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High Capacity CDMA and Collaborative Techniques High Capacity CDMA and Collaborative Techniques By Indu Lal Shakya A Thesis submitted for the degree of Doctor of Philosophy School of Science and Technology University of Sussex April 2008 Declarations I, Indu Lal Shakya hereby certify that the materials presented in this thesis are my own work, except where indicated. Any information or materials that are not my own creations are indicated explicitly as references including their origins, publication dates. I also declare that this thesis has not been submitted, either in the same or different form to this or any other university for a degree. Indu Lal Shakya, Brighton, UK i University of Sussex Indu Lal Shakya Thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy High Capacity CDMA and Collaborative Techniques Summary The thesis investigates new approaches to increase the user capacity and improve the error performance of Code Division Multiple Access (CDMA) by employing adaptive interference can- cellation and collaborative spreading and space diversity techniques. Collaborative Coding Multi- ple Access (CCMA) is also investigated as a separate technique and combined with CDMA. The advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the uplink and downlink are proposed and evaluated. Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM) property of digital signals to blindly suppress interference during the despreading process and ob- tain amplitude estimation with minimum mean squared error for use in cancellation stages. Two new blind adaptive receiver designs employing successive and parallel interference cancellation architectures using the CM algorithm (CMA) referred to as ‘CMA-SIC’ and ‘BA-PIC’, respec- tively, are presented. These techniques have shown to offer near single user performance for large number of users. It is shown to increase the user capacity by approximately two fold compared with conventional IC receivers. The spectral efficiency analysis of the techniques based on output signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore, an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI without any knowledge of channel amplitudes and allows large number of users compared with equal gain and maximum ratio combining techniques normally used in practice. New user collaborative schemes are proposed and analysed theoretically and by simulations in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First, a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA are designed and analysed. Next, a new user collaborative scheme with successive interference cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce MAI and achieve improved diversity. To further improve the performance of C-SIC under high system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed. It is shown to minimize the residual MAI, leading to improved user capacity and a more robust system. It is known that collaborative diversity schemes incur loss in throughput due to the need of orthogonal time/frequency slots for relaying source’s data. To address this problem, finally a novel near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is ii proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time encoding methods. At the receiver collaborative joint detection is performed to separate each paired users’ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth as inter-user channel SNR becomes high. A novel approach of ‘User Collaboration’ is introduced to increase the user capacity of CDMA for both the downlink and uplink. First, collaborative group spreading technique for the downlink of overloaded CDMA system is introduced. It allows the sharing of the same single spreading sequence for more than one user belonging to the same group. This technique is referred to as Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collabora- tive coding is used for each group to form a composite codeword signal of the users and then a single orthogonal sequence is used for the group. At each user’s receiver, decoding of composite codeword is carried out to extract the user’s own information while maintaining a high SINR per- formance. To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions, Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collabora- tive coding multiple access and space-time coding principles. A new scheme for uplink of CDMA using the ‘User Collaboration’ approach, referred to as CS-CDMA-UL is presented next. When users’ channels are independent (uncorrelated), significantly higher user capacity can be achieved by grouping multiple users to share the same spreading sequence and performing MUD on per group basis followed by a low complexity ML decoding at the receiver. This approach has shown to support much higher number of users than the available sequences while also maintaining the low receiver complexity. For improved performance under highly correlated channel conditions, T-user collaborative coding is also investigated within the CS-CDMA-UL system . iii Acknowledgements First of all, I would like to express my sincere gratitude to Dr. Falah Ali for allowing me to pursue this research study under his supervision within the Communications Research Group. The ideas and techniques proposed in this thesis are the results of countless number of discussions we had on many topics of contemporary multiuser wireless communication systems. I am particularly thankful to him for his insights and innovative thinking which have given a solid shape to this thesis. He has shown a lot of care and concern on the progress of my research study and been an excellent mentor throughout the period of research. Also, I am very much grateful to Dr. Elias Stipidis for his supervision, motivations and support. He has been very generous and enthusiastic to facilitate our research activities and organizing various group occasions. The COMMS group has been a truly inspiring and friendly place to carry out the research and I am very glad to have support from so many friends and colleagues and wish to thank them all. In no particular order: Dr. Xiadong Cai, Dr. Arun Baburam, Dr. David Annalingam, Dr. Rohan Goonwardene, Dr. Periklis, Dr. George, Dr. Ireri, Shahin , Obi, Ian, Matt, Panos, Fuad, Antonio, Bilal, Walid and Marwan. Particularly, I am very thankful to my friend Dr. Cai for taking his time to read the draft of some of the chapters of this thesis. His invaluable comments and suggestions have greatly improved the presentation of the thesis. The path to this long journey has been the true test of my abilities and self determinations. The first milestone of the journey has now completed and I have now reached to the stage where I can contemplate my past and acknowledge the relentless support I received from my family. First of all, it is the dream and passion of my parents Mr. Shrawan Lal Shakya and Mrs. Sanu Shakya to see me succeed and achieve this very important goal of completing the PhD. The words in this thesis will be very little to express my gratitude and love to their selfless and everlasting support. My brother Kundan has made my study for an MS in engineering in Russia successful by supporting me with all means that paved the way for the further study. My sisters Nilu, Minu and Sumi, Kundan bro and Surendra bhinaju have established my route to come to the UK and pursue the graduate study. I always counted on them for all support and not worry about any hardship. Also Puni bhauju has been very supportive throughout so many years. Guddi and Reiya always cheer me up. Finally, the words are reserved exclusively for the one and only who is the most important part of my life: my lovely and wonderful wife Prerana. I would not have been in this stage without her coherent and restless support. I appreciate her endurance and tolerance to see me spending almost all of my time towards building the thesis but not to be with her. This thesis will not be enough to show my dedication to her, but still, I feel that this is something I could have ever been able to offer. iv List of Publications • I. Shakya, F. H. Ali, E. Stipidis, “Improved Successive Interference Cancellation for DS CDMA Using Constant Modulus Algorithm”, International Symposium on Communica- tions Theory and Applications (ISCTA), Ambleside, UK, July 2007 • I. Shakya, F. H. Ali, E. Stipidis, “Robust Blind PIC with Adaptive Despreader and Improved Interference Estimator”, To appear in International Journal of Communications Systems • I. Shakya, F. H. Ali, E. Stipidis, “Blind Adaptive Subcarrier Combining Technique for MC- CDMA Receiver in Mobile Rayleigh Channel”, Submitted to IET Electronics Letters in July 2008 • F. H. Ali, I. Shakya, E. Stipidis, “User Cooperation Diversity for Multiuser CCMA”, IEEE Symposium on Personal Indoor and Mobile Radio Communications (PIMRC), Athens, Greece, September 2007 • I. Shakya, F. H. Ali, E. Stipidis, “Performance of Joint User Cooperation Diversity and Successive Interference Cancellation for CDMA”, IEE Workshop on Smart Antennas and Cooperative Communications, London, UK, October 2007 • I. Shakya, F. H. Ali, E. Stipidis, “Performance of Cooperative CDMA with Successive Inter- ference Cancellation”, To appear in the 5th International Conference on Broadband Com- munications, Networks and Systems (BROADNETS 2008) • I.
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