the output frequency of the reference oscillator is controlled to be increased or decreased is stored as movement information, and synchronization is detected within a certain period. If not, it is determined that the deviation of the reference oscillator has exceeded the pull-in range, and the initial value set as the median value of the control voltage output to the reference oscillator is switched based on this movement information. Is. For this reason, in the conventional AFC control method, it is possible to draw in a shift in reception frequency that would be outside the pull-in range, and it is possible to expand the pull-in range.

  As described above, according to the present invention, when the AFC control is performed, the direction in which the output frequency of the reference oscillator is moved is stored as movement information, and the synchronization is not detected within a certain period. Since the median value of the control voltage output to the reference oscillator is switched based on this movement information, an effect that the pull-in range can be expanded can be obtained.

  As described above, while the stability of crystal parts such as VCTCXO is about 0.9 ppm, the control voltage and frequency deviation width of VCTCXO are sufficiently wide as several tens of k, and the ability to draw a deviation of about 1200 Hz is not enough. You will be well prepared. Therefore, in the present invention, the AFC pull-in range is expanded by using the ability of the VCXO while using the conventional method of controlling the VCXO from the phase shift of I and Q.

  Next, embodiments of the present invention will be described in detail with reference to the drawings.

  First, the configuration of a mobile station for realizing the AFC control method of one embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, the mobile station in the present embodiment includes an RF unit 101, a mixer 102, an A / D converter 103, mixers 104 and 105, a phase shifter 106, a detection unit 107, An AFC circuit 108 and a VCTCXO 109 functioning as a reference oscillator are provided.

  The RF unit 101 selects and amplifies only a signal having a specific frequency from signals received by the antenna, and outputs it as a high frequency signal. The mixer 102 multiplies the high-frequency signal from the RF unit 101 by the local oscillation signal generated by the VCTCXO 103 to convert the high-frequency signal into a chip rate baseband signal. The TCXO 109 outputs a signal whose frequency is controlled by the control voltage from the AFC circuit 108 as a local oscillation signal. The A / D converter 103 A / D converts the chip rate baseband signal generated by the mixer 102 into a digital signal.

  Then, the digital signals converted by the A / D converter 103 are converted into I and Q signals by being multiplied by signals having phases different by π / 2 by the mixers 104 and 105, respectively. The I and Q signals are demodulated by the detection unit 107 to become reception I and Q data, respectively.

  The AFC circuit 108 detects the shift amount of the reception frequency from the I and Q data from the detection unit 107 and adjusts the control voltage output to the VCTCXO 109 so that the detected shift amount of the reception frequency is reduced. The output frequency of the VCTCXO 109 is controlled.

  Then, the AFC circuit 108 stores movement information indicating whether the output frequency of the VCTCXO 109 is controlled in the direction of increasing (+ direction) or controlled in the direction of decreasing (− direction), and synchronization is performed within a certain period. If it is determined that the detected movement information has not been detected, the stored movement information indicates that the output frequency is controlled to increase, and the initial value set as the median value of the control voltage output to the VCTCXO 109 is a constant frequency When the stored movement information indicates that the output frequency is controlled to decrease, the initial value set as the median value of the control voltage output to the VCTCXO 109 is decreased by a certain frequency.

  Next, the AFC control method of the present embodiment performed in the AFC circuit 108 in FIG. 1 will be described in detail with reference to the flowchart of FIG.

  First, when the AFC circuit 108 is executed, first, the presence / absence of past movement information is confirmed (step 201). Here, the movement information is information indicating whether the output frequency of the VCTCXO 109 is controlled in the + or − direction as the AFC. However, since the movement information is not stored when the AFC circuit 108 is executed for the first time, the normal initial value is set as the median value of the control voltage output to the VCTCXO 109 (step 202).

  Next, the AFC circuit 108 checks the sign bits of the received I and Q data from the detection unit 107, checks in which quadrant on the Gaussian plane the received I and Q data are present, and the second to fourth quadrants. If it is, the conversion is performed so as to return the quadrant information to the first quadrant, and all received symbols are collected in one quadrant. The AFC circuit 108 calculates the received symbol angle by obtaining the arc tangent of the I and Q data returned to the first quadrant (step 206). Then, the AFC circuit 108 obtains a reception frequency shift amount from the absolute value of the difference between the calculated reception symbol angle and 45 degrees (step 207).

  Here, the details of the method for obtaining the shift amount of the reception frequency from the reception symbol angle are shown in FIGS.

  In a CDMA communication system, QPSK (Quadrature Phase Shift Keying: Orthogonal PSK) is used as a modulation method of primary modulation performed before spread modulation, and thus each symbol is 2-bit data. 0, 0), (0, 1), (1, 0), or (1, 1). FIG. 3 shows these values on a vector diagram. Here, the horizontal axis indicates the magnitude of the in-phase component (I: In-phase component), and the vertical axis indicates the magnitude of the quadrature component (Q: Quadrature component). For example, in an ideal state where there is no reception frequency error, the symbol angle of the reception symbol (0, 0) in the first quadrant is 45 degrees. Therefore, even when the received symbols in the second to fourth quadrants are turned back to the first quadrant, the received symbol angle is 45 degrees.

Here, as shown in FIG. 4, when the I component of a certain received symbol 401 is a and the Q component is b, the received symbol angle θ of the received symbol 402 is θ = tan ⁻¹ (b / a ).

  The AFC circuit 108 converts the obtained information on the received frequency deviation amount into control information for controlling the VCTCXO 109, and affects the control even if the information fluctuates instantaneously due to uncertain factors. Filtering processing is performed so as not to give (step 208). The filtered control information is added to an integrator (not shown), and AFC control is performed as a control voltage of the VCTCXO 109 (step 209). At this time, the AFC circuit 108 stores, as movement information, whether the control voltage of the VCTCXO 109 has been controlled in the + direction (in which the output frequency is increased) or in the-direction (in which the output frequency is decreased). (Step 210).

  Then, it is determined whether or not synchronization is achieved within a certain period (step 211). If synchronization is achieved, AFC control is performed by continuing the processing of steps 206 to 210. The reception frequency shift is reduced. Here, the method for determining whether or not synchronization has been achieved is not particularly limited, and the determination criterion may be that a synchronization word is detected by a CODEC or the like arranged in a subsequent stage.

  If no synchronization is detected for a certain period in the processing of step 211, the AFC circuit 108 determines that the deviation of the VCTCXO 109 has exceeded the pull-in range, and sets the initial value of the control voltage output to the VCTCXO 109 as the median value. Change the value. Specifically, if it is determined in step 211 that synchronization cannot be detected, the AFC circuit 108 determines presence / absence of movement information (step 201). If movement information exists, the AFC circuit 108 determines its movement direction ( Step 203). The AFC circuit 108 sets an initial value shifted by 600 Hz to the + side when the moving direction is the + direction (step 204), and sets an initial value shifted by 600 Hz to the − side when the moving direction is the − direction. (Step 205).

  By performing the control in this manner, the conventional AFC control method can extend the pull-in range up to ± 1200 Hz, which can be pulled only up to ± 600 Hz of reception frequency shift. That is, in the conventional AFC control method, only a quarter of the symbol rate could be drawn as AFC. However, according to the AFC control method of the present embodiment, the pull-in range can be reduced to half the symbol rate. It becomes possible to enlarge.

It is a block diagram which shows the structure of the mobile communication apparatus for implement | achieving the AFC control method of one Embodiment of this invention. It is a flowchart which shows the AFC control method by the AFC circuit 108 in FIG. It is a figure which shows the received symbol in the ideal state with no receiving frequency error. It is a figure for demonstrating the method of calculating a reception symbol angle.

Explanation of symbols

101 RF unit 102 Mixer 103 A / D converter 104, 105 Mixer 106 Phase shifter 107 Detection unit 108 AFC circuit 109 VCTCXO
201-211 Step 401 Received symbol

Claims (3)

An AFC control method for adjusting a reception frequency in a mobile communication device to a transmission frequency of a base station, calculating a reception frequency deviation amount from reception I and Q data;
Based on the calculated deviation amount of the reception frequency, the output frequency of the reference oscillator is controlled, and movement information indicating whether the output frequency of the reference oscillator is controlled to increase or decrease is stored. Steps,
If it is determined that synchronization cannot be detected within a certain period, the stored movement information indicates that the output frequency is controlled to increase, and the median value of the control voltage output to the reference oscillator When the initial value to be set is increased by a certain frequency and the stored movement information indicates that the output frequency is controlled to decrease, set as the median value of the control voltage output to the reference oscillator An initial value to be reduced by a certain frequency, and an AFC control method. Means for calculating a deviation amount of the reception frequency from the reception I and Q data in the AFC circuit for matching the reception frequency in the mobile communication device with the transmission frequency of the base station ;
Based on the calculated deviation amount of the reception frequency, the output frequency of the reference oscillator is controlled, and movement information indicating whether the output frequency of the reference oscillator is controlled to increase or decrease is stored . Means,
When it is determined that synchronization cannot be detected within a certain period of time, if the stored movement information indicates that the output frequency is controlled to increase, the median value of the control voltage output to the reference oscillator When the initial value to be set is increased by a certain frequency and the stored movement information indicates that the output frequency is controlled to decrease, set as the median value of the control voltage output to the reference oscillator Means for reducing the initial value to be reduced by a certain frequency. A mobile communication device that performs AFC control to match a reception frequency with a transmission frequency of a base station,
A reference oscillator that outputs a signal whose frequency is controlled by the input control voltage as a local oscillation signal;
With an AFC circuit,
The AFC circuit is
Means for calculating a received frequency deviation amount from received I and Q data ;
Based on the calculated shift amount of the reception frequency, the output frequency of the reference oscillator is controlled, and movement information indicating whether the output frequency of the reference oscillator is controlled to increase or decrease is stored. Means to
When it is determined that synchronization cannot be detected within a certain period of time, if the stored movement information indicates that the output frequency is controlled to increase, the median value of the control voltage output to the reference oscillator When the initial value to be set is increased by a certain frequency and the stored movement information indicates that the output frequency is controlled to decrease, set as the median value of the control voltage output to the reference oscillator And a means for decreasing the initial value by a certain frequency.

Images