A communication method of a wireless power receiver for receiving power in a wireless manner, includes generating information of the wireless power receiver based on a load modulation within a slot being allocated to the wireless power receiver while wireless power is received from a wireless power transmitter; receiving the wireless power from to the wireless power transmitter; generating first information of the wireless power receiver based on the load modulation within a first unallocated slot among the plurality of slots, wherein the first information is in collision with second information generated by another wireless power receiver; demodulating a collision related signal of the wireless power transmitter based on a frequency shift keying (FSK); and executing a collision resolution mechanism.