## 1 Introduction  
Bandwidth maximization, lower complexity, and reliability are the key attributes that characterize orthogonal frequency division multiplexing (OFDM) as one of the most promising multicarrier transmission schemes since the deployment of fourth generation (4G) services. The apparent importance of OFDM is also evident in the adoption of OFDM and OFDM-based multicarrier schemes for the efficient implementation of the current fifth generation (5G) networks [1] and its supporting technologies [2; 3; 4; 5]. In addition, recent research geared towards the standardization of sixth generation (6G) and beyond networks suggests that OFDM and its optimized modulation variants will not be neglected in the actualization of a 6G connected world [6; 7]. Therefore, there is no gainsaying the number of immeasurable opportunities that abound if OFDM and its variants are leveraged by current and future wireless communications. OFDM variants are broadly classified into cyclic prefix (CP) OFDM, known symbol padding (KSP) OFDM [8; 9; 10; 11], zero padding (ZP) OFDM [12; 13], and unique word (UW) OFDM [14; 15]. They all have been the focus of investigation among researchers for the last couple of decades. CP-OFDM has been adopted as standard for digital video broadcasting (DVB) in Europe, while KSP-OFDM, also known as time domain synchronous (TDS) OFDM, has governed the Chinese DVB system [16; 17; 18]. The major difference in these alternatives is the structure of the OFDM block. CP-OFDM uses cyclic prefix as the guard interval (GI) (sequence), which varies from block to block and combats inter-block interference (IBI), while ZP-OFDM uses zeros for this purpose. CP-OFDM is considered to be a promising scheme for fast-fading underwater channels, 5G new radio [19; 20], and short packet communications, wherein a prefix and a suffix is used as GI in CP-OFDM [21]. The KSP/TDS-OFDM system uses the PN sequence as a guard interval, which has multi-fold advantages. Apart from IBI removal, it is also suitable for channel estimation and block synchronization due to its high self-correlation properties [22; 23; 24; 25]. UW-OFDM is also considered to be superior to CP-OFDM based on its lower bit error rate (BER) performance and lower out-of-band radiations, specifically in the frequency-selective Rayleigh fading channel [26]. Although both the UW-OFDM and KSP-OFDM systems use the known PN sequence, the former is a frequency domain method. The speedy time domain synchronization and channel estimation make KSP-OFDM superior to UW-OFDM. The usage of their guard intervals is limited by all of the aforementioned variants of multicarrier schemes; for instance, although each OFDM block's cyclic prefix in CP-OFDM is unique, the receiver discards it to prevent inter-block interference (IBI), which severely restricts the usage of a variable CP in each block. ZP-OFDM uses zero padding for this purpose and performs identically to CP-OFDM [26]. UW-OFDM [27] and KSP-OFDM use a fixed PN sequence and have some additional benefits of synchronization and channel estimation along with IBI removal. Some other techniques, such as subcarrier number modulation [28], filter bank (FB) OFDM-based index modulation (IM) and hybrid number and index modulation [29; 30] schemes, are also reported to improve the BER performance; nevertheless, these schemes assume the CP-OFDM block structure and exploit the number/index of active subcarriers in the block. In contrast to these schemes, our proposed enhanced orthogonal frequency division multiplexing (eOFDM) employs variable guard sequence, which does not require the insertion of pilot subcarriers in each block as in CP-OFDM but still provides scattered data subcarriers in the block for channel estimation. It should be noted that if the number/index of our proposed scheme is also exploited, the gain in BER or throughput as reported by [28; 29; 30] will be an added advantage.