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Unlike the basic adaptive array antenna (AAA) , a tapped delay line adaptive array antenna (TDL-AAA) is effective in mitigating interference from user signals or multipath waves which impinge on the array close to the direct wave. This is also useful for reducing the number of antenna elements under a multipath propagation environment. Furthermore, this adaptive antenna system is also effective for multi-input multi-output (MIMO) spatial multiplexing techniques under frequency selective fading channels. However, a method of designing the required number of antenna elements or tap stages has not been studied. We propose a design equation for TDL-AAA when a number of users and maximum delay time are given. We assume that multipath waves impinge on the array successively with symbol period interval and individual waves have a different direction of arrival (DoA). The influence of the roll-off factor of a cosine roll-off filter is also studied by computer simulation. Simulation results show that the proposed design equation is applicable in case of a roll-off factor of 1.0. On the other hand, simulation results of a roll-off factor of 0.5 show that one can determine the number of antenna elements.