Patent ID: 7016425

Claim:
An orthogonal frequency division multiplexing (OFDM) system for transmission of a multi-amplitude digital modulated signal from a transmitter to a receiver via a radio channel, wherein said transmitter is adapted to process a coded data stream with a block frame structure consisting in each frame of a sequence of n quadrature phase shift keyed (QPSK) control symbols followed by m sixteen quadrature amplitude modulated (16 QAM) data symbols, where n and m are integers greater than zero, the transmitter comprising: a demultiplexer means ( 1 ) for separating said control symbols and said data symbols, and for feeding said control signals to a QPSK modulator means ( 2 ) for mapping of two bits each into QPSK complex symbols and for supplying said data symbols to a 16 QAM modulator means ( 3 ) for mapping of four bits each into 16 QAM complex symbols, first multiplexing means ( 4 ) for multiplexing said QPSK complex symbols and said 16 QAM complex symbols and for supplying the resulting data stream to a phase differential encoder means ( 5 ) for differential modulation of the phases of respectively subsequent symbols on a same subcarrier and keeping the respective amplitudes of said symbols unchanged, second multiplexing means ( 7 ) for transferring the resulting data symbol stream into OFDM signals that are subject to an inverse fast Fourier transformation ( 8 ) to produce a baseband OFDM-modulated signal, quadrature modulator means ( 10 ) for modulating said baseband signal with carriers having 90° phase difference and supplying the thus modulated signal to D/A-converter means ( 11 ) followed by an up-converter means ( 11 ) to hook the analog D/A-converter output signal to a given carrier frequency for transmission; and wherein said receiver has differential phase decoding means ( 21 ) for demodulating said respectively subsequent symbols on said same subcarrier; and means for coherent processing of the amplitudes of said OFDM signals.