A system comprising - a communication station (3, 4) for communicating with a number of radio tags (7 - 14) using a time slot communication method, in which a number of time slots (Z1 - ZN) are provided per time slot cycle in a repeating sequence for communication purposes and each time slot (Z1 - ZN) is characterized by a unique time slot symbol (ZS1 - ZSN), wherein the communication station is designed to transmit a synchronisation data signal which comprises a time slot symbol (Z1 - ZN) for the current time slot (Z1 - ZN) at the beginning of the respective time slot (Z1 - ZN) and wherein - a radio tag (7 - 14) is designed: + to switch from a sleep state into an active state at a wake-up time and + to receive the synchronization data signal in the active state and + to detect its synchronism with the communication station solely due to the fact of recognizing the time slot symbol which occurs at the time anticipated by it or in the expected-time window and indicates the time slot specified for it and + when the received time slot symbol (ZS1 - ZSN) indicates a time slot (Z1 - ZN) specified for it, to define a new wake-up time which corresponds to the next occurrence of the time slot (Z1 - ZN) specified for it in a time slot cycle following the current time slot cycle and + once the radio tag (7 - 14) has detected its synchronism, to switch back to the sleep state and remain there until there is once again a wake-up and switch from the sleep state to the active state at the new wake-up time in the next time slot cycle. - a communication station (3, 4) for communicating with a number of radio tags (7 - 14) using a time slot communication method, in which a number of time slots (Z1 - ZN) are provided per time slot cycle in a repeating sequence for communication purposes and each time slot (Z1 - ZN) is characterized by a unique time slot symbol (ZS1 - ZSN), wherein the communication station is designed to transmit a synchronisation data signal which comprises a time slot symbol (Z1 - ZN) for the current time slot (Z1 - ZN) at the beginning of the respective time slot (Z1 - ZN) and wherein - a radio tag (7 - 14) is designed: + to switch from a sleep state into an active state at a wake-up time and + to receive the synchronization data signal in the active state and + to detect its synchronism with the communication station solely due to the fact of recognizing the time slot symbol which occurs at the time anticipated by it or in the expected-time window and indicates the time slot specified for it and + when the received time slot symbol (ZS1 - ZSN) indicates a time slot (Z1 - ZN) specified for it, to define a new wake-up time which corresponds to the next occurrence of the time slot (Z1 - ZN) specified for it in a time slot cycle following the current time slot cycle and + once the radio tag (7 - 14) has detected its synchronism, to switch back to the sleep state and remain there until there is once again a wake-up and switch from the sleep state to the active state at the new wake-up time in the next time slot cycle. + to switch from a sleep state into an active state at a wake-up time and + to receive the synchronization data signal in the active state and + to detect its synchronism with the communication station solely due to the fact of recognizing the time slot symbol which occurs at the time anticipated by it or in the expected-time window and indicates the time slot specified for it and + when the received time slot symbol (ZS1 - ZSN) indicates a time slot (Z1 - ZN) specified for it, to define a new wake-up time which corresponds to the next occurrence of the time slot (Z1 - ZN) specified for it in a time slot cycle following the current time slot cycle and + once the radio tag (7 - 14) has detected its synchronism, to switch back to the sleep state and remain there until there is once again a wake-up and switch from the sleep state to the active state at the new wake-up time in the next time slot cycle.