Random access method, network device, and terminal device

1. A random access method, the method comprising:

receiving, by a terminal device, a system message from a network device on a first downlink carrier, wherein the system message comprises first information for performing random access on a first uplink carrier and second information for performing random access on a second uplink carrier, wherein the second information further comprises a first Physical Random Access Channel (PRACH) power threshold, and wherein a frequency of the first uplink carrier is higher than a frequency of the second uplink carrier;

determining, by the terminal device based on a reference signal received power of the first downlink carrier and the first PRACH power threshold, an uplink carrier for sending a random access request message, wherein:

when the reference signal received power of the first downlink carrier is greater than or equal to the first PRACH power threshold, the uplink carrier for sending the random access request message is the first uplink carrier; and

when the reference signal received power of the first downlink carrier is less than the first PRACH power threshold, the uplink carrier for sending the random access request message is the second uplink carrier;

sending, by the terminal device, the random access request message on the uplink carrier for sending the random access request message; and

receiving, by the terminal device, a random access response message from the network device on the first downlink carrier.

2. The method according to claim 1 , wherein:

the first information comprises a center frequency, a bandwidth, and physical random access channel (PRACH) resource configuration information of the first uplink carrier; and

the second information comprises a center frequency, a bandwidth, and PRACH resource configuration information of the second uplink carrier.

3. The method according to claim 1 , wherein before the sending, by the terminal device, a random access request message on the first uplink carrier or on the second uplink carrier, the method further comprises:

determining, by the terminal device, whether to send the random access request message on the first uplink carrier or on the second uplink carrier.

4. The method according to claim 3 , wherein the determining, by the terminal device, whether to send the random access request message on the first uplink carrier or on the second uplink carrier comprises:

determining, by the terminal device and based on the second information comprised in the system message, sending the random access request message on the second uplink carrier.

5. The method according to claim 3 , wherein the system message further comprises access indication information, and wherein the access indication information is used to instruct the terminal device to send the random access request message on the second uplink carrier; and

wherein the determining, by the terminal device, whether to send the random access request message on the first uplink carrier or on the second uplink carrier comprises:

determining, by the terminal device and based on the access indication information, sending the random access request message on the second uplink carrier.

6. The method according to claim 3 , wherein the second information further comprises PRACH power threshold information, wherein the PRACH power threshold information is third information, and wherein the third information is used to instruct the terminal device to send the random access request message on the second uplink carrier; and

wherein the determining, by the terminal device, whether to send the random access request message on the first uplink carrier or on the second uplink carrier comprises:

determining, by the terminal device and based on the third information, sending the random access request message on the second uplink carrier.

7. The method according to claim 1 , wherein the sending, by the terminal device, a random access request message on the first uplink carrier or on the second uplink carrier comprises:

sending, by the terminal device, the random access request message to the network device by using a resource that is determined based on the first information and that is on the first uplink carrier, or by using a resource that is determined based on the second information and that is on the second uplink carrier.

8. The method according to claim 1 , wherein a frequency of the first downlink carrier is the same as the frequency of the first uplink carrier.

9. The method according to claim 1 , wherein a value range of the first PRACH power threshold is same as a value range of the reference signal received power of the terminal device.

10. The method according to claim 1 , further comprising:

sending, by the terminal device, a radio resource control (RRC) connection request message on the uplink carrier for sending the random access request message.

11. A terminal device, the terminal device comprising:

at least one processor; and

a memory storing instructions executable by the at least one processor, the instructions instructing the at least one processor to:

receive a system message from a network device on a first downlink carrier, wherein the system message comprises first information for performing random access on a first uplink carrier and second information for performing random access on a second uplink carrier, wherein the second information further comprises a first Physical Random Access Channel (PRACH) power threshold, and wherein a frequency of the first uplink carrier is higher than a frequency of the second uplink carrier;

determine, based on a reference signal received power of the first downlink carrier and the first PRACH power threshold, an uplink carrier for sending a random access request message, wherein:

when the reference signal received power of the first downlink carrier is greater than or equal to the first PRACH power threshold, the uplink carrier for sending the random access request message is the first uplink carrier; and

when the reference signal received power of the first downlink carrier is less than the first PRACH power threshold, the uplink carrier for sending the random access request message is the second uplink carrier;

send the random access request message on the uplink carrier for sending the random access request message; and

receive a random access response message from the network device on the first downlink carrier.

12. The terminal device according to claim 11 , wherein:

the first information comprises a center frequency, a bandwidth, and physical random access channel (PRACH) resource configuration information of the first uplink carrier; and

the second information comprises a center frequency, a bandwidth, and PRACH resource configuration information of the second uplink carrier.

13. The terminal device according to claim 11 , wherein the instructions further instruct the at least one processor to:

determine whether to send the random access request message on the first uplink carrier or on the second uplink carrier.

14. The terminal device according to claim 13 , wherein the instructions further instruct the at least one processor to:

determine, based on the second information comprised in the system message, sending the random access request message on the second uplink carrier.

15. The terminal device according to claim 11 , wherein the instructions further instruct the at least one processor to:

send the random access request message to the network device by using a resource that is determined based on the first information and that is on the first uplink carrier, or by using a resource that is determined based on the second information and that is on the second uplink carrier.