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Collision mitigation is one of main research issues to improve the system performance in wireless local area networks (WLANs). Our previous study showed that our proposed multi-user multiple-input multiple-output (MIMO)-based collision mitigation scheme is very efficient for uplink throughput performance. However, for infrastructure-based WLANs, there still exists a downlink and uplink collision problem, since the multi-user MIMO-based collision mitigation scheme only can partially resolve uplink collisions. In order to mitigate this kind of collision, in this paper, we propose a relay-assisted downlink and uplink collision mitigation scheme and analyze the corresponding performance. Moreover, when it comes to the coexistence of the uplink and downlink traffic in WLANs, due to a random access nature of the WLAN systems, the access point (AP) may have less channel access opportunities than stations(STAs). Thus, we need to take into account a downlink and uplink throughput balancing problem. Numerical results show that the proposed scheme yields better throughput performance than the conventional multi-user MIMO-based collision mitigation scheme, especially when we consider the uplink and downink throughput balancing.