A wireless power transmitter for performing communication with a wireless power receiver, the wireless power transmitter comprising: a power conversion unit including a plurality of coils and being configured to transfer a wireless power signal to the wireless power receiver; and a power transmission control unit configured to: - perform communication with the wireless power receiver based on a frame having a fixed time length which starts with a synchronization pattern, the synchronization pattern having an information field indicating the frame i) as a slotted frame having a plurality of time slots, wherein the plurality of time slots have the same preset time duration, or ii) as a free-format frame in which no time slot is defined, characterized in that the power transmission control unit is configured to perform the communication with the wireless power receiver using: -- the free-format frame Z r to perform communication of data packets above 5 bytes or to select any one of the plurality of coils, when the wireless power receiver is detected, and -- the slotted frame to receive a first packet from the wireless power receiver within one time slot of the plurality of time slots and to transmit an ACK signal to the wireless power receiver in response to the first packet. a power conversion unit including a plurality of coils and being configured to transfer a wireless power signal to the wireless power receiver; and a power transmission control unit configured to: - perform communication with the wireless power receiver based on a frame having a fixed time length which starts with a synchronization pattern, the synchronization pattern having an information field indicating the frame i) as a slotted frame having a plurality of time slots, wherein the plurality of time slots have the same preset time duration, or ii) as a free-format frame in which no time slot is defined, - perform communication with the wireless power receiver based on a frame having a fixed time length which starts with a synchronization pattern, the synchronization pattern having an information field indicating the frame i) as a slotted frame having a plurality of time slots, wherein the plurality of time slots have the same preset time duration, or ii) as a free-format frame in which no time slot is defined, characterized in that the power transmission control unit is configured to perform the communication with the wireless power receiver using: -- the free-format frame Z r to perform communication of data packets above 5 bytes or to select any one of the plurality of coils, when the wireless power receiver is detected, and -- the slotted frame to receive a first packet from the wireless power receiver within one time slot of the plurality of time slots and to transmit an ACK signal to the wireless power receiver in response to the first packet. -- the free-format frame Z r to perform communication of data packets above 5 bytes or to select any one of the plurality of coils, when the wireless power receiver is detected, and -- the slotted frame to receive a first packet from the wireless power receiver within one time slot of the plurality of time slots and to transmit an ACK signal to the wireless power receiver in response to the first packet.