Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

MC-CDMA 71Thedataofthedesireduserk are detected by applying interference cancellation withreliability information. Before detection of user k’s data in the lowest path of Figure 2-8with an appropriate single-user detection technique, the contributions of the K − 1interferingusers g,g = 0,...,K − 1 with g ≠ k, are detected with single-user detection andsubtracted from the received signal. The principle of parallel or successive interferencecancellation or combinations of both can be applied within a soft interference cancellationscheme.In the following, we focus on the contribution of the interfering user g with g ≠ k. Thesoft decided values w (g)[j] are obtained after single-user detection, symbol de-mapping,and de-interleaving. The corresponding log-likelihood ratios (LLRs) for channel decodingare given by the vector l (g)[j] . LLRs are the optimum soft decided values which can beexploited in a Viterbi decoder (see Section 2.1.8). From the subsequent soft-in/soft-outchannel decoder, besides the output of the decoded source bits, reliability information inthe form of LLRs of the coded bits can be obtained. These LLRs are given by the vector(g) [j]Ɣ κ,out = ln(g) [j]l out(g) [j]= (Ɣ(g) [j]0,out,Ɣ(g) [j]1,out,...,ƔL b −1,out )T . (2.53)In contrast to the LLRs of the coded bits at the input of the soft-in/soft-out channeldecoder, the LLRs of the coded bits at the output of the soft-in/soft-out channel decoder()P {b (g)κ =+1|w (g) [j] }P {b (g)κ =−1|w (g) [j] }, κ = 0,...,L b − 1, (2.54)are the estimates of all the other soft decided values in the sequence w (g)[j] about this(g) [j]coded bit, and not only of one received soft decided value w κ . For brevity, the indexκ is omitted since the focus is on the LLR of one coded bit in the sequel. To avoid errorpropagation, the average value of coded bit b (g) is used, which is the so-called soft bit(g) [j]w [21]. The soft bit is defined asoutw (g)[j]out = E{b (g) |w (g)[j] }= (+1)P {b (g) =+1|w (g)[j] }+(−1)P {b (g) =−1|w (g)[j] }. (2.55)With Equation (2.54), the soft bit results in(g) [j]w out = tanh( )(g) [j] Ɣ out. (2.56)2(g) [j]The soft bit w out can take on values in the interval [−1, +1]. After interleaving, the softbits are soft symbol mapped such that the reliability information included in the soft bitsis not lost. The obtained complex-valued data symbols are spread with the user-specificspreading code and each chip is pre-distorted with the channel coefficient assigned to thesub-carrier on which the chip has been transmitted. The total reconstructed multiple accessinterference is subtracted from the received signal r. After canceling the interference, thedata of the desired user k are detected using single-user detection. However, in contrast tothe initial detection stage, in further stages the equalizer coefficients given by the matrixG (k)[j] and the LLRs given by the vector l (k)[j] after soft interference cancellation areadapted to the quasi multiple access interference-free case.