Patent Publication Number: US-2019174471-A1

Title: Signal Transmission Mode Switching Method, Base Station, and Terminal Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Patent Application No. PCT/CN2016/094900, filed on Aug. 12, 2016, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention relate to the field of LTE technologies, and in particular, to a signal transmission mode switching method, a base station, and a terminal device. 
     BACKGROUND 
     With continuous development of the science and technology, users impose higher requirements on a signal transmission rate and the like. However, signal coverage depth that can be implemented by using an existing LTE technology cannot meet user requirements. For example, in some remote regions, a signal transmitted by a base station by using a downlink channel cannot be received by a terminal device, and the terminal device cannot send a data signal to the base station by using an uplink channel. Enhanced machine type communication (enhanced Machine Type Communication, eMTC for short) may be used, so that the signal coverage depth can meet requirements of more users. The eMTC is an Internet of Things technology in a protocol release LTE R13. In the technology, coverage is mainly improved by using channel repetition and frequency hopping technologies, and the coverage depth may be improved by a maximum of 15 db according to a protocol requirement. In other words, the existing LTE technology can support normal coverage (Normal Coverage, NC for short), but cannot support deeper coverage. However, the eMTC technology can support coverage enhanced (Coverage Enhanced, CE for short). 
     In the prior art, specifically as shown in  FIG. 1 , it is assumed that a terminal device accesses a network in a normal coverage area of LTE. As shown in  FIG. 1( a ) , in this case, the terminal device is in an NC mode. Because the terminal device does not know existence of an eMTC broadcast, the terminal device reads only an LTE broadcast but does not read the eMTC broadcast. When the terminal device moves to a weak coverage area of LTE, as shown in  FIG. 1( b ) , the terminal device needs to enter a CE mode. Although a base station may deliver radio resource control (Radio Resource Control, RRC for short) signaling, to instruct the terminal device to switch from the NC mode to the CE mode, the terminal device certainly does not receive the eMTC broadcast because the terminal device does not know existence of the eMTC broadcast during network access. Consequently, the terminal device cannot work in the CE mode. According to a current protocol, the terminal device can only be “offline” as a signal becomes weak. After being offline, the terminal device automatically chooses, based on downlink signal strength, to access the network in the CE mode, so as to read the eMTC broadcast, and then work in the CE mode. In this way, although the terminal device can switch from the NC mode to the CE mode, the switching performed by the terminal device is not active switching. Instead, the terminal device passively switches, based on the downlink signal strength, to the CE mode for network access only when the terminal device moves to the weak coverage area of LTE and be “offline” as the signal becomes weak. 
     Therefore, how to implement switching of a terminal device between an NC mode and a CE mode becomes a problem that needs to be resolved in the present invention. 
     SUMMARY 
     Embodiments of the present invention provide a signal transmission mode switching method, a base station, and a terminal device. 
     According to a first aspect, a signal transmission mode switching method is provided. The method includes:
         when it is detected that signal transmission with a terminal device is abnormal, sending, to the terminal device, control instruction information used to configure a signal transmission mode, where the control instruction information is used to instruct the terminal device to switch a current signal transmission mode on the terminal device side to a to-be-switched-to signal transmission mode; and switching the current signal transmission mode to the to-be-switched-to signal transmission mode, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode.       

     When a base station detects that the signal transmission between the base station and the terminal device is abnormal, the base station immediately sends, to the terminal device, the control instruction information used to switch a signal transmission mode, so that the terminal device switches the signal transmission mode, to avoid unsmooth signal transmission between the terminal device and the base station. The base station side also needs to switch the signal transmission mode, to synchronize the signal transmission mode with the terminal device. 
     With reference to the first aspect, in a first possible implementation of the first aspect, that the signal transmission with the terminal device is abnormal includes: it is detected that a received signal strength value of the terminal device is less than a first predetermined threshold; or it is detected that a rate at which the terminal device sends an uplink signal is less than a second predetermined threshold. 
     It should be understood that only two cases in which signal transmission is abnormal are enumerated in the present invention, but it does not represent that only the two cases exist. A signal transmission mode also needs to be switched in another abnormal case, and details are not described herein. 
     Before switching the signal transmission mode on the terminal device based on the control instruction information, the terminal device further needs to configure the to-be-switched-to signal transmission mode, and a manner of obtaining configuration information may include the following several aspects. Specifically, the manner includes a second possible implementation of the first aspect and a third possible implementation of the first aspect. Two other manners are described in detail on the terminal device side, and details are not described herein. 
     With reference to the first aspect or the first possible implementation of the first aspect, in the second possible implementation of the first aspect, the control instruction information includes a new broadcast information element, so that the terminal device determines, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     With reference to the first aspect or the first possible implementation of the first aspect, in the third possible implementation of the first aspect, before the switching the current signal transmission mode to the to-be-switched-to signal transmission mode, the method further includes:
         adding a new broadcast information element to/deleting a new broadcast information element from a first broadcast corresponding to the current signal transmission mode, and using an obtained broadcast as a second broadcast, so that the terminal device determines, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode.       

     With reference to any one of the first aspect to the third possible implementation of the first aspect, in a fourth possible implementation of the first aspect, before the switching the current signal transmission mode to the to-be-switched-to signal transmission mode, the method further includes: sending uplink transmission resource indication information to the terminal device, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the fourth possible implementation of the first aspect, in a fifth possible implementation of the first aspect, the method further includes: receiving the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, and switching the current signal transmission mode to the to-be-switched-to signal transmission mode based on the response message. 
     With reference to the fifth possible implementation of the first aspect, in a sixth possible implementation of the first aspect, a specific moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode is as follows:
         a moment of receiving the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is used as a start moment, and the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1.       

     With reference to the fifth possible implementation of the first aspect, in a seventh possible implementation of the first aspect, a specific moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode is as follows:
         if the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is received within a first predetermined time period, the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, to the terminal device, the control instruction information used to configure a signal transmission mode; and if the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is not received within the first predetermined time period, the current signal transmission mode is maintained.       

     With reference to any one of the first aspect to the third possible implementation of the first aspect, in an eighth possible implementation of the first aspect, after the switching the current signal transmission mode to the to-be-switched-to signal transmission mode, the method further includes: sending uplink transmission resource indication information to the terminal device, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the eighth possible implementation of the first aspect, in a ninth possible implementation of the first aspect, the method further includes: receiving the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is received. 
     With reference to any one of the first aspect to the third possible implementation of the first aspect, in a tenth possible implementation of the first aspect, after the sending, to the terminal device, control instruction information used to configure a signal transmission mode, the method further includes:
         separately sending uplink transmission resource indication information to the terminal device by using the current signal transmission mode and the second signal transmission mode, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode.       

     With reference to the tenth possible implementation of the first aspect, in an eleventh possible implementation of the first aspect, the method further includes: separately detecting a first signal transmission channel corresponding to the current signal transmission mode and a second signal transmission channel corresponding to the second signal transmission mode, to determine that the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device is able to be received through one of the first signal transmission channel or the second signal transmission channel; and
         using a moment of receiving the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device as a start moment, and switching the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1.       

     After sending the control instruction information to the terminal device, the base station further needs to designate the uplink transmission resource for the terminal device for transmitting the response message indicating successful configuration of the to-be-switched-to signal transmission mode. Therefore, the base station further needs to send the uplink transmission resource indication information to the terminal device. The terminal device determines the uplink transmission resource based on the uplink transmission resource indication information and transmits, by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. A specific sending time may be before the base station switches the current signal transmission mode to the to-be-switched-to signal transmission mode, or after the base station switches the current signal transmission mode to the to-be-switched-to signal transmission mode, or after the base station sends, to the terminal device, the control instruction information used to configure a signal transmission mode. This is specifically determined according to an actual situation, and is not limited herein. Regardless of a manner, a purpose is to ensure consistency between a signal transmission mode used by the base station to transmit the uplink transmission resource indication information and a signal transmission mode used by the terminal device side to transmit the response message indicating successful configuration of the to-be-switched-to signal transmission mode. In other words, the messages can be received in a signal transmission mode agreed between the base station and the terminal device, so that the base station and the terminal device can successfully receive the messages. 
     According to a second aspect, an embodiment of the present invention further provides another signal transmission mode switching method. The method includes: receiving control instruction information that is used to configure a signal transmission mode and that is sent by a base station; and switching a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. 
     When the base station detects that the signal transmission between the base station and a terminal device is abnormal, the base station immediately sends, to the terminal device, the control instruction information used to switch a signal transmission mode. After receiving the control instruction information, the terminal device switches the signal transmission mode, to avoid unsmooth signal transmission between the terminal device and the base station. 
     Before switching the signal transmission mode on the terminal device based on the control instruction information, the terminal device further needs to configure the to-be-switched-to signal transmission mode, and a manner of obtaining configuration information may include the following several aspects. Specifically, the manner includes a first possible implementation of the second aspect to a fourth possible implementation of the second aspect. 
     With reference to the second aspect, in the first possible implementation of the second aspect, the control instruction information includes a new broadcast information element; and the method further comprises:
         determining, by the terminal device based on the new broadcast information element and a first broadcast, configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode; and   configuring the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the second aspect, in the second possible implementation of the second aspect, before the switching a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information, the method further includes:
         receiving a second broadcast sent by the base station, where the second broadcast is a broadcast obtained after a new broadcast information element is added to/deleted from a first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode; determining, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode; and configuring the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the second aspect, in the third possible implementation of the second aspect, after the receiving control instruction information that is used to configure a signal transmission mode and that is sent by a base station, the method further includes:
         receiving a second broadcast sent by the base station, where the second broadcast is a broadcast corresponding to the to-be-switched-to signal transmission mode, the second broadcast includes an information element the same as an information element in a first broadcast or an information element having a same property as an information element in at least one of a first broadcast or a new broadcast information element relative to the first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode; determining the configuration information of the to-be-switched-to signal transmission mode based on the second broadcast; and configuring the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the second aspect, in the fourth possible implementation of the second aspect, before the switching the current signal transmission mode to the to-be-switched-to signal transmission mode based on the control instruction information, the method further includes:
         separately receiving a first broadcast and a second broadcast, and determining, from the first broadcast and the second broadcast, a broadcast corresponding to the to-be-switched-to signal transmission mode; determining, based on the broadcast corresponding to the to-be-switched-to signal transmission mode, the configuration information corresponding to the to-be-switched-to signal transmission mode; and configuring the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to any one of the second aspect to the fourth possible implementation of the second aspect, in a fifth possible implementation of the second aspect, before the switching a current signal transmission mode to a to-be-switched-to signal transmission mode, the method further includes: receiving uplink transmission resource indication information sent by the base station; determining an uplink transmission resource based on the uplink transmission resource indication information; and transmitting, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the fifth possible implementation of the second aspect, in a sixth possible implementation of the second aspect, a specific moment of switching the signal transmission mode is as follows: a moment of receiving, by the base station, the response message indicating successful configuration of the to-be-switched-to signal transmission mode is used as a start moment, and the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1. 
     With reference to the fifth possible implementation of the second aspect, in a seventh possible implementation of the second aspect, a specific moment of switching the signal transmission mode is as follows: if the base station receives, within a first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the base station, the control instruction information used to configure a signal transmission mode; and if the base station does not receive, within the first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is maintained. 
     With reference to any one of the second aspect to the fourth possible implementation of the second aspect, in an eighth possible implementation of the second aspect, after the switching a current signal transmission mode to a to-be-switched-to signal transmission mode, the method further includes:
         receiving uplink transmission resource indication information sent by the base station;   determining an uplink transmission resource based on the uplink transmission resource indication information; and   transmitting, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode.       

     After receiving the uplink transmission resource indication information sent by the base station, the terminal device further needs to determine the uplink transmission resource based on the uplink transmission resource indication information, and transmit, by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. A specific time of receiving the uplink transmission resource indication information may be before the terminal device switches the current signal transmission mode to the to-be-switched-to signal transmission mode or after the terminal device switches the current signal transmission mode to the to-be-switched-to signal transmission mode. This is specifically determined according to an actual situation, and is not limited herein. Regardless of a manner, a purpose is to ensure consistency between a signal transmission mode used by the base station to transmit the uplink transmission resource indication information and a signal transmission mode used by the terminal device side to transmit the response message indicating successful configuration of the to-be-switched-to signal transmission mode. In other words, the messages can be received in a signal transmission mode agreed between the base station and the terminal device, so that the base station and the terminal device can successfully receive the messages. 
     According to a third aspect, an embodiment of the present invention further provides a base station. The base station includes:
         a detection unit, configured to detect whether signal transmission between the base station and a terminal device is abnormal;   a sending unit, configured to: when the detection unit detects that the signal transmission between the base station and the terminal device is abnormal, send, to the terminal device, control instruction information used to configure a signal transmission mode, where the control instruction information is used to instruct the terminal device to switch a current signal transmission mode on the terminal device side to a to-be-switched-to signal transmission mode; and   a processing unit, configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode.       

     When the detection unit detects that the signal transmission between the base station and the terminal device is abnormal, the sending unit immediately sends, to the terminal device based on a detection result of the detection unit, the control instruction information used to switch a signal transmission mode, so that the terminal device switches the signal transmission mode, to avoid unsmooth signal transmission between the terminal device and the base station. The base station side also needs to switch the signal transmission mode, to synchronize the signal transmission mode with the terminal device. 
     With reference to the third aspect, in a first possible implementation of the third aspect, the detection unit is specifically configured to:
         detect whether a received signal strength value of the terminal device is less than a first predetermined threshold; or   detect whether a rate at which the terminal device sends an uplink signal is less than a second predetermined threshold.       

     It should be understood that only two cases in which signal transmission is abnormal are enumerated in the present invention, but it does not represent that only the two cases exist. A signal transmission mode also needs to be switched in another abnormal case, and details are not described herein. 
     Before switching the signal transmission mode on the terminal device based on the control instruction information, the terminal device further needs to configure the to-be-switched-to signal transmission mode, and a manner of obtaining configuration information may include the following several aspects. Specifically, the manner includes a second possible implementation of the third aspect and a third possible implementation of the third aspect. Two other manners are described in detail on the terminal device side, and details are not described herein. 
     With reference to the third aspect or the first possible implementation of the third aspect, in the second possible implementation of the third aspect, the control instruction information includes a new broadcast information element, so that the terminal device determines, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     With reference to the third aspect or the first possible implementation of the third aspect, in the third possible implementation of the third aspect, the processing unit is further configured to: add a new broadcast information element to/delete a new broadcast information element from a first broadcast corresponding to the current signal transmission mode, and use an obtained broadcast as a second broadcast, so that the terminal device determines, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     With reference to any one of the third aspect to the third possible implementation of the third aspect, in a fourth possible implementation of the third aspect, the sending unit is further configured to: before the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send uplink transmission resource indication information to the terminal device, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the fourth possible implementation of the third aspect, in a fifth possible implementation of the third aspect, the base station further includes: a receiving unit, configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, where the processing unit is further configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode based on the response message. 
     With reference to the fifth possible implementation of the third aspect, in a sixth possible implementation of the third aspect, a specific moment of switching, by the processing unit, the current signal transmission mode to the to-be-switched-to signal transmission mode is as follows:
         a moment of receiving, by the receiving unit, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is used as a start moment, and the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1.       

     With reference to the fifth possible implementation of the third aspect, in a seventh possible implementation of the third aspect, a specific moment of switching, by the processing unit, the current signal transmission mode to the to-be-switched-to signal transmission mode is as follows:
         if the receiving unit receives, within a first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the sending unit to the terminal device, the control instruction information used to configure a signal transmission mode; and   if the receiving unit does not receive, within the first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processing unit maintains the current signal transmission mode.       

     With reference to any one of the third aspect to the third possible implementation of the third aspect, in an eighth possible implementation of the third aspect, the sending unit is further configured to:
         after the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send uplink transmission resource indication information to the terminal device, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode.       

     With reference to the eighth possible implementation of the third aspect, in a ninth possible implementation of the third aspect, the base station further includes a receiving unit, configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     With reference to any one of the third aspect to the third possible implementation of the third aspect, in a tenth possible implementation of the third aspect, the sending unit is further configured to: after sending, to the terminal device, the control instruction information used to configure a signal transmission mode, separately send uplink transmission resource indication information to the terminal device by using the current signal transmission mode and the second signal transmission mode, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the tenth possible implementation of the third aspect, in an eleventh possible implementation of the third aspect, the detection unit is further configured to: separately detect a first receiving unit corresponding to the current signal transmission mode and a second receiving unit corresponding to the second signal transmission mode, to determine that one of the first receiving unit or the second receiving unit can receive the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device; and the processing unit is further configured to: use a moment of receiving the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device as a start moment, and switch the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1. 
     According to a fourth aspect, an embodiment of the present invention further provides a terminal device. The terminal device includes:
         a receiving unit, configured to receive control instruction information that is used to configure a signal transmission mode and that is sent by a base station; and   a processing unit, configured to switch a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode.       

     With reference to the fourth aspect, in a first possible implementation of the fourth aspect, the control instruction information includes a new broadcast information element; and
         the processing unit is configured to: determine, based on the new broadcast information element and a first broadcast, configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the fourth aspect, in a second possible implementation of the fourth aspect, the receiving unit is further configured to: before the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode based on the control instruction information, receive a second broadcast sent by the base station, where the second broadcast is a broadcast obtained after a new broadcast information element is added to/deleted from a first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode; and
         the processing unit is further configured to: determine, based on the second broadcast, configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the fourth aspect, in a third possible implementation of the fourth aspect, the receiving unit is further configured to: after receiving the control instruction information that is used to configure the signal transmission mode and that is sent by the base station, receive a second broadcast sent by the base station, where the second broadcast is a broadcast corresponding to the to-be-switched-to signal transmission mode, the second broadcast includes an information element the same as an information element in a first broadcast or an information element having a same property as an information element in a first broadcast, and/or a new broadcast information element relative to the first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode; and
         the processing unit is further configured to: determine configuration information of the to-be-switched-to signal transmission mode based on the second broadcast; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the fourth aspect, in a fourth possible implementation of the fourth aspect, the receiving unit is further configured to: before the processing unit configures, based on the control instruction information, the to-be-switched-to signal transmission mode, separately receive a first broadcast and a second broadcast, and determine, from the first broadcast and the second broadcast, a broadcast corresponding to the to-be-switched-to signal transmission mode; and
         the processing unit is further configured to: determine, based on the broadcast corresponding to the to-be-switched-to signal transmission mode, configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to any one of the fourth aspect to the fourth possible implementation of the fourth aspect, in a fifth possible implementation of the fourth aspect, the base station further includes a sending unit; the receiving unit is further configured to: before the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive uplink transmission resource indication information sent by the base station; the processing unit is further configured to determine an uplink transmission resource based on the uplink transmission resource indication information; and the sending unit is configured to transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the fifth possible implementation of the fourth aspect, in a sixth possible implementation of the fourth aspect, a specific moment of switching, by the processing unit, the signal transmission mode is as follows: a moment of receiving, by the base station, the response message indicating successful configuration of the to-be-switched-to signal transmission mode is used as a start moment, and the signal transmission mode is switched at a moment after an X th  subframe signal is received, where X is a positive integer greater than or equal to 1. 
     With reference to the fifth possible implementation of the fourth aspect, in a seventh possible implementation of the fourth aspect, a specific moment of switching, by the processing unit, the signal transmission mode is as follows:
         if the base station receives, within a first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the base station, the control instruction information used to configure a signal transmission mode; and   if the base station does not receive, within the first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is maintained.       

     With reference to any one of the fourth aspect to the fourth possible implementation of the fourth aspect, in an eighth possible implementation of the fourth aspect, the terminal device further includes a sending unit;
         the receiving unit is further configured to: after the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive uplink transmission resource indication information sent by the base station;   the processing unit is further configured to determine an uplink transmission resource based on the uplink transmission resource indication information; and   the sending unit is configured to transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode.       

     According to a fifth aspect, an embodiment of the present invention further provides a base station. The base station includes:
         a detector, configured to detect whether signal transmission between the base station and a terminal device is abnormal;   a transmitter, configured to: when the detector detects that the signal transmission between the base station and the terminal device is abnormal, send, to the terminal device, control instruction information used to configure a signal transmission mode, where the control instruction information is used to instruct the terminal device to switch a current signal transmission mode on the terminal device side to a to-be-switched-to signal transmission mode; and   a processor, configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode.       

     When the detector detects that the signal transmission between the base station and the terminal device is abnormal, the transmitter immediately sends, to the terminal device based on a detection result of the detector, the control instruction information used to switch a signal transmission mode, so that the terminal device switches the signal transmission mode, to avoid unsmooth signal transmission between the terminal device and the base station. The base station side also needs to switch the signal transmission mode, to synchronize the signal transmission mode with the terminal device. 
     With reference to the fifth aspect, in a first possible implementation of the fifth aspect, the detector is specifically configured to:
         detect whether a received signal strength value of the terminal device is less than a first predetermined threshold; or   detect whether a rate at which the terminal device sends an uplink signal is less than a second predetermined threshold.       

     It should be understood that only two cases in which signal transmission is abnormal are enumerated in the present invention, but it does not represent that only the two cases exist. A signal transmission mode also needs to be switched in another abnormal case, and details are not described herein. 
     Before switching the signal transmission mode on the terminal device based on the control instruction information, the terminal device further needs to configure the to-be-switched-to signal transmission mode, and a manner of obtaining configuration information may include the following several aspects. Specifically, the manner includes a second possible implementation of the fifth aspect and a third possible implementation of the fifth aspect. Two other manners are described in detail on the terminal device side, and details are not described herein. 
     With reference to the fifth aspect or the first possible implementation of the fifth aspect, in the second possible implementation of the fifth aspect, the control instruction information includes a new broadcast information element, so that the terminal device determines, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     With reference to the fifth aspect or the first possible implementation of the fifth aspect, in the third possible implementation of the fifth aspect, the processor is further configured to: add a new broadcast information element to/delete a new broadcast information element from a first broadcast corresponding to the current signal transmission mode, and use an obtained broadcast as a second broadcast, so that the terminal device determines, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     With reference to any one of the fifth aspect to the third possible implementation of the fifth aspect, in a fourth possible implementation of the fifth aspect, the transmitter is further configured to: before the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send uplink transmission resource indication information to the terminal device, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the fourth possible implementation of the fifth aspect, in a fifth possible implementation of the fifth aspect, the base station further includes:
         a receiver, configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, where the processor is further configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode based on the response message.       

     With reference to the fifth possible implementation of the fifth aspect, in a sixth possible implementation of the fifth aspect, a specific moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode by the processor is as follows:
         a moment of receiving, by the receiver, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is used as a start moment, and the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1.       

     With reference to the fifth possible implementation of the fifth aspect, in a seventh possible implementation of the fifth aspect, a specific moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode by the processor is as follows:
         if the receiver receives, within a first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the transmitter to the terminal device, the control instruction information used to configure a signal transmission mode; and   if the receiver does not receive, within the first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processor maintains the current signal transmission mode.       

     With reference to any one of the fifth aspect to the third possible implementation of the fifth aspect, in an eighth possible implementation of the fifth aspect, the transmitter is further configured to:
         after the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send uplink transmission resource indication information to the terminal device, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode.       

     With reference to the eighth possible implementation of the fifth aspect, in a ninth possible implementation of the fifth aspect, the base station further includes a receiver, configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     With reference to any one of the fifth aspect to the third possible implementation of the fifth aspect, in a tenth possible implementation of the fifth aspect, the transmitter is further configured to: after sending, to the terminal device, the control instruction information used to configure a signal transmission mode, separately send uplink transmission resource indication information to the terminal device by using the current signal transmission mode and the second signal transmission mode, where the uplink transmission resource indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the tenth possible implementation of the fifth aspect, in an eleventh possible implementation of the fifth aspect, the detector is further configured to: separately detect a first receiver corresponding to the current signal transmission mode and a second receiver corresponding to the second signal transmission mode, to determine that the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device is able to be received through one of the first receiver or the second receiver; and the processor is further configured to: use a moment of receiving the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device as a start moment, and switch the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1. 
     According to a sixth aspect, an embodiment of the present invention further provides a terminal device. The terminal device includes:
         a receiver, configured to receive control instruction information that is used to configure a signal transmission mode and that is sent by a base station; and   a processor, configured to switch a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode.       

     With reference to the sixth aspect, in a first possible implementation of the sixth aspect, the control instruction information includes a new broadcast information element; and
         the processor is configured to: determine, based on the new broadcast information element and a first broadcast, configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the sixth aspect, in a second possible implementation of the sixth aspect, the receiver is further configured to: before the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode based on the control instruction information, receive a second broadcast sent by the base station, where the second broadcast is a broadcast obtained after a new broadcast information element is added to/deleted from a first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode; and
         the processor is further configured to: determine, based on the second broadcast, configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the sixth aspect, in a third possible implementation of the sixth aspect, the receiver is further configured to: after receiving the control instruction information that is used to configure the signal transmission mode and that is sent by the base station, receive a second broadcast sent by the base station, where the second broadcast is a broadcast corresponding to the to-be-switched-to signal transmission mode, the second broadcast includes an information element the same as an information element in a first broadcast or an information element having a same property as an information element in a first broadcast, and/or a new broadcast information element relative to the first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode; and
         the processor is further configured to: determine configuration information of the to-be-switched-to signal transmission mode based on the second broadcast; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to the sixth aspect, in a fourth possible implementation of the sixth aspect, the receiver is further configured to: before the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode based on the control instruction information, separately receive a first broadcast and a second broadcast, and determine, from the first broadcast and the second broadcast, a broadcast corresponding to the to-be-switched-to signal transmission mode; and
         the processor is further configured to: determine, based on the broadcast corresponding to the to-be-switched-to signal transmission mode, configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information.       

     With reference to any one of the sixth aspect to the fourth possible implementation of the sixth aspect, in a fifth possible implementation of the sixth aspect, the receiver is further configured to: before the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive uplink transmission resource indication information sent by the base station; the processor is further configured to determine an uplink transmission resource based on the uplink transmission resource indication information; and the transmitter is further configured to transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     With reference to the fifth possible implementation of the sixth aspect, in a sixth possible implementation of the sixth aspect, a specific moment of switching the signal transmission mode by the processor is as follows: a moment of receiving, by the base station, the response message indicating successful configuration of the to-be-switched-to signal transmission mode is used as a start moment, and the signal transmission mode is switched at a moment after an X th  subframe signal is received, where X is a positive integer greater than or equal to 1. 
     With reference to the fifth possible implementation of the sixth aspect, in a seventh possible implementation of the sixth aspect, a specific moment of switching the signal transmission mode by the processor is as follows:
         if the base station receives, within a first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the base station, the control instruction information used to configure a signal transmission mode; and   if the base station does not receive, within the first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is maintained.       

     With reference to any one of the sixth aspect to the fourth possible implementation of the sixth aspect, in an eighth possible implementation of the sixth aspect, the receiver is further configured to:
         after the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive uplink transmission resource indication information sent by the base station;   the processor is further configured to determine an uplink transmission resource based on the uplink transmission resource indication information; and   the transmitter is further configured to transmit, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode.       

     In the signal transmission mode switching method provided in the embodiments of the present invention, the terminal device obtains, in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode; determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. In addition, the base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural diagram in which a terminal device passively switches from an NC mode to a CE mode in the prior art; 
         FIG. 2  is a schematic flowchart of a signal transmission mode switching method according to Embodiment 1 of the present invention; 
         FIG. 3  is a schematic flowchart of another signal transmission mode switching method according to Embodiment 2 of the present invention; 
         FIG. 4  is a schematic flowchart of a method for implementing cognitive consistency between a base station and a terminal device about a mode switching time period according to an embodiment of the present invention; 
         FIG. 5  is a schematic flowchart of another method for implementing cognitive consistency between a base station and a terminal device about a mode switching time period according to an embodiment of the present invention; 
         FIG. 6  is a schematic flowchart of another method for implementing cognitive consistency between a base station and a terminal device about a mode switching time period according to an embodiment of the present invention; 
         FIG. 7  is a schematic flowchart of another method for implementing cognitive consistency between a base station and a terminal device about a mode switching time period according to an embodiment of the present invention; 
         FIG. 8  is a schematic structural diagram of a base station according to Embodiment 3 of the present invention; 
         FIG. 9  is a schematic structural diagram of a terminal device according to Embodiment 4 of the present invention; 
         FIG. 10  is a schematic structural diagram of another base station according to Embodiment 5 of the present invention; and 
         FIG. 11  is a schematic structural diagram of another terminal device according to Embodiment 6 of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following describes the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are some rather than all of the embodiments of the present invention. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention. 
     There may be a plurality of signal transmission modes. However, to better describe the technical solutions of the present invention, only two transmission modes are used as an example for description in the following specific embodiments. In addition, in specific examples enumerated in the following, a first transmission mode is defined as an NC mode and a second transmission mode is defined as a CE mode. Certainly, when ordinal numbers such as “first” and “second” are mentioned in the embodiments of the present invention, they are merely used for distinguishing unless they definitely show a sequence according to context. 
     Embodiment 1 of the present invention provides a schematic flowchart  200  of a signal transmission mode switching method. The method is mainly performed by a base station. Specifically, as shown in  FIG. 2 , the method includes the following steps. 
     Step  210 : When it is detected that signal transmission with a terminal device is abnormal, send, to the terminal device, control instruction information used to configure a signal transmission mode. 
     Specifically, there may be a plurality of cases in which the signal transmission between a base station and the terminal device is abnormal. For example, it is detected that a received signal strength value of the terminal device is less than a first predetermined threshold, or it is detected that a rate at which the terminal device sends an uplink signal is less than a second predetermined threshold. Optionally, the control instruction information may be radio resource control (Radio Resource Control, RRC for short) information. 
     Usually, when the terminal device accesses a network from a “near point”, the terminal device is in normal signal coverage, and has a relatively strong signal receiving capability. However, when the terminal device accesses the network from a “far point”, the terminal device is in relatively weak signal coverage, and consequently has a relatively weak signal receiving capability. Therefore, the base station first needs to detect a received signal strength value when the terminal device accesses the network, to determine whether the terminal device of a user accesses the network from the “near point” or the “far point”. Certainly, it should be understood that both the “near point” and the “far point” are relative concepts. In coverage of a signal sent by the base station, a point at which a signal receiving capability is relatively strong is the “near point”, and a location at an edge of the coverage of the signal sent by the base station or a location at which the signal receiving capability is relatively weak or is even 0 may be defined as the “far point”. For example, as shown in  FIG. 1( a ) , the terminal device accesses the network from the “near point”; and as shown in  FIG. 1( b ) , the terminal device accesses the network from the “far point”. When the terminal device accesses the network from the “far point”, a received signal strength value is usually less than the first predetermined threshold. Alternatively, when the terminal device accesses the network from the “far point”, a rate of an uplink signal received by the base station side may also be less than the second predetermined threshold. Therefore, the base station needs to send, to the terminal device, the control instruction information used to configure a signal transmission mode. The control instruction information is used to instruct to switch a current signal transmission mode on the terminal device side to a to-be-switched-to signal transmission mode. 
     Certainly, the foregoing enumerated “abnormal cases” are merely examples, and it cannot be concluded the abnormal cases as all “abnormal cases”. There may be a plurality of cases in which the signal transmission between the base station and the terminal device is abnormal. For brief description, details are not described herein. 
     Step  220 : Switch a current signal transmission mode to a to-be-switched-to signal transmission mode. 
     Specifically, the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. 
     Before switching the signal transmission mode, the terminal device first needs to determine configuration information corresponding to the to-be-switched-to signal transmission mode, for example, basic cell information corresponding to the to-be-switched-to signal transmission mode. The configuration information needs to be obtained by using a broadcast sent by the base station. As described in the background, for example, the terminal device accesses the network from the “near point”. In this case, a corresponding signal transmission mode is NC. When the terminal device needs to switch to a CE mode, the terminal device does not know existence of an eMTC broadcast. In other words, when the terminal device needs to switch the current signal transmission mode to the to-be-switched-to signal transmission mode, the terminal device does not know broadcast content corresponding to the to-be-switched-to signal transmission mode, and therefore cannot determine, based on the broadcast content corresponding to the to-be-switched-to signal transmission mode, the configuration information corresponding to the to-be-switched-to signal transmission mode. Therefore, the terminal device cannot configure the to-be-switched-to signal transmission mode. In this case, the present invention provides the following several manners of obtaining the configuration information. 
     In a first manner, when the control instruction information used to configure a signal transmission mode is sent to the terminal device, a new broadcast information element may be added to/deleted from the control instruction information, so that the terminal device may determine, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. 
     It should be understood that both the broadcast corresponding to the current signal transmission mode and a broadcast corresponding to the to-be-switched-to signal transmission mode are broadcasts periodically sent by the base station. When the terminal device knows existence of the broadcasts and needs to receive the broadcasts, the terminal device can regularly receive the broadcasts. 
     In a second manner, a new broadcast information element is added to/deleted from a first broadcast corresponding to the current signal transmission mode, and an obtained broadcast is used as a second broadcast, so that the terminal device determines, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. 
     In a specific example, if the first broadcast is a Long Term Evolution (Long Term Evolution, LTE for short) broadcast, and the second broadcast is an eMTC broadcast, there may be two types of new broadcast information elements. 
     One type is a most important broadcast, namely, a MIB (Master Information Block) broadcast, and the new broadcast information element is schedulingInfoSIB1-BR-r13. 
     The other type is a system broadcast, namely, a SIB (System Information Block) broadcast, and new broadcast content is:
         an information element such as “bandwidthReducedAccessRelatedInfo-r13”, “CellSelectionInfoCE-r13”, or “t-ReselectioNCUTRA-CE-r13”.       

     Two other manners are described in detail when signal transmission mode switching performed on the terminal device side is described in the following, and details are not described herein. 
     It should be noted that the second broadcast may include a new information element and/or an information element exactly the same as an information element in the first broadcast, or include a new information element and/or an information element having a same property as an information element in the first broadcast. Specific content in the second broadcast may be determined according to an actual situation, and is not limited herein. 
     After sending the control instruction information, the base station needs to receive a response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. Therefore, before this, the base station needs to send uplink transmission resource indication information to the terminal device. The indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     After receiving the response message, the base station switches the current signal transmission mode to the to-be-switched-to signal transmission mode. 
     The base station side may determine a specific moment of switching the signal transmission mode, to implement consistency between the base station side and the terminal device side during signal transmission mode switching. The specific moment of switching the signal transmission mode may include the following. 
     A moment of receiving the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is used as a start moment, and the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1. 
     Alternatively, if the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is received within a first predetermined time period (it is assumed that the first time period equals a time of receiving K subframe signals), the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first time period starts from a moment of sending, by the base station to the terminal device, the control instruction information used to configure a signal transmission mode. To be specific, a moment upon reception of K+N subframe signals after the moment of sending, by the base station to the terminal device, the control instruction information used to configure a signal transmission mode is used as a moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode. However, if the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is not received within the first predetermined time period, the current signal transmission mode is maintained. Both K and N are positive integers greater than or equal to 1. 
     In another case, the base station and the terminal device agree on that after the base station successfully sends the control instruction information to the terminal device, the base station and the terminal device separately switch the current signal transmission mode to the to-be-switched-to signal transmission mode. Certainly, after receiving the control instruction information, the terminal device sends, to the base station, a response message indicating successful receiving of the control instruction information. Then the base station and the terminal device switch the signal transmission mode. When sending the uplink transmission resource indication information to the terminal device, the base station transmits the uplink transmission resource indication information on a signal transmission channel corresponding to the second signal transmission mode. Similarly, the uplink transmission resource indication information is used to instruct the terminal device to determine the uplink transmission resource and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     The base station side also receives, on the signal transmission channel corresponding to the second signal transmission mode, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     In another case, the base station and the terminal device side do not agree on a time of simultaneously switching the signal transmission mode. After receiving the control instruction information that is used to configure a signal transmission mode and that is sent by the base station, the terminal device side configures the to-be-switched-to signal transmission mode, and switches the signal transmission mode after configuration succeeds, that is, switches the current signal transmission mode to the to-be-switched-to signal transmission mode. Before receiving the response message that is used to indicate successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the base station side does not know when the terminal device switches the signal transmission mode. Therefore, the base station side separately sends the uplink transmission resource indication information to the terminal device by using the current signal transmission mode and the second signal transmission mode. 
     In addition, the base station separately detects a first signal transmission channel corresponding to the current signal transmission mode and a second signal transmission channel corresponding to the second signal transmission mode, to ensure that the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device is able to be received through one of the first signal transmission channel or the second signal transmission channel. 
     A moment of switching, by the base station side, the signal transmission mode is as follows: A moment of receiving the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device is used as a start moment, and the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1. 
     In the signal transmission mode switching method provided in the foregoing Embodiment 1 of the present invention, the base station side sends, to the terminal device in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode, so that the terminal device determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. In addition, the base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
     Embodiment 2 of the present invention further provides a schematic flowchart  300  of another signal transmission mode switching method. The method is mainly performed by a terminal device. Specifically, as shown in  FIG. 3 , the method includes the following steps: 
     Step  310 : Receive control instruction information that is used to configure a signal transmission mode and that is sent by a base station. 
     Step  320 : Switch a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information. 
     Specifically, the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. Before configuring the to-be-switched-to signal transmission mode based on the control instruction information, the terminal device first needs to obtain configuration information corresponding to the to-be-switched-to signal transmission mode. A specific manner of obtaining the configuration information corresponding to the to-be-switched-to signal transmission mode includes the following. 
     In a first manner, when sending, to the terminal device, the control instruction information used to configure a signal transmission mode, the base station may add a new broadcast information element to/delete a new broadcast information element from the control instruction information. The terminal device determines, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. The terminal device further needs to configure the to-be-switched-to signal transmission mode based on the configuration information. 
     It should be understood that both the broadcast corresponding to the current signal transmission mode and a broadcast corresponding to the to-be-switched-to signal transmission mode are broadcasts periodically sent by the base station. When the terminal device knows existence of the broadcasts and needs to receive the broadcasts, the terminal device can regularly receive the broadcasts. 
     Alternatively, in a second manner, the terminal device receives a second broadcast sent by the base station. The second broadcast is actually a broadcast obtained after a new broadcast information element is added to/deleted from a first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode. The terminal device may determine, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information. 
     In a specific example, if the first broadcast is an LTE broadcast, and the second broadcast is an eMTC broadcast, there may be two types of new broadcast information elements. 
     One type is a MIB broadcast, and the new broadcast information element is schedulingInfoSIB1-BR-r13. 
     The other type is a SIB broadcast, and new broadcast content is:
         an information element such as “bandwidthReducedAccessRelatedInfo-r13”, “CellSelectionInfoCE-r13”, or “t-ReselectioNCUTRA-CE-r13”.       

     In a third manner, if the terminal device clearly knows existence of a second broadcast corresponding to the to-be-switched-to signal transmission mode, the terminal device may directly receive the second broadcast. The second broadcast includes an information element the same as an information element in a first broadcast or an information element having a same property as an information element in a first broadcast, and/or a new broadcast information element relative to the first broadcast. The first broadcast is a broadcast corresponding to the current signal transmission mode. The terminal device may determine the configuration information of the to-be-switched-to signal transmission mode based on the second broadcast; and configure the to-be-switched-to signal transmission mode based on the configuration information. The terminal device receives the second broadcast after receiving the control instruction information that is used to configure a signal transmission mode and that is sent by the base station. To be specific, the terminal device may receive the broadcast before switching the mode or after switching the mode. 
     In a fourth manner, when it is not determined that the current signal transmission mode of the terminal device is which signal transmission mode, the terminal device may separately obtain a first broadcast and a second broadcast. Then the terminal device determines, from the first broadcast and the second broadcast, a broadcast corresponding to the to-be-switched-to signal transmission mode. The terminal device determines, based on the broadcast corresponding to the to-be-switched-to signal transmission mode, configuration information corresponding to the to-be-switched-to signal transmission mode; and then configures the to-be-switched-to signal transmission mode based on the configuration information. 
     Certainly, only the second broadcast may be read. In this case, the second broadcast may include a new broadcast information element and/or an information element exactly the same as an information element in the first broadcast. Therefore, even if the to-be-switched-to signal transmission mode is a signal transmission mode corresponding to the first broadcast, configuration information corresponding to the signal transmission mode that is corresponding to the first broadcast may be obtained only after the new information element is deleted or added. 
     It should be noted that, it can also be learned from the foregoing description that the second broadcast may include a new information element and/or an information element exactly the same as an information element in the first broadcast, or include a new information element and/or an information element having the same property as an information element in the first broadcast. Specific content in the second broadcast may be determined according to an actual situation, and is not limited herein. 
     Certainly, after successfully configuring the to-be-switched-to signal transmission mode, the terminal device further needs to send, to the base station, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. Therefore, before this, the terminal device first needs to receive uplink transmission resource indication information, determine an uplink transmission resource based on the uplink transmission resource indication information, and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     After sending, to the base station side, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the terminal device may switch the signal transmission mode. A specific switching moment includes the following: A moment of receiving, by the base station, the response message indicating successful configuration of the to-be-switched-to signal transmission mode is used as a start moment, and the signal transmission mode is switched at a moment after an X th  subframe signal is received, where X is a positive integer greater than or equal to 1. 
     Alternatively, if the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is received within a first predetermined time period (it is assumed that the first time period equals a time of receiving K subframe signals), the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first time period starts from a moment of sending, by the base station to the terminal device, the control instruction information used to configure a signal transmission mode. To be specific, a moment upon reception of K+N subframe signals after the moment of sending, by the base station to the terminal device, the control instruction information used to configure a signal transmission mode is used as a moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode. However, if the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is not received within the first predetermined time period, the current signal transmission mode is maintained. Similarly, the terminal device also needs to maintain an original status. Both K and N are positive integers greater than or equal to 1. 
     In another case, the base station and the terminal device agree on that after the base station successfully sends the control instruction information to the terminal device, the base station and the terminal device separately switch the current signal transmission mode to the to-be-switched-to signal transmission mode. 
     In this case, the terminal device performs the following operations after the terminal device switches the signal transmission mode: receiving uplink transmission resource indication information sent by the base station, determining an uplink transmission resource based on the uplink transmission resource indication information, and transmitting, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     In another case, the base station and the terminal device side do not agree on a time of simultaneously switching the signal transmission mode. After receiving the control instruction information that is used to configure a signal transmission mode and that is sent by the base station, the terminal device side configures the to-be-switched-to signal transmission mode, and switches the signal transmission mode after configuration succeeds, that is, switches the current signal transmission mode to the to-be-switched-to signal transmission mode. Similar to the foregoing case, the terminal device also performs the following operations after the terminal device switches the signal transmission mode: receiving uplink transmission resource indication information sent by the base station, determining an uplink transmission resource based on the uplink transmission resource indication information, and transmitting, to the base station by using the uplink transmission resource, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     In the another signal transmission mode switching method provided in Embodiment 2 of the present invention, the terminal device obtains, in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode; determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. The base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
     For the foregoing two embodiments, how to implement, through negotiation, signal transmission mode switching synchronization or a synchronization between the base station and the terminal device is specifically described in the following by using an example. It is assumed that the foregoing current signal transmission mode is NC and the foregoing to-be-switched-to signal transmission mode is CE. To be specific, mode switching is performed between the NC and the CE. It is assumed that control signaling that is used to switch a signal transmission mode and that is sent by the base station to the terminal device is RRC signaling. 
     When the base station detects that the signal transmission between the base station and the terminal device is abnormal, for example, the terminal device accesses a network from a “near point”, specifically as shown in  FIG. 1 , the terminal device needs to switch from the NC mode to the CE mode, so that the terminal device can normally receive a signal when moving to a weak coverage area of LTE. The NC mode corresponds to an LTE broadcast, and the CE mode corresponds to an eMTC broadcast. 
     The base station first sends the RRC signaling to the terminal device, to notify the terminal device that a signal transmission mode needs to be switched. The terminal device switches the signal transmission mode from the NC to the CE. 
     After receiving the control signaling, the terminal device needs to configure a to-be-switched-to signal transmission mode. A specific configuration parameter is obtained by using the eMTC broadcast. 
     The eMTC broadcast may be obtained in the following four manners. 
     In a first manner, the base station sends a new information element to the terminal device by using the RRC signaling that is used to instruct to switch a signal transmission mode and that is sent by the base station to the terminal device. Alternatively, the base station may send a new information element to the terminal device by using other RRC signaling. The new information element is a new broadcast information element in the eMTC broadcast relative to the LTE broadcast. A specific new broadcast information element is described in Embodiment 1 and Embodiment 2, and details are not described herein again. 
     In a second manner, a new information element is added to an LTE broadcast sent by the base station. In this way, even if the terminal device accesses a network from a “near point”, the terminal device can still obtain configuration information corresponding to the CE mode. 
     In a third manner, after the terminal device receives the RRC signaling, the terminal device directly reads the eMTC broadcast. The terminal device triggers, based on the RRC signaling, switching from the NC mode to the CE mode. Certainly, a process of reading the broadcast may be before the NC mode is switched to the CE mode or after the NC mode is switched to the CE mode. It should be noted that if the eMTC broadcast is read in the CE mode, a protocol constraint needs to be released because the terminal device in a connected mode of the CE mode cannot read the broadcast according to stipulation in a current protocol (the protocol LTE R13). 
     In a fourth manner, regardless of whether the terminal device accesses a network in the NC mode or the CE mode, the terminal device reads both the LTE broadcast and the eMTC broadcast when accessing the network. Alternatively, the terminal device reads only the eMTC broadcast because the eMTC broadcast is obtained based on the LTE broadcast. It can be learned from the foregoing that the eMTC broadcast includes a new information element and a broadcast information element having same content or a same property as a broadcast information element in the LTE broadcast. When only the eMTC broadcast is read, a premise is that content in the eMTC broadcast except the new information element needs to be the same as content in the LTE broadcast. 
     By using any one of the foregoing four manners, the content in the eMTC broadcast can be read when the NC mode is switched to the CE mode, and then the CE mode is configured based on the content in the eMTC broadcast. After configuration is completed, a response message indicating successful configuration is sent to the base station side. 
     Before the response message indicating successful configuration is sent to the base station side, the base station side further needs to send uplink transmission resource indication information to the terminal device, to instruct the terminal device to determine an uplink transmission resource and send the response message to the base station on the uplink transmission resource. 
     However, during configuration of the CE mode performed by the terminal device, a mode switching time period of the terminal device may be inconsistent with a mode switching time period of the base station. Consequently, when the base station side sends the uplink transmission resource indication information to the terminal device side, the terminal device fails in receiving the uplink transmission resource indication information, or the base station side fails in receiving the response message indicating successful configuration. To avoid the foregoing problem, the base station and the terminal device may set a criterion for switching a signal transmission mode, to ensure that the terminal device can successfully receive the uplink transmission resource indication information, or that the base station side can successfully receive the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     Specifically, the following four manners are included. 
     In a first manner, specifically, as shown in  FIG. 4 , the base station side sends, in the NC mode, the RRC signaling and the uplink transmission resource indication information to the terminal device. After receiving the RRC signaling, the terminal device first obtains the eMTC broadcast in any one of the foregoing four manners of obtaining the eMTC broadcast, and configures the CE mode. After completing configuration, the terminal device determines the uplink transmission resource based on the uplink transmission resource indication information, and then sends, by using the uplink transmission resource, the response message indicating successful configuration. A moment of receiving, by the base station, the response message indicating successful configuration is used as a start moment, and the terminal device and the base station simultaneously switch the signal transmission mode from the NC mode to the CE mode at a moment after an X th  subframe signal is received, where X is a positive integer greater than or equal to 1. 
     In a second manner, specifically, as shown in  FIG. 5 , the base station side sends, in the NC mode, the RRC signaling and the uplink transmission resource indication information to the terminal device. After receiving the RRC signaling, the terminal device first obtains the eMTC broadcast in any one of the foregoing four manners of obtaining the eMTC broadcast, and configures the CE mode. After completing configuration, the terminal device determines the uplink transmission resource based on the uplink transmission resource indication information, and then sends, by using the uplink transmission resource, the response message indicating successful configuration. If the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is received within a time period of receiving K subframe signals after a moment of sending, by the base station to the terminal device, control instruction information used to configure a signal transmission mode, a current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after the K subframe signals are received, where N is a positive integer greater than or equal to 1. To be specific, the base station and the terminal device simultaneously switch from the NC mode to the CE mode at a moment upon reception of K+N subframe signals after the moment of sending, by the base station to the terminal device, the control instruction information used to configure a signal transmission mode. However, if the response message that indicates successful configuration and that is sent by the terminal device is not received within the time period of receiving the K subframe signals, the base station and the terminal device maintain the current NC mode. Both K and N are positive integers greater than or equal to 1. 
     In a third manner, specifically, as shown in  FIG. 6 , the base station and the terminal device agree on that the base station and the terminal device separately switch the signal transmission mode from the NC mode to the CE mode after the base station successfully sends the RRC signaling to the terminal device. Specifically, after the terminal device receives the RRC signaling, the terminal device sends a response message indicating successful receiving of the RRC signaling. Then the terminal device and the base station switch the signal transmission mode. The base station sends, in the CE mode, the uplink transmission resource indication information to the terminal device. After receiving the uplink transmission resource indication information, and after successfully configuring the CE mode, the terminal device sends, in the CE mode to the base station by using the uplink transmission resource, the response message indicating successful configuration. 
     In a fourth manner, specifically, as shown in  FIG. 7 , the base station and the terminal device do not agree on a time of simultaneously switching the signal transmission mode. In this case, after receiving the RRC signaling sent by the base station, the terminal device configures the CE mode according to a normal procedure and the configuration information in the eMTC broadcast, and directly switches to the CE mode after configuration succeeds. Then the terminal device sends, to the base station, the response message indicating successful configuration. However, the base station side does not know when the terminal can successfully perform configuration, and switches the signal transmission mode. Therefore, the base station may send the uplink transmission resource indication information to the terminal device separately on signal transmission channels in the NC mode and the CE mode. In this way, the terminal device can receive the indication information in both modes, determine the uplink transmission resource based on the indication information, and then transmit, by using the uplink transmission resource, the response message indicating successful configuration. The base station side also separately receives, by using the signal transmission channels in the NC mode and the CE mode, the response message indicating successful configuration; and uses a moment of receiving the response message that indicates successful configuration and that is sent by the terminal device as a start moment, and switches a current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1. 
     Therefore, the foregoing problem can be resolved: Because the mode switching time period of the terminal device is inconsistent with the mode switching time period of the base station, the terminal device fails in receiving the uplink transmission resource indication information when the base station side sends the uplink transmission resource indication information to the terminal device side, or the base station side fails in receiving the response message indicating successful configuration. 
     It should be understood that the foregoing description is merely an implementation of a specific embodiment. When the signal transmission mode is switched from the CE mode to the NC mode, a broadcast obtaining process is a reverse process. For example, in the first manner of obtaining a broadcast, the LTE broadcast is obtained. Therefore, although the RRC configuration signaling also includes the new information element, the terminal device needs to delete the new information element from the eMTC broadcast to obtain the LTE broadcast. Then the terminal device configures the NC, and finally switches from the CE mode to the NC mode. Other manners each are similar to this manner, and details are not described herein. Certainly, only a switching process between the CE and the NC mode is described in the specific example. A principle of a method for switching between other modes is the same or similar, and details are not described herein. 
     Corresponding to the signal transmission mode switching method provided in the foregoing Embodiment 1, Embodiment 3 of the present invention further provides a base station. Specifically,  FIG. 8  is a schematic structural diagram of the base station  800  according to Embodiment 3 of the present invention. The base station includes a detection unit  801 , a sending unit  802 , and a processing unit  803 . 
     The detection unit  801  is configured to detect whether signal transmission between the base station and a terminal device is abnormal. 
     Specifically, the detection unit  801  is specifically configured to: detect whether a received signal strength value of the terminal device is less than a first predetermined threshold; or detect whether a rate at which the terminal device sends an uplink signal is less than a second predetermined threshold. 
     It should be understood that only two cases in which signal transmission is abnormal are enumerated in the present invention, but it does not represent that only the two cases exist. A signal transmission mode also needs to be switched in another abnormal case, and details are not described herein. 
     The sending unit  802  is configured to: when the detection unit  801  detects that the signal transmission between the base station and the terminal device is abnormal, send, to the terminal device, control instruction information used to configure a signal transmission mode, where the control instruction information is used to instruct the terminal device to switch a current signal transmission mode on the terminal device side to a to-be-switched-to signal transmission mode. 
     The processing unit  803  is configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode, where the to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. 
     When the detection unit  801  detects that the signal transmission between the base station and the terminal device is abnormal, the sending unit  802  immediately sends, to the terminal device based on a detection result of the detection unit, the control instruction information used to switch a signal transmission mode, so that the terminal device switches the signal transmission mode, to avoid unsmooth signal transmission between the terminal device and the base station. The base station side also needs to switch the signal transmission mode, to synchronize the signal transmission mode with the terminal device. 
     Before switching the signal transmission mode on the terminal device based on the control instruction information, the terminal device further needs to configure the to-be-switched-to signal transmission mode, and configuration information may be obtained in the following two manners. Two other manners are described in detail on the terminal device side, and details are not described herein. 
     In a first manner, when the sending unit  802  sends, to the terminal device, the control instruction information used to configure a signal transmission mode, a new broadcast information element may be added to/deleted from the control instruction information, so that the terminal device may determine, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. 
     It should be understood that both the broadcast corresponding to the current signal transmission mode and a broadcast corresponding to the to-be-switched-to signal transmission mode are broadcasts periodically sent by the base station. When the terminal device knows existence of the broadcasts and needs to receive the broadcasts, the terminal device can regularly receive the broadcasts. 
     In a second manner, the processing unit  803  adds a new broadcast information element to/deletes a new broadcast information element from a first broadcast corresponding to the current signal transmission mode, and uses an obtained broadcast as a second broadcast, so that the terminal device determines, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. 
     After the sending unit  802  sends the control instruction information to the terminal device, the base station needs to receive a response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. Therefore, before this, the sending unit  802  needs to send uplink transmission resource indication information to the terminal device. The indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     Therefore, in one case, the sending unit  802  is further configured to: before the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send the uplink transmission resource indication information to the terminal device. 
     Correspondingly, the base station further includes a receiving unit  804 , configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     The processing unit  803  is further configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode based on the response message. 
     Specifically, a specific moment of switching, by the processing unit  803 , the current signal transmission mode to the to-be-switched-to signal transmission mode is as follows: A moment of receiving, by the receiving unit  804 , the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is used as a start moment, and the processing unit  803  switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1. 
     Alternatively, a specific moment of switching, by the processing unit  803 , the current signal transmission mode to the to-be-switched-to signal transmission mode may be as follows: If the receiving unit  804  receives, within a first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processing unit  803  switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the sending unit  802  to the terminal device, the control instruction information used to configure a signal transmission mode. If the receiving unit  804  does not receive, within the first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processing unit  803  maintains the current signal transmission mode. 
     In another case, the sending unit  802  is further configured to: after the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send the uplink transmission resource indication information to the terminal device. The uplink transmission resource indication information is used to instruct the terminal device to determine the uplink transmission resource and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     The receiving unit  804  is configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     In another case, the sending unit  802  is further configured to: after sending, to the terminal device, the control instruction information used to configure a signal transmission mode, separately send the uplink transmission resource indication information to the terminal device by using the current signal transmission mode and the second signal transmission mode. After transmitting the uplink transmission resource indication information by using two different signal transmission modes, the base station further needs to use the detection unit  801  to detect a first receiving unit corresponding to the current signal transmission mode and a second receiving unit corresponding to the second signal transmission mode, to determine that one of the first receiving unit or the second receiving unit can receive the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device. The processing unit  803  is further configured to: use a moment of receiving the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device as a start moment, and switch the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1. 
     When the base station provided in Embodiment 3 of this application runs, the base station performs the method provided in Embodiment 1 of this application. For operating details of the base station, refer to the method provided in Embodiment 1 of this application. 
     In the base station provided in Embodiment 3 of the present invention, the sending unit sends, to the terminal device in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode, so that the terminal device determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. The base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
     Corresponding to the another signal transmission mode switching method provided in Embodiment 2, Embodiment 4 of the present invention further provides a terminal device. Specifically,  FIG. 9  is a schematic structural diagram of the terminal device according to Embodiment 4 of the present invention. The terminal device includes a receiving unit  901  and a processing unit  902 . 
     The receiving unit  901  is configured to receive control instruction information that is used to configure a signal transmission mode and that is sent by a base station. 
     The processing unit  902  is configured to switch a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information. The to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. 
     Specifically, before configuring the to-be-switched-to signal transmission mode based on the control instruction information, the terminal device first needs to obtain configuration information corresponding to the to-be-switched-to signal transmission mode. A specific manner of obtaining the configuration information corresponding to the to-be-switched-to signal transmission mode includes the following. 
     In a first manner, when sending, to the terminal device, the control instruction information used to configure a signal transmission mode, the base station may add a new broadcast information element to/delete a new broadcast information element from the control instruction information. 
     The processing unit  902  is configured to: determine, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information. 
     In a second manner, before the processing unit  902  switches the current signal transmission mode to the to-be-switched-to signal transmission mode based on the control instruction information, the receiving unit  901  receives a second broadcast sent by the base station. The second broadcast is a broadcast obtained after a new broadcast information element is added to/deleted from a first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     The processing unit  902  is further configured to: determine, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information. 
     In a third manner, after receiving the control instruction information that is used to configure a signal transmission mode and that is sent by the base station, the receiving unit  901  receives a second broadcast sent by the base station. The second broadcast is a broadcast corresponding to the to-be-switched-to signal transmission mode, the second broadcast includes an information element the same as an information element in a first broadcast or an information element having a same property as an information element in a first broadcast, and/or a new broadcast information element relative to the first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     The processing unit  902  determines the configuration information of the to-be-switched-to signal transmission mode based on the second broadcast; and configures the to-be-switched-to signal transmission mode based on the configuration information. 
     In a fourth manner, before the processing unit  902  configures, based on the control instruction information, the to-be-switched-to signal transmission mode, the receiving unit  901  separately receives a first broadcast and a second broadcast, and determines, from the first broadcast and the second broadcast, a broadcast corresponding to the to-be-switched-to signal transmission mode. 
     The processing unit  902  determines, based on the broadcast corresponding to the to-be-switched-to signal transmission mode, the configuration information corresponding to the to-be-switched-to signal transmission mode; and configures the to-be-switched-to signal transmission mode based on the configuration information. 
     In addition, the receiving unit  901  is further configured to receive uplink transmission resource indication information sent by the base station, to determine an uplink transmission resource based on the uplink transmission resource indication information and send, to the base station, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. Therefore, the terminal device further includes a sending unit  903 . 
     Therefore, in one case, the receiving unit  901  is further configured to: before the processing unit  902  switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive the uplink transmission resource indication information sent by the base station. The processing unit  902  is further configured to determine the uplink transmission resource based on the uplink transmission resource indication information. The sending unit  903  is configured to transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     A specific moment of switching, by the processing unit  902 , the signal transmission mode is as follows: A moment of receiving, by the base station, the response message indicating successful configuration of the to-be-switched-to signal transmission mode is used as a start moment, and the signal transmission mode is switched at a moment after an X th  subframe signal is received, where X is a positive integer greater than or equal to 1. 
     Alternatively, if the base station receives, within a first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the base station, the control instruction information used to configure a signal transmission mode. 
     If the base station does not receive, within the first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is maintained. 
     In another case, the receiving unit  901  is further configured to: after the processing unit switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive the uplink transmission resource indication information sent by the base station. The processing unit  902  is further configured to determine the uplink transmission resource based on the uplink transmission resource indication information. The sending unit  903  is configured to transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     When the terminal provided in Embodiment 4 of this application runs, the terminal performs the method provided in Embodiment 2 of this application. For operating details of the terminal, refer to the method provided in Embodiment 2 of this application. 
     In the terminal device provided in Embodiment 4 of the present invention, the receiving unit obtains, in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode. The processing unit determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. The base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
     Corresponding to the signal transmission mode switching method provided in the foregoing Embodiment 1, Embodiment 5 of the present invention further provides a base station. Specifically,  FIG. 10  is a schematic structural diagram of the base station  1000  according to Embodiment 5 of the present invention. The base station includes a detector  1001 , a transmitter  1002 , and a processor  1003 . 
     The detector  1001  is configured to detect whether signal transmission between the base station and a terminal device is abnormal. 
     Specifically, the detector  1001  is specifically configured to: detect whether a received signal strength value of the terminal device is less than a first predetermined threshold; or detect whether a rate at which the terminal device sends an uplink signal is less than a second predetermined threshold. It should be understood that only two cases in which signal transmission is abnormal are enumerated in the present invention, but it does not represent that only the two cases exist. A signal transmission mode also needs to be switched in another abnormal case, and details are not described herein. 
     The transmitter  1002  is configured to: when the detector  1001  detects that the signal transmission between the base station and the terminal device is abnormal, send, to the terminal device, control instruction information used to configure a signal transmission mode. The control instruction information is used to instruct the terminal device to switch a current signal transmission mode on the terminal device side to a to-be-switched-to signal transmission mode. 
     The processor  1003  is configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode. The to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. 
     When the detector  1001  detects that the signal transmission between the base station and the terminal device is abnormal, the transmitter  1002  immediately sends, to the terminal device based on a detection result of the detector  1001 , the control instruction information used to switch a signal transmission mode, so that the terminal device switches the signal transmission mode, to avoid unsmooth signal transmission between the terminal device and the base station. The base station side also needs to switch the signal transmission mode, to synchronize the signal transmission mode with the terminal device. 
     Before switching the signal transmission mode on the terminal device based on the control instruction information, the terminal device further needs to configure the to-be-switched-to signal transmission mode, and configuration information may be obtained in the following two manners. Two other manners are described in detail on the terminal device side, and details are not described herein. 
     In a first manner, when sending, to the terminal device, the control instruction information used to configure a signal transmission mode, the transmitter  1002  may add a new broadcast information element to/delete a new broadcast information element from the control instruction information, so that the terminal device may determine, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. 
     It should be understood that both the broadcast corresponding to the current signal transmission mode and a broadcast corresponding to the to-be-switched-to signal transmission mode are broadcasts periodically sent by the base station. When the terminal device knows existence of the broadcasts and needs to receive the broadcasts, the terminal device can regularly receive the broadcasts. 
     In a second manner, the processor  1003  adds a new broadcast information element to/deletes a new broadcast information element from a first broadcast corresponding to the current signal transmission mode, and uses an obtained broadcast as a second broadcast, so that the terminal device determines, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode. The first broadcast is a broadcast corresponding to the current signal transmission mode. 
     After the transmitter  1002  sends the control instruction information to the terminal device, the base station needs to receive a response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. Therefore, before this, the transmitter  1002  needs to send uplink transmission resource indication information to the terminal device. The indication information is used to instruct the terminal device to determine an uplink transmission resource and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     Therefore, in one case, the transmitter  1002  is further configured to: before the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send the uplink transmission resource indication information to the terminal device. 
     The base station further includes a receiver  1004 , configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     The processor  1003  is further configured to switch the current signal transmission mode to the to-be-switched-to signal transmission mode based on the response message. 
     Specifically, a specific moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode by the processor  1003  is as follows: A moment of receiving, by the receiver  1004 , the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device is used as a start moment, and the processor  1003  switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an X th  subframe signal, where X is a positive integer greater than or equal to 1. 
     Alternatively, a specific moment of switching the current signal transmission mode to the to-be-switched-to signal transmission mode by the processor  1003  may be as follows: If the receiver  1004  receives, within a first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processor  1003  switches the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the transmitter  1002  to the terminal device, the control instruction information used to configure a signal transmission mode. If the receiver  1004  does not receive, within the first predetermined time period, the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device, the processor  1003  maintains the current signal transmission mode. 
     In another case, the transmitter  1002  is further configured to: after the processor  1003  switches the current signal transmission mode to the to-be-switched-to signal transmission mode, send the uplink transmission resource indication information to the terminal device. The uplink transmission resource indication information is used to instruct the terminal device to determine the uplink transmission resource and transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     Correspondingly, the base station further includes a receiver  1004 , configured to receive the response message that indicates successful configuration of the to-be-switched-to signal transmission mode and that is sent by the terminal device. 
     In another case, the transmitter  1002  is further configured to: after sending, to the terminal device, the control instruction information used to configure a signal transmission mode, separately send the uplink transmission resource indication information to the terminal device by using the current signal transmission mode and the second signal transmission mode. After transmitting the uplink transmission resource indication information by using two different signal transmission modes, the base station further needs to use the detector  1001  to separately detect a first receiver corresponding to the current signal transmission mode and a second receiver corresponding to the second signal transmission mode, to determine that the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device is able to be received through one of the first receiver or the second receiver. The processor  1003  is further configured to: use a moment of receiving the response message that indicates successful configuration of the signal transmission mode and that is sent by the terminal device as a start moment, and switch the current signal transmission mode to the to-be-switched-to signal transmission mode at a moment upon reception of a J th  subframe signal, where J is a positive integer greater than or equal to 1. 
     When the base station provided in Embodiment 5 of this application runs, the base station performs the method provided in Embodiment 1 of this application. For operating details of the base station, refer to the method provided in Embodiment 1 of this application. 
     In the base station provided in Embodiment 5 of the present invention, the transmitter sends, to the terminal device in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode, so that the terminal device determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. In addition, the base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
     Corresponding to the another signal transmission mode switching method provided in the foregoing Embodiment 2, Embodiment 6 of the present invention further provides a terminal device. Specifically,  FIG. 11  is a schematic structural diagram of the terminal device  1100  according to Embodiment 6 of the present invention. The terminal device includes a receiver  1101  and a processor  1102 . 
     The receiver  1101  is configured to receive control instruction information that is used to configure a signal transmission mode and that is sent by a base station. 
     The processor  1102  is configured to switch a current signal transmission mode to a to-be-switched-to signal transmission mode based on the control instruction information. The to-be-switched-to signal transmission mode is a second signal transmission mode other than the current signal transmission mode. 
     Specifically, before configuring the to-be-switched-to signal transmission mode based on the control instruction, the terminal device first needs to obtain configuration information corresponding to the to-be-switched-to signal transmission mode. A specific manner of obtaining the configuration information corresponding to the to-be-switched-to signal transmission mode includes the following. 
     In a first manner, when the base station sends, to the terminal device, the control instruction information used to configure a signal transmission mode, the base station may add a new broadcast information element to/delete a new broadcast information element from the control instruction information. 
     The processor  1102  is configured to: determine, based on the new broadcast information element and a first broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode, where the first broadcast is a broadcast corresponding to the current signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information. 
     In a second manner, before the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode based on the control instruction information, the receiver  1101  receives a second broadcast sent by the base station. The second broadcast is a broadcast obtained after a new broadcast information element is added to/deleted from a first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     The processor  1102  is further configured to: determine, based on the second broadcast, the configuration information corresponding to the to-be-switched-to signal transmission mode; and configure the to-be-switched-to signal transmission mode based on the configuration information. 
     In a third manner, after receiving the control instruction information that is used to configure a signal transmission mode and that is sent by the base station, the receiver  1101  receives a second broadcast sent by the base station. The second broadcast is a broadcast corresponding to the to-be-switched-to signal transmission mode, the second broadcast includes an information element the same as an information element in a first broadcast or an information element having a same property as an information element in a first broadcast, and/or a new broadcast information element relative to the first broadcast, and the first broadcast is a broadcast corresponding to the current signal transmission mode. 
     The processor  1102  determines the configuration information of the to-be-switched-to signal transmission mode based on the second broadcast; and configures the to-be-switched-to signal transmission mode based on the configuration information. 
     In a fourth manner, before the processor configures, based on the control instruction information, the to-be-switched-to signal transmission mode, the receiver  1101  separately receives a first broadcast and a second broadcast, and determines, from the first broadcast and the second broadcast, a broadcast corresponding to the to-be-switched-to signal transmission mode. 
     The processor  1102  determines, based on the broadcast corresponding to the to-be-switched-to signal transmission mode, configuration information corresponding to the to-be-switched-to signal transmission mode; and configures the to-be-switched-to signal transmission mode based on the configuration information. 
     In addition, the receiver  1101  is further configured to receive uplink transmission resource indication information sent by the base station, to determine an uplink transmission resource based on the uplink transmission resource indication information and send, to the base station, a response message indicating successful configuration of the to-be-switched-to signal transmission mode. Therefore, the terminal device further includes a transmitter  1103 . 
     Therefore, in one case, the receiver  1101  is further configured to: before the processor  1102  switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive the uplink transmission resource indication information sent by the base station. The processor  1102  is further configured to determine the uplink transmission resource based on the uplink transmission resource indication information. The transmitter  1103  is configured to transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     A specific moment of switching the signal transmission mode by the processor  1102  is as follows: A moment of receiving, by the base station, the response message indicating successful configuration of the to-be-switched-to signal transmission mode is used as a start moment, and the signal transmission mode is switched at a moment after an X th  subframe signal is received, where X is a positive integer greater than or equal to 1. 
     Alternatively, if the base station receives, within a first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is switched to the to-be-switched-to signal transmission mode at a moment upon reception of an N th  subframe signal after an end moment of the first predetermined time period, where N is a positive integer greater than or equal to 1, and the first predetermined time period starts from a moment of sending, by the base station, the control instruction information used to configure a signal transmission mode. 
     If the base station does not receive, within the first predetermined time period, the response message indicating successful configuration of the to-be-switched-to signal transmission mode, the current signal transmission mode is maintained. 
     In another case, the receiver  1101  is further configured to: after the processor switches the current signal transmission mode to the to-be-switched-to signal transmission mode, receive the uplink transmission resource indication information sent by the base station. The processor  1102  is further configured to determine the uplink transmission resource based on the uplink transmission resource indication information. The transmitter  1103  is configured to transmit, to the base station by using the uplink transmission resource, the response message indicating successful configuration of the to-be-switched-to signal transmission mode. 
     When the terminal provided in Embodiment 6 of this application runs, the terminal performs the method provided in Embodiment 2 of this application. For operating details of the terminal, refer to the method provided in Embodiment 2 of this application. 
     In the terminal device provided in Embodiment 6 of the present invention, the receiver obtains, in a plurality of manners, the broadcast corresponding to the to-be-switched-to signal transmission mode. The processor determines, based on the broadcast, the configuration information of the to-be-switched-to signal transmission mode; configures the to-be-switched-to signal transmission mode based on the configuration information; and then switches the current signal transmission mode to the to-be-switched-to signal transmission mode. The base station and the terminal device separately switch, in an agreed time period, the current signal transmission mode to the to-be-switched-to signal transmission mode, to avoid a problem of cognitive inconsistency between the base station and the terminal device about a transmission mode. 
     A person skilled in the art may be further aware that, in combination with the examples described in the embodiments disclosed in this specification, units and algorithm steps can be implemented by electronic hardware, computer software, or a combination thereof. To clearly describe the interchangeability between the hardware and the software, the foregoing has generally described compositions and steps of each example based on functions. Whether the functions are performed by hardware or software depends on particular applications and design constraints of the technical solutions. A person skilled in the art may use different methods to implement the described functions for each particular application, but it should not be considered that the implementation goes beyond the scope of the present invention. 
     A person of ordinary skill in the art may understand that all or some steps in the method in the foregoing embodiments may be completed by a program instructing a processor. The program may be stored in a computer readable storage medium. The storage medium is a non-transitory (non-transitory) medium, for example, a random access memory, a read-only memory, a flash memory, a hard disk, a solid-state drive, a magnetic tape (magnetic tape), a floppy disk (floppy disk), an optical disc (optical disc), or any combination thereof. 
     The foregoing descriptions are merely examples of specific implementations of the present invention, but are not intended to limit the protection scope of the present invention. Any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in the present invention shall fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.