Sidelink channel will be used for device to device (D2D) communication. Most probably Side link channel will be based on SC-FDMA and intention is to enable D2D with minimal/no changes in radio interface. TDD mode would be preferred choice for Side Link channel.
Side link channel will be used only if device is not in communication with eNodeB. In other words, if Uplink hardware is not being used for eNodeB communication.