alized mode, maps the DFT precoded symbols to adjacent subcarriers in the frequency domain. DFT-precoded OFDM is a particular kind of single-carrier transmission with cyclic prefix (CP) that has been introduced in LTE for two major reasons. First, single-carrier transmission reduces the power fluctuations of the output waveforms at the transmitter and allows a better usage of the power amplifier. Second, the DFT precoding spreads the information across the assigned set of subcarriers and enables the use of multipath diversity. Currently available DFT cores for FPGAs have a fixed number of points [XX].* In order to realize a variable bandwidth, one must be able to change the number of points at run time. A variable DFT with a selected number of points has been realized with variable up-sampling in front of a custom-made 1.200-point DFT (see Figure 11.5). In order to realize DFTs of size N = 1.200/F, where F is an inte- ger, the input signal is F-times up-sampled with F-1 zeros inserted between input values. This gives F periodic blocks after the 1.200-point DFT, where each block is identical to the desired N-point DFT output. One of these blocks is further processed, mapped to the desired resources, and fed into the IFFT of the inner OFDM transmitter. Note that this solution lacks flexibility and is more complex than a variable DFT. On the other hand, at the time of writing this book, this is the only functional FPGA-based DFT implementation for which even multiuser multiplexing is feasible [16]. 11.1.4.1 Power Scaling Issues Care must be taken with the scaling after the DFT before the signal is fed into the inner OFDM transmitter. At first, the output must be scaled depending on bandwidth used. Signals are fed with the same mean amplitude per carrier into the IFFT. Furthermore, signals must be decimated properly after the 23-b DFT output before passing them into the 12-b IFFT input. Because 1,200 is not a power The fixed-size 1.200-point DFT soft core needs as many resources in the FPGA as four 2.048- point FFT c