# MIMO Self-Heterodyne OFDM Using Band Selection Technique
## 1 Introduction  
Due to the rapid increase in the number of users and the need for higher data rates, free bands and more bandwidth are required in the new 5G technology. This attracted the researchers' attention to utilizing the millimeter-wave (mm-wave) bands in 5G [1]. The mm-wave communication is characterized by the high data rate as it occupies wide bandwidth at the high-frequency range (30 GHz-300 GHz) [2, 3]. Despite the advantages of using the mm-wave band in 5G communication [4], however, it is difficult to design an oscillator for the mm-wave band with the same frequency stability of the UHF or VHF bands [5]. Moreover, the radio frequency (RF) components will create phase noise at the receiver and hence, techniques to handle these issues are required [6]. In order to manage these issues, the researchers in [7] and [8] suggested utilizing the self-heterodyne receiver in mm-wave communication. At the transmitter side of the self-heterodyne system, the modulated signals along with the local carrier are transmitted through the mm-wave band for the up-conversion. At the receiver side, the down-conversion is done to estimate the transmitted symbols using a square-law device, in which, the received local carrier is mixed with the modulated signals to get the estimated symbols [9]. Hence, by using a self-heterodyne receiver, the problem of phase noise and frequency offset are evaded. Moreover, at the self-heterodyne receiver, no local oscillator or phased locked loop (PLL) is required. Therefore, the system is more stable and has a lower cost and complexity of the transmitter [9]. In [9], a single-input-single-output (SISO) self-heterodyne orthogonal frequency division multiplexing (OFDM) system is proposed. The data symbols in [9] are transmitted on half of the available bandwidth, while the carrier frequency is transmitted on the other half, leaving a guard band between OFDM subcarriers and the RF carrier. Although the guard band is necessary to eliminate the interference caused by the nonlinear device, however, it diminishes the efficiency of bandwidth to the half. Based on this, researchers in [10; 11] used the suggested receiver in [9] to replace the conventional super-heterodyne receiver. The space time block code (STBC) is widely used by researchers in various research area with different systems and techniques, such as multiple input multiple output (MIMO) system [12; 13; 14], Orthogonal Transform Division Multiplexing (OTDM) technique [15], Generalized Spatial Modulation (GSM) and antenna grouping system [16], dual-hop full-duplex relay networks [17], visible light communications system [18], and heterogeneous system [19]. The used of the STBC eliminates the multipath effects and hence, enhance the performance of these systems in terms of bit error rate. In [12], a STBC MIMO self-het OFDM system is considered in which, two transmitting antennas and two receiving antennas are used. Moreover, the STBC is used in order to remove the multipath effects and improve stability of the system. In this system, Alamouti code [20; 21] was used with MIMO self-het OFDM, and the results show that the system performance is decreased when compared to the conventional MIMO STBC OFDM system. This degradation in system performance occurs due to the decrease in bandwidth efficiency, by transmitting the symbols on half of the bandwidth [12]. Moreover, the proposed system in [12] solved the problem of the phase noise at the receiver and the receiver complexity at the expense of the system BER performance. In [22], FSTBC is used with MIMO self-het OFDM system. The authors in [22] try to solve the problem of the bandwidth efficiency degradation that found in [12] by using the same technique but for four transmitting antennas and transmitting the symbols on the whole bandwidth. The symbols are transmitted alternatively on different half sub bands. In [22], four symbols are transmitted in single time slot. While in [12] only two symbols are transmitted per time slot.