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The design of bit-interleaved turbo-coded modulation (BITCM) in OFDM-based wireless LAN systems is studied. It is observed that for the frame size commonly used in wireless LAN applications, a larger constraint length in turbo codes does not necessarily introduce significant improvement in the BITCM performance in the signal-to-noise ratio range of interest, and it even leads to performance degradation in some cases. An explanation is provided from the viewpoint of weight distribution of turbo codes. Extensive performance evaluation indicates that BITCM with a constraint length of four has a good tradeoff between the performance and complexity for the OFDM-based wireless LAN system.