All digital communication channels are subject to inter-symbol interference (ISI). To achieve the desired system performance, at receiver end, the effect of ISI must be compensated and the task of the equalizer is to combat the degrading effects of ISI on the transmission. Due to the demand of high data transmission rate, the multicarrier modulation (MCM) technique implemented with the orthogonal frequency division multiplexing (OFDM) has been adopted in many modern communications systems for block transmission. In block transmission systems, transmitter-included redundancy using finite-impulse response (FIR) filterbanks can be utilized to suppress inter-block-interference (IBI). However, the length of redundancy will affect the system performance, which is highly dependent on the length of channel impulse response. To deal with the effect of ISI, many equalizing schemes have been proposed, among them the FIR zero-forcing (ZF) equalizer with the non-orthogonal projector provides a useful transceiver design structure for suppressing the IBI and ISI, simultaneously. In this thesis, we propose a new equalizing scheme; it combines the FIR-ZF equalizer with non-orthogonal projector as well as the Bayesian decision feedback equalizer (DFE) for IBI and ISI suppression. The Bayesian DFE is known to be one of the best schemes to achieve the desired performance for eliminating ISI. It can be employed to achieve the full potential of symbol-by-symbol equalizer. That is, after removing the effect of IBI with the non-orthogonal projector, the Bayesian DFE is employed for eliminating the ISI, simultaneously. For comparison, the system performance, in term of bit error rate (BER) is investigated, and compared with the minimum mean square error (MMSE)-IBI-DFE. The advantage of the new proposed equalizing scheme is verified via computer simulation under condition of insufficient redundancy.

In orthogonal frequency division multiplexing (OFDM) systems, when transmitted blocks of signal through the multipath channel, the present transmitted blocks of signal will be interfered by the previous one due to the effect of channel delay spread. In order to solve this problem, conventionally we introduce a guard interval in transmitted blocks of signal to avoid inter-block interference (IBI). The most popular methods of the guard interval insertion are zero-padding (ZP) and cyclic prefixing (CP). ZP insert the guard interval at the end of the transmitted blocks of signal, in which all elements are zeros. The length of this interval must be equal to or greater then the channel order. In this way we can avoid IBI, at the expense of lossing the orthogonality between subcarrier due to the multipath channel. However, we can use CP to cope whit this problem. In CP we copy the 25% length at the end of the transmitted blocks of signal, and put it in front of the transmitted block to suppress the inter-block interference and inter-carrier interference. In this thesis, we consider OFDM system without the guard interval, hence the received signal contains IBI and ICI. In our proposed scheme, we use an oblique projector (OB) to suppress IBI at first. Then we combine it with a generalized sidelobe canceller (GSC) based equalizer [1] for ICI supppression and demodulate the receiver signal. In computer simulations, we use the GSC-based equalizer with oblique projector to suppress IBI/ICI in the time-invariant channel and time-varying channel. As we will see, in the CP-free OFDM system, the oblique projector can suppress IBI exactly. Moreover we can get better performance in channel with larger order.

Orthogonal frequency division multiplexing (OFDM) modulator with redundancy has been adopted in many wireless communication systems for higher data rate transmissions. The introduced redundancy at the transmitter allows us to overcome serious inter-block interference (IBI) problems due to highly dispersive channel. However, the selection of redundancy length will affect the system performance and spectral efficiency, and is highly dependent on the length of channel impulse response. In this thesis, based on the pseudorandom postfix (PRP) OFDM scheme we propose a novel block-based OFDM transceiver framework. Since in the PRP-OFDM system the PRP can be employed for semi-blind channel estimation with order-one statistics of the received signal. Hence, for sufficient redundancy case the PRP-OFDM system with the Bayesian decision feedback equalizer (DFE) is adopted for suppressing the IBI and ISI simultaneously. However, for the insufficient redundancy case (the length of redundancy is less than the order of channel), we first propose a modified scheme for channel estimation. To further reduce the complexity of receiver, the maximum shortening signal-to-noise-ratio time domain equalizer (MSSNR TEQ) with the Bayesian DFE is developed for suppressing the IBI and ISI, separately. That is, after knowing the channel state information (CSI) and removing the effect of IBI with MSSNR TEQ, the Bayesian DFE is applied for eliminating the ISI. Via computer simulation, we verify that performance improvement, in terms of bit error rate (BER), compared with the conventional block-based minimum mean square error (MMSE)-DFE can be achieved.