| 摘要 |
<p>A sub-optimal method is disclosed for calculating the reliability values (soft values) for the bits of a multilevel signal. The log-likelihood values are approximated using only the dominant terms, so called max-log approximation, that is for each bit position only the two closest signal symbols of opposite bit value (S8,S6) are considered in the sum. The used modulation scheme is 16-QAM together with Gray-labelling. Two versions of approximation are proposed: one version consists of using the two distances between the received value and the two closest symbols of opposite bit value ( δ1, δ2). In order to simplify and speed up the calculation, the second version consists of using the distance between the two closest symbols (δ3) to approximate the distance between the second closest symbol and the received value. Furthermore, precalculated results are stored in look-up tables to speed up the calculation. Possible applications are especially bit interleaved coded modulation (BICM) together with soft-input decoding. It is also of interest for TCM and BCM schemes.</p> |
