JP2015056868A - Base station and communication control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a base station and a communication control method that are able to improve accuracy in estimation and correction of a frequency offset by preventing feedback of an inaccurate frequency offset due to erroneous determination.SOLUTION: A configuration of a base station 110 according to the invention comprises: a search processing unit 114a that receives control signals from a plurality of peripheral base stations 120a-120d and selects a peripheral base station from which the reception intensity of the control signal is the highest as a synchronization destination base station; and a synchronization processing unit 114b that repeats reception of the control signal from the synchronization destination base station and synchronizes frame timing with the synchronization destination base station. The search processing unit individually estimates frequency offsets at the time of demodulating the control signals from the plurality of peripheral base stations. The synchronization processing unit demodulates the control signal received from the synchronization destination base station by using the frequency offset estimated for the synchronization destination base station by the search processing unit.

Description

  The present invention relates to a base station that performs wireless communication with a wireless terminal and frame synchronization with peripheral base stations existing in the vicinity, and a communication control method in the base station.

  In a wireless communication system, a frequency offset (frequency error) may occur due to a difference in oscillation frequency of an oscillator (clock) between a transmission-side base station and a receiving-side base station. When a frequency offset occurs, the received signal is distorted in the base station on the receiving side, resulting in a decrease in reception quality. In order to prevent this, the base station performs frequency offset correction that estimates a frequency offset, corrects the offset amount to a received signal, and demodulates the received signal. For example, in Patent Document 1, when an actual received signal and a signal (reference signal) that is known to be received in advance are compared and frequency offset estimation is performed using the difference, the average is repeatedly estimated during wireless communication between base stations. A method of updating an offset estimated value by a conversion process or the like is disclosed.

  By the way, in PHS (Personal Handy phone System), frame synchronization for aligning frame timing (transmission / reception timing) is performed. In frame synchronization, reference base stations serving as frame timing references are interspersed in each predetermined area, and neighboring base stations receive control signals sequentially around the reference base stations, demodulate the control signals, and demodulate them. This is a process of determining that the received timing is the timing at which the neighboring base station has transmitted the control signal and matching the frame timing.

  Frame synchronization processing is broadly divided into search processing and synchronization processing. In the search process, control signals from a plurality of neighboring base stations are received, and a neighboring base station having the highest received strength of such control signals is selected as a synchronization destination base station. In the synchronization process, the frame timing is synchronized with the synchronization destination base station by repeatedly receiving the control signal from the synchronization destination base station selected in the search process. After these processes are completed, the base station transmits a radio signal in accordance with the frame timing.

JP 2001-285161 A

  In order to improve the accuracy of control signal reception processing, the above-described frequency offset correction is also performed during frame synchronization. At that time, in order to improve the accuracy of frequency offset estimation and correction, the frequency offset is estimated and corrected every time the control signal is repeatedly received, and the result is fed back (taken over) at the next estimation and correction, thereby averaging the frequency offset. Etc.

  Here, in frame synchronization, signals of a plurality of peripheral base stations are received, but the clocks of the peripheral base stations are synchronized with a clock supplied from the PHS network. Therefore, since the frequency offset of each neighboring base station normally has the same value, the frequency offset estimated by the control signal received from one neighboring base station is fed back to the frequency offset estimating process of the received signal of the other neighboring base station. Is basically not a problem.

  Further, as described above, the base station repeatedly performs reception processing in the search processing, and searches for timings at which the control signal can be demodulated. At this time, in an area where the installation density of the peripheral base stations is low, there is a timing when none of the peripheral base stations transmits a control signal. Even when the control signal is not transmitted, if the base station performs control signal reception processing, noise is received. Normally, even if such noise is received, it cannot be demodulated, resulting in a reception error. As described in Patent Document 1, since it is determined that the frequency offset cannot be correctly estimated in the case of a reception error (CRC error), in such a case, the frequency offset is estimated and corrected for the next frequency offset. Is not fed back (cannot be taken over).

  However, in the CRC error check, even if the noise is supposed to be a CRC error, an erroneous determination may occur due to the coincidence of the CRC. Then, since the frequency offset that is not correctly estimated is fed back to the estimation and correction of the next frequency offset, the accuracy of the estimation and correction of the next frequency offset is lowered.

  The present invention has been made in view of such problems, and it is an object of the present invention to provide a base station and a communication control method capable of preventing feedback of a frequency offset with low accuracy due to erroneous determination and improving the accuracy of frequency offset estimation and correction. And

  In order to solve the above problems, a representative configuration of a base station according to the present invention is a base station that performs radio communication with a radio terminal and frame synchronization with neighboring base stations existing in the vicinity, and includes a plurality of base stations. A search processing unit that receives a control signal from a neighboring base station and selects a neighboring base station having the highest control signal reception strength as a synchronization destination base station, and repeatedly receives control signals from the synchronization destination base station, A synchronization processing unit that synchronizes the frame timing with the base station, the search processing unit individually estimates a frequency offset when demodulating control signals of a plurality of neighboring base stations, and the synchronization processing unit The control signal received from the synchronization destination base station is demodulated using the frequency offset estimated for the synchronization destination base station.

  In order to solve the above-described problem, another configuration of the base station according to the present invention is a base station that performs radio communication with a radio terminal and frame synchronization with peripheral base stations existing in the vicinity, and a plurality of peripheral stations A search processing unit that receives a control signal from a base station and selects a peripheral base station having the highest control signal reception intensity as a synchronization destination base station, and repeatedly receives control signals from the synchronization destination base station, A synchronization processing unit that synchronizes the frame timing with the station, and the search processing unit uses the frequency offset estimated for the peripheral base station whose reception strength of the control signal is equal to or higher than a predetermined value to The base station control signal is demodulated to estimate the frequency offset of the other neighboring base stations, and the synchronization processing unit uses the frequency offset estimated for the synchronization destination base station in the search processing unit. Characterized by demodulating the control signal received from.

  The synchronization processing unit may store the frequency offset estimated for the synchronization destination base station, and use the stored frequency offset for the second and subsequent demodulation of the control signal.

  In order to solve the above problems, a typical configuration of a communication control method according to the present invention is a communication control method in a base station that performs radio communication with a radio terminal and frame synchronization with neighboring base stations existing in the vicinity. And receiving a control signal from a plurality of neighboring base stations, performing a search process for selecting a neighboring base station having the highest received strength of the control signal as a synchronizing destination base station, and receiving a control signal from the synchronizing destination base station. Is repeated to synchronize the frame timing with the synchronization destination base station. In the search process, the frequency offset is estimated individually when demodulating the control signals of a plurality of neighboring base stations. The control signal received from the synchronization destination base station is demodulated using the frequency offset estimated for the synchronization destination base station.

  In order to solve the above problems, another configuration of the communication control method according to the present invention is a communication control method in a base station that performs radio communication with a radio terminal and frame synchronization with neighboring base stations existing in the vicinity. Receive a control signal from a plurality of neighboring base stations, perform a search process to select a neighboring base station having the highest received strength of the control signal as a synchronizing destination base station, and receive a control signal from the synchronizing destination base station. Repeatedly, the synchronization processing is performed to synchronize the frame timing with the synchronization destination base station, and the search processing uses other frequency offsets estimated for the neighboring base stations whose control signal reception strength is equal to or higher than a predetermined value. Demodulate the control signal of the neighboring base station and estimate the frequency offset of the other neighboring base stations. In the synchronization process, the frequency offset estimated for the synchronization destination base station in the search process Characterized by demodulating the control signal received from the destination base station using.

  ADVANTAGE OF THE INVENTION According to this invention, the base station and communication control method which can prevent the feedback of the frequency offset with a low precision by misjudgment, and can raise the precision of estimation and correction | amendment of a frequency offset can be provided.

It is a figure explaining the base station concerning this embodiment, and the radio | wireless communications system containing the same. It is a flowchart explaining the operation | movement of the search process of 1st Embodiment. It is a flowchart explaining the operation | movement of the synchronous process of 1st Embodiment. It is a flowchart explaining operation | movement of 2nd Embodiment of a base station. It is a flowchart explaining the operation | movement of the search process of 3rd Embodiment. It is a flowchart explaining the operation | movement of the synchronous process of 3rd Embodiment. It is a flowchart explaining the operation | movement of the search process of 4th Embodiment. It is a flowchart explaining the operation | movement of the synchronous process of 4th Embodiment.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The dimensions, materials, and other specific numerical values shown in the embodiment are merely examples for facilitating understanding of the invention, and do not limit the present invention unless otherwise specified. In the present specification and drawings, elements having substantially the same function and configuration are denoted by the same reference numerals, and redundant description is omitted, and elements not directly related to the present invention are not illustrated. To do.

  FIG. 1 is a diagram illustrating a base station 110 according to the present embodiment and a radio communication system 100 including the base station 110, and FIG. 1A illustrates a radio communication system 100 including the base station 110 according to the present embodiment. FIG. 1B is a functional block diagram illustrating the configuration of the base station 110 according to this embodiment.

  In the embodiments described below, after describing a configuration common to the first to fourth embodiments, characteristic operations in the respective embodiments will be described in detail. In the present embodiment, the case where four peripheral base stations 120a, 120b, 120c, and 120d are arranged around the base station 110 will be described as an example, but the present invention is limited to this. The number of neighboring base stations can be set as appropriate.

  As shown to Fig.1 (a), the radio | wireless communications system 100 concerning this embodiment is comprised including the base station 110 and several peripheral base station 120a-120d which exists in the periphery. Both the base station 110 and the peripheral base stations 120a to 120d perform wireless communication with the wireless terminal 102 (see FIG. 1B). In addition, the base station 110 performs frame synchronization using any one of the neighboring base stations 120a to 120d as a synchronization destination.

  As shown in FIG. 1B, the base station 110 of the present embodiment includes a control unit 112, a storage unit 116, and a wireless communication unit 118. The control unit 112 manages and controls the entire base station 110 by a semiconductor integrated circuit including a central processing unit (CPU). The storage unit 116 includes a ROM, a RAM, an EEPROM, a nonvolatile RAM, a flash memory, an HDD, and the like, and stores a program processed by the control unit 112. The wireless communication unit 118 wirelessly communicates with the wireless terminal 102 such as a PHS terminal via the communication antenna 118a and a frame with the peripheral base stations 120a to 120d (the peripheral base station 120b is illustrated in FIG. 2) existing in the vicinity. Synchronize.

  In the present embodiment, the control unit 112 also functions as the search processing unit 114a and the synchronization processing unit 114b. The search processing unit 114a receives control signals from the plurality of peripheral base stations 120a to 120d, and selects a peripheral base station having the highest control signal reception intensity as a synchronization destination base station (search processing). The synchronization processing unit 114b repeatedly receives control signals from the synchronization destination base station, and synchronizes the frame timing with the synchronization destination base station (synchronization processing).

(First embodiment)
2 and 3 are flowcharts for explaining the operation of the first embodiment of the base station 110, FIG. 2 is a flowchart for explaining the search processing operation of the first embodiment, and FIG. It is a flowchart explaining the operation | movement of the synchronous process of embodiment. In the first to fourth embodiments described below, the operation of the base station 110 is described, and the communication control method in the base station 110 is also described. Also, in the flowchart of FIG. 3, the same processing as in FIG.

  As shown in FIG. 2, in the search processing of the first embodiment, in the base station 110, the control unit 112 functions as the search processing unit 114a, and the wireless communication unit 118 receives a control signal from the neighboring base station 120a ( In step S202), a frequency offset when demodulating the control signal is set to an initial value (for example, 0 Hz) (step S204). Then, the search processing unit 114a demodulates the control signal (step S206), and estimates the frequency offset at that time (step S208). If the control signal can be demodulated (YES in step S210), the storage unit 116 stores the reception intensity when the control signal is received, the reception timing, and the data of the frequency offset when the control signal is demodulated (step S210). S212).

  After the data is stored (step S212) and when the control signal cannot be demodulated (NO in step S210), the search processing unit 114a determines whether reception of the control signal is completed at all timings (step S214). . If reception of the control signal has not been completed at all timings (NO in step S214), the search processing unit 114a changes the reception timing (step S216), and repeats steps S202 to S216. As a result, the base station further receives control signals from the neighboring base stations 120b to 120d, and estimates frequency offsets for the neighboring base stations 120a to 120d, respectively.

  That is, in the search processing of the first embodiment, the frequency offset estimated when the control signal of the neighboring base station 120a is received is not fed back to the demodulation of the control signal of the next received neighboring base station 120b (not taken over). ). The same applies to demodulation of control signals of peripheral base stations after the peripheral base station 120c. In other words, in the first embodiment, the search processing unit 114a estimates the frequency offset individually when demodulating the control signals of the plurality of neighboring base stations 120b to 120d.

  When reception of the control signal is completed at all timings (YES in step S214), the search processing unit 114a selects a neighboring base station having the highest received strength of the control signal among the neighboring base stations 120b to 120d that have received the control signal. A synchronization destination base station is selected (step S218). Hereinafter, the case where the peripheral base station 120b is selected as the synchronization destination base station will be described as an example.

  Subsequently, the search processing unit 114a refers to the storage unit 116 and determines whether or not the reception intensity of the control signal received from the synchronization destination base station is equal to or higher than a predetermined value (step S220). If the received intensity is equal to or greater than the predetermined value (YES in step S220), the search processing unit 114a uses the frequency offset estimated when demodulating the control signal of the neighboring base station 120b that has become the synchronization destination base station (estimated during the search process). Is determined as a frequency offset at the time of the synchronization process performed after the search process (step S222). On the other hand, if the reception intensity is not greater than or equal to the predetermined value (NO in step S220), the frequency offset at the time of synchronization processing is set to an initial value (for example, 0 Hz) (step S224).

  As described above, in the search process of the first embodiment, the frequency offset estimated when demodulating the control signal is fed back to the frequency offset at the time of the synchronization process according to the reception strength of the control signal received from the synchronization destination base station. Judgment whether or not. If the reception intensity is equal to or higher than a predetermined value, that is, the reception intensity is high, it is estimated that the estimated value of the frequency offset is close to the true value even when a reception error (CRC) occurs. Therefore, when the reception strength of the control signal is high, even if the frequency offset is fed back due to the erroneous determination of the CRC error check, it is considered that the influence on the estimation accuracy of the frequency offset during the synchronization processing is extremely small.

  On the other hand, if the control signal reception strength is low, feeding back the frequency offset estimated when demodulating the control signal to the frequency offset at the time of synchronization processing may adversely affect the estimation of the frequency offset at the time of synchronization processing. There is. Therefore, when the control signal reception strength is low, the frequency offset estimated in the search process is not fed back, and the frequency offset is set to the initial value, thereby reducing the estimation accuracy of the frequency offset during the synchronization process. It becomes possible to prevent.

  In the present embodiment, in the search process, whether or not the frequency offset estimated when demodulating the control signal is fed back to the frequency offset at the time of the synchronization process based on the reception strength of the control signal received from the synchronization destination base station. Although the process (step S220-step S224) which judges whether is provided, it is not limited to this, It is good also as not including those processes. In this embodiment, since the neighboring base station having the highest control signal reception strength is selected as the synchronization destination base station (step S218), it is considered that the control signal reception strength exceeds a predetermined value. It is.

  Subsequently, as shown in FIG. 3, in the synchronization processing of the first embodiment, in the base station 110, the control unit 112 functions as the synchronization processing unit 114b, refers to the storage unit 116, and performs synchronization processing at the synchronization destination base station. In this embodiment, the reception timing when the control signal from the neighboring base station 120b is received is set as the synchronization timing (step S232). Then, the synchronization processing unit 114b receives the control signal from the synchronization destination base station by the wireless communication unit 118 at the timing (step S234).

  When the control signal is received (step S234), the synchronization processing unit 114b determines whether or not the received control signal is a control signal received for the first time in the synchronization processing (step S236). If the received control signal is the first control signal (YES in step S236), the synchronization processing unit 114b uses the frequency offset determined in the search process (step S222 or step S224 in FIG. 2) as the control signal. The frequency offset used for demodulation is set (step S238).

  In step S238, in the first embodiment, if the reception intensity of the control signal received from the synchronization destination base station in the search process described above is equal to or higher than a predetermined value, the control signal of the peripheral synchronization destination base station is demodulated. The frequency offset estimated at the time of processing (frequency offset estimated at the time of the search processing) is set to the frequency offset used when demodulating the control signal received for the first time at the time of the synchronization processing. On the other hand, when the reception intensity of the control signal received from the synchronization destination base station in the search process described above is less than a predetermined value, an initial value (for example, 0 Hz) of the frequency offset is set. In the present embodiment, the former case will be described as an example.

  Subsequently, the synchronization processing unit 114b demodulates the control signal received from the synchronization destination base station using the frequency offset estimated for the synchronization destination base station during the search process (step S242), and the frequency offset at that time Is estimated (step S244). When the control signal can be demodulated (YES in step S210), the synchronization processing unit 114b adjusts the frame timing of the own station to match the frame timing of the synchronization destination base station (step S246).

  Then, the synchronization processing unit 114b determines whether or not the reception intensity of the control signal received in step S234 is greater than or equal to a predetermined value (step S220). If the received intensity is equal to or greater than the predetermined value (YES in step S220), the synchronization processing unit 114b uses the frequency offset stored in the storage unit 116 (the frequency offset estimated during the search process when receiving the first control signal). The frequency offset estimated in step S244 is updated (step S248). If the reception intensity is less than the predetermined value (NO in step S220), the frequency offset stored in the storage unit 116 (the frequency offset estimated during the search process when receiving the first control signal) is not updated. .

  After updating the frequency offset (step S248), if the control signal cannot be demodulated (NO in step S210), and if the received intensity of the control signal received in step S234 is less than a predetermined value (NO in step S220) ), The synchronization processing unit 114b determines whether or not the synchronization processing (frame timing synchronization) of the base station 110 has been completed (step S250). When the synchronization process is completed (YES in step S250), the synchronization processing unit 114b ends the synchronization process. If the synchronization process has not been completed (NO in step S250), the synchronization processing unit 114b repeats the above-described steps S234 to S250.

  When step S234 to step S250 are repeated, in the next step S236, the control signal received in step S234 is not the first control signal in the synchronization process, but the control signal for the second and subsequent times (NO in step S236). Therefore, the synchronization processing unit 114b sets the frequency offset updated in step S248 (updated frequency offset) as the frequency offset for demodulating the control signal in step S242 (step S240).

  As described above, in the operation of the first embodiment of the base station 110 and the communication control method of the first embodiment, in the search process, when the control signals of the plurality of neighboring base stations 120b to 120d are demodulated individually. Estimate the frequency offset. Thereby, in the search process, when the control signals from the plurality of frequency base stations 120b to 120d are demodulated, the feedback of the frequency offset with low accuracy due to the erroneous determination of the CRC error check is prevented. Therefore, the accuracy of frequency offset estimation and correction can be improved. In the synchronization process, the reception performance in the synchronization process is improved by demodulating the control signal received from the synchronization destination base station using the frequency offset estimated for the synchronization destination base station in the search process. It becomes possible.

  In the embodiment described above, in order to facilitate understanding, a configuration (step S248) in which the frequency offset stored in the storage unit 116 is updated to the frequency offset estimated in step S244 in the synchronization process is illustrated. However, the present invention is not limited to this. In actual operation, it is preferable to average the frequency offset estimated every time step S234 to step S250 are repeated in the synchronization process, and use the averaged frequency offset as the update value in step S248.

(Second Embodiment)
In the first embodiment, in the search process, the frequency offset is individually estimated when demodulating the control signals of the plurality of neighboring base stations 120b to 120d. In the synchronization process, the frequency offset is estimated for the synchronization destination base station in the search process. The frequency offset is used to demodulate the control signal received from the synchronization destination base station. On the other hand, in the second embodiment, in the search process, the control signal of another neighboring base station is used by using the frequency offset estimated for the neighboring base station whose reception strength of the control signal is equal to or higher than a predetermined value. And estimate the frequency offset. In the synchronization process, as in the first embodiment, the control signal received from the synchronization destination base station is demodulated using the frequency offset estimated for the synchronization destination base station in the search process. Therefore, since only the search process is different between the first embodiment and the second embodiment, the description of the synchronization process is omitted in the following description.

  FIG. 4 is a flowchart for explaining the operation of the second embodiment of the base station, and particularly shows the operation of the search processing of the second embodiment. In addition, about the process which overlaps with the search process of 1st Embodiment, duplication description is avoided by attaching | subjecting the same code | symbol. As shown in FIG. 4, in the search processing of the second embodiment, in the base station 110, the control unit 112 functions as the search processing unit 114a, and after the wireless communication unit 118 receives a control signal from the neighboring base station 120a. (Step S202), it is determined whether or not the control signal is the control signal received for the first time in the search process (Step S302).

  If the control signal is the first control signal (YES in step S302), the search processing unit 114a sets the initial value (for example, 0 Hz) when estimating the frequency offset (step S204), and stores the data in the storage unit 116. Processing up to storage (step S212) is performed. Thereafter, the search processing unit 114a determines whether or not the reception intensity of the control signal received in step S202 is equal to or higher than a predetermined value (step S306).

  If the reception intensity of the control signal is equal to or higher than the predetermined value (YES in step S306), the search processing unit 114a uses the frequency offset stored in the storage unit 116 (initial value when receiving the first control signal) in step S208. The frequency offset estimated in step S308 is updated (step S308). If the reception intensity is less than the predetermined value (NO in step S306), the frequency offset is not updated. Thereafter, the search processing unit 114a repeats Steps S202 to S216 until reception of the control signal is completed at all timings (NO in Step S214).

  When step S202 to step S216 are repeated, in the next step S302, the control signal received in step S202 is not the first control signal in the search process, but the second and subsequent control signals (NO in step S302). Therefore, the search processing unit 114a sets the frequency offset updated in step S308 (updated frequency offset) as a frequency offset for demodulating the control signal in step S206 (step S304). In step S206, the search processing unit 114a searches for the frequency offset updated in step S308, that is, among the control signals received before the control signal received this time, the control signal whose reception intensity is equal to or higher than a predetermined value. The control signal is demodulated using the frequency offset estimated for the transmitted neighboring base station.

  When reception of the control signal is completed at all timings (YES in step S214), the search processing unit 114a selects a neighboring base station having the highest received strength of the control signal among the neighboring base stations 120a to 120d that have received the control signal. A synchronization destination base station is selected (step S218). Then, the search processing unit 114a determines the frequency offset estimated in step S208 when demodulating the last received control signal (frequency offset estimated during the search process) as the frequency offset during the synchronization process performed after the search process. (Step S310).

  As described above, in the operation of the second embodiment of the base station 110 and the communication control method of the second embodiment, the control signal received before the second time or later in the search process is demodulated in the search process. A frequency offset estimated for a neighboring base station that has transmitted a control signal having a received strength equal to or higher than a predetermined value is used. Thereby, it is possible to improve the reception performance in the search process as compared with the case where the frequency offset is estimated individually for each of the neighboring base stations 120b to 120d.

(Third embodiment)
5 and 6 are flowcharts for explaining the operation of the third embodiment of the base station 110, FIG. 5 is a flowchart for explaining the search processing operation of the third embodiment, and FIG. It is a flowchart explaining the operation | movement of the synchronous process of embodiment. In addition, about the process which overlaps with 1st Embodiment and 2nd Embodiment, duplication description is avoided by attaching | subjecting the same code | symbol.

  The operation of the third embodiment is characterized in that the past frequency offset stored in the storage unit 116 for the neighboring base stations is used in the search process. Specifically, as shown in FIG. 5, in the search processing of the third embodiment, in the base station 110, the control unit 112 functions as the search processing unit 114a and receives the same reception intensity as the control signal received in step S202. It is determined whether or not the data of the neighboring base station that has transmitted the control signal having the strength is stored in the storage unit 116 (step S402). When the data of the peripheral base station is not stored in the storage unit 116 (NO in step S402), the search processing unit 114a sets an initial value (for example, 0 Hz) for estimating the frequency offset (step S204).

  In the search process, the base station ID of the neighboring base station that transmitted the control signal is not known only by receiving the control signal. Therefore, the data of the neighboring base station is stored in the storage unit 116 using the base station ID as the search ID. I can't search. Here, since the positions of the neighboring base stations 120a to 120d are basically fixed, the reception intensity does not vary greatly from the past frame synchronization. Therefore, by using the reception intensity as a search key as described above, it is possible to determine whether or not data of neighboring base stations is stored in the storage unit 116.

  When the data of the peripheral base station is stored in the storage unit 116, the search processing unit 114 a is a frequency offset estimated for the peripheral base station in the past stored in the storage unit 116 in association with the peripheral base station. Is set to the frequency offset used when demodulating the control signal in step S202 (step S404). Thereafter, the search processing unit 114a performs processing from demodulation of the control signal using the frequency offset set in step S404 (step S206) to storage of data in the storage unit 116 (step S212).

  After storing the data, the search processing unit 114a determines whether or not the reception intensity of the control signal received in step S202 is equal to or higher than a predetermined value (step S406). If the reception intensity of the control signal is equal to or greater than the predetermined value (YES in step S406), the search processing unit 114a uses the frequency offset previously estimated for the neighboring base stations stored in the storage unit 116 in step S208. The frequency offset estimated in step S408 is updated (step S408). If the reception intensity is less than the predetermined value (NO in step S406), the frequency offset is not updated.

  Thereafter, the search processing unit 114a repeats Steps S202 to S216 until reception of the control signal is completed at all timings (NO in Step S214). As a result, the base station 110 further receives control signals from the neighboring base stations 120b to 120d, and individually estimates the current frequency offset for each of the neighboring base stations 120a to 120d. If the reception strength of the control signals from the neighboring base stations 120a to 120d is equal to or higher than the predetermined value (YES in step S406), the storage unit 116 transmits the control signals of the plurality of neighboring base stations 120a to 120d. The past frequency offset estimated for each is individually updated (stored) in step S408.

  When reception of the control signal is completed at all timings (YES in step S214), the search processing unit 114a selects a neighboring base station having the highest received strength of the control signal among the neighboring base stations 120a to 120d that have received the control signal. A synchronization destination base station is selected (step S218). Then, the search processing unit 114a applies the frequency offset (frequency offset estimated during the search process) updated in step S408 to the peripheral base station 120b that has become the synchronization destination base station at the time of the synchronization process performed after the search process. A frequency offset is determined (step S410).

  Subsequently, as shown in FIG. 6, in the synchronization processing of the third embodiment, in the base station 110, the control unit 112 functions as the synchronization processing unit 114b, and updates the frequency offset from the setting of the synchronization timing (step S232). Processing up to (step S248) is performed. In the update of the frequency offset in step S248, the frequency offset stored in the storage unit 116 is updated (stored) to the frequency offset estimated for the control signal of the synchronization destination base station in step S244.

  If the synchronization process has not been completed (NO in step S250), the synchronization processing unit 114b repeats the above-described steps S234 to S250. At this time, in the next step S236, the control signal received in step S234 is not the first control signal in the synchronization process, but the second and subsequent control signals (NO in step S236). Accordingly, the synchronization processing unit 114b sets the frequency offset updated in step S248 and stored in the storage unit 116 as a frequency offset for demodulating the second and subsequent control signals in step S242 (step S240).

  When the synchronization processing is completed (YES in step S250), the synchronization processing unit 114b stores data related to the synchronization destination base station such as the frequency offset stored in the storage unit 116 and the reception strength when the control signal is received during the synchronization processing. Is updated (step S412). After the data update, the synchronization processing unit 114b ends the synchronization process.

  As described above, in the operation of the third embodiment of the base station 110 and the communication control method of the third embodiment, each of the plurality of neighboring base stations 120a to 120d is stored in the storage unit 116 during the past synchronization processing. The estimated frequency offset is individually stored, and in the search process, the control signals of the plurality of neighboring base stations 120a to 120d are demodulated using the stored past frequency offset. Thereby, even if there is a variation in the frequency offset for each of the neighboring base stations 120a to 120d, the past frequency offset for each of the neighboring base stations 120a to 120d stored in the storage unit 116 is used for demodulation of the control signal. Therefore, it is possible to improve the reception performance during the search process.

(Fourth embodiment)
7 and 8 are flowcharts for explaining the operation of the fourth embodiment of the base station 110, FIG. 7 is a flowchart for explaining the search processing operation of the fourth embodiment, and FIG. It is a flowchart explaining the operation | movement of the synchronous process of embodiment. In addition, about the process which overlaps with 1st Embodiment, duplication description is avoided by attaching | subjecting the same code | symbol.

  As shown in FIG. 7, in the search process of the fourth embodiment, if the control signal can be demodulated (YES in step S210), the storage unit 116 stores the reception timing when the control signal is received (step S502). ). When reception of the control signal is completed at all timings (YES in step S214), the search processing unit 114a, among the peripheral base stations 120b to 120d that have received the control signal, has the highest reception strength of the control signal. The station is selected as a synchronization destination base station (step S218), and the search process is terminated.

  In the first embodiment, since the frequency offset for demodulating the control signal received for the first time in the synchronization process after the search process is determined by determining the reception strength, the control signal is received in the data storage process. The received signal strength, reception timing, and frequency offset when the control signal is demodulated are stored as data. On the other hand, in the fourth embodiment, the reception strength is not judged after reception of the control signal at all timings is completed (YES in step S214). Therefore, in the data storage process (step S502), only the reception timing needs to be stored.

  That is, in the search process of the fourth embodiment, the frequency offset when demodulating the control signals of the plurality of neighboring base stations 120a to 120d is always set to an initial value (for example, 0 Hz) (step S204). In the search process of the fourth embodiment, the frequency offset used for the demodulation of the initial control signal at the time of the synchronization process performed thereafter is not set.

  Therefore, in the synchronization processing of the fourth embodiment, as shown in FIG. 8, when the control signal from the neighboring base stations 120a to 120d is received (step S234), the synchronization processing unit 114b demodulates the control signal. Is set to an initial value (for example, 0 Hz) (step S504). Then, after adjusting the frame timing (step S246), the synchronization processing unit 114b repeats steps S232 to S250 until the synchronization processing is completed (NO in step S250). At this time, in the synchronization processing of the fourth embodiment, the frequency offset is not updated after the frame timing is adjusted. Therefore, in the fourth embodiment, the frequency offset used for demodulation of the control signal is always set to an initial value (for example, 0 Hz) even in the synchronization process (step S504).

  As described above, in the operation of the fourth embodiment of the base station 110 and the communication control method of the fourth embodiment, the initial value is always used for demodulation of the control signal in both the search process and the synchronization process. That is, in the fourth embodiment, in the search process and the synchronization process, the frequency offset estimated when demodulating the control signal is not fed back to the frequency offset used when demodulating the next control signal. Therefore, it is possible to prevent a decrease in frequency offset estimation accuracy due to frequency offset feedback with low accuracy due to erroneous determination. Further, since the processing is simplified as compared with the first to third embodiments, the processing time can be shortened.

  In the first to third embodiments, the process of using the reception intensity of the control signal as a criterion for feedback to the frequency offset when demodulating the next control signal is included. However, the present invention is not limited to this. Not what you want. For example, if the difference between the estimated value of the past frequency offset (the value that received feedback) and the estimated value in the received frame is greater than or equal to a predetermined value, the credibility of the frequency offset value estimated in the received frame is low. It is also possible to adopt a configuration that does not consider feedback.

  Also, as a criterion for feedback to frequency offset when demodulating the next control signal, EVM (Error Vector Magnitude), CIR (desired signal to interference signal ratio), synchronization position (unique word signal (Detection position) can also be adopted (however, the synchronization position determination can be applied only to the synchronization process). In the case of EVM, when the EVM after detection is larger than a predetermined value, it is considered that the accuracy of the estimated frequency offset is low, so feedback to the frequency offset when demodulating the next control signal is not performed.

  In the case of CIR, if the CIR is smaller than a predetermined value, it is considered that the accuracy of the estimated frequency offset is low, so feedback to the frequency offset when demodulating the next control signal is not performed. In the case of the synchronization position, if the synchronization position estimation result at the time of reception deviates by a predetermined symbol from the conventional synchronization position, it is considered that the accuracy of the estimated frequency offset is low. There is no feedback to the offset.

  In the present embodiment, the base station 110 and its communication control method are used. However, the present invention can also be applied to a mobile station (wireless terminal 102). The mobile station searches for a connected base station after turning on the power. Since the mobile station estimates the frequency offset at the time of this search, the present invention may be adopted for the processing at that time. In addition, GPS synchronization is generally used in wireless communication systems such as WiMAX and LTE, but the present invention can be suitably applied when a GPS receiver is removed for the purpose of cost reduction.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to this example. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

  INDUSTRIAL APPLICABILITY The present invention can be used as a base station that performs wireless communication with a wireless terminal and frame synchronization with peripheral base stations existing in the vicinity, and a communication control method in such a base station.

DESCRIPTION OF SYMBOLS 100 ... Wireless communication system, 102 ... Wireless terminal, 110 ... Base station, 112 ... Control part, 114a ... Search processing part, 114b ... Synchronization processing part, 116 ... Memory | storage part, 118 ... Wireless communication part, 118a ... Communication antenna, 120a ... Neighboring base station, 120b ... Neighboring base station, 120c ... Neighboring base station, 120d ... Neighboring base station

Claims (5)

A base station that performs wireless communication with a wireless terminal and frame synchronization with neighboring base stations existing in the vicinity,
A search processing unit that receives control signals from a plurality of neighboring base stations, and selects a neighboring base station having the highest received strength of the control signal as a synchronization destination base station;
A synchronization processor that repeats reception of a control signal from the synchronization destination base station, and synchronizes frame timing with the synchronization destination base station;
With
The search processing unit estimates a frequency offset individually when demodulating control signals of a plurality of the neighboring base stations,
The base station characterized in that the synchronization processing unit demodulates a control signal received from the synchronization destination base station using the frequency offset estimated for the synchronization destination base station in the search processing unit. A base station that performs wireless communication with a wireless terminal and frame synchronization with neighboring base stations existing in the vicinity,
A search processing unit that receives control signals from a plurality of neighboring base stations, and selects a neighboring base station having the highest received strength of the control signal as a synchronization destination base station;
A synchronization processor that repeats reception of a control signal from the synchronization destination base station, and synchronizes frame timing with the synchronization destination base station;
With
The search processing unit demodulates the control signals of the other neighboring base stations using a frequency offset estimated for the neighboring base station whose reception strength of the control signal is equal to or greater than a predetermined value, Estimate the frequency offset of other neighboring base stations,
The base station characterized in that the synchronization processing unit demodulates a control signal received from the synchronization destination base station using the frequency offset estimated for the synchronization destination base station in the search processing unit.   The synchronization processing unit stores a frequency offset estimated for the synchronization destination base station, and uses the stored frequency offset for demodulation of the control signal for the second and subsequent times. The base station according to 1 or 2. A communication control method in a base station that performs radio communication with a radio terminal and frame synchronization with neighboring base stations existing in the vicinity,
Receiving a control signal from a plurality of the peripheral base stations, performing a search process to select a peripheral base station having the highest received strength of the control signal as a synchronization destination base station,
Repeat the reception of the control signal from the synchronization destination base station, perform a synchronization process to synchronize the frame timing with the synchronization destination base station,
In the search process, a frequency offset is estimated individually when demodulating a plurality of control signals of the neighboring base stations,
In the synchronization processing, the control signal received from the synchronization destination base station is demodulated using the frequency offset estimated for the synchronization destination base station in the search processing. A communication control method in a base station that performs radio communication with a radio terminal and frame synchronization with neighboring base stations existing in the vicinity,
Receiving a control signal from a plurality of the peripheral base stations, performing a search process to select a peripheral base station having the highest received strength of the control signal as a synchronization destination base station,
Repeat the reception of the control signal from the synchronization destination base station, perform a synchronization process to synchronize the frame timing with the synchronization destination base station,
In the search process, the control signal of another neighboring base station is demodulated using the frequency offset estimated for the neighboring base station whose reception strength of the control signal is equal to or greater than a predetermined value, and the other Estimate the frequency offset of neighboring base stations
In the synchronization processing, the control signal received from the synchronization destination base station is demodulated using the frequency offset estimated for the synchronization destination base station in the search processing.

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