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Signal modulation has an important impact on the system performance in optical communication. Two attractive modulation schemes are pulse position modulation (PPM) and multipulse PPM (MPPM), which have the advantage of average energy efficiency and bandwidth efficiency, respectively. An important practical issue is to employ an efficient channel coding of reasonable (decoding) complexity adapted to these modulations. In this view, we consider the use of a classic binary convolutional code together with iterative soft demodulation and channel decoding at the receiver. In particular, we discuss the impact of bit-symbol mapping on the iterative receiver performance and provide design rules for optimal mapping in the case of MPPM.