Over-The-Air Phase Synchronization Mechanism for LTE Small Cells in Interference-Limited Scenarios

Phase synchronization is one of the most important features needed in some of the techniques envisaged for LTE-Advanced, such as Cooperative Multi-Point (CoMP) or enhanced inter-cell interference coordination (eICIC), among others. Phase synchronization can be accomplished through the use of IEEE 1588v2 or GPS receivers. However when dealing with small cells deployed in an uncoordinated manner, such as Home-eNBs, the use of IEEE 1588v2 is not always feasible due to delay and jitter impairments which can be present in xDSL lines. Moreover, GPS receivers are not operative in indoors. The present paper describes an over-the-air synchronization mechanism whereby already synchronized LTE macro cells transmit a new physical signal on a subset of yet unused resource elements, so that small cells can acquire suitable phase synchronization. As the involved resources are reserved and not used for transmission, interference is much lower and signal to noise ratio can be improved compared to traditional network listening modes. Simulations employing a simple proposed receiver structure show very good results in channels where the delay spread is within the length of the cyclic prefix.