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Precoding has been widely investigated for multiple-input multiple-output (MIMO) and inter-symbol interference (ISI) channels. Most related work so far has been on uncoded systems and the impact of forward-error-correction (FEC) codes has not been well studied. In this paper, we present an optimized precoding technique that takes into account iterative joint LMMSE detection and FEC decoding. The proposed scheme has some interesting features. First, the design problem is convex. Second, the proposed scheme achieves water-filling gain when channel state information at the transmitter (CSIT) is available and diversity gain when CSIT is not available, as well as multi-user gain in multi-user environments. Third, the water-filling gain is achieved using a single code (instead of multiple codes as in conventional approaches), which greatly simplifies the design and implementation problems. Both simulation and evolution analyses are provided to show that significant performance improvement can be achieved with the proposed scheme.