Method for reducing the out-of-band emission in am transmitters for digital transmission

The problem associated with digital transmission using existing AM transmitters is that the out-of-band emission that occurs must be reduced in order to comply with the ITU mask, as the shoulder distances that can be achieved by the AM transmitters are not sufficient for a satisfactory compensation. The signals that are required to control the AM transmitters, (amplitude signal and phase-modulated RF signal), are formed from the digital modulation signal by a Cartesian } polar transformation, whereby the bandwidth for the amplitude signal and RF signal reaches a value, which causes an unacceptable out-of-band emission. To prevent this, the invention provides methods for digital modulation, which avoid the zero point by a wide margin in their vector diagram representations, i.e. which form a "hole" around the 0/0 point. Modulation methods of this type are referred to as offset modulations and coded modulations. A variant of 16APSK is particularly suitable, as a higher net data throughput can be achieved with only 16 set points and negligible error protection coding is required. To transmit an OFDM signal, the "hole" in the vector diagram is obtained by offsetting the zero crossings for the I(t) and Q(t) signals and the latter is achieved by the insertion of a pulse into the subsequent component, e.g. I(t), if the other component Q(t) already falls short of a defined threshold value. A BPSK test sequence for measuring the radio channel can be used by means a modification, in which a modulation signal is formed by inserting a pulse between two pulses with a change of sign bit, said modulation signal having a "hole" around the 0/0 point. The size of the hole can be adjusted by the inserted pulse.