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

204 Implementation IssuesSSPAThe inversion of the SSPA with the above specified parameters leads to the normalizedfunctionz n (t)r n (t) =(1 − z n (t) 10 ) 1/10 ,ϕ(t) = ψ(t). (4.94)Performance of analytical pre-distortionFor performance evaluation, the total degradation (TD) for a given BER is considered asthe main criterion. It is given as follows:TD = SNR − SNR AWGN + OBO, (4.95)where SNR is the signal-to-noise ratio in the presence of non-linear distortions andSNR AWGN is the signal-to-noise ratio in the case of a linear channel, i.e. only Gaussiannoise.One can see that as the OBO decreases, the HPA is more efficient, but, on the otherhand, the non-linear distortion effects increase and a higher SNR is needed to compensatefor this effect compared to a linear channel. For high OBO values the HPA works in itslinear zone and there are no distortions. However, the loss of HPA efficiency throughthe high OBO value is taken into account in the total degradation expression. Since thetotal degradation changes from a decreasing to an increasing function, we can expect anoptimal value for the OBO that minimizes the total degradation. In order to be independentof the channel coding, we consider an uncoded BER of 10 −2 and 10 −3 for our analysis.The performance of analytical pre-distortion for the MC-CDMA system is presented inFigure 4-65. As Table 4-8 shows, for the uplink one can achieve about 2.1 dB gain withTotal Degradation in dB14.012.010.08.06.04.0Uplink (K = 1), BER = 10E − 2Uplink (K = 1), BER = 10E − 3Downlink, K = 32, BER = 10E − 2Downlink, K = 32, BER = 10E − 3Downlink, K = 64, BER = 10E − 2Downlink, K = 64, BER = 10E − 32.00.00.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0Output back-off (OBO) in dBFigure 4-65Performance of pre-distortion technique for MC-CDMA, TWTA