A system of RFID reader units, comprising a plurality of reader units (11.1, 11.2, 11.3, 11.4, 11.i) at least comprising a first reader unit (11.1) and at least one second reader unit, the system being arranged such that the first reader unit (11.1) and the at least one second reader unit (11.2, 11.3, 11.4), in use, each generate their own carrier wave independently from each other, the first reader unit (11.1) is arranged to modulate the carrier wave generated by the first reader unit with interrogation data to generate and is provided with a transmitter for transmitting an electromagnetic interrogation signal comprising the carrier wave modulated by the first reader unit for interrogating an RFID tag; and the at least one second reader unit (11.2, 11.3, 11.4) is arranged to modulate the carrier wave generated by the second reader unit with interrogation data and is provided with a transmitter for transmitting an electromagnetic interrogation signal comprising a carrier wave modulated by the at least one second reader unit for interrogating an RFID tag, wherein the system is arranged to receive a reply signal generated by an RFID tag in response to a carrier wave modulated with the interrogation signal received by the tag, characterized in that the system is arranged such that with the first reader unit (11.1) and the at least one second reader unit (11.2, 11.3, 11.4), in use, the first reader unit and the at least one second reader unit each transmit identical and synchronized interrogation data, wherein either according to a first option the system is provided with a control unit which generates the interrogation data, wherein the control unit is communicatively connected (91, 92, 93) with the first reader unit (11.2) and the at least one second reader unit (11.3, 11.4) for submitting the interrogation data to these reader units or according to a second option the first reader unit (11.1) is configured to act as a master, wherein the first reader unit is arranged to generate the interrogation data and the at least one second RFID reader unit (11.2, 11.3, 11.4) is configured to act as a slave, and the system is arranged for distributing the interrogation data generated by the first reader unit to the at least one second reader unit. the first reader unit (11.1) is arranged to modulate the carrier wave generated by the first reader unit with interrogation data to generate and is provided with a transmitter for transmitting an electromagnetic interrogation signal comprising the carrier wave modulated by the first reader unit for interrogating an RFID tag; and the at least one second reader unit (11.2, 11.3, 11.4) is arranged to modulate the carrier wave generated by the second reader unit with interrogation data and is provided with a transmitter for transmitting an electromagnetic interrogation signal comprising a carrier wave modulated by the at least one second reader unit for interrogating an RFID tag, wherein the system is arranged to receive a reply signal generated by an RFID tag in response to a carrier wave modulated with the interrogation signal received by the tag, characterized in that the system is arranged such that with the first reader unit (11.1) and the at least one second reader unit (11.2, 11.3, 11.4), in use, the first reader unit and the at least one second reader unit each transmit identical and synchronized interrogation data, wherein either according to a first option the system is provided with a control unit which generates the interrogation data, wherein the control unit is communicatively connected (91, 92, 93) with the first reader unit (11.2) and the at least one second reader unit (11.3, 11.4) for submitting the interrogation data to these reader units or according to a second option the first reader unit (11.1) is configured to act as a master, wherein the first reader unit is arranged to generate the interrogation data and the at least one second RFID reader unit (11.2, 11.3, 11.4) is configured to act as a slave, and the system is arranged for distributing the interrogation data generated by the first reader unit to the at least one second reader unit.