# An FPGA-Oriented Baseband Modulator Architecture for 4G/5G Communication Scenarios
## 3 Baseband Modulator Architecture
### Baseband Modulation for 5G Waveform Candidates  
The waveforms supported by the baseband modulator are OFDM, FBMC, and UFMC. These multi-carrier waveforms efficiently perform waveform synthesis using the Inverse Fast Fourier Transform (IFFT) operation. The differences between the selected waveforms are mainly related to the techniques adopted for time-domain windowing (pulse-shaping in the frequency domain) and/or time-domain filtering (equivalent to frequency domain windowing). OFDM is the most prominent waveform in current wireless communications, and it is characterized by the orthogonality between subcarriers, which eliminates inter-carrier interference. Every OFDM symbol is prepended with a Cyclic Prefix (CP), which mitigates inter-symbol interference, but contributes to the degradation of spectral efficiency. Currently, 4G LTE systems improve the frequency response of Cyclic Prefix OFDM (CP-OFDM) by applying time-domain windowing of the CP-extended OFDM symbols and overlapping the edge transition of adjacent symbols: Weighted Overlap and Add (WOLA). The OFDM modulator implemented here follows a CP-OFDM with WOLA approach, and its datapath structure is shown in Figure 1. The main baseband parameters involved in this waveform are the IFFT size \(N\), which is equivalent to the number of subcarriers per OFDM symbol, the CP length \(L_{\text{CP}}\), and the number of time-domain samples used for WOLA- \(W\). Due to its sinc-pulse shapes transmission, OFDM does not provide genuinely band-limited signals, and the high side lobe power levels can cause unwanted interference with adjacent spectrum bands. FBMC achieves better spectral containment by filtering each subcarrier individually. This eliminates the need for a guard interval like the cyclic prefix and contributes to a higher spectral efficiency. Quite often, the improved spectral efficiency of FBMC systems comes at the cost of relaxed signal orthogonality. In these cases, Offset QAM (OQAM) is employed to ensure real-part orthogonality of FBMC symbols: OQAM-FBMC. In this work, FBMC modulation (Figure 2) follows the approach from [30], where frequency spreading is applied before the IFFT. The frequency spreading operation, which is characterized by the overlapping factor \(K\), comprises an up-sampler module and an FIR filter with \(2\times K-1\) non-zero coefficients. Due to the frequency spreading operation, the waveform synthesis is performed with an IFFT of size \(K\times N\), where \(N\) is the number of subcarriers. Finally, \(K\) IFFT output blocks are overlapped and added to create an FBMC output multi-carrier symbol. UFMC is a waveform with better out-of-band suppression than CP-OFDM and a better multi-antenna compatibility than FBMC [28]. This multi-carrier scheme divides the \(N\) available subcarriers that represent the whole frequency band into blocks of subcarriers--Physical Resource Blocks (PRB)--that represent individual sub-bands. Usually, only a part of the PRBs is used for transmission: active PRBs. Then, for each active PRB, IFFT and bandpass \(L\)-order FIR filtering are  
Figure 1: Datapath structure for OFDM baseband modulation. Figure 2: Datapath structure for frequency spreading Filter-Bank Multi-Carrier modulation. performed. The same filter can be applied to all sub-bands, but its center frequency must be shifted. At the end, the filtered sub-bands are superimposed to form the UFMC symbol to be transmitted. The classic UFMC modulation scheme from [31] considers an \(N\)-point IFFT and frequency-shifted FIR filters with complex coefficients for each sub-band. To counteract this increased computational complexity, [PERSON] et al. [29] proposed an algorithm that combines a reduced size \(N^{\prime}\)-point IFFT with \(\frac{N}{N^{\prime}}\) upsampling and keeps real-valued coefficient FIR filters by performing frequency shifting after filtering (Figure 3). The transition from 4G to 5G will not be as abrupt as in previous generations.