Performance and DSP implementation of soft bit-level demapper for M-QAM-OFDM systems

Parallel implementation of a simple suboptimal bit-level demapper algorithm of M-ary QAM orthogonal frequency division multiplexing (OFDM) by using a TMS320C5515 digital signal processor (DSP) is presented. The performance of both bit-level demapper in hard and soft decision for M-QAM-OFDM is compared in terms of bit error rate (BER) and DSP computational time at a 100 MHz clock rate. The FFT operation takes the major computational time at the receiver for both hard and soft decision. The soft decision requires a Viterbi decoder, which requires the additional computing time to that of hard decision. The BER performance of this bit-level algorithm is shown to be the same as that of the optimal ML method theoretically, when E_b/N_o is high. Moreover, soft bit values for soft decoding are derived from this bit-level demapper algorithm. At BER=10^-3 and rate ½, the BER of the coded 64-QAM-OFDM significantly outperforms the coded 256-QAM-OFDM by 6 dB in COST207 typical urban (TU) fading channels. At BER=10^-4 and rate 2/3, the BER of the coded QPSK-OFDM significantly outperforms the coded 16-QAM-OFDM by 8 dB in COST207 TU fading channels.