Millimeter wave relay link discovery

1. A method for wireless communication at a first user equipment (UE) in a wireless communication network, the method comprising:

receiving a message from a base station over a first link upon an occurrence of an external event;

performing beam training with a second UE in response to the message to select a directional beam pair link with the second UE; and

communicating with the second UE using the directional beam pair link via a relay link.

2. The method of claim 1 , further comprising:

receiving a signal from the second UE via at least one pseudo omni-directional beam to discover that the first UE is within a range of the second UE; and

determining a cell identifier (ID) associated with the second UE based on the signal.

3. The method of claim 2 , further comprising:

conveying the cell ID of the second UE to the base station to report a presence of the first UE within the range of the second UE; and

storing the cell ID of the second UE.

4. The method of claim 2 , wherein the signal comprises at least one synchronization signal.

5. The method of claim 1 , further comprising:

receiving beam training information, wherein the beam training information comprises reference signal resources, a timing offset, and at least one beam pair link option to utilize for beam training.

6. The method of claim 5 , wherein the at least one beam pair link option further comprises a list of beam pair link options, and further comprising:

selecting the directional beam pair link for the relay link from the list of beam pair link options based on an order of the beam pair link options in the list.

7. The method of claim 5 , wherein the reference signal resources for the beam training comprise a set of common resources.

8. The method of claim 5 , wherein the reference signal resources for the beam training comprise a set of UE-specific resources.

9. The method of claim 1 , further comprising:

initiating a random access procedure with the second UE to establish a random access link with the second UE; and

performing beam training with the second UE over the random access link to select the directional beam pair link.

10. The method of claim 1 , further comprising:

communicating with the second UE over the relay link utilizing a millimeter wave carrier frequency.

11. The method of claim 1 , wherein the external event comprises blockage of a second link between the base station and the second UE, fading of the second link between the base station and the second UE, or initiation of assisted communication for the second link using the relay link.

12. The method of claim 1 , wherein the external event comprises a beam failure event associated with a second link between the base station and the second UE, and the message is a broadcast message to a plurality of UEs comprising the first UE, the broadcast message requesting assistance from the plurality of UEs in establishing respective relay links with the second UE.

13. The method of claim 1 , further comprising:

establishing the relay link with the second UE using the directional beam pair link; and

relaying information between the second UE and the base station via the first link and the relay link.

14. A first user equipment (UE), comprising:

a wireless transceiver;

one or more memories; and

one or more processors coupled to the wireless transceiver and the one or more memories, wherein the one or more processors are configured to:

receive a message from a base station over a first link upon an occurrence of an external event;

perform beam training with a second UE in response to the message to select a directional beam pair link with the second UE; and

communicate with the second UE using the directional beam pair link via a relay link.

15. The first UE of claim 14 , wherein the one or more processors are further configured to:

receive a signal from the second UE via at least one pseudo omni-directional beam to discover that the first UE is within a range of the second UE; and

determine a cell identifier (ID) associated with the second UE based on the signal.

16. The first UE of claim 15 , wherein the one or more processors is further configured to:

convey the cell ID of the second UE to the base station to report the presence of the first UE within the range of the second UE; and

store the cell ID of the second UE within the memory.

17. The first UE of claim 14 , wherein the one or more processors are further configured to:

receive beam training information, wherein the beam training information comprises reference signal resources, a timing offset, and at least one beam pair link option to utilize for beam training.

18. The first UE of claim 17 , wherein the at least one beam pair link option further comprises a list of beam pair link options, and wherein the processor is further configured to:

select the directional beam pair link for the relay link from the list of beam pair link options based on an order of the beam pair link options in the list.

19. The first UE of claim 14 , wherein the message comprises a broadcast message, and the one or more processors are further configured to:

initiate a random access procedure with the second UE in response to receiving the broadcast message to establish a random access link with the second UE;

perform beam training with the second UE over the random access link to select the directional beam pair link; and

establish the relay link utilizing the directional beam pair link.

20. A method of wireless communication at a first user equipment (UE) in a wireless communication network, the method comprising:

broadcasting a signal via at least one pseudo-omnidirectional beam to enable a second UE to discover that the second UE is within a range of the first UE;

performing beam training with the second UE to select a directional beam pair link for communication with the second UE via a relay link upon an occurrence of an external event; and

communicating with the second UE via the relay link.