## 1 Introduction  
As technology advances in wireless communications, more innovative solutions have been appearing. These solutions should be able to handle the increase in the number of devices connected to the network and also the number of mobile traffic services that are growing day by day. Newer generations are expected to offer better system robustness, higher mobility and better spectral and energy efficiency than previous ones. Current fifth-generation (5G) technology is capable, efficient and flexible, but can be improved further. 5G technology works in bands below 7 GHz, which corresponds to the Frequency Range 1 (FR1) of 5G New Radio (5G NR) [1], and above 24 GHz, corresponding to Frequency Range 2 (FR2) of 5G NR, and can accommodate many more users than the last generation counterpart. It uses the massive multiple-input multiple-output (MIMO) concept which allows achieving 100x higher efficiency without requiring more base stations (BSs). Massive MIMO and nonorthogonal multiple access (NOMA) are widely used in wideband networks and can accommodate a higher number of users and streams but fail to achieve a higher spectral efficiency (SE) that is required to meet the challenges and requirements of beyond 5G (B5G) and future generations (6G) [2; 3]. Regarding recent research work that has been done in the physical layer, one of the potential candidates to improve future and current networks is the use of index modulation (IM) schemes [4; 5]. IM has surged in recent years as a promising technique to convey additional information bits, named index bits, that are distributed through a certain order and select certain resource elements such as antennas, subcarriers, slots and channels. This capability of activating certain resources of communications, through the indices of the building blocks, can result in improvements in energy efficiency (EE) and SE and also reduces the complexity of the receivers [3; 6]. IM can be applied together with current technologies such as massive MIMO and can be used in high-mobility scenarios, such as mobile communications [5]. Spatial modulation (SM) and orthogonal frequency division multiplexing with IM (OFDM-IM) schemes are well-known examples of IM schemes [4; 7]. SM was first introduced in order to simplify the MIMO schemes to a lower computational complexity and implementation, as it only requires a single RF chain to convey the index bits [8]. This type of scheme can be combined with conventional modulation techniques such as amplitude and phase modulation (APM), phase-shift keying (PSK) or quadrature amplitude modulation (QAM). Although SM only uses one active transmitter, in [9] the concept of SM was generalized to enable multiple active antennas simultaneously, which was named generalized SM (GSM). IM schemes such as SM and GSM are usually more suitable in large-scale communications, allowing superior EE and better BER results than conventional MIMO [5; 9; 10]. The addition of precoding techniques allows the implementation of SM relying on the activation of antennas at the receiver instead of the transmitter, enabling lower implementation complexity at the receiver [10]. OFDM-IM uses an alternative approach as it explores IM applied at the frequency domain instead of the spatial domain, relying on the indices of subcarriers of conventional OFDM transmission [11; 12]. Like SM/GSM, in OFDM-IM only a portion of subcarriers are activated according to their indices, and only those convey \(M\)-ary modulated signals [13]. Various authors have modified and improved the basic OFDM-IM scheme. An example is using precoding techniques to improve further the communications in downlink between BSs and receivers. This results in reduced complexity at the receiver side, which tends to lower the interference [14]. The use of interleavers on OFDM subcarriers is another possible approach for achieving considerable improvements. In [15], the authors compared the performance of OFDM-IM with a subcarrier-level block interleaver (OFDM-ISIM) against traditional OFDM and OFDM-IM.