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In this paper, we consider the joint code-rate and spreading-factor optimisation of turbo-style iterative joint detection and decoding assisted single-carrier interleave division multiple access (SC-IDMA) systems using different-rate convolutional codes and extrinsic information transfer (EXIT) charts, when communicating over additive white Gaussian noise (AWGN) channels. More explicitly, we study the extrinsic information exchange between two serial concatenated components and maximise the number of users supported by the SC-IDMA system under the constraint of a fixed bandwidth expansion factor, while maintaining a predefined bit error ratio (BER) versus Eb/N0 performance. We found that an optimum code-rate and spreading-factor combination can be found for the SC-IDMA system at low Eb/N0 values, where maintaining a low BER inevitably requires the employment of channel coding. By contrast, at high Eb/N0 the system performs best, when no channel coding is used, i.e. DS-spreading is the only means of bandwidth expansion.