The paper considers wireless collision detection at the transmitter for wireless local area network systems. One of the major challenges to achieve such collision detection is the self-interference of very high power, which decrease the effective quantization level for the collision packet to be detected. In order to cope with the interference, we employ a self-interference canceller for each receiving antenna and combine the outputs of the cancelers with maximal ratio combining. Moreover, we utilize the correlation between the combiner output and the preamble of the packet for the detection of the existence of the collision packets. Performance of the proposed scheme is demonstrated via computer simulations.