Sidelink synchronization in telecommunication systems

An apparatus comprising:

• at least one processor;
• and
• at least one memory including computer program code;
• the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to:
• receive, at a first network access node in a wireless network and from a first user equipment, one or more measurement reports;
• perform, based on the one or more measurement reports, a process to handover the first user equipment from the first network access node to a second network access node, wherein the first network access node is in a first wireless network that uses a first synchronization source and the second network access node is in a second network that uses a second synchronization source different from the first synchronization source, and wherein the first user equipment and one or more second user equipment use timing of one of the first or second synchronization sources to maintain a sidelink communication over a sidelink between the first user equipment and the one or more second user equipment;
• and
• receive at the first network access node a message from the second network indicating at least a time difference between the first and second synchronization sources;
• and
• send one or more messages from the first network access node to the second user equipment to allow the second user equipment to synchronize to the second synchronization source,
• wherein the at least one processor and the computer program code configured to send the one or more messages comprises the at least one processor and the computer program code configured to:
• send timing for the sidelink based on the information for a new synchronization signal block corresponding to the second synchronization source;
• and
• send the one or more messages comprises sending information indicating at least a time difference between the first and second synchronization source.

An apparatus comprising:

• at least one processor;
• and
• at least one memory including computer program code;
• the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to:
• send via a sidelink one or more messages from a first user equipment to one or more second user equipment indicating the first user equipment is changing to a different network access node and therefore might be changing a synchronization source, which the one or more second user equipment use to maintain a sidelink communication between the first and second user equipment;
• perform by the first user equipment a handover process from a first network access node in a first wireless network to the different network access node in a second wireless network, the second wireless network using the second synchronization source;
• and
• attempt by the first user equipment to synchronize the one or more second user equipment to the second synchronization source,
• wherein the at least one processor and the computer program code are further configured to:
• receive by the first user equipment a message from at least one of the one or more second user equipment that there is a synchronization failure by the at least one second user equipment to the second synchronization source;
• and
• the handover process comprises a random access channel procedure performed between the first user equipment and the different network access node, wherein the first user equipment receives timing information for the second synchronization source during the random access channel procedure,
• wherein the at least one processor and the computer program code are further configured to:
• perform, after the handover is complete, a synchronization process, using timing for the second synchronization source and over the sidelink, by the first user equipment with the one or more second user equipment using one or more resources indicated by the synchronization information for communication over the sidelink with corresponding ones of the one or more second user equipment;
• receive by the first user equipment a message from at least one of the one or more second user equipment that synchronization to the second synchronization source is complete;
• set a timer by the first user equipment and in response to the sending the message indicating at least synchronization information for the second synchronization source;
• and
• determine by the first user equipment that synchronization with the second synchronization source has failed for one of the one or more second user equipment in response to the timer having expired prior to synchronization completion for the one second user equipment.