Since recent times the importance of wireless communications has experienced a phenomenal growth, thus incurring a considerable increase in usage of the frequency spectrum available for communications and hence creating the need for efficient methods for its utilisation. Multiple-access is one efficient method, which enables several users to share a common frequency bandwidth. A multiple-access technique known as Collaborative Coding Multiple Access (CCMA) has been theoretically investigated and has been shown to give an increase in channel capacity and bandwidth efficiency. This technique uses a special type of codes known as collaborative codes to enable multiple users to communicate simultaneously over a common frequency bandwidth, thus eliminating the use of time, frequency and orthogonal code divisions. However to implement this technique practically, requires a multiple access channel (MAC) that coherently combines the signals originating from different users to give a composite signal. The combining of signals to implement a CCMA scheme can be easily achieved in baseband channels, where signals can be represented as voltages or currents which can add, to give uniquely identifiable code combinations. However when extended to bandpass carrier channels, the combining of signals over radio channels involves the addition of signals of differing, and, in the mobile situation particularly, varying relative phases and amplitudes. This can give rise to a major detection problem due to the resulting destructive signal combining and thus making CCMA non- practical over radio channels. This thesis describes a novel multiple-access scheme known as Complex-Valued Collaborative Coding Multiple Access (CV-CCMA), to provide a practical system over radio channels that achieves the theoretical promises of CCMA. CV_CCMA overcomes the problem associated with the non-coherent combining of multiple signals over radio channels by employing a complex-valued collaborative code and a combined detection and channel estimation process to estimate the data of each user as well as their associated amplitude and phase variations induced by the channel. The research is initiated with the principles and the theoretical foundations of the CV_CCMA technique. A combined signal detection and channel estimation process is presented. The choice of a suitable collaborative coding scheme is investigated. The conditions for the selection of valid codesets are established for a given number of users, modulation levels and code length. In addition, the effect of channel fading is also investigated. AT-user M-PSK CV_CCMA system operating in Rayleigh and Rician fading channels, is modelled and simulated to establish the error-performance. The results from this investigation are obtained for different number of users and modulation levels. The effect of varying signal power levels is also investigated. Observing the obtained results an additional code selection condition is established for a Rician channel, with an improved error performance. The channel capacity of the CV_CCMA scheme operating in a flat Rayleigh fading channel is obtained. In a given T-user M-PSK CV_CCMA scheme the channel outputs result in multi-level signals. Therefore the channel capacity of CV_CCMA is compared with equivalent multi-level single access schemes. It is shown that the channel capacity of CV_CCMA is greater than the corresponding multi-level schemes. The theoretical channel capacity limits of CV_CCMA is also compared with the limits of TDMA, FDMA and CDMA operating in a flat Rayleigh fading channel. The enhancement of the error performance of CV_CCMA in fading channels is investigated by using space diversity reception and error-control coding. It is found that by using these techniques the error performance of CV_CCMA can be greatly enhanced. For example with a dual-diversity scheme in a flat Rayleigh fading channel, the achieved gain is in the order of 20 dB. Using error-control coding, a coding gain is achieved depending on the type of code used. It is also found that using both of these techniques contributes towards further performance gains depending on the number of diversity paths and the coding scheme. The thesis is finally concluded with an overview of the contributions of this thesis, the achieved results and their implications and suggestions for future work in this subject.

Thesis (Ph.D.)--Auburn University, 2007. / Abstract. Includes bibliographic references (ℓ. 216-223)

No description available.

Thesis (M.S.)--University of Missouri--Rolla, 2007. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed December 7, 2007) Includes bibliographical references (p. 171-173).

Thesis (M.S.)--Youngstown State University, 2009. / Includes bibliographical references (leaves 100-103). Also available via the World Wide Web in PDF format.

Thesis (M.S.)--University of Texas at Dallas, 2008. / Includes vita. Includes bibliographical references (leaves 51-52)

No description available.

As the demand for wireless communication services grows quickly, spectrum scarcity has been on the rise sharply. In this context, cognitive radio (CR) is being viewed as a new intelligent technology to solve the deficiency of fixed spectrum assignment policy in wireless communications. Spectrum sensing is one of the most fundamental technologies to realise dynamic spectrum access in cognitive radio networks. It requires high accuracy as well as low complexity. In this thesis, a novel adaptive threshold setting algorithm is proposed to optimise the trade-off between detection and false alarm probability in spectrum sensing while satisfying sensing targets set by the IEEE 802.22 standard. The adaptive threshold setting algorithm is further applied to minimise the error decision probability with varying primary users' spectrum utilisations. A closed-form expression for the error decision probability, satisfied SNR value, number of samples and primary users' spectrum utilisation ratio are derived in both fixed and the proposed adaptive threshold setting algorithms. By implementing both Welch and wavelet based energy detectors, the adaptive threshold setting algorithm demonstrates a more reliable and robust sensing result for both primary users (PUs) and secondary users (SUs) in comparison with the conventional fixed one. Furthermore, the wavelet de-noising method is applied to improve the sensing performance when there is insu cient number of samples. Finally, a novel database assisted spectrum sensing algorithm is proposed for a secondary access of the TV White Space (TVWS) spectrum. The proposed database assisted sensing algorithm is based on the developed database assisted approach for detecting incumbents like Digital Terrestrial Television (DTT) and Programme Making and Special Events (PMSE), but assisted by spectrum sensing to further improve the protection to primary users. Monte-Carlo simulations show a higher SUs' spectrum efficiency can be obtained for the proposed database assisted sensing algorithm than the existing stand-alone database assisted or sensing models.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: precision personnel locator; digital systems; embedded systems; waveform generation; data communication. Includes bibliographical references (leaves 108-110).

Includes bibliographical references (p. 84-87).