Maximum likelihood decoding of M-ary orthogonal modulated signals for multi-carrier spread-spectrum systems

For a multi-carrier spread-spectrum (MC-SS) system, the inner spreading can be optimized by applying M-ary orthogonal modulation. We investigate the concatenation of maximum likelihood (ML) Viterbi (1995) decoding with M-ary orthogonal modulation in a MC-SS system. The system operates over a frequency-selective Rayleigh fading indoor channel in the uplink. We first evaluate the bit specific log-likelihood ratio for ML Viterbi decoding and present an estimation of the ratio exploiting the MC technique. Furthermore, the trade-off between channel coding, M-ary orthogonal modulation and simple spreading is considered by Monte-Carlo simulations. The results are always compared with BPSK performance and they emphasize for the concerned indoor transmission scenario that a moderate bit-error-rate can only be achieved if M-ary orthogonal modulation is employed. All simulations are related to the European Hiperlan/2 standardization whereas the results are generally valid