JP4679644B2 - Wireless terminal device - Google Patents

Description

  The present invention relates to a wireless terminal device, and more particularly to a wireless terminal device that performs frequency correction of an uplink signal when performing communication with a base station.

  In 3GPP (3rd Generation Partnership Project), EUTRAN (Evolved UTRAN), which is currently being studied as a next generation system, has SC-FDMA (Single Carrier-Frequency Division Multiple Access) as a modulation method for UL (Up Link) signals. (For example, refer nonpatent literature 1). Multiple terminals perform user multiplexing by transmitting signals using different frequencies and times, and the base station uses FFT (Fast Fourier Transform) to collectively collect signals from multiple terminals multiplexed in space. Demodulate.

  A terminal that moves at high speed receives a Doppler shift and receives a DL (Down Link) signal having a frequency offset from the base station. Since the terminal transmits in synchronization with the carrier frequency of the UL signal, the UL signal of the terminal moving at high speed already includes a frequency offset at the time of transmission, and the UL signal received by the base station is Further, a Doppler shift in the UL is added to this.

FIG. 10 is a diagram for explaining the frequency offset in the Doppler shift. In the figure, a base station 121 and a moving terminal 122 are shown. Here, the carrier frequency of the DL signal by the base station 121 transmits to f _c. The frequency deviation of the oscillator for generating a carrier frequency f _c of the base station 121 and Delta] f _BS1. The frequency of the DL signal transmitted from the base station 121 is as shown in the following equation (1).

f _c + Δf _BS1 (1)
The moving terminal 122 receives the DL signal having the frequency offset from the base station 121 in response to the Doppler shift. Therefore, the frequency of the DL signal received by the terminal 122 is as shown in the following equation (2), where the maximum Doppler frequency is f _D. Δf _UE indicates a frequency deviation of the oscillator included in the terminal 122.

f _c + Δf _BS1 + Δf _UE + f _D (2)
The terminal 122 transmits a UL signal to the base station 121 in synchronization with the frequency shown in Equation (2). For this reason, in the base station 121, a Doppler shift is further added to Formula (2), and UL signal is received. Therefore, the carrier frequency of the UL signal received by the base station 121 is as shown in the following equation (3).

f _c + Δf _BS1 + Δf _UE + 2f _D (3)
Thus, the frequency offset in the base station 121 is as shown in equation (3) the following equation (4), except for the carrier frequency f _c from.

f _OS = Δf _BS1 + Δf _UE + 2f _D (4)
As shown in Expression (4), the frequency offset of the base station 121 includes a double Doppler shift in addition to the frequency deviation between the base station 121 and the terminal 122.

  By the way, in the base station, when receiving a plurality of UL signals having different frequency offsets, inter-carrier interference occurs due to a loss of orthogonality between carriers, and reception quality deteriorates.

  Therefore, a method has been proposed in which the frequency offset amount at the base station is reduced by correcting the transmission carrier frequency of the terminal by control from the base station (see, for example, Non-Patent Document 2). In this method, the base station estimates the frequency offset amount based on the UL signal from each terminal. Then, the base station notifies each terminal of the correction amount of the frequency of the UL signal as a DL control signal to each terminal so that the frequency offset amount becomes small. Each terminal performs frequency correction of the UL signal based on this control information. Thereby, in the base station, the influence of the Doppler shift of the UL signal can be removed, and deterioration of the reception quality can be suppressed.

  FIG. 11 is a diagram for explaining the frequency offset control. In the figure, a base station 131 and a moving terminal 132 are shown. The frequency of the DL signal transmitted from the base station 131 is as shown in the following equation (5), similar to equation (1) in FIG.

f _c + Δf _BS1 (5)
Further, the frequency of the DL signal received by the terminal 132 is as shown in the following equation (6) as in the equation (2).

f _c + Δf _BS1 + Δf _UE + f _D (6)
The base station 131 transmits the DL signal control signal including the correction amount f _CNT so as to reduce the frequency offset amount. The terminal 132 transmits the UL signal to the base station 131 in synchronization with the frequency shown in Expression (6), but transmits it in synchronization with the frequency obtained by subtracting the correction amount _fCNT transmitted from the base station 131. That is, the UL signal is transmitted in synchronization with the frequency shown in the following equation (7) obtained by subtracting the correction amount _fCNT from the equation (6).

(F _c + Δf _BS1 + Δf _UE + f _D ) −f _CNT (7)
Here, the value of the correction amount f _CNT transmitted by the base station 131 is Δf _UE + 2f _D. The correction amount f _CNT can be obtained by subtracting the frequency deviation Δf _BS1 of the base station 131 itself from the frequency offset equation (4). Therefore, Expression (7) becomes as shown in the following Expression (8).

(F _c + Δf _BS1 + Δf _UE + f _D ) − (Δf _UE + 2f _D )
= F _c + Δf _BS1 −f _D (8)
The frequency shown in equation (7) is further subjected to Doppler shift during UL. Therefore, the carrier frequency of the UL signal received by the base station 131 is as shown in the following equation (9).

f _c + Δf _BS1 (9)
As shown in Expression (9), the influence of Doppler shift is removed from the frequency of the UL signal received by the base station 131. In this way, the base station 131 corrects the frequency offset due to the Doppler shift, thereby suppressing carrier interference between the terminals and suppressing reception quality deterioration.

When the terminal moves in the area of multiple overlapping cells, the DL signal from each base station received by the terminal has a different frequency offset. In addition, UL signals from terminals received at the respective base stations also have different frequency offsets. Therefore, the frequency offset control after the handover needs to be performed by the handover destination base station.
3GPP TR25.814 3GPP TSG-RAN WG1 # 47 R1-063475

  However, the random access signal that the terminal first transmits in the handover destination cell during handover cannot be subjected to frequency offset control from the handover destination base station. This is because the handover destination base station cannot know the correction amount of the frequency offset amount of the terminal until it receives a random access signal from the terminal. Therefore, the random access signal has a large frequency offset (Doppler shift), and there is a problem that the reception quality at the base station deteriorates.

  The present invention has been made in view of such points, and provides a wireless terminal device capable of suppressing the frequency offset amount of a random access signal received by a base station to be small and suppressing deterioration in reception quality of the random access signal. With the goal.

  In the present invention, in order to solve the above problem, in the wireless terminal device 1 that performs wireless communication with the base station as shown in FIG. 1, the downlink frequency of the handover source base station 2 and the handover source base station 2 are notified. Frequency calculation means 1a for calculating the Doppler frequency in the handover destination base station 3 from the correction amount of the uplink frequency and the downlink frequency of the handover destination base station 3, and the transmission to the handover destination base station 3 based on the Doppler frequency There is provided a radio terminal device 1 having frequency correction means 1b for correcting the frequency of a random access signal.

  According to such a wireless terminal device 1, the downlink frequency of the handover source base station 2, the correction amount of the uplink frequency notified from the handover source base station 2, and the downlink frequency of the handover destination base station 3 The Doppler frequency in the handover destination base station 3 is calculated, and the frequency of the random access signal transmitted to the handover destination base station 3 is corrected based on the calculated Doppler frequency.

  Thereby, the frequency offset amount of the random access signal received by the handover destination base station 3 can be suppressed small.

  In the radio terminal apparatus of the present invention, the Doppler in the handover destination base station is calculated from the downlink frequency of the handover source base station, the correction amount of the uplink frequency notified from the handover source base station, and the downlink frequency of the handover destination base station. The frequency is calculated, and the frequency of the random access signal transmitted to the handover destination base station is corrected based on the calculated Doppler frequency.

  As a result, the amount of frequency offset of the random access signal received by the handover destination base station can be kept small, and deterioration of the reception quality of the random access signal at the handover destination base station can be suppressed.

  These and other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings which illustrate preferred embodiments by way of example of the present invention.

It is the figure which showed the outline | summary of the radio | wireless terminal apparatus. It is the figure which showed the communication system of the terminal which concerns on 1st Embodiment. FIG. 3 is a sequence diagram of handover in LTE. It is the figure which showed the example of arrangement | positioning of a random access channel. It is the figure which showed the data structural example of the random access signal. It is the figure which showed another data structural example of the random access signal. It is a functional block diagram of a base station. It is a functional block diagram of a terminal. It is the figure which showed the communication system of the terminal which concerns on 2nd Embodiment. It is a figure explaining the frequency offset in a Doppler shift. It is a figure explaining frequency offset control.

Hereinafter, the principle of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram illustrating an outline of a wireless terminal device. As shown in the figure, the wireless terminal device 1 has a frequency calculation means 1a and a frequency correction means 1b. It is assumed that the wireless terminal device 1 moves from the handover source base station 2 to the cell of the handover destination base station 3 and performs handover.

Since the wireless terminal device 1 receives the DL signal from the handover source base station 2 and transmits the UL signal in synchronization with the frequency, the wireless terminal device 1 can recognize the DL frequency of the handover source base station 2. Further, since the handover source base station 2 transmits the control signal of the DL signal including the correction amount (f _CNT ) at the time of UL, the wireless terminal device 1 can recognize the correction amount of the UL frequency. Further, since the wireless terminal device 1 synchronizes the DL with the handover destination base station 3 at the time of the handover, the wireless terminal device 1 can recognize the DL frequency of the handover destination base station 3.

  The frequency calculation means 1a uses the DL frequency of the handover source base station 2, the correction amount of the UL frequency notified from the handover source base station 2, and the DL frequency of the handover destination base station 3 to determine the frequency at the handover destination base station 3. Calculate the Doppler frequency.

  Specifically, the frequency calculation means 1a subtracts the DL frequency of the handover destination base station 3 from the DL frequency of the handover source base station 2, and further calculates the correction amount of the UL frequency notified from the handover source base station 2. By subtracting, the Doppler frequency in the handover destination base station 3 can be calculated.

  The frequency correction unit 1b corrects the frequency of the random access signal transmitted to the handover destination base station 3 based on the Doppler frequency in the handover destination base station 3 calculated by the frequency calculation unit 1a.

  Thus, the wireless terminal device 1 calculates the Doppler frequency in the handover destination base station 3. Thereby, the Doppler shift of the random access signal transmitted to the handover destination base station 3 can be reduced, and deterioration of the reception quality of the random access signal at the handover destination base station 3 can be suppressed.

Next, a first embodiment of the present invention will be described in detail with reference to the drawings.
FIG. 2 is a diagram illustrating a communication system of a terminal according to the first embodiment. In the figure, base stations 11 and 12 and a terminal 21 are shown. The terminal 21 is a wireless terminal such as a mobile phone, for example. The terminal 21 is assumed to move from the cell of the base station 11 to the cell of the base station 12 and perform a handover.

  The terminal 21 transmits a random access signal to the handover destination base station 12 at the time of handover. At this time, the terminal 21 estimates the frequency offset amount of the random access signal received by the handover destination base station 12, and controls the frequency offset for the transmitted random access signal based on this estimated value. Subsequent frequency offset control of the UL signal subsequent to the random access signal is performed based on a control signal fed back from the handover destination base station 12 as described with reference to FIG.

  The terminal 21 performs frequency synchronization with the DL signal of the handover destination base station 12 at the time of handover. The terminal 21 calculates the difference between the frequency of the DL signal received from the handover source base station 11 and the frequency of the DL signal from the handover destination base station 12.

The frequency of the DL signal received from the handover source base station 11 is expressed by the following equation (10) as described in equation (2).
f _c + Δf _BS1 + Δf _UE + f _D1 (10)
Further, the frequency of the DL signal received from the handover destination base station 12 is expressed by the following equation (11).

f _c + Δf _BS2 + f _D2 (11)
Δf _BS1 and Δf _BS2 in the equations (10) and (11) indicate the frequency deviations of the base stations 11 and 12, respectively. f _D1 represents the Doppler frequency in the base station 11, and f _D2 represents the Doppler frequency in the base station 12. It is assumed that the carrier frequencies of the base stations 11 and 12 are the same at _fc .

Therefore, the difference between the frequency of the DL signal received from the handover source base station 11 and the frequency of the DL signal from the handover destination base station 12 is expressed by the following equation (12).
(F _c + Δf _BS1 + Δf _UE + f _D1 ) − (f _c + Δf _BS2 + f _D2 )
= (Δf _BS1 −Δf _BS2 ) + (f _D1 −f _D2 ) + Δf _UE (12)
As shown in equation (12), by subtracting the expression (11) from equation (10), the carrier frequency _{f c} is erased.

As described with reference to FIG. 11, in a system that controls frequency offset correction in a base station, a terminal receives a frequency offset correction amount from a handover source base station using a DL signal control signal. Terminal 21 by the correction amount _{f CNT} and wherein receiving from the handover source base station 11 (12), perform the computation of the following equation (13) can be estimated Doppler frequency _{f D2.} If the terminal 21 can estimate the Doppler frequency f _D2 in the handover destination base station 12, the influence of the Doppler shift can be eliminated in the handover destination base station 12 by including the correction amount of f _D2 in the frequency of the random access signal. Because.

{(Δf _BS1 −Δf _BS2 ) + (f _D1 −f _D2 ) + Δf _UE } −f _CNT / 2 (13)
Since f _CNT is Δf _UE + 2f _D1 as described with reference to FIG. 11, Equation (13) is represented by the following Equation (14).

(Δf _BS1 −Δf _BS2 ) + (f _D1 −f _D2 ) + Δf _UE − {f _D1 + (Δf _UE / 2)}
= (Δf _BS1 −Δf _BS2 ) −f _D2 + Δf _UE / 2 (14)
In the expression (12), the term f _D1 −f _D2 remains. From this, as shown in Expression (13), by subtracting the correction amount f _CNT / 2 including f _D1 , the Doppler frequency f _D1 of the handover source base station 11 is erased as shown in Expression (14). The Doppler frequency f _{D2 at} the handover destination base station 12 remains.

The frequency deviation of the base station is usually 0.05 ppm or less, and the frequency deviation after the terminal pulls in the frequency is usually 0.1 ppm or less. When the carrier frequency is 2 GHz, the maximum deviation according to the term of Δf _BS1 −Δf _BS2 in Expression (14) is 200 Hz. In addition, the maximum deviation according to the term Δf _UE / 2 is 100 Hz. Therefore, the error of the Doppler frequency _{f D2} according to formula (14) is equal to or less than ± 300 Hz.

That is, Expression (14) is obtained by the calculations of Expressions (12) and (13), and the Doppler frequency f _D2 can be estimated.
As described above, the terminal 21 estimates the frequency offset amount (Doppler frequency f _D2 ) of the access random signal received by the handover destination base station 12, and based on this estimated value, the frequency of the random access signal to be transmitted is determined. Perform offset correction. Since the DL carrier frequency f _DL and the UL carrier frequency f _UL of the base station 12 are different in frequency, the terminal 21 corrects the frequency offset for the random access signal according to the difference between the DL and UL carrier frequencies. . The carrier frequency of the random access signal transmitted by the terminal 21 is as shown in the following equation (15).

f _UL -2f _D2 (f _UL / f _DL ) (15)
F _{UL in} Expression (15) indicates the UL carrier frequency of the handover destination base station 12. f _DL indicates the DL carrier frequency of the handover destination base station 12.

When the random access signal is transmitted to the handover destination base station 12, it is transmitted at the frequency of f _UL , but the f _D2 estimated by the equation (14) is corrected. The reason _{why fD2} is doubled is because Doppler shift in DL and UL is taken into consideration. The reason why ₂ f _D2 is multiplied by f _UL / f _DL is that the frequency of _DL carrier frequency f _DL is different from the frequency of _UL carrier frequency f _UL .

  In this way, the terminal 21 estimates the Doppler frequency of the handover destination base station 12, corrects the Doppler shift of the random access signal, and transmits it. Thereby, the handover destination base station 12 can suppress the frequency offset amount of the received random access signal to be small, and can suppress the deterioration of the reception quality of the random access signal.

  Further, after transmitting the random access signal, the terminal 21 corrects the UL signal based on the correction amount fed back from the handover destination base station 12, as in FIG. Since the frequency offset amount of the random access signal received by the handover destination base station 12 has already been corrected by the terminal 21, the base station 12 calculates a correction amount relative to the random access signal. That is, the base station 12 does not need to transmit the absolute correction amount to the terminal 21 in the frequency offset control after receiving the random access signal, and can transmit the correction amount to the terminal 21 with a small number of bits.

Next, a handover procedure in LTE (Long Term Evolution) will be described.
FIG. 3 is a sequence diagram of handover in LTE. In the figure, a sequence of a terminal, a handover (HO) source base station, and a handover destination base station is shown.

In step S1, the terminal transmits the level measurement result of the neighboring cell to the handover source base station.
In step S2, the handover source base station determines the handover destination base station of the terminal based on the received level measurement result. The handover source base station makes a handover request to the determined hand base station.

  In step S3, the handover destination base station that has received the handover request returns confirmation of the handover request to the handover source base station if there is no problem with the handover.

In step S4, when receiving the confirmation of the handover request from the handover destination base station, the handover source base station issues a handover command to the terminal.
In step S5, when receiving a handover command from the handover source base station, the terminal receives the DL signal of the handover destination base station and performs synchronization processing. For example, the terminal performs DL synchronization processing using a synchronization channel or a common reference signal. Further, the terminal estimates the Doppler frequency in the handover destination base station as described in the equations (10) to (14), and calculates the carrier frequency of the random access signal as described in the equation (15).

In step S6, the terminal transmits a random access signal to the handover destination base station based on the calculated carrier frequency.
In step S7, the handover destination base station transmits radio resource scheduling information and TA (Timing Advance) information to the terminal.

In step S8, the terminal notifies the handover destination base station of the completion of the handover.
Next, a random access signal will be described. The random access signal is a signal transmitted by the terminal when the terminal is not synchronized with the base station. For example, it is transmitted when the terminal is powered on or handed over. The base station can perform wireless communication by synchronizing with the terminal using a random access signal.

  FIG. 4 is a diagram illustrating an arrangement example of random access channels. The vertical direction in the figure indicates frequency, and the horizontal direction indicates time. As shown in the figure, the terminal transmits a random access signal at every transmission interval of the random access channel. For example, the terminal selects one of two channels (RACH1, RACH2) of the system bandwidth and transmits a random access signal.

  FIG. 5 is a diagram illustrating a data configuration example of a random access signal. The random access signal is composed of a cyclic prefix, a preamble sequence, and a guard time. The Cyclic prefix and Guard time are set to be larger than the maximum transmission delay variation of the random access signal.

  The Preamble sequence is changed for each terminal. For the preamble sequence, a code sequence having good autocorrelation and cross-correlation characteristics is used so that the random access signal can be identified even if the random access signal is transmitted to the base station. For example, a CAZAC sequence or a Zadoff-Chu sequence is used.

  FIG. 6 is a diagram illustrating another data configuration example of the random access signal. The random access signal in FIG. 6 has a shorter preamble sequence than the random access signal in FIG.

  As a method for improving reception characteristics even for a random access signal having a large frequency offset, a method using a short preamble sequence as shown in FIG. 6 has been proposed (for example, 3GPP TSG-RAN WG1 # 47 R1-062387). . This is because shortening the Preamble sequence length improves the autocorrelation and cross-correlation characteristics when there is a frequency offset, and can improve reception characteristics.

However, the above method has a problem that the number of sequences that can be used for the preamble sequence is reduced because the preamble sequence length is shortened.
On the other hand, the terminal 21 described with reference to FIG. 2 estimates the Doppler frequency of the handover destination base station 12, corrects the Doppler shift of the random access signal, and transmits. Thereby, the reception characteristic of the random access signal of the handover destination base station 12 can be improved without shortening the preamble sequence length.

Next, functions of the base station and the terminal will be described.
FIG. 7 is a functional block diagram of the base station. The transmission buffer 31 temporarily stores a plurality of data 1, 2,... Transmitted to each user (terminal).

  The base station cannot transmit all user data at once. The DLSC (DL scheduler) 32 schedules which user data is to be transmitted in a radio frame at the present time.

When it is determined by the DLSC 32 which user's data is to be transmitted, the user multiplexing unit 33 takes out the corresponding data from the transmission buffer 31 and multiplexes the data.
The ULSC 34 performs UL signal scheduling.

Common control information common to each user, individual individual control information, and UL schedule information are input to the control channel multiplexing unit 35. The individual control information includes a correction amount (f _CNT ) for correcting the frequency offset amount. The control channel multiplexing unit 35 multiplexes common control information, individual control information, and UL scheduling information on the multiplexed data output from the user multiplexing unit 33.

  An IFFT (Inverse Fast Fourier Transform) 36 performs IFFT processing on the data output from the control channel multiplexing unit 35. The base station modulates the data into OFDM (Orthogonal Frequency Division Multiplexing) and transmits it to the terminal.

A CP (Cyclic Prefix) adding unit 37 adds a guard interval to data output from the IFFT 36.
The oscillator 38 outputs a high-frequency signal for up-converting data output from the CP assigning unit 37. A UC (Up Conversion) 39 up-converts data output from the CP assigning unit 37 based on a signal output from the oscillator 38. The amplifier 40 amplifies the up-converted signal and outputs it to the antenna duplexer 41. The antenna duplexer 41 outputs the signal amplified by the amplifier 40 to the antenna.

  The antenna duplexer 41 outputs a signal from the terminal received by the antenna to the amplifier 42. The amplifier 42 amplifies the signal output from the antenna duplexer 41. A DC (Down Conversion) 43 down-converts the signal output from the amplifier 42 by the signal output from the oscillator 38.

  The CP removal unit 44 removes the guard interval of data output from the DC 43. The FFT 45 performs an FFT process on the signal output from the CP removal unit 44. The conversion unit 46 performs sampling conversion of data frequency-converted by FFT processing. An IDFT (Inverse Discrete Fourier Transform) 47 performs IDFT processing on the data output from the conversion unit 46 and converts it into the time domain.

The demodulator 48 and the error corrector 49 demodulate the data output from the IDFT 47 and perform error correction. Thereby, the data 1, 2,... Received from each terminal are obtained.
The offset measuring unit 50 measures the frequency offset as described with reference to FIG. The offset control unit 51 obtains the correction amount f _CNT from the frequency offset measured by the offset measurement unit 50 as described with reference to FIG. The correction amount f _CNT is output to the individual information control unit 52 and is included in the individual control information as described above.

  FIG. 8 is a functional block diagram of the terminal. The antenna duplexer 61 outputs the OFDM signal from the base station received by the antenna to the amplifier 62. The amplifier 62 amplifies the signal output from the antenna duplexer 61. The DC 64 down-converts the signal output from the amplifier 62 based on the signal output from the oscillator 63. The CP removing unit 65 removes the guard interval of the data output from the DC 64. The FFT 66 performs FFT processing on the output data from the CP removal unit 65. The demodulator 67 demodulates data output from the FFT.

As described with reference to FIG. 2, the Doppler estimation unit 68 estimates the Doppler frequency in the handover destination base station when the terminal is handed over. The control information extraction unit 69 extracts the correction amount _fCNT included in the control information transmitted from the base station. The error correction unit 70 performs error correction on the data output from the demodulation unit 67.

The encoding unit 71 encodes data to be transmitted to the base station. The modulation unit 72 modulates the encoded data.
The multiplexing unit 73 multiplexes the random access signal transmitted when the terminal is handed over to the data output from the modulation unit 72.

  The DFT 74 performs DFT processing on the data output from the multiplexing unit 73. The conversion unit 75 performs sampling conversion of data frequency-converted by the DFT 74 process. The IFFT 76 performs IFFT processing on the data output from the conversion unit 75 and converts it into the time domain.

The correction unit 77 performs frequency offset correction of data output from the IFFT 76. The correcting unit 77 corrects the frequency offset using the correction amount f _CNT output from the control information extracting unit 69. At the time of handover, the frequency offset is corrected with the Doppler frequency at the handover destination base station estimated by the Doppler estimation unit 68.

  The CP assigning unit 78 adds a guard interval to the signal output from the correcting unit 77. The UC 79 up-converts the signal output from the CP assigning unit 78 based on the signal output from the oscillator 63. The amplifier 80 amplifies the signal output from the UC 79. The antenna duplexer 61 outputs the signal output from the amplifier 80 to the antenna.

  In this way, the terminal 21 estimates the Doppler frequency of the handover destination base station 12, corrects the Doppler shift of the random access signal, and transmits it. As a result, the handover destination base station 12 can suppress the frequency offset amount of the received random access signal to be small, and can reduce the degradation of the reception quality of the random access signal.

  Further, the handover destination base station 12 does not need to transmit the absolute correction amount to the terminal 21 in frequency offset control after receiving the random access signal, and can transmit the correction amount to the terminal 21 with a small number of bits. Become.

  Next, a second embodiment of the present invention will be described in detail with reference to the drawings. In the first embodiment, the carrier frequency of the handover source base station and the handover destination base station are the same. In the second embodiment, estimation of the Doppler frequency when the carrier frequency of the handover source base station and the handover destination base station are different will be described.

  FIG. 9 is a diagram illustrating a communication system of a terminal according to the second embodiment. In the figure, base stations 91 and 92 and a terminal 101 are shown. The carrier frequencies of the DL signals of the base station 91 and the base station 92 are different.

The terminal 101 is a wireless terminal such as a mobile phone, for example. The terminal 101 moves from the cell of the base station 91 to the cell of the base station 92 and performs a handover.
The terminal 101 transmits a random access signal to the handover destination base station 92 at the time of handover. At this time, the terminal 101 estimates the Doppler frequency in the handover destination base station 92 based on the correction amount _fCNT received from the handover source base station 91, and corrects the random access signal. Subsequent frequency offset control of the UL signal subsequent to the random access signal is performed based on a control signal fed back from the handover destination base station 92 as in the description of FIG.

In the case of handover at a different frequency, the equation (13) shown in the first embodiment cannot be used to calculate the Doppler frequency. This is because the frequency deviation of the oscillator in the terminal free-run state usually has a large value. Therefore, the terminal estimates the Doppler frequency f _D2 in the handover destination base station 92 using the correction amount f _CNT transmitted from the handover source base station 91 as shown in the following equation (16).

f _D2 = −f _CNT / 2 (16)
Since f _CNT is Δf _UE + 2f _D1 , Equation (16) becomes as shown in the following Equation (17).

f _D2 = − (Δf _UE + 2f _D1 ) / 2 = −f _D1 −Δf _UE / 2 (17)
f _D1 is a Doppler frequency in the handover source base station 91. Δf _UE is a frequency deviation of the terminal 101.

The frequency deviation Δf _UE after the frequency pull-in of the terminal 101 is usually 0.1 ppm. If the carrier frequency is 2 GHz, the error of f _D2 is f _D1 ± 100 Hz or less.
At the time of handover, since the terminal 101 is considered to approach the handover destination base station 92, the Doppler frequencies in the handover source base station 91 and the handover destination base station 92 are usually reversed in polarity. The magnitude (absolute value) of the Doppler frequency depends on the positional relationship between the two base stations 91 and 92 and the terminal 101 and the moving direction of the terminal, and is not always the same.

  For example, when the terminal 101 moves on the straight line between the base station 91 and the base station 92, the magnitude of the Doppler frequency is considered to be the same, but is not on the straight line between the base station 91 and the base station 92, When the terminal 101 is moving in the direction (for example, the terminal 101 is moving diagonally right upward in FIG. 9), the magnitudes of the Doppler frequencies are not the same.

Since it is not necessary to completely correct the Doppler frequency f _D2 in the handover destination base station 92 and it is considered that it should be suppressed to a certain level or less, the value (f _D2 ≈− f _D1 ) is used. Terminal 101 transmits a random access signal at a frequency shown in the following equation (18).

f _UL -2f _D2 (18)
f _UL indicates the UL carrier frequency.
Note that the base stations 91 and 92 have the same functional blocks as in FIG. The terminal 101 has the same functional blocks as in FIG. 8, but the function of the Doppler estimation unit 68 is different. In the case of the terminal 101, the Doppler estimation unit 68 estimates the Doppler frequency based on Expression (16).

  Thus, the terminal 101 estimates the Doppler frequency of the handover destination base station 92 based on the correction amount transmitted from the handover source base station 91, corrects the Doppler shift of the random access signal, and transmits it. As a result, the handover destination base station 92 can suppress the frequency offset amount of the received random access signal to be small, and can suppress the deterioration of the reception quality of the random access signal.

  The above merely illustrates the principle of the present invention. In addition, many modifications and changes can be made by those skilled in the art, and the present invention is not limited to the precise configuration and application shown and described above, and all corresponding modifications and equivalents may be And the equivalents thereof are considered to be within the scope of the invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Wireless terminal device 1a Frequency calculation means 1b Frequency correction means 2 Handover source base station 3 Handover destination base station

Claims (5)

In a wireless terminal device that performs wireless communication with a base station,
Frequency calculation for calculating the Doppler frequency in the handover destination base station from the downlink frequency of the handover source base station, the correction amount of the uplink frequency notified from the handover source base station, and the downlink frequency of the handover destination base station Means,
Frequency correcting means for correcting the frequency of a random access signal to be transmitted to the handover destination base station based on the Doppler frequency;
A wireless terminal device comprising:   The frequency calculation means subtracts the downlink frequency of the handover destination base station from the downlink frequency of the handover source base station, and further subtracts the correction amount to calculate the Doppler frequency. The wireless terminal device according to 1.   The frequency correction means corrects the frequency of the random access signal based on the Doppler frequency, the downlink frequency of the handover destination base station, and the uplink frequency of the handover destination base station. The wireless terminal device according to claim 1.   In a wireless terminal device that performs wireless communication with a base station,
  A frequency calculation means for calculating a Doppler frequency in the handover destination base station from a correction amount of the frequency offset notified from the handover source base station;
  Frequency correcting means for correcting the frequency of a random access signal to be transmitted to the handover destination base station based on the Doppler frequency;
  A wireless terminal device comprising:   5. The radio terminal apparatus according to claim 4, wherein the frequency calculation means calculates the Doppler frequency by dividing the correction amount by two.

Images