A communication system comprising a plurality of wireless field devices (A, B, C, R) configured to measure process quantities related to a plant and wirelessly transmit measurement data and/or to wirelessly receive manipulation data and perform manipulation process quantities related to a plant, the plurality of wireless field devices (A, B, C, R) being arranged in the plant; a terminal device connected to the system management device and capable of setting time slot allocation information and frequency channel allocation information for each link involving one of the plurality of wireless field devices (A, B, C, R) to be supplied to the wireless field devices (A, B, C, R); a system management device configured to supply time slot allocation information and channel allocation information to each of the wireless field devices (A, B, C, R); and wherein the plurality of wireless field devices (A, B, C and R) comprises at least one wireless field device with relay function between the system management device and another one of said plurality of wireless field devices (A, B, C) using a multi-hop wireless communication by communicating in the time slots and the frequency channels allocated to each link of the respective wireless field devices, said wireless field device with relay function comprising: a plurality of antennas (1, 2) including at least a first and a second directional antenna (1, 2) arranged in accordance with a direction where a communication partner of the wireless field devices without relay function or the system management device is installed; one or more communication circuits configured to perform the wireless communication and to transmit and receive signals to and from the other wireless field devices (A, B, C) and/or the system management device as the communication partners; and a control unit configured to refer to the time slot allocation information and channel allocation information for each communication link to the other wireless field devices (A, B, C) and/or the system management device, select an antenna (1, 2) that is to be used in the wireless communication of the corresponding communication link in the direction towards the wireless field devices (A, B, C) without relay function or the system management device and to use the frequency channel for each time slot allocated to the communication link and control the communication circuits to transmit and receive the measurement data and/or manipulation data via the selected antenna (1, 2). a plurality of antennas (1, 2) including at least a first and a second directional antenna (1, 2) arranged in accordance with a direction where a communication partner of the wireless field devices without relay function or the system management device is installed; one or more communication circuits configured to perform the wireless communication and to transmit and receive signals to and from the other wireless field devices (A, B, C) and/or the system management device as the communication partners; and a control unit configured to refer to the time slot allocation information and channel allocation information for each communication link to the other wireless field devices (A, B, C) and/or the system management device, select an antenna (1, 2) that is to be used in the wireless communication of the corresponding communication link in the direction towards the wireless field devices (A, B, C) without relay function or the system management device and to use the frequency channel for each time slot allocated to the communication link and control the communication circuits to transmit and receive the measurement data and/or manipulation data via the selected antenna (1, 2).