 tion. A third memory is required to store the samples prior to generating the final block by performing the transpose or customized allocation. The demodulator has a similar architec- ture in reversed order. The N4-FFT block is enabled or disabled depending on the domain of the equalized signal and the configuration of the demodulator. For instance, if the input is an FD equalized signal and the demodulator is configured in TD, this block needs to be con- figured as N-IFFT. The allocation memory is used to store the received signal in columns and forward it in rows to the next block. The samples at the output of the demodulator cor- respond to the columns of D in the TD and to the rows in the FD configuration. Usually, FD equalization is used. Thus, the modem can be configured as TD modulator and FD demod- ulator. In this way, the overall modulation and equalization processing requires only one N-FFT for the equalizer. On the other hand, including N4-FFT preserves the symmetry of the architecture. Therefore, with fast reconfiguration, the modulator can be reconfigured as demodulator. This solution is typical for low-cost time division duplex transmission. 4.3 GFDM for OFDM Enhancement Recalling from Section 4.2.1.2, the TD block can be expressed where x is the FD generated with FD modulator. Consequently, the FD block can be seen as precoded data corresponding to the OFDM symbol of length N. However, the waveform is influenced by the FD modulation matrix A. The idea can be extended to fit a small-size GFDM block within a subband that consists of a group of OFDM subcarriers. 4.3.1 Transmitter Let N be the number of OFDM subcarriers used to allocate the GFDM signal, with the parameters M, Ku, gu, and Nu = The corresponding FD GFDM block is Xu = Audu E CNux1. TD block with critical sampling The upsampling and filtering preserve the spectral properties of the = i.e. N -Nu zeros are inserted in the middle of Xu Therefore, the upsampled and filtered signal corresponding to XU with upsampling N/ 2 A frequency shift of 