Signal interference is automatically avoided between product proximity subsystems that emit signals through mutually facing presentation windows of different workstations. A magnetometer in each workstation determines a direction faced by a respective presentation window. A controller in each workstation controls a respective product proximity subsystem to emit and receive a signal with a first signal characteristic when the respective magnetometer determines that the respective presentation window faces a first direction, and controls a respective product proximity subsystem to emit and receive a signal with a different second signal characteristic when the respective magnetometer determines that the respective presentation window faces a different second direction.