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

RF Issues 205Table 4-8Minimum total degradation for different transmission schemes, BER = 10 −2Transmission scheme Downlink, K = 32 Downlink, K = 64 UplinkMC-CDMA withoutpre-distortionDS-CDMA withoutpre-distortionMC-CDMA withpre-distortionSSPA TWTA SSPA TWTA SSPA TWTA2.6 dB 4.0 dB 3.2 dB 4.7 dB 1.25 dB 3.0 dB5.0 dB 5.5 dB 5.3 dB 5.8 dB 0.9 dB 1.1 dB– 2.5 dB – 3.1 dB – 0.9 dBrespect to a non-pre-distorted scheme using the TWTA. Similar gains for the downlinkcase for different BER cases have been obtained.In Table 4-8 the minimum total degradations TD min at a BER of 10 −2 for MC-CDMAand DS-CDMA systems are summarized. The lower degradation of the MC-CDMA systemfor the downlink is due to its lower peak signal amplitude compared to the DS-CDMAsignal and to the presence of frequency interleaving. In the case of TWTAs, the gain providedby pre-distortion for MC-CDMA is more than 1.5 dB for both the uplink and thedownlink. From this table we conclude that with respect to non-linearities MC-CDMA isa good choice for the downlink and MC-DS-CDMA with a small number of sub-carrierscould be adequate for the uplink.Appropriate selection of spreading codesAs we have seen before, MC-CDMA is sensitive to amplifier non-linearity effects. Wehave used Walsh–Hadamard codes rather than Gold codes for the downlink in the case ofmulti-carrier transmission. In this section we highlight the effects of the spreading codeselection and detail the appropriate choice for the uplink and the downlink [66, 69].In Section 2.1.4.2, upper bounds for the PAPR of different spreading codes have beenpresented for MC-CDMA systems. Both uplink and downlink have been analyzed and it isshown that different codes are optimum with respect to the PAPR for up- and downlinks.Exemplary values for these upper bounds are given in Table 4-9 for different spreadingcode lengths in the uplink. As shown in Table 4-9, Zadoff–Chu codes have the lowestPAPR for the uplink.Table 4-9 PAPR upper bounds for differentspreading codes in the uplinkSpreading code L = 16 L = 64Walsh–Hadamard 32 128Gold ∼ 15.5 ∼ 31.5Golay 4 4Zadoff–Chu 2 2