Praise for Fundamentals of WiMAX

4.3 An Example: OFDM in WiMAX 123Time DomainnXL-ptIDFTxP/SAddCPh[n]+DeleteCPS/PyL-ptDFTYFEQX^A circular channel: y = hx + nFrequency DomainFigure 4.7 An OFDM system in vector notation.3. In order for the IFFT/FFT to decompose the ISI channel into orthogonal subcarriers, acyclic prefix of length v must be appended after the IFFT operation. The resulting L+vsymbols are then sent in serial through the wideband channel.4. At the receiver, the cyclic prefix is discarded, and the L received symbols are demodulated,using an FFT operation, which results in L data symbols, each of the form Yl = HlXl + Nlfor subcarrier l.5. Each subcarrier can then be equalized via an FEQ by simply dividing by the complexchannel gain Hi [] for that subcarrier. This results in Xˆ l= X l+ N l/ H l.We have neglected a number of important practical issues thus far. For example, we haveassumed that the transmitter and the receiver are perfectly synchronized and that the receiverperfectly knows the channel, in order to perform the FEQ. In the next section, we present theimplementation issues for OFDM in WiMAX.4.3 An Example: OFDM in WiMAXTo gain an appreciation for the time- and frequency-domain interpretations of OFDM, WiMAXsystems can be used as an example. Although simple in concept, the subtleties of OFDM can beconfusing if each signal-processing step is not understood. To ground the discussion, we considera passband OFDM system and then give specific values for the important systemparameters.Figure 4.8 shows a passband OFDM modulation engine. The inputs to this figure are Lindependent QAM symbols (the vector X), and these L symbols are treated as separate subcarriers.These L data-bearing symbols can be created from a bit stream by a symbol mapper andserial-to-parallel convertor (S/P). The L-point IFFT then creates a time-domain L-vector x that iscyclic extended to have length L(1 + G), where G is the fractional overhead. This longer vectoris then parallel-to-serial (P/S) converted into a wideband digital signal that can be amplitudemodulated with a single radio at a carrier frequency of = ω /2π.f cc