A communications apparatus (100; 200), comprising: a first radio module (110; 210), being configured to communicate with a first communications device in compliance with a first protocol, wherein the first communications device is a peer communications device; a second radio module (120; 220), communicating with a second communications device in compliance with a second protocol, wherein the second communications device is a peer communications device; and an antenna array (140; 240), coupled to the first radio module and the second radio module and comprising a plurality of antennas, wherein when the first radio module and the second radio module operate at the same time, the first radio module (110; 210) is configured to transmit a first message to the first communications device to trigger a procedure for changing an amount of antenna(s) used by the first radio module, so that the first radio module can use the amount of antenna(s) to communicate with the first communications device, and the remaining antenna(s) of the plurality of antennas, which is/are not used by the first radio module (110; 210) can be reserved for the second radio module (120; 220) to communicate with the second communications device, and wherein the first radio module (110; 210) is further configured to transmit a second message to the first communications device to change an amount of currently supported spatial stream(s) after the second radio module (120; 220) has finished operating with the at least one of the remaining antenna(s), so that the amount of antenna(s) to be used by the first radio module (110; 210) after the transmission of the second message is increased. a first radio module (110; 210), being configured to communicate with a first communications device in compliance with a first protocol, wherein the first communications device is a peer communications device; a second radio module (120; 220), communicating with a second communications device in compliance with a second protocol, wherein the second communications device is a peer communications device; and an antenna array (140; 240), coupled to the first radio module and the second radio module and comprising a plurality of antennas, wherein when the first radio module and the second radio module operate at the same time, the first radio module (110; 210) is configured to transmit a first message to the first communications device to trigger a procedure for changing an amount of antenna(s) used by the first radio module, so that the first radio module can use the amount of antenna(s) to communicate with the first communications device, and the remaining antenna(s) of the plurality of antennas, which is/are not used by the first radio module (110; 210) can be reserved for the second radio module (120; 220) to communicate with the second communications device, and wherein the first radio module (110; 210) is further configured to transmit a second message to the first communications device to change an amount of currently supported spatial stream(s) after the second radio module (120; 220) has finished operating with the at least one of the remaining antenna(s), so that the amount of antenna(s) to be used by the first radio module (110; 210) after the transmission of the second message is increased.