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

74 MC-CDMA and MC-DS-CDMAEqual Gain Transmision (EGT)G l,l = H ∗l,l|H l,l | . (2.64)Zero Forcing (ZF)G l,l =H ∗l,l√ √√√√√ L|H l,l | 2 L−1∑n=0. (2.65)1|H n,n | 2Quasi Minimum Mean Square Error (MMSE) Pre-EqualizationH ∗ √ √√√√√l,lLG l,l =|H l,l | 2 + σ 2 L−1∑H ∗ ∣ . (2.66)n, n ∣∣∣2∣|H n, n | 2 + σ 2We call this technique quasi MMSE pre-equalization, since this is an approximation. Theoptimum technique requires a very high computational complexity, due to the powerconstraint condition.As with the single-user detection techniques presented in Section 2.1.5.1, controlledpre-equalization can be applied. Controlled pre-equalization applies zero forcing preequalizationon sub-carriers where the amplitude of the channel coefficients exceeds apredefined threshold a th . All other sub-carriers apply equal gain transmission for preequalization.2.1.6.2 UplinkIn an MC-CDMA uplink scenario, pre-equalization is performed in the terminal stationof user k according tos (k) = G (k) s (k) . (2.67)The received signal at the base station after inverse OFDM operation results inn=0K−1∑r = H (k) s (k) + nk=0K−1∑= H (k) G (k) s (k) + n. (2.68)k=0The pre-equalization techniques presented in Equations (2.63) to (2.66) are applied in theuplink individually for each terminal station; i.e. G (k)l,l and H (k)l,lhave to be applied insteadof G l,l and H l,l respectively.Finally, knowledge about the channel in the transmitter can be exploited, not only toperform pre-equalization but also to apply adaptive modulation per sub-carrier in orderto increase the capacity of the system (see Chapter 4).