Thesis (M.S.)--University of Missouri-Columbia, 2004. / Typescript. Includes bibliographical references (leaves 110-112). Also available on the Internet.

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Supervisor: Gustavo de Veciana. Vita. Includes bibliographical references. Also available from UMI.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 150-160). Also available in electronic version.

After establishing itself in the radio domain, Spread spectrum code-division multiplexing/multiple-access (CDMA) has seen a recent upsurge in optical domain as well. Due to its fairness, flexibility, service differentiation and increased inherent security, CDMA is proved to be more suitable for the bursty nature of local area networks than synchronous multiplexing techniques like Frequency/Wavelength Division Multiplexing (F/WDM) and Time Division Multiplexing (TDM). In optical domain, CDMA techniques are commonly known as Optical-CDMA (O-CDMA). All optical CDMA systems are plagued with the problem of multiple-access interference (MAI). Spectral amplitude coding (SAC) is one of the techniques used in the literature to deal with the problem of MAI. The choice of spreading code in any CDMA system is another way to ensure the successful recovery of data at the receiving end by minimizing the effect of MAI and it also dictates the hardware design of the encoder and decoder. This thesis focuses on the efficient design of encoding and decoding hardware. Perfect difference codes (PDC) are chosen as spreading sequences due to their good correlation properties. In most of the literature, evaluation of error probability is based on the assumptions of ideal conditions. Such assumptions ignore major physical impairments such as power splitting losses at the multiplexers of transmitters and receivers, and gain losses at the receivers, which may in practice be an overestimate or underestimate of the actual probability of error. This thesis aims to investigate thoroughly with the consideration of practical impairments the applications of PDCs and other spreading sequences in optical communications systems based on spectral-amplitude coding and utilizing codedivision as multiplexing/multiple-access technique. This work begins with a xix general review of optical CDMA systems. An open-ended practical approach has been used to evaluate the actual error probabilities of OCDM/A systems under study. It has been concluded from results that mismatches in the gains of photodetectors, namely avalanche photodiode (APDs), used at the receiver side and uniformity loss in the optical splitters results in the inaccurate calculation of threshold level used to detect the data and can seriously degrade the system bit error rate (BER) performance. This variation in the threshold level can be compensated by employing techniques which maintain a constant interference level so that the decoding architecture does not have to estimate MAI every time to make a data bit decision or by the use of balanced sequences. In this thesis, as a solution to the above problem, a novel encoding and decoding architecture is presented for perfect difference codes based on common zero code technique which maintains a constant interference level at all instants in CDM system and thus relieves the need of estimating interference. The proposed architecture only uses single multiplexer at the transmitters for all users in the system and a simple correlation based receiver for each user. The proposed configuration not only preserves the ability of MAI in Spectral-Amplitude Coding SAC-OCDM system, but also results in a low cost system with reduced complexity. The results show that by using PDCs in such system, the influence of MAI caused by other users can be reduced, and the number of active users can be increased significantly. Also a family of novel spreading sequences are constructed called Manchestercoded Modified Legendre codes (MCMLCs) suitable for SAC based OCDM systems. MCMLCs are designed to be used for both single-rate and Multirate systems. First the construction of MCMLCs is presented and then the bit error rate performance is analyzed. Finally the proposed encoding/decoding architecture utilizing perfect difference codes is applied in wireless infrared environment and the performance is found to be superior to other codes.

Loosely Synchronized (LS) codes can be used as spreading codes in quasi­ synchronous code-division multiple-access (QS-CDMA) systems. In such CDMA systems, close control of synchronization is achieved at the chip level, interme­ diate between that in synchronous CDMA and that in asynchronous CDMA. The LS code can then capitalize on zero correlation in a limited synchronization window to reduce code correlations and so reduce interference. LS codes are {O, +1, -1} codes constructed using Hadamard matrices and Golay pairs. A variation of LS codes inserts short strings of zeros between the components of the Golay pairs to increase the number of codewords, with only limited dete­ rioration in the correlations. These strings of zeros are known as internal padding. One of the advantages normally claimed for CDMA systems is resistance to eavesdropping and jamming. It might appear at first sight that the structure of LS codes is rather predictable in comparison with codes constructed using linear feedback shift registers, such as m-sequences or Gold codes. One way to overcome any such difficulty would be to evolve the code very quickly, in such a way that by the time a generation of the code is determined (or determined to a moderate correlation value) it is too late to exploit it. This thesis explores the way that LS codes can be evolved in order to achieve resistance to eavesdropping and jamming. The thesis starts with a detailed account of the necessary background and of the construction of Loosely Synchronized codes. The early part of the thesis then concentrates on showing that many generations of LS code can be constructed in such a way that the correlation between distinct generations is small. This prevents one observed generation of the code from being used for jamming or prediction in another generation. Specifically: •The construction of Golay pairs is investigated and a search is carried out over all possible Golay pairs and their mates to find a set of pairs that leads to the satisfaction of a suitable correlation criterion; •Bent functions, almost bent functions and other second order Boolean functions are used to create sets of Hadamard matrices that are guaranteed to satisfy the same correlation criterion; •A sequential search method to generate a set of arrangements of the internal padding that satisfies the same correlation criterion is described. Later in the thesis this approach is replaced by a recency list approach. This ensures that the correlation criterion is satisfied against recently used generations of the code, in place of all generations of the code; •The way in which these evolutions of the components combine together is also explored. Attention turns in the second part of the thesis to the mechanisms for evolution and the way that these might be predicted by a third party observer. Transform methods that the third party might use are described. Detailed simulations quantify the ability of the third party to identify the code during the transmission of a single bit. It is shown that theoretical resistance to early code prediction is not possible, although it might be possible to demonstrate security arising from the relative speed of the necessary computations for the user and the observer. This would require a detailed hardware study, and this is listed as future work. In fact it is shown here that LS codes are actually better than linear feedback shift register codes, as a result of the Berlekamp-Massey algorithm. Attention is also focussed on the scenario in which details of the algorithms of one user are obtained by the third party. Only the Hadamard matrix provides protection against this scenario, as all other components of the construction are shared between all users. From this second viewpoint the true weakness of LS codes becomes apparent. Although the Hadamard matrix constructions are satisfactory if the order of the Hadamard matrix is not too small, it seems that the sequence of Hadamard matrix rows of each user must be computed centrally and distributed to users as private keys if this scenario is not to remain a major concern. The volume of private key distribution necessary may seem unattractive to operators. Ultimately it seems that evolution of the Golay pairs may have little real role except to increase the workload of the observer. The recency list based evolution of internal padding can take the main role in ensuring low correlation between close generations of the code. The evolution of the Hadamard matrix should be designed to concentrate on the second viewpoint, where the third party has obtained details of the algorithms of one user.

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This thesis develops simulation techniques for evaluating the performance of future wireless digital multiple access standards for fast and slow moving vehicles in outdoor environments. Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) are both multiple access techniques for wireless systems that can support significantly more users per cell than the present analog FDMA system. Both CDMA and TDMA use digital modulation, and so performance is compared based on bit and packet error rates resulting from the simulations. The primary contribution of this thesis is the development of fast and accurate algorithms for channel simulations, and flexible structured implementation of error correction coding. Previous simulation techniques have resulted in extremely high computational complexity, limiting the number of design options which may be explored. This thesis presents a multirate simulation technique which allows an order of magnitude reduction in simulation times for digital systems on multipath channels. The simulations are carried out for randomly generated data as well as coded voice data. The data is then processed according to the standard selected (IS-54 TDMA or IS-95 CDMA.) The coded bits are transmitted over a simulated baseband channel. Receiver implementations are also examined. Applications of this research include rapid software prototyping of new systems. If the performance of a new system can be accurately evaluated on a computer without building hardware, implementation costs can be significantly reduced. / Master of Science

Interference is a major problem in modern wireless communications systems. No longer are background noise and average power loss the limiting factors in system capacity corruption of the available spectrum by multiple access and nearby interference provides the upper limit to system capacity. If the exponential growth of commercial wireless communications is to continue, systems must effectively deal with the increasingly crowded and corrupted spectrum. Direct Sequence Spread Spectrum modulation (DS-SS) combined with Time Dependent Processing represents a valid approach to meeting the needs of future communications systems. Traditionally, the exploitation of cyclostationarity in digital communications signals has been reserved for the hostile communication environments faced by the military. However, the advent of cost-effective, high-speed DSP chips and associated processing hardware have made Time Dependent Processing a viable commercial technology. This thesis presents several forms of the Time Dependent Adaptive Filter (TDAF) which are able to fully exploit the cyclostationarity and high degree of spectral correlation in certain DS-SS signals. It is shown that these optimal TDAFs are able to combat interference from noise, multipath, signals with dissimilar modulation, and signals with similar modulation (multiple access interference). Performance gains are achieved without a knowledge of the specific type of interference and depend solely on the high degree of spectral correlation in DS-SS signals. It is shown that properly designed DS-SS CDMA systems that utilize the TDAF can achieve spectral efficiencies which are within 10% of FDM/TDM systems. Furthermore, these systems retain the benefits of wideband modulation and universal frequency reuse traditionally associated with CDMA systems. The net result is a tremendous increase in system user capacity and signal reception quality. / Master of Science