The rapid developments of wireless communications have brought forward changes in the world of mobile wireless communication technology and Wireless Local Area Networks (WLAN). This development leads to the fourth generation (4G) as the upcoming wireless communication systems that can provide comprehensive Internet Protocols (IP) solutions where any signal can be provided to the user at anytime,anywhere basis and at higher data rates compared to first (1G), second (2G), and third generations (3G). In order to achieve the vision of 4G, a higher modulation scheme is needed; Orthogonal Frequency Division Multiplexing (OFDM) has been considered as a good candidate for 4G due to its good spectrum efficiency and being robust to Inter-Symbol Interference (ISI). However, in high speed digital wireless applications, the ISI channel may have spectral nulls, which can degrade the efficiency of the OFDM systems because the Fourier transform of the ISI channel needs to be inverted for each subcarrier at the receiver. Also the data rate overhead of the cyclic prefix insertion is high when the ISI channel has many taps. Moreover, OFDM system is sensitive to phase noise; Carrier Frequency Offsets (CFO) and high Peak to Average Power Ratio (PAPR) which decreases the power efficiency of the Radio Frequency (RF) amplifier.Precoded-OFDM was proposed to improve the Bit Error Rate (BER) performance of OFDM system in ISI channel with spectral nulls and reduce the overhead data rate transmission due to the insertion of Cyclic Prefix (CP). But due to the insertion of zeros between V consecutive information symbols, precoded-OFDM has high PAPR. A Vector OFDM (VOFDM) was presented to reduce the cyclic prefix data rate overhead. VOFDM system is mainly from the precoded OFDM systems, where no zeros inserted between two sets of V consecutive information symbols but each V consecutive information symbols are blocked together as a vector sequence and the data rate overhead of the CP is not expanded. Also the ISI channel is blocked into a square matrix with the length reduced compared to the original ISI channel length. A rotated precoder has been proposed to reduce the high PAPR of precoded-OFDM system, although the BER performance was similar, was unable to achieve the value less than the conventional OFDM system.This study proposed a hybridization of precoded/vector OFDM and Carrier Interferometry (CI) techniques to improve the Bit Error Rate (BER) performance and reduce the PAPR. The CI spreading codes has been also introduced in the rotated precoder based OFDM and in the suboptimal based VOFDM systems. In the proposed system, each information symbol in the vector sequence is modulated onto all of the N carriers. To ensure the separation of information symbols at the receiver side, the transmitter applies a unique orthogonal spreading code to each information symbol. The phase offset that makes the symbol separation at the receiver protects the symbols from adding coherently at the same time hence lower PAPR. The simulation results described that the proposed system was able to reduce the value of PAPR.In order to achieve better BER performance of the vector OFDM, we proposed equalization scheme to match with vector sequences. The simulation result confirms that the performance outperforms the conventional vector OFDM even in the presence of CFO errors.The study investigated the effects of CFO and proposed a frequency estimator and the derivation of the proposed estimator is based on the principles of nonlinear least squares and phase difference. The simulation results show that the BER performance of the proposed system outperforms the precoded-OFDM and achieved a low PAPR compared to the conventional OFDM system.