## 1 Introduction  
With the development of household and industrial applications, wireless data accesses between devices and users have become a concern on demand. Among various wireless communication schemes, multiple-input, multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) technology [1] are superior in transmission performances and are widely employed in numerous standards, such as WiFi (IEEE 802.11n/ac/ax), Long Term Evolution (LTE), and advanced 5G New Radio (NR) [2; 3; 4; 5]. In practical applications, to access data from various devices, the user side must structure MIMO-OFDM communication according to the standard employed by the individual device. Therefore, developing a user-side transceiver, which can perform a multistandard MIMO-OFDM transmission for different devices, is useful (Figure 1).  
Figure 1: Scheme illustration of multistandard multiple-input, multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) data access applications.  
For all MIMO-OFDM communication between users and devices, Figure 2 shows the block diagram of a conventional \(M\times M\) MIMO-OFDM transmission model. A transmitter (TX) sends \(M\) streams of data symbols through \(M\) antennas. To reduce intersymbol interference (ISI) in OFDM communication, a guard interval (GI) was applied to the symbols by copying the number of samples from a symbol end to a symbol head. A receiver (RX) then uses \(M\) antennas to receive signals, which was obtained through the \(M\times M\) MIMO channel. Figure 2 shows that the RX part contains \(M\) sets of radio frequencies (RF), analog-to-digital convertors (ADC), fast Fourier transforms (FFTs) and the following equalization and demodulation blocks. For the given \(N\) samples in an OFDM symbol, an \(M\)\(\times\)\(M\) MIMO-OFDM receiver requires \(M\)-stream \(N\)-point FFT operations. The TX requires to perform \(M\)-stream \(N\)-point inverse FFT (IFFT) operations. Therefore, a set of \(M\)-stream \(N\)-point FFT (IFFT) processors are essential components in the MIMO-OFDM system. In general, the maximum \(M\) and \(N\) values for standard MIMO-OFDM FFT are 8 and 2048, respectively [6,7], and they can be higher in 5G applications [5]. Therefore, the cost of hardware and operation throughput for the required FFT/IFFT processors are significant design concerns. Further considering the FFT/IFFT design for multistandard MIMO-OFDM applications, implementing an effective scheme to develop the multimode hardware configuration is a difficult task. In this study, an FFT kernel design, which could be structured as a base module for reconfigurable FFT/IFFT processors, was proposed in a multistandard MIMO-OFDM system. The proposed FFT kernel could perform 4/5/6-stream 64/128-point FFT operations by efficiently using GI duration with resource-sharing and operation-rescheduling schemes. Therefore, the presented FFT kernel supports area-efficient and high-throughput development for multistandard MIMO-OFDM FFT/IFFT processors. The remainder of the paper is organized as follows. Section 2 outlines the design considerations for the FFT design and literature. Section 3 presents the hardware architecture of the proposed FFT kernel. Section 4 reports the implementation and comparison of the proposed FFT kernel design. Finally, conclusions are presented in Section 5.