Altering transmission parameters in radio transmitter for high security

The transmitters send information simultaneously to the receivers over a common radio frequency. Information from a transmitter is distributed over one or more blocks. All blocks for each of the transmitters form a frame. Information in each block is addressable to a specific receiver or group of receivers, a specific part of the blocks being not wholly used by the information. A specific transmitter, upon transmission of test information, is allocated a specific block in a frame for identifying the transmitter for the receivers. Only one transmitter transmits test information in the part of the specific block which is not wholly used for other information.