The increasing demands for the higher data rates, the variations of the radio channel, and the inherent traffic asymmetry between the uplink and downlink impose many new challenges on spectrum and radio resource management in wireless networks. Time division duplex (TDD) is an efficient approach to combat the traffic asymmetry issues via adapting the resource (time slots) for uplink and downlink. However, asymmetric traffic will cause cross-slot co-channel interference, which can seriously degrade the system performance. This paper proposes a novel link-proportional dynamical channel assignment (LP-DCA) scheme with directional antennas to alleviate the impact of cross-slot interference in the code division multiple access (CDMA) system. The proposed scheme partitions the whole users in a sector into several groups according to their received link quality. Basically, the LP-DCA assigns the time slots with higher potential cross-slot interference to the group of users with better link quality, or vice versa. Through the simulation, we demonstrate that the LP-DCA can reduce the cross-slot interference significantly, thereby improving the system capacity.