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We quantify the improvement in the transmission quality for DPSK transmission by using convolutional error correction coding. To avoid bandwidth-limitation problems from, e.g., chromatic dispersion, the convolutional coding is combined with bandwidth efficient modulation. The modulation schemes used are return-to-zero differential quadrature phase shift keying (RZ-DQPSK) and RZ-ASK/DSPK modulation which is a combination of amplitude shift keying (ASK) and differential phase shift keying (DPSK). For uncoded transmission, RZ-DPSK modulation is used as a benchmark. The RZ-DQPSK transmission scheme offers a clearly better coding gain than the RZ-ASK/DPSK scheme.