Multilevel Coding: Use of Hard Decisions in Multistage Decoding

| Multilevel coding (MLC) schemes with powerful softly decodable component codes like turbo codes allow very power and bandwidth e cient digital communication [WH95, FHW96, WFH97a]. But the use of the non{ quantized channel output in each stage of the multistage decoding procedure leads to highly complex receivers. In this paper, the employment of hard decision decoding at several coding levels is proposed as a very e cient way to save complexity. It turns out that, for Ungerbock's labeling of signal points, the performance loss compared to soft decision decoding (in terms of capacity of the equivalent binary channel as well as in simulations) is only about 0.13 dB for ASK transmission over the AWGN channel, when hard decision decoding is employed at all but the lowest level. If error{correction techniques are exclusively employed, an MLC approach with ` individual binary error{correcting codes is recommended.