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

68 MC-CDMA and MC-DS-CDMAMaximum Likelihood Symbol-by-Symbol Estimation (MLSSE)MLSSE minimizes the symbol error probability, which is equivalent to maximizing theconditional probability P {d µ (k) |r} that d(k) µ was transmitted given the received sequence r.The estimate of d (k) obtained by MLSSE isˆd (k) = arg maxd (k)µP {d µ (k) |r}. (2.46)If the noise N l is additive white Gaussian the most likely transmitted data symbol is∑(ˆd (k) = arg maxexp − 1 )d µ(k)σ 2 2 (d µ , r) . (2.47)∀dµwith samerealization of d (k) µThe increased complexity with MLSSE compared to MLSE can be observed in the comparisonof (2.47) with (2.45). An advantage of MLSSE compared to MLSE is that MLSSEinherently generates reliability information for detected data symbols that can be exploitedin a subsequent soft decision channel decoder.Block Linear EqualizerThe block linear equalizer is a sub-optimum, low complex multi-user detector whichrequires knowledge about the system matrix A in the receiver. Two criteria can be appliedto use this knowledge in the receiver for data detection.Zero Forcing Block Linear EqualizerJoint detection applying a zero forcing block linear equalizer delivers at the output of thedetector the soft decided data vectorwhere (·) H is the Hermitian transposition.v = (A H A) −1 A H r = (v (0) ,v (1) ,...,v (K−1) ) T , (2.48)MMSE Block Linear EqualizerAn MMSE block linear equalizer delivers at the output of the detector the soft decideddata vectorv = (A H A + σ 2 I) −1 A H r = (v (0) ,v (1) ,...,v (K−1) ) T . (2.49)Hybrid combinations of block linear equalizers and interference cancellation schemes(see the next sub-section) are possible, resulting in block linear equalizers with decisionfeedback.Interference CancellationThe principle of interference cancellation is to detect and subtract interfering signals fromthe received signal before detection of the signal of interest. It can be applied to reduceintra-cell and inter-cell interference. Most detection schemes focus on intra-cell interference,which will be further discussed in this sub-section. Interference cancellation schemes