# An FPGA-Oriented Baseband Modulator Architecture for 4G/5G Communication Scenarios
## 3 Baseband Modulator Architecture
### Baseband Modulation for 5G Waveform Candidates
Instead, 5G should enable the coexistence and tight interworking between different radio access technologies in order to facilitate the gradual penetration of 5G systems [2]. Thus, the waveform numerologies adopted in our work are based on the 4G LTE standards. In particular, OFDM Modes 1 and 2 correspond to LTE 5 MHz and 10 MHz channelizations, respectively. Like [18], we assume that a primary user communicates using OFDM and that secondary users opportunistically transmit using OFDM, FBMC, or UFMC. Thus, the numerologies for FBMC and UFMC should be compatible with the OFDM numerologies. Table 1 presents the modes of operation and numerologies supported in this work, and Figure 4 depicts the combination of periodograms for Mode 1 OFDM, FBMC, and UFMC baseband signals, in what would be a scenario where these waveforms coexist by sharing a portion of the spectrum band. In all cases, the 16-QAM constellation scheme was used for digital modulation. Regarding the hardware implementations for the baseband modulator datapaths, we will describe the UFMC modulator design in more detail, and for OFDM and FBMC, we refer to our previous works [22; 26]. The implemented modulator datapaths have AXI4-Stream-compatible input/output data interfaces, and all arithmetic operations are done in fixed-point precision, considering real and imaginary parts represented in the Q5.11 format. The UFMC modulator architecture follows the algorithm description from [29], also illustrated in Figure 3. The first module of a sub-band branch is the QAM mapper. For the 16-QAM case, the module is simply implemented with a 16:1 multiplexer: a four-bit input signal selects the corresponding complex value out of 16 values that compose the constellation. The subcarrier mapping module maps the 12 PRB subcarriers to the central bins of an \(N^{\prime}\)-element array and zeroes the remaining \(N^{\prime}-12\) subcarriers. This module comprises a double buffer of \(2\times N^{\prime}\) elements and read/write control engines. The double buffer is implemented using dual-port block RAMs embedded in the FPGA logic fabric and allows for simultaneous read and write of consecutive \(N^{\prime}\)-element arrays without causing any data conflicts. Figure 3: Datapath structure for UFMC baseband modulation. The IFFT computation involves complex arithmetics, and it is replicated for each sub-band processing branch. Consequently, the design choice for the IFFT module should consider a balanced trade-off between performance and resource usage. There are two dominant categories of IFFT/FFT architectures: pipelined and memory-based. In pipelined architectures, the IFFT datapath is tightly synchronized in time and can simultaneously execute transform calculations on the current data frame, load the next input data frame, and unload the results from the previous data frame. This allows \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Mode 1** & **Mode 2** \\ \hline \# subcarriers, \(N\) (IFFT size) & 512 & 1024 \\ CP length, \(L_{CP}\) & 40 (1 st slot symb.) & 80 (1 st slot symb.) \\
36 (other symb.) & 72 (other symb.) \\ WOLA samples, \(W\) & 4 & 6 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Waveform numerologies. Figure 4: Periodograms for OFDM, FBMC, and UFMC baseband signals. for the continuous flow of data along the datapath at the cost of a higher resource utilization. In turn, memory-based architectures are characterized by an iterative processing nature, and the input data loading and results unloading operations cannot occur simultaneously with transform processing. Compared with pipelined architectures, memory-based architectures consume less circuit area/resources, but provide a lower performance. Although pipelined-based IFFT modules were adopted in the OFDM and FBMC modulators, we chose a memory-based approach to design the IFFT modules in the UFMC modulator, motivated by three aspects. First, UFMC is a preferred waveform for short-burst transmissions.