A wireless communication device includes a first wireless communication system and a second wireless communication system. Regarding the first wireless communication system, an up-conversion circuit up-converts a first transmit (TX) signal in a baseband to generate a second TX signal with a first carrier frequency, and a front-end circuit transmits the second TX signal to another wireless communication device. Regarding the second wireless communication system, a first down-conversion circuit down-converts a first receive (RX) signal with a second carrier frequency to generate a second RX signal with a third carrier frequency, and a second down-conversion circuit down-converts the second RX signal with the third carrier frequency to generate a third RX signal in the baseband. The third carrier frequency is different from all fundamental frequencies included in a band combination that is employed at the first wireless communication system and is supported by another wireless communication device.