Apparatus and method for configuring synchronization signal for communication between user devices

A system comprising:

• one or more base stations configured to transmit a downlink signal;
• and
• a first wireless user device configured to:
• transmit, to the one or more base stations, an uplink signal;
• transmit, to a second wireless user device, one or more wireless signals for direct communication;
• synchronize with a synchronization source based on a synchronization signal received from the synchronization source;
• determine, based on the synchronization source and from a plurality of integer values, a root index of a primary synchronization signal, wherein each of the plurality of integer values is different from root index values associated with synchronization signals transmitted from the one or more base stations;
• generate, based on the determined root index, the primary synchronization signal;
• determine one of two different conditions;
• map a secondary synchronization signal in two Single Carrier-Frequency Division Multiplexing (SC-FDM) symbols in a same subframe, wherein mapping the secondary synchronization signal comprises mapping, based on the one of the two different conditions, a first m-sequence of the secondary synchronization signal to first sub-carriers of the two SC-FDM symbols or to second sub-carriers of the two SC-FDM symbols;
• and
• transmit, to the second wireless user device, a second synchronization signal comprising the primary synchronization signal and the secondary synchronization signal,
• wherein indexes of the first sub-carriers are even numbers;
• and
• wherein indexes of the second sub-carriers are odd numbers.

A system comprising:

• one or more base stations to transmit a downlink signal;
• and
• a wireless user device configured to:
• receive, from a second wireless user device, one or more wireless signals for direct communication;
• receive a synchronization signal transmitted from the second wireless user device, the synchronization signal comprising a primary synchronization signal;
• determine, based on the received primary synchronization signal, a root index associated with the received primary synchronization signal, wherein the root index associated with the received primary synchronization signal has one of a plurality of integer values, and each of the plurality of integer values is different from root index values associated with synchronization signals transmitted from the one or more base stations;
• select the second wireless user device to be a synchronization source of the wireless user device;
• generate, using the root index associated with the received primary synchronization signal, a primary synchronization signal to be transmitted to a third wireless user device;
• map a secondary synchronization signal in two Single Carrier-Frequency Division Multiplexing (SC-FDM) symbols in a same subframe, wherein mapping the secondary synchronization signal comprises mapping, based on one of two different conditions, a first m-sequence of the secondary synchronization signal to first sub-carriers of the two SC-FDM symbols or to second sub-carriers of the two SC-FDM symbols;
• and
• transmit, to the third wireless user device, the generated primary synchronization signal and the secondary synchronization signal,
• wherein indexes of the first sub-carriers are even numbers;
• and
• wherein indexes of the second sub-carriers are odd numbers.