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

70 MC-CDMA and MC-DS-CDMAinitial detection stage, the data symbols of all K active users are detected in parallel bysingle-user detection, i.e.ˆd (k)[0] = Q{c (k)∗ G (k)[0] r T },k = 0,...,K − 1, (2.50)where G (k)[0] denotes the equalization coefficients assigned to the initial stage. The followingdetection stages work iteratively by using the decisions of the previous stage toreconstruct the interfering contribution in the received signal. The obtained interferenceis subtracted, i.e. cancelled from the received signal, and the data detection is performedagain with reduced multiple access interference. Thus, the second and further detectionstages apply⎧ ⎛⎞⎪⎨K−1ˆd (k) [j] = Q c (k)∗ G (k)[j] ⎜⎝ r − ∑H (g) d (g) [j−1] c (g) ⎟⎠T ⎫ ⎪ ⎬, j = 1,...,J it , (2.51)⎪ ⎭⎪⎩g≠kg=0where, except for the final stage, the detection has to be applied for all K users.Parallel interference cancellation can be applied with different detection strategies in theiterations. Starting with EGC in each iteration [17, 18] various combinations have beenproposed [6, 25, 30]. Promising results are obtained with MMSE equalization adaptedin the first iteration to the actual system load and in all further iterations to MMSEequalization adapted to the single-user case [24]. The application of MRC seems theoreticallyto be of advantage for the second and further detection stages, since MRC isthe optimum detection technique in the multiple access interference free case, i.e. in thesingle-user case. However, if one or more decision errors are made, MRC has a poorperformance [25].Successive Interference CancellationSuccessive interference cancellation detects and subtracts the interfering signals in theorder of the interfering signal power. First, the strongest interferer is cancelled, beforethe second strongest interferer is detected and subtracted, i.e.ˆd (k) [j] = Q {c (k)∗ G (k) [j] (r − H (g) (d (g)[j−1] c (g) )) T }, (2.52)where g is the strongest interferer in the iteration j,j = 1,...,J it . This procedure iscontinued until a predefined stop criterion applies. Successive interference cancellation issuitable for systems with large power variations between the interferers [6].Soft Interference CancellationInterference cancellation can use reliability information about the detected interferencein the iterative process. These schemes can be without [41] and with [21, 28] channeldecoding in the iterative process, and are termed soft interference cancellation. If reliabilityinformation about the detected interference is taken into account in the cancellationscheme, the performance of the iterative scheme can be improved since error propagationcan be reduced compared to schemes with hard decided feedback. The block diagramof an MC-CDMA receiver with soft interference cancellation is illustrated in Figure 2-8.