This paper proposes a new Acknowledge packet transmission scheme for bidirectional two-hop wireless network coding systems based on IEEE802.11a standard in order to achieve higher TCP throughput. The proposed scheme utilizes the subcarriers that carry zero-padding bits added to MAC protocol data unit (MPDU) for a relay node to identify which node transmits ACK packet against the XOR-ed packet. Computer simulations verify that the proposed scheme can improve TCP throughput performance by 3.4 Mbps when adaptive modulation scheme is applied.