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

198 Implementation Issues10 0 2310 −1phase noise without correctionphase noise with correctionwithout phase noise10 −2BER10 −310 −410 −520 21 2224E b /N 0 in dB25 26 27Figure 4-60 Effects of phase noise with and without common phase error correction for a DVB-Ttransmission with 64-QAM in a Ricean fading channelrequires higher output back-off values to keep an acceptable performance in the presenceof non-linear amplifiers. This is due to the presence of a high peak-to-average power ratio(PAPR) in an OFDM signal, leading to severe clipping effects. The PAPR of an OFDMsignal is defined asPAPR OFDM = max |z i| 2, i = 0,...,NN1 ∑ c −1c − 1, (4.86)|z i | 2N ci=0where z i are the transmitted time samples of an OFDM symbol.The transmitted samples z i result from the IFFT operation. Following the central limittheorem, the samples z i have a complex Gaussian distribution for large numbers of subcarriersN c , where their amplitudes |z i | are Rayleigh distributed. At the output of theHPA, all OFDM signal points with amplitudes higher than the saturation amplitude A satwill be mapped to a point in a circle with radius A sat , which causes high interference,i.e. high degradation of the transmitted signal.The aim of this section is to analyze the influence of the effects of the non-linearitydue to a traveling waves tube amplifier (TWTA) and a solid state power amplifier (SSPA)in an MC-CDMA system. First, we present the influence of non-linear distortions onthe downlink and uplink transmissions and then we examine some techniques to reducethe effects of non-linear distortions. These techniques are based on pre-distortion or theappropriate selection of the spreading codes to reduce the PAPR for an MC-CDMAtransmission system.