ering the multi carrier modulation will be multiplexed onto N channels where each of these blocks is modulated by a different carrier signal. The carrier signals (1 O2) are carefully selected subject to various conditions and they differ from one scheme to another. Since the focus is on OFDM, these carrier signals will be orthogonal in time. In OFDM, the sub-carrier frequencies are chosen SO that the sub-carriers orthogonal to each other, meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not required [2-4]. modulator modulator Serial Parallel ON(t) ,(N(t) modulator Figure 2.2 MCM scheme. Reproduced with permission from Dawid and Rethnakaran © 2003 Synopsys Next Generation Mobile Communications Ecosystem Principles of OFDM OFDM is a combination of modulation and multiplexing: Modulation - a process of conveying a message signal inside another signal that can be physically transmitted. Modulation is the systematic variation of some attribute of the carrier, such as amplitude, phase, or frequency, in accordance with a function of the message signal. Multiplexing - a process where multiple digital data streams are combined into one signal over a shared medium. The letter O in OFDM implies orthogonality among the sub-carriers that results in the elimination of guard bands required by FDM. Once the sub carriers are made orthogonal to each other, the interference among them is also eliminated. The term orthogonality means that the sub-carriers are perpendicular to each other in a mathematical sense allowing the spectrum of each subcarrier to overlap another without interfering with it. Figure 2.3 shows the effect of orthogonality by reducing the bandwidth required as compared to FDM. The bandwidth is reduced by removing guard bands and allowing their spectra to overlap each other. 2.3.1 OFDM System Model OFDM is a multicarrier block modulation scheme where data symbols are grouped and transmitted in parallel by employing a large number of orthogonal sub-carriers. OFDM is re