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

202 Implementation Issues10 −110 −2UplinkDownlink (32 users)Downlink (64 users)Probability10 −310 −40.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0Input AmplitudeFigure 4-63Signal amplitude distribution with MC-CDMAdistributed. Compared to the uplink of a DS-CDMA system, the MC-CDMA signal has ahigher peak amplitude. This high signal amplitude leads to a higher degradation for smallOBO values and results in severe clipping effects.In the same figure, the signal amplitude distribution (derived by simulations) for thedownlink case with L = 64,N c = 512, and K = 32 (respectively K = 64) active users ispresented. This signal is also Rayleigh distributed with average power = 10 log(K) +E c = 7 dB (respectively 10 dB). Comparing this distribution to that of the downlink of aDS-CDMA system, one can see that the MC-CDMA system has a lower peak amplitude.Therefore, in this example the MC-CDMA signal is more resistant to non-linearities thanthe DS-CDMA signal in the downlink.For low OBO values, the signal y(t) at the output of the amplifier is highly disturbed,which leads to a non-linear channel with memory characterized by wrapped output chipscontaining clusters, which results in inter-chip/inter-carrier interference.4.7.2.2 Reducing the Influence of HPA Non-linearitiesIn this section two techniques to reduce the effects of non-linear amplifiers are discussed.The first one is to adapt the transmitted signal to the HPA characteristics by pre-correction,i.e. pre-distortion methods. The second method concentrates on the choice of the spreadingcode to reduce the transmitted signal PAPR.Pre-distortion techniquesThrough the above analysis, we have seen that a high OBO might be needed for thedownlink to guarantee a given bit error rate. To make better use of the available HPApower, compensation techniques can be used at the transmitter side. Several methods