In this thesis, a novel transmission structure is proposed for the Direct Sequence Ultra Wide-Band (DS-UWB) systems. The main purpose of the proposed structure is to eliminate the inter-symbol interference caused by the multi-path environment. In DS-UWB systems, shortening the guard interval is one of the possible ways to achieve higher data rates. However, interference will increase inversely with the length of the guard interval because the signal delay spread caused by the multi-path effect will induce inter-symbol interference. In this thesis, a novel transmission structure that utilizes the autocorrelation properties of the perfect sequence is proposed for interference cancellation in DS-UWB systems. Both computer simulation and mathematical analysis are provided for performance evaluation.

Almost p-ary perfect and nearly perfect sequences are equivalent to certain relative difference sets and direct product difference sets, respectively. This feature enables Chee, Tan and Zhou to determine the existence status of those sequences by using the tools of Design Theory. In particular, they determined the existence status of almost p-ary perfect and nearly perfect sequences of period n+1 for n 100, except some open cases in [6]. In this thesis, we obtained a set of Diophantine equations in integers while observing relative difference sets, and proved nonexistence of almost p-ary perfect sequences of period n + 1 for n (50,76,94,99,100).Also, we observed that it was possible to extend Diophantine equations that we used for relative difference sets to the direct product difference sets, thereby proved the nonexistence of almost p-ary nearly perfect sequences of type II of period n + 1 for p = 2, p = 3 and p = 5 at certain values of n. As a result, we answered two questions posed by Chee, Tan and Zhou in [6].

Selected mapping (SLM) schemes are commonly employed to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It has been shown that the computational complexity of the traditional SLM scheme can be substantially reduced by adopting conversion vectors obtained by using the inverse fast Fourier transform (IFFT) of the phase rotation vector in place of the conventional IFFT operations [21]. Unfortunately, however, the elements of these phase rotation vectors of the conversion vectors in [21] do not generally have an equal magnitude, and thus a significant degradation in the bit error rate (BER) performance is incurred. This problem can be remedied by utilizing conversion vectors having the form of a perfect sequence. This paper presents three novel classes of perfect sequence, each of which comprises certain base vectors and their cyclic-shifted versions. Three novel low-complexity SLM schemes are then proposed based upon the unique structures of these perfect sequences. It is shown that while the PAPR performances of the proposed schemes are marginally poorer than that of the traditional SLM scheme, the three schemes achieve an identical BER performance and have a substantially lower computational complexity.

Selected mapping (SLM) schemes are commonly employed to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It has been shown that the computational complexity of the traditional SLM scheme can be substantially reduced by adopting conversion vectors obtained by using the inverse fast Fourier transform (IFFT) of the phase rotation vectors in place of the conventional IFFT operations. To ensure that the elements of these phase rotation vectors have an equal magnitude, conversion vectors should have the form of a perfect sequence. This study firstly presents three novel classes of perfect sequence, each of which comprises certain base vectors and their cyclically shifted versions. Three novel low-complexity SLM schemes are then proposed based upon the unique structures of these perfect sequences. It is shown that while the PAPR reduction performances of the proposed schemes are marginally poorer than that of the traditional SLM scheme, the three schemes achieve a substantially lower computational complexity. Since the three proposed PAPR reduction schemes cannot be utilized in the orthogonal frequency division multiple access (OFDMA) system. A low-complexity scheme for PAPR reduction in OFDMA uplink systems using either an interleaved or a sub-band sub-carrier assignment strategy is also proposed in the second part of this study. The proposed scheme requires just one IFFT operation. The PAPR reduction performance of the proposed scheme is only marginally poorer than that of the traditional SLM scheme. However, the proposed schemes have significantly lower computational complexities. Besides, multiple-input multiple-output (MIMO) OFDM systems with space-frequency block coding (SFBC) are well-known for their robust performance in time selective fading channels. However, SFBC MIMO-OFDM systems have a high computational complexity since the number of IFFTs required scales in direct proportion to the number of antennas at the transmitter. Furthermore, SFBC MIMO-OFDM systems have a high PAPR. Accordingly, a low-complexity PAPR reduction scheme for SFBC MIMO OFDM systems with the Alamouti encoding scheme is proposed in this study. Extending this scheme obtains two low-complexity transmitter architectures for SFBC MIMO-OFDM systems with a general encoding matrix and an arbitrary number of transmitter antennas. The proposed schemes achieve a significant reduction in computational complexity by fully exploiting the time-domain signal properties of the transmitted signal. In addition, a PAPR reduction scheme is presented based on the proposed transmitter schemes. It is shown that the PAPR reduction performance of the proposed scheme is almost as good as that of the traditional SLM scheme, but is achieved with a substantially lower computational complexity.