JP2010232919A - Radio terminal, and radio communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radio terminal capable of preventing degradation of communication performance due to inter-code interference while utilizing an OFDM system when responding to both a CDMA system and the OFDM system. <P>SOLUTION: The radio terminal 100A includes: a communication part 120 configured to execute radio communication by selecting either the CDMA system or the OFDM system as a used communication method using a cell formed in a radio base station; a measurement part 140A configured to measure reception quality related to a time difference between proceeding waves and delayed waves of a reception signal of the OFDM system; a measurement part 140B configured to measure reception quality of a reception signal of the CDMA system; and a control part 160A configured to cause the measurement part 140B to measure reception quality of the reception signal of the CDMA system when the reception quality related to the time difference obtained in the measurement part 140 is degraded relative to a threshold corresponding to a guard interval used in the OFDM system in the case in which the OFDM method is selected as a used communication method in an idle state. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a wireless terminal and a wireless communication method for performing wireless communication using either a CDMA method or an OFDM method.

  Currently, third generation (or 3.5 generation) mobile phone systems using a CDMA (Code Division Multiple Access) system are widely used.

  In recent years, an OFDM (Orthogonal Frequency Division Multiplexing) scheme that transmits data in parallel using a plurality of subcarriers orthogonal to each other has attracted attention as a new communication scheme. The OFDM method can be used in fourth generation (or 3.9 generation) mobile phone systems and the like because it can exhibit higher communication performance than the CDMA method.

  In wireless communication systems such as mobile phone systems, wireless base stations corresponding to new communication methods are gradually installed. Therefore, in the transition period from the third generation to the fourth generation, both the CDMA method and the OFDM method are used. Wireless terminals that are compatible with the system are expected to become widespread.

  Conventionally, a wireless terminal (so-called dual terminal) that supports two communication methods uses a communication method used for wireless communication with a wireless base station (hereinafter, used communication method) using the following method. Switching. For example, the dual terminal compares the received power measured in each communication method, and switches the used communication method from the communication method with the lower reception power to the communication method with the higher reception power (see Patent Document 1).

JP 2009-50056 A

  By the way, the receiving side in the wireless communication system receives a combined wave of a plurality of radio waves (multipath waves) having different paths from the transmitting side. For this reason, in the OFDM system, the transmission side adds a guard interval for absorbing the time difference between the preceding wave and the delayed wave for each OFDM symbol.

  However, when a delayed wave exceeding the guard interval time length occurs, interference (so-called intersymbol interference) occurs between temporally preceding and succeeding OFDM symbols in the received signal on the receiving side, and communication performance deteriorates.

  Here, since the conventional dual terminal simply selects the communication method with higher reception power, the OFDM method is used if the reception power of the OFDM method is higher than the CDMA method. However, when intersymbol interference occurs in the OFDM system, there is a problem that not only the original communication performance of the OFDM system cannot be exhibited, but also the communication performance is lower than that of the CDMA system.

  Accordingly, an object of the present invention is to provide a wireless terminal and a wireless communication method capable of preventing deterioration in communication performance due to intersymbol interference while utilizing the OFDM method in the case of supporting both the CDMA method and the OFDM method. And

  In order to solve the above-described problems, the present invention has the following features. First, the first feature of the wireless terminal of the present invention is that a communication unit (communication) that performs wireless communication by selecting either a CDMA method or an OFDM method as a communication method to be used in a cell formed in a wireless base station Unit 120), a first measurement unit (measuring unit 140A) for measuring reception quality related to a time difference (Tdmax) between a preceding wave and a delayed wave of the received signal of the OFDM scheme, and reception quality of the received signal of the CDMA scheme And when the OFDM scheme is selected as the communication method used in the idle state, the reception quality related to the time difference obtained by the first measurement unit is: When the threshold value corresponding to the guard interval (Tg) used in the OFDM method is deteriorated, the reception quality of the reception signal of the CDMA method is measured by the second measurement unit. And summarized in that and a that controller (controller 160A).

  According to such a feature, the control unit causes the second measurement unit to measure the reception quality of the reception signal of the CDMA method when the reception quality related to the time difference between the preceding wave and the delayed wave is deteriorated below the threshold value. Therefore, it is possible to prepare for switching to the CDMA system in a situation where it is considered that intersymbol interference has occurred. Further, since the OFDM method is used until the reception quality related to the time difference between the preceding wave and the delayed wave is deteriorated below the threshold value, the communication performance of the OFDM method can be achieved. Therefore, in the case of supporting both the CDMA scheme and the OFDM scheme, a wireless terminal is provided that can prevent a decrease in communication performance due to intersymbol interference while utilizing the OFDM scheme.

  A second feature of the wireless terminal according to the present invention relates to the first feature, and when the measurement result of the second measurement unit is good, the control unit changes the used communication method from the OFDM method to the CDMA. The gist is to control the communication unit to switch to a method.

  A third feature of the wireless terminal of the present invention relates to the first or second feature, wherein the first measurement unit includes, as reception quality related to the time difference, an OFDM symbol included in the received signal of the OFDM scheme, The gist is to measure a value indicating a difference from a reference point of the OFDM symbol.

  A fourth feature of the wireless terminal of the present invention relates to any one of the first to third features, and when adaptive modulation is used for the wireless communication, the control unit applies a modulation scheme applied to the wireless communication. The gist is to set the threshold based on the above.

  A fifth feature of the wireless terminal of the present invention relates to any one of the first to fourth features, wherein the guard interval is added to each OFDM symbol included in the received signal of the OFDM scheme, The first measurement unit measures the voltage waveform state of the received signal of the OFDM scheme at the measurement timing corresponding to the guard interval, and the voltage waveform state measured at the current measurement timing is the current measurement timing. The gist is to measure the reception quality related to the time difference only when it is different from the state of the voltage waveform measured at the previous measurement timing.

  A sixth feature of the wireless terminal according to the present invention relates to any one of the first to fifth features. When the used communication method is the CDMA method, the communication unit converts the received signal of the CDMA method into a received signal. The gist is to perform a process of combining the preceding wave and the delayed wave included.

  The wireless communication method of the present invention is characterized in that a wireless communication is performed by selecting either a CDMA method or an OFDM method as a communication method used in a cell formed in a wireless base station, and reception of the OFDM method. The step of measuring the reception quality related to the time difference between the preceding wave and the delayed wave of the signal, and when the OFDM method is selected as the used communication method in the idle state, the reception quality related to the time difference is used in the OFDM method. And a step of measuring the reception quality of the reception signal of the CDMA system when it deteriorates below a threshold corresponding to the guard interval.

  According to the present invention, it is possible to provide a wireless terminal and a wireless communication method capable of preventing a decrease in communication performance due to intersymbol interference while utilizing the OFDM method when supporting both the CDMA method and the OFDM method.

1 is an overall schematic diagram of a wireless communication system according to a first embodiment of the present invention. It is a figure for demonstrating the state of the synthetic wave which the radio | wireless terminal which concerns on 1st Embodiment of this invention receives. It is a figure for demonstrating the guard interval in an OFDM system. It is a block diagram which shows the structure of the radio | wireless terminal which concerns on 1st Embodiment of this invention. 4 is a flowchart showing an operation of the radio terminal according to the first embodiment of the present invention. It is a figure for demonstrating the waveform measurement process by the measurement part (1st measurement part) which concerns on 1st Embodiment of this invention. It is a figure for demonstrating the EVM measurement process by the measurement part (1st measurement part) which concerns on 1st Embodiment of this invention. It is a block diagram which shows the structure of the radio | wireless terminal which concerns on 2nd Embodiment of this invention. It is a block diagram which shows the structure of the wireless base station which concerns on 2nd Embodiment of this invention. It is a sequence diagram which shows operation | movement of the radio | wireless terminal and radio | wireless base station which concern on 2nd Embodiment of this invention.

  Next, a first embodiment, a second embodiment, and other embodiments of the present invention will be described with reference to the drawings. In the description of the drawings in the following embodiments, the same or similar parts are denoted by the same or similar reference numerals.

[First Embodiment]
In the first embodiment, (1) an outline of a radio communication system, (2) a configuration of a radio terminal, (3) detailed operation of the radio terminal, and (4) operational effects will be described.

(1) Overview of Radio Communication System FIG. 1 is an overall schematic diagram of a radio communication system 10 according to the first embodiment. As shown in FIG. 1, the radio communication system 10 includes a radio terminal 100A, a radio base station 200A, and a radio base station 300.

  The radio terminal 100A is compatible with both the CDMA scheme and the OFDM scheme. In FIG. 1, a mobile phone terminal is illustrated as the wireless terminal 100A. However, the wireless terminal 100A is not limited to a mobile phone terminal, and may be a terminal equipped with a CDMA and OFDM communication device. The radio base station 200A corresponds to the OFDM system. In this specification, the OFDM scheme includes an OFDMA (Orthogonal Frequency Division Multiplexing Access) scheme. The radio base station 300 is compatible with a CDMA system such as a cdma2000 system or a W-CDMA system. Radio base stations 200A and 300 each form a cell which is a communication area.

  The radio communication system 10 has a configuration based on LTE (Long Term Evolution) standardized by, for example, 3GPP (3rd Generation Partnership Project). In LTE, OFDM is used for downlink communication, and Single-Carrier Frequency-Division Multiple. Access (SC-FDMA) is used for uplink communication. In the following, mainly downlink communication will be described.

  The OFDM system is a system in which data is distributed to a plurality of subcarriers orthogonal to each other and each subcarrier is modulated. On the transmission side, each subcarrier is subjected to multiphase PSK modulation or multilevel QAM modulation, and then each subcarrier is subjected to inverse fast Fourier transform (IFFT) to generate an OFDM signal. The receiving side performs demodulation by performing a fast Fourier transform (FFT) on the OFDM signal.

  When wireless communication is performed in an environment where the wireless terminal 100A and the wireless base station 200A are not directly visible, the antenna of the wireless terminal 100A has a plurality of radio waves (multipath waves) with different paths as shown in FIG. Receive. In the example of FIG. 2A, the path P1 directly reaching the antenna of the radio terminal 100A between the antenna of the radio base station 200A and the antenna of the radio terminal 100A and the radio terminal 100A after reflection by a building or the ground Routes P2 and P3 reaching the antenna are formed.

  The radio wave received via the path P1 by the antenna of the wireless terminal 100A is a preceding wave (direct wave). The radio wave received by the antenna of the radio terminal 100A via the paths P2 and P3 is a delayed wave delayed from the preceding wave.

  As shown in FIG. 2B, the radio waves of each path have different delay times. In the example of FIG. 2B, the antenna of the wireless terminal 100A receives the radio wave (direct wave) of the path P1 with the delay time τ1, receives the radio wave (reflected wave) of the path P2 with the delay time τ2, and the path P3. Radio wave (reflected wave) is received with a delay time τ3. The antenna of the radio terminal 100A receives these radio waves as a synthesized wave.

  In the OFDM scheme, in order to absorb such a delay time difference caused by multipath, the transmission side adds a redundant signal section called a guard interval to each symbol.

  FIG. 3A is a diagram illustrating a symbol configuration in the OFDM scheme. As shown in FIG. 3 (a), symbols in the OFDM scheme (hereinafter referred to as OFDM symbols) are a guard symbol obtained by copying a finite time effective symbol section generated by IFFT and a part of the effective symbol section. It consists of an interval.

  By using the guard interval, as shown in FIG. 3B, the time difference (hereinafter referred to as “delay time difference”) Tdmax between the time when the preceding wave is received and the time when the latest delayed wave is received is If it falls within the guard interval time length (hereinafter referred to as “guard interval length”) Tg, the FFT on the receiving side functions normally, and the occurrence of intersymbol interference can be avoided.

  On the other hand, when a delayed wave exceeding the guard interval length Tg occurs, intersymbol interference occurs, the FFT on the receiving side does not function normally, and a large distortion occurs, resulting in a decrease in communication performance.

  Therefore, the wireless terminal 100A according to the first embodiment switches the communication method used from the OFDM method to the CDMA method when it is estimated that intersymbol interference has occurred.

(2) Configuration of Radio Terminal Next, the configuration of the radio terminal 100A will be described in the order of (2.1) schematic configuration, (2.2) configuration of communication unit, and (2.3) configuration of control unit.

(2.1) Schematic Configuration FIG. 4 is a block diagram illustrating a configuration of the wireless terminal 100A. As illustrated in FIG. 4, the wireless terminal 100A includes an antenna 101, a communication unit 120, a measurement unit 140, a control unit 160A, and a storage unit 180. Note that the number of antennas 101 is not limited to one and may be a plurality.

  The communication unit 120 performs radio communication with the radio base station 200 </ b> A or the radio base station 300 via the antenna 101. The communication unit 120 performs radio communication with the radio base station 200A using either the CDMA scheme or the OFDM scheme.

  The measurement unit 140A measures the reception quality of the combined wave of the preceding wave and the delayed wave received by the communication unit 120.

  140 A of measurement parts are the values (EVM (Error Vector Magnitude)) which show the difference between the OFDM symbol (specifically effective symbol area) contained in the synthetic wave which the communication part 120 received, and the reference point of OFDM symbol ) As reception quality. Details of the EVM will be described later.

  In addition, the measurement unit 140A measures the voltage waveform state of the synthesized wave received by the communication unit 120 at the measurement timing corresponding to each guard interval.

  The measurement unit 140B measures the reception quality of the radio signal corresponding to the CDMA system received by the communication unit 120.

  The control unit 160A is configured using, for example, a CPU, and controls various functions provided in the wireless terminal 100A. The control unit 160A controls the communication unit 120 to switch the used communication method between the OFDM method and the CDMA method.

  The storage unit 180 is configured using, for example, a memory, and stores various types of information used for control in the control unit 160A.

(2.2) Configuration of Communication Unit Next, the configuration of the communication unit 120 will be described. The communication unit 120 includes a modulation unit 121, a switch SW1, a CDMA transmission unit 122A, an OFDM transmission unit 122B, a switch unit 123, a CDMA reception unit 124A, an OFDM reception unit 124B, and a demodulation unit 125.

  The modulation unit 121 modulates and encodes transmission data to the radio base station 200A. The modulation unit 121 has a configuration corresponding to adaptive modulation. In adaptive modulation, a plurality of modulation schemes are determined in advance depending on the combination of the modulation level and the coding rate. The modulation scheme is also called a modulation class or an MCS level. The modulation unit 121 modulates and encodes transmission data using any one of a plurality of modulation schemes.

  The switch SW1 inputs the transmission data output from the modulation unit 121 to either the CDMA transmission unit 122A or the OFDM transmission unit 122B according to control by the control unit 160A. The switch SW1 inputs transmission data to the CDMA transmission unit 122A when the communication method used is the CDMA method, and inputs transmission data to the OFDM transmission unit 122B when the communication method used is the OFDM method.

  The CDMA transmitter 122A spreads the input transmission data according to the CDMA system, and performs conversion to the radio frequency band and amplification processing to generate a CDMA signal (hereinafter referred to as CDMA wave) in the radio frequency band. The generated CDMA wave is transmitted to the radio base station 200A via the switch unit 123 and the antenna 101.

  The OFDM transmitter 122B multi-carrier-modulates the input transmission data according to the OFDM scheme, and generates an OFDM signal (hereinafter referred to as an OFDM wave) in the radio frequency band by performing conversion to the radio frequency band and amplification processing. . The generated OFDM wave is transmitted to the radio base station 200A via the switch unit 123 and the antenna 101.

  The switch unit 123 inputs the CDMA wave generated by the CDMA transmission unit 122A or the OFDM wave generated by the OFDM transmission unit 122B to the antenna 101 under the control of the control unit 160A. The switch unit 123 inputs the CDMA wave to the antenna 101 when the used communication method is the CDMA method, and inputs the OFDM wave to the antenna 101 when the used communication method is the OFDM method.

  On the other hand, at the time of reception, the switch unit 123 inputs the radio signal received by the antenna 101 to the CDMA reception unit 124A and the OFDM reception unit 124B.

  The CDMA receiving unit 124A performs conversion to the baseband and amplification processing on the input CDMA wave, and also performs despreading according to the CDMA method. Further, the CDMA receiving unit 124A performs RAKE reception, which is a process for combining the preceding wave and the delayed wave included in the CDMA wave received from the radio base station 200A. In RAKE reception, the reception quality is improved by combining the preceding wave and the delayed wave with the same phase. The reception data obtained in this way is input to the demodulation unit 125.

  The OFDM receiving unit 124B performs conversion to baseband and amplification processing on the input OFDM wave, and multicarrier demodulation according to the OFDM scheme. Further, the OFDM receiver 124B removes the guard interval included in the OFDM wave. The reception data obtained in this way is input to the demodulation unit 125.

  The demodulator 125 demodulates and decodes the input received data. The demodulator 125 demodulates and decodes the received data by a method corresponding to any one of a plurality of modulation schemes. Further, the demodulation unit 125 performs symbol determination on the input received data.

(2.3) Configuration of Control Unit Next, the configuration of the control unit 160A will be described. The control unit 160A includes a determination unit 161, an acquisition unit 162, a switching control unit 163, and a cell selection unit 164.

  The determination unit 161 is a process for determining whether or not the delay time difference Tdmax exceeds the guard interval length Tg based on the EVM measured by the measurement unit 140A when the communication method used is the OFDM method in the idle state. (Hereinafter, determination processing) is performed. The determination unit 161 determines whether or not the EVM measured by the measurement unit 140A is equal to or greater than the EVM threshold. In the present embodiment, EVM is reception quality related to a time difference between a preceding wave and a delayed wave of an OFDM reception signal. In the present embodiment, the EVM threshold is a threshold corresponding to the guard interval.

  For example, the EVM threshold value is set in advance to the value of EVM when the delay time difference Tdmax exceeds the guard interval length Tg. The value of EVM when the delay time difference Tdmax exceeds the guard interval length Tg can be obtained experimentally or empirically.

  The acquisition unit 162 acquires, from the storage unit 180, an EVM threshold used by the determination unit 161 for determination based on a modulation scheme applied to wireless communication (specifically, downlink communication). The acquisition unit 162 sets the acquired EVM threshold value in the determination unit 161.

  The switching control unit 163 controls the switch SW1 and the switch unit 123 of the communication unit 120. The switching control unit 163 controls the communication unit 120 (specifically, the switch SW1 and the switch unit 123) to switch the used communication method from the OFDM method to the CDMA method.

  The cell selection unit 164 selects a cell that is a communication area formed by the radio base station 300 that supports the CDMA scheme. For example, when the reception quality (EVM) from a radio base station (radio base station in the vicinity) corresponding to the OFDM scheme is lower than a predetermined threshold (EVM threshold), the cell selection unit 164 is formed by the radio base station 300. Reselect the cell to be saved.

(3) Detailed Operation of Wireless Terminal Next, detailed operation of the wireless terminal 100A will be described. FIG. 5 is a flowchart showing the operation of the wireless terminal 100A when the communication method used is the OFDM method. This operation flow is executed in a period in which the wireless terminal 100A is in an idle state, for example, a period in which the wireless terminal 100A is not communicating with a communication partner other than the wireless base station and is waiting.

  In step S101, measurement unit 140A measures the voltage waveform state of the OFDM wave received by OFDM reception unit 124B.

  FIG. 6 shows a waveform measurement process performed by the measurement unit 140A. In the example of FIG. 6, the delay time difference is τ as shown in FIG. 6A, and the preceding wave shown in FIG. 6B and the delayed wave shown in FIG. The OFDM wave shown in d) is received. For example, the measurement unit 140A specifies the guard interval period of the preceding wave according to the result of the symbol synchronization, and measures the voltage waveform state (voltage value) of the OFDM wave at the measurement timing corresponding to the guard interval period. The measurement is performed at each measurement timing corresponding to each guard interval period.

  In step S102, the measurement unit 140A determines that the voltage waveform state measured at the current measurement timing T (n) is a measurement timing before the current measurement timing (hereinafter referred to as “previous measurement timing”) T (n−1). It is determined whether or not it is equal to the voltage waveform state measured in (1). When the voltage waveform state measured at the current measurement timing T (n) is equal to the voltage waveform state measured at the previous measurement timing T (n−1), the process proceeds to step S103, where n is incremented in step S103, Processing moves to the next voltage waveform measurement. On the other hand, if the voltage waveform state measured at the current measurement timing T (n) is different from the voltage waveform state measured at the previous measurement timing T (n−1), the process proceeds to step S104.

  In step S104, measurement unit 140A measures the EVM of the OFDM symbol corresponding to current measurement timing T (n). EVM is called modulation accuracy, and represents the amount of phase / amplitude deviation of the observed symbol point S from the intended symbol reference point Sref as shown in FIG. Specifically, EVM is the effective value of the error vector and is expressed as a percentage of the square root of the average power of the ideal signal. FIG. 7C shows a calculation formula for EVM.

  In step S <b> 105, the acquisition unit 162 acquires an EVM threshold value from the storage unit 180. As illustrated in FIG. 7B, the storage unit 180 stores a table in which the modulation scheme and the EVM threshold are associated with each other. As the modulation method is capable of high-speed communication (the modulation method having a larger number of bits per symbol), the restrictions on the phase / amplitude error become more severe. For this reason, the EVM threshold is set lower as the modulation method enables higher-speed communication. Note that, at the time of initial connection, since the modulation scheme (for example, QPSK) having the lowest communication speed is used, the EVM threshold value corresponding to the modulation scheme may be fixedly used.

  In step S106, the determination unit 161 determines whether or not the EVM measured in step S104 is greater than or equal to the EVM threshold acquired and set in step S105. If it is determined that the EVM measured in step S104 is greater than or equal to the EVM threshold acquired and set in step S105, the process proceeds to step S107. On the other hand, if it is determined that the EVM measured in step S104 is less than the EVM threshold acquired and set in step S105, the process proceeds to step S103.

  In step S107, the cell selection unit 164 causes the measurement unit 140B to measure the reception quality of the radio signal corresponding to the CDMA scheme. When the measurement result is a good result in step S108 (for example, when the measured RSSI is equal to or greater than a predetermined threshold), the cell selection unit 164 reselects the cell formed by the radio base station 300 in step S109. In step S110, the switching control unit 163 controls the communication unit 120 (specifically, the switch SW1 and the switch unit 123) to switch the used communication method from the OFDM method to the CDMA method. Note that the communication unit 120 transmits a connection request to a CDMA-compatible radio base station for which the measurement result of the measurement unit 140B is good when the communication method used is switched from the OFDM method to the CDMA method. When there are a plurality of radio base stations to perform, it is preferable to transmit to the one with the best measurement result. If the measured EVM from each OFDM-compatible radio base station in the vicinity of the radio terminal 100A is equal to or greater than the EVM threshold, step S107 is executed. Steps S107 to S110 may be processing compliant with a standard such as 3GPP TS36.300.

(4) Operational Effect According to the first embodiment, the control unit 160A confirms that the reception quality of the reception quality of the CDMA scheme is good in a situation where the delay time difference Tdmax is estimated to exceed the guard interval length Tg. After that, the communication unit 120 is controlled to switch the used communication method from the OFDM method to the CDMA method.

  For this reason, it is possible to avoid intersymbol interference in the OFDM scheme, and to prevent a decrease in communication performance. In addition, until it is estimated that the delay time difference Tdmax exceeds the guard interval length Tg, the excellent communication performance of the OFDM method can be utilized.

  Therefore, in the case of supporting both the CDMA scheme and the OFDM scheme, a wireless terminal is provided that can prevent a decrease in communication performance due to intersymbol interference while utilizing the OFDM scheme.

  In the first embodiment, the cell selection unit 164 causes the measurement unit 140B to measure the reception quality of a radio signal corresponding to the CDMA scheme when the measured EVM is equal to or greater than the EVM threshold.

  EVM can be measured with a small amount of computation and has a short measurement time compared to other reception quality indicators such as SNR (Signal to Noise ratio), BER (Bit Error Rate), or channel estimation value. Has characteristics.

  For this reason, it is possible to easily and immediately determine whether or not the delay time difference Tdmax exceeds the guard interval length Tg by using the EVM as the reception quality index. Therefore, the processing load and power consumption of the radio terminal 100A can be reduced and the period during which the communication performance is deteriorated due to intersymbol interference can be reduced as compared with the case of using other reception quality indicators.

  In the first embodiment, when the communication unit 120 performs wireless communication using adaptive modulation, the acquisition unit 162 sets an EVM threshold based on a modulation scheme applied to wireless communication. For this reason, even when adaptive modulation is used, the EVM threshold can be appropriately set.

  In the first embodiment, the measurement unit 140A receives the reception quality only when the voltage waveform state measured at the current measurement timing T (n) is different from the voltage waveform state measured at the previous measurement timing T (n-1). (EVM) is measured.

  If the voltage waveform state measured at the current measurement timing T (n) is equal to the voltage waveform state measured at the measurement timing T (n−1) before the current measurement timing, the multipath state changes. It can be regarded as not. Therefore, in such a case, the processing load and power consumption of the wireless terminal 100A can be reduced by omitting the EVM measurement and determination processing.

  Further, when the determination is performed using only the reception quality (EVM), if the EVM changes due to a factor other than the change in the multipath state (for example, a circuit factor), there is a possibility that an erroneous determination is made in the determination process. There is. For this reason, only when the state of the voltage waveform measured at the current measurement timing T (n) is different from the state of the voltage waveform measured at the previous measurement timing T (n−1), the determination accuracy is improved by measuring the EVM. Can be improved.

  In the first embodiment, when the used communication method is the CDMA method, the CDMA receiving unit 124A performs RAKE reception that combines the preceding wave and the delayed wave included in the CDMA wave received from the radio base station 200A. That is, in the CDMA system, when the time difference between the preceding wave and the delayed wave is large, a path diversity effect by RAKE reception can be obtained.

  Therefore, by switching the communication system used from the OFDM system to the CDMA system, it is possible to effectively suppress the degradation of communication performance by utilizing the characteristics of the CDMA system.

[Second Embodiment]
In the second embodiment, determination processing is performed on the radio base station side. In the following, (1) the configuration of the radio terminal, (2) the configuration of the radio base station, (3) the operation of the radio communication system, and (4) the operational effects will be described. However, only a different point from 1st Embodiment is demonstrated and the overlapping description is abbreviate | omitted.

(1) Configuration of Radio Terminal FIG. 8 is a block diagram showing the configuration of the radio terminal 100B according to the second embodiment. As illustrated in FIG. 8, the control unit 160B of the wireless terminal 100B does not include the determination unit 161 and the acquisition unit 162 described in the first embodiment. Instead, the control unit 160B includes a notification processing unit 165 that exchanges various notifications with the radio base station 200B (see FIG. 9) according to the second embodiment. The notification processing unit 165 notifies the radio base station 200 </ b> B of reception quality information indicating the EVM measured by the measurement unit 140 using the communication unit 120.

(2) Configuration of Radio Base Station FIG. 9 is a block diagram showing a configuration of the radio base station 200B according to the second embodiment. As illustrated in FIG. 9, the radio base station 200B includes an antenna 201, a communication unit 220, a control unit 240, and a storage unit 260.

  The communication unit 220 performs wireless communication with the wireless terminal 100B via the antenna 201. The control unit 240 is configured using, for example, a CPU, and controls various functions provided in the radio base station 200B. The storage unit 260 is configured using a memory, for example, and stores various types of information used for control in the control unit 240.

  The communication unit 220 receives reception quality information indicating the EVM measured by the wireless terminal 100B from the wireless terminal 100B.

  The control unit 240 includes an acquisition unit 241, a determination unit 242, and an instruction unit 243.

  The determination unit 242 determines whether the EVM indicated by the reception quality information received by the communication unit 220 is equal to or greater than the EVM threshold.

  The acquisition unit 241 acquires the threshold used by the determination unit 242 for determination from the storage unit 260 based on a modulation scheme applied to wireless communication (specifically, downlink communication). The acquisition unit 241 sets the acquired threshold value in the determination unit 242.

  When the determination unit 242 determines that the EVM is equal to or greater than the EVM threshold, the instruction unit 243 generates a measurement instruction that instructs the reception quality of the OFDM scheme reception quality to be measured.

  The measurement instruction generated by the instruction unit 243 is transmitted to the wireless terminal 100B by the communication unit 220.

(3) Operation of Radio Communication System FIG. 10 is a sequence diagram showing operations of the radio terminal 100B and the radio base station 200B.

  In step S201, the communication unit 220 of the radio base station 200B transmits an OFDM wave to the radio terminal 100B. The OFDM receiver 124B of the wireless terminal 100B receives the OFDM wave.

  In step S202, the measurement unit 140 of the radio terminal 100B measures the voltage waveform state of the OFDM wave received by the OFDM reception unit 124B at the measurement timing corresponding to each guard interval. When the voltage waveform state measured at the current measurement timing (T (n)) is different from the voltage waveform state measured at the measurement timing (T (n-1)) before the current measurement timing, the measurement unit 140 As long as EVM is measured.

  In step S203, the notification processing unit 165 of the radio terminal 100B notifies the radio base station 200B of the reception quality information indicating the EVM measured by the measurement unit 140 using the OFDM transmission unit 122B of the communication unit 120.

  In step S204, the determination unit 242 of the radio base station 200B determines whether or not the EVM indicated by the reception quality information received by the communication unit 220 is greater than or equal to the EVM threshold. Here, it is assumed that EVM is determined to be equal to or greater than the EVM threshold.

  In step S205, the communication unit 220 of the radio base station 200B transmits the measurement instruction generated by the instruction unit 243 to the radio terminal 100B. The OFDM receiver 124B of the wireless terminal 100B receives the measurement instruction. The measurement instruction received by the OFDM receiving unit 124B is transmitted to the notification processing unit 165.

  In step S206, the cell selection unit 164 of the radio terminal 100B causes the measurement unit 140B to measure the reception quality of the CDMA reception signal in accordance with the measurement instruction transmitted to the notification processing unit 165.

(4) Effects According to the second embodiment, the processing load and power consumption of the radio terminal 100B can be reduced by performing the determination process on the radio base station 200B side as compared with the first embodiment.

[Other Embodiments]
As mentioned above, although this invention was described by embodiment, it should not be understood that the description and drawing which form a part of this indication limit this invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art.

  In the above-described embodiment, an example in which the CDMA communication unit (CDMA transmission unit 122A and CDMA reception unit 124A) and the OFDM communication unit (OFDM transmission unit 122B and OFDM reception unit 124B) are individually provided has been described. However, the CDMA communication unit and the OFDM communication unit may be configured as one communication unit. For example, in a wireless terminal called a cognitive terminal, the communication method can be switched in software by downloading software (SDR BB, Tunable RF) corresponding to the communication method to be used.

  In order to reduce power consumption, the switching control unit 163 uses the CDMA communication unit (CDMA transmission unit 122A, CDMA reception unit 124A) and OFDM communication unit (OFDM transmission unit 122B, OFDM reception unit 124B). You may be comprised so that the electric power supply to the communication part which is not a communication system may be stopped.

  In the above-described embodiment, EVM is used as the reception quality indicator, but other reception quality indicators (for example, SNR, BER, or channel estimation value) may be used.

  In the embodiment described above, LTE has been described as an example of a wireless communication system using the OFDM method, but is not limited to LTE, UMB (Ultra Mobile Broadband) standardized in 3GPP2, WiMAX standardized in IEEE 802.16, Alternatively, it may be a next generation PHS or the like.

  Thus, it should be understood that the present invention includes various embodiments and the like not described herein. Therefore, the present invention is limited only by the invention specifying matters in the scope of claims reasonable from this disclosure.

  DESCRIPTION OF SYMBOLS 10 ... Wireless communication system, 100A, 100B ... Wireless terminal, 101 ... Antenna, 120 ... Communication part, 121 ... Modulation part, 122A ... CDMA transmission part, 122B ... OFDM transmission part, 123 ... Switch part, 124A ... CDMA reception part, 124B: OFDM receiver, 125 ... demodulator, 140 ... measurement unit, 140A, 140B ... measurement unit, 160A, 160B ... control unit, 161 ... determination unit, 162 ... acquisition unit, 163 ... switching control unit, 164 ... cell selection 165 ... notification processing unit, 180 ... storage unit, 200A, 200B, 300 ... radio base station, 201 ... antenna, 220 ... communication unit, 240 ... control unit, 241 ... acquisition unit, 242 ... determination unit, 243 ... instruction Part, 260 ... storage part

Claims (7)

A communication unit that performs radio communication by selecting either a CDMA method or an OFDM method as a communication method used in a cell formed in a radio base station;
A first measurement unit that measures reception quality related to a time difference between a preceding wave and a delayed wave of the received signal of the OFDM system;
A second measurement unit for measuring reception quality of the CDMA reception signal;
When the OFDM method is selected as the used communication method in the idle state, the reception quality related to the time difference obtained by the first measurement unit is deteriorated from a threshold corresponding to the guard interval used in the OFDM method. And a control unit that causes the second measurement unit to measure reception quality of the CDMA reception signal.   2. The wireless terminal according to claim 1, wherein the control unit controls the communication unit to switch the used communication method from the OFDM method to the CDMA method when a measurement result of the second measurement unit is good.   The first measurement unit measures a value indicating a difference between an OFDM symbol included in the received signal of the OFDM scheme and a reference point of the OFDM symbol as reception quality related to the time difference. The wireless terminal described in 1.   The wireless terminal according to any one of claims 1 to 3, wherein, when adaptive modulation is used for the wireless communication, the control unit sets the threshold based on a modulation scheme applied to the wireless communication. The guard interval is added to each OFDM symbol included in the received signal of the OFDM scheme,
The first measurement unit includes:
At the measurement timing corresponding to the guard interval, measure the voltage waveform state of the received signal of the OFDM method,
The reception quality related to the time difference is measured only when the voltage waveform state measured at the current measurement timing is different from the voltage waveform state measured at the measurement timing before the current measurement timing. The radio | wireless terminal as described in any one.   The said communication part performs the process which synthesize | combines the preceding wave and the delay wave which are contained in the received signal of the said CDMA system, when the said used communication system is the said CDMA system. The wireless terminal described. A wireless communication method used in a wireless terminal,
A step of performing wireless communication by selecting either a CDMA method or an OFDM method as a used communication method used in a cell formed by a wireless base station;
Measuring reception quality related to a time difference between a preceding wave and a delayed wave of the received signal of the OFDM scheme;
When the OFDM scheme is selected as the used communication scheme in an idle state, when the reception quality related to the time difference is deteriorated below a threshold corresponding to the guard interval used in the OFDM scheme, the received signal of the CDMA scheme A wireless communication method comprising: measuring reception quality.

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