an indirect optical free - space communications system provides broadband transmission of high - speed data in transportation vehicles such as aircraft . the system includes a transmitter with a light source that can be modulated and a receiver with a photo detector , adapted to receive light that is emitted from the transmitter and to convert it into an electric signal . the transmitter and the receiver are directed onto at least one common surface , which reflects the light emitted from the transmitter before it reaches the receiver . the free space communications system is cellular and includes several cells , each of which has a transmitter and at least one corresponding receiver . the cells are configured in such a way that crosstalk with a respective adjoining cell is prevented . the cells are thus independent of one another .