A method of selecting a synchronization signal, the method comprising: determining, by a first wireless user device (410, 430, 440, 450, 460, 470, 900, 950), a plurality of of synchronization signals transmitted from a plurality of synchronization sources, wherein the plurality of synchronization sources comprise an evolved NodeB, eNB (400, 420), a Global Navigational Satellite System, GNSS, a device capable of synchronizing with an eNB, and a device capable of synchronizing with a GNSS; characterized by synchronizing, by the first wireless user device (410, 430, 440, 450, 460, 470, 900, 950), a timing of a first sidelink synchronization signal selected from the determined synchronization signals, wherein the timing of the first sidelink synchronization signal is associated with a synchronization timing of the GNSS, and wherein the first sidelink synchronization signal is transmitted from a second wireless user device, wherein the synchronizing the timing of the first sidelink synchronization signal comprises: determining a subframe to which a primary sidelink synchronization signal, PSSS, of the first sidelink synchronization signal is mapped; and determining a secondary sidelink synchronization signal, SSSS, of the first sidelink synchronization signal from 62 consecutive subcarriers of two consecutive single frequency-frequency division multiple access, SC-FDMA, symbols in the determined subframe, wherein the SSSS comprises a first length-31 sequence generated based on m 1 and a second length-31 sequence generated based on m 0, where m 1 is greater than m 0, wherein based on a first mapping scheme of two mapping schemes, a portion of the first length-31 sequence is mapped to a subcarrier having a lowest index among the 62 consecutive subcarriers and a portion of the second length-31 sequence is mapped to a subcarrier having a highest index among the 62 consecutive subcarriers, and wherein the first mapping scheme distinguishes the first sidelink synchronization signal from sidelink synchronization signals that use a second mapping scheme of the two mapping schemes. determining, by a first wireless user device (410, 430, 440, 450, 460, 470, 900, 950), a plurality of of synchronization signals transmitted from a plurality of synchronization sources, wherein the plurality of synchronization sources comprise an evolved NodeB, eNB (400, 420), a Global Navigational Satellite System, GNSS, a device capable of synchronizing with an eNB, and a device capable of synchronizing with a GNSS; characterized by synchronizing, by the first wireless user device (410, 430, 440, 450, 460, 470, 900, 950), a timing of a first sidelink synchronization signal selected from the determined synchronization signals, wherein the timing of the first sidelink synchronization signal is associated with a synchronization timing of the GNSS, and wherein the first sidelink synchronization signal is transmitted from a second wireless user device, wherein the synchronizing the timing of the first sidelink synchronization signal comprises: determining a subframe to which a primary sidelink synchronization signal, PSSS, of the first sidelink synchronization signal is mapped; and determining a secondary sidelink synchronization signal, SSSS, of the first sidelink synchronization signal from 62 consecutive subcarriers of two consecutive single frequency-frequency division multiple access, SC-FDMA, symbols in the determined subframe, wherein the SSSS comprises a first length-31 sequence generated based on m 1 and a second length-31 sequence generated based on m 0, where m 1 is greater than m 0, wherein based on a first mapping scheme of two mapping schemes, a portion of the first length-31 sequence is mapped to a subcarrier having a lowest index among the 62 consecutive subcarriers and a portion of the second length-31 sequence is mapped to a subcarrier having a highest index among the 62 consecutive subcarriers, and wherein the first mapping scheme distinguishes the first sidelink synchronization signal from sidelink synchronization signals that use a second mapping scheme of the two mapping schemes. determining a subframe to which a primary sidelink synchronization signal, PSSS, of the first sidelink synchronization signal is mapped; and determining a secondary sidelink synchronization signal, SSSS, of the first sidelink synchronization signal from 62 consecutive subcarriers of two consecutive single frequency-frequency division multiple access, SC-FDMA, symbols in the determined subframe, wherein the SSSS comprises a first length-31 sequence generated based on m 1 and a second length-31 sequence generated based on m 0, where m 1 is greater than m 0, wherein based on a first mapping scheme of two mapping schemes, a portion of the first length-31 sequence is mapped to a subcarrier having a lowest index among the 62 consecutive subcarriers and a portion of the second length-31 sequence is mapped to a subcarrier having a highest index among the 62 consecutive subcarriers, and wherein the first mapping scheme distinguishes the first sidelink synchronization signal from sidelink synchronization signals that use a second mapping scheme of the two mapping schemes.