In this paper, binary linear block codes with very low encoding and decoding complexity are introduced for the use as outer codes in a serial concatenated coding scheme employing differential modulations as inner codes. Using extrinsic information transfer chart based analysis and optimization the performance of the new concatenated codes is found to be within less than 1 dB of the pertinent capacity limit for the additive white Gaussian noise channel. This compares favorably with more complex benchmark schemes using e.g. outer low density parity check codes proposed recently. For decoding without channel phase estimation a low-complexity noncoherent decoder is presented, which achieves performance close to that of a coherent decoder in channels with moderate phase noise.