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For radio access systems based on time division duplex (TDD) mode, like the time division-synchronous code division multiple access (TD-SCDMA) radio system, frame synchronization among base stations (BSs) is a fundamental feature to minimize interference and therefore optimize the offered traffic capacity. In particular, radio frame synchronization ensures that uplink and downlink transmission directions are positioned, at least for adjacent cells, in the same time intervals, thus preventing interference between adjacent downlink and/or uplink slots. We propose a method to realize on-air frame synchronization between two BSs, applicable to both the TDD-low chip rate (TDD-LCR) and the TD-SCDMA for mobile (TSM) standards. The main operation performed in the proposed frame synchronization procedure is correlation between the received signal and the known synchronization sequences transmitted in the frame by each BS. The correlation process is repeated on a chosen number of consecutive radio frames to obtain better synchronization performance. The method presented allows a higher reliability and accuracy with respect to the on-air synchronization methods described in the UTRAN standard for TDD-LCR. The proposed technique can be generalized to all time division duplex digital cellular communication systems which make use of two (or more) synchronization sequences with a known relative time position.