隨著行動通訊技術持續的發展，傳輸量的需求是愈來愈大。在現有的多重存取系統中，大多數都是基於正交多重存取（orthogonal multiple access，OMA）所設計的，然而，這些技術都無法充分使用頻譜資源。因此在第五代行動通訊標準中，其中一項技術為非正交多重存取，比較於正交多重存取，此技術目的是能夠將每個頻譜資源同時分配給多位使用者以提供更高傳輸量，但是，這將造成多重存取干擾。而在檢測方面多以連續干擾消除（successive interference cancellation，SIC）作為偵測訊號方法。
本文將探討可應用於上行傳輸系統的非正交多重存取（non-orthogonal multiple access，NOMA）技術。在稀疏碼多重存取（sparse code multiple access，SCMA）中，不會分配所有資源給單一用戶。本文提出一項新的設計，用戶可以利用其餘資源傳送另一筆資料，此筆資料將在下一個時間槽再次傳送，目的是讓所有傳送區塊都與前後區塊有關聯，因此在偵測前可由前一區塊算出的資訊作為事前資訊，而在通道編碼部分採用低密度奇偶檢查碼（low-density parity-check，LDPC），另一項研究乃是搭配外部資訊傳遞演算法（extrinsic information transfer，EXIT）找出符合偵測端與解碼端之最佳化低密度奇偶檢查碼。在錯誤率的表現上比稀疏碼多重存取還要好，且經過設計之碼也是優於標準碼的。

In recent years, the demand for high data transmission throughput grows significantly with the progress of the technologies for mobile communications. Most of these technologies are based on the idea of orthogonal multiple access (OMA) in current wireless systems. However, it is concerned that spectrum resources may not be fully exploited when considering conventional OMA schemes. Consequently, the non-orthogonal multiple access (NOMA), which is one of the promising radio access technique for 5G communication systems, is proposed. Comparing to OMA-based systems, each spectrum resource can be allocated to more than one users, and hence yields a significant improvement in throughput performances.
In this thesis, we propose a novel scheme for the sparse code multiple access (SCMA), which is one of the NOMA-based techniques. In the proposed scheme, the users are allowed to transmit their data not only through the resources assigned for themselves in the current transmission block, but also through other resources, which are originally assigned for other users, in the next transmission block. As a result, each of the transmission blocks is coupled with the previous and the afterward transmission block. Consequently, the \textit{a priori} information for each of the transmission blocks can be obtained before its own detection process, from the detection process of the previous transmission block. In addition, the low-density parity-check (LDPC) codes, which are considered as the outer channel codes, can be optimized by using the extrinsic information transfer (EXIT) technique. It is shown through simulations that the bit-error rate (BER) the performance of the proposed scheme is superior to that of the original SCMA scheme.

Abstract I
中文摘要 II
第一章 簡介 1
第二章 知識背景 4
第三章 偵測演算法與新型架構介紹 29
第四章 低密度奇偶檢查碼之設計 50
第五章 模擬結果 75
第六章 結論 93

[1] L. Dai, B.Wang, Y. Yuan, S. Han, C. l. I, and Z.Wang, “Non-orthogonal multiple access for 5G: solutions, challenges, opportunities, and future research trends,” IEEE Communications Magazine, vol. 53, no. 9, pp. 74-81, Sept 2015.
[2] P. Wang, J. Xiao, and L. P, “Comparison of orthogonal and non-orthogonal approaches to future wireless cellular systems,” IEEE Vehicular Technology Magazine, vol. 1, no. 3, pp. 4-11, Sept 2006.
[3] A. Benjebbour, Y. Saito, Y. Kishiyama, A. Li, A. Harada, and T. Nakamura, “Concept and practical considerations of non-orthogonal multiple access (NOMA) for future radio access,” in 2013 International Symposium on Intelligent Signal Processing and Communication Systems, Nov 2013, pp. 770-774.
[4] S. Vanka, S. Srinivasa, Z. Gong, P. Vizi, K. Stamatiou, and M. Haenggi, “Superposition coding strategies: Design and experimental evaluation,” IEEE Transactions on Wireless Communications, vol. 11, no. 7, pp. 2628-2639, July 2012.
[5] B. Wang, K. Wang, Z. Lu, T. Xie, and J. Quan, “Comparison study of non-orthogonal multiple access schemes for 5G,” in 2015 IEEE International Symposium on Broadband Multimedia Systems and Broadcasting, June 2015, pp. 1-5.
[6] J. van de Beek and B. M. Popovic, “Multiple access with low-density signatures,” in GLOBECOM 2009 - 2009 IEEE Global Telecommunications Conference, Nov 2009, pp. 1-6.
[7] R. Hoshyar, F. P. Wathan, and R. Tafazolli, “Novel low-density signature for synchronous cdma systems over awgn channel,” IEEE Transactions on Signal Processing, vol. 56, no. 4, pp. 1616-1626, April 2008.
[8] H. Nikopour and H. Baligh, “Sparse code multiple access,” in 2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), Sept 2013, pp. 332-336.
[9] M. Taherzadeh, H. Nikopour, A. Bayesteh, and H. Baligh, “SCMA codebook design,” in 2014 IEEE 80th Vehicular Technology Conference (VTC2014-Fall), Sept 2014, pp. 1-5.
[10] G. Song, X. Wang, and J. Cheng, “Signature design of sparsely spread CDMA based on superposed constellation distance analysis,” CoRR, vol. abs/1604.04362, 2016. [Online]. Available: http://arxiv.org/abs/1604.04362
[11] S. Zhang, X. Xu, L. Lu, Y. Wu, G. He, and Y. Chen, “Sparse code multiple
access: An energy e_cient uplink approach for 5G wireless systems,” in 2014 IEEE Global Communications Conference, Dec 2014, pp. 4782-4787.
[12] Y. Wu, S. Zhang, and Y. Chen, “Iterative multiuser receiver in sparse code multiple access systems,” in 2015 IEEE International Conference on Communications (ICC), June 2015, pp. 2918-2923.
[13] B. Xiao, K. Xiao, S. Zhang, Z. Chen, B. Xia, and H. Liu, “Iterative detection and decoding for SCMA systems with LDPC codes,” in 2015 International Conference on Wireless Communications Signal Processing (WCSP), Oct 2015, pp. 1-5.
[14] S. ten Brink, G. Kramer, and A. Ashikhmin, “Design of low-density parity-check codes for modulation and detection,” IEEE Transactions on Communications, vol. 52, no. 4, pp. 670-678, April 2004.
[15] J. Proakis and M. Salehi, Digital Communications. McGraw Hill, 2007.
[16] R. Gallager, “Low-density parity-check codes,” IRE Transactions on Information Theory, vol. 8, no. 1, pp. 21-28, January 1962.
[17] W. Ryan and S. Lin, Channel Codes: Classical and Modern. Cambridge University Press, September 2009.
[18] S. Kotz and S. Nadarajah, Extreme Value Distributions: Theory and Applications. London: Imperial College Press, 2000.
[19] S. ten Brink and B. M. Hochwald, “Detection thresholds of iterative MIMO processing,” in Proceedings IEEE International Symposium on Information Theory, 2002, pp. 22-.
[20] S. ten Brink, “Convergence behavior of iteratively decoded parallel concatenated codes,” IEEE Transactions on Communications, vol. 49, no. 10, pp. 1727-1737, Oct 2001.
[21] “IEEE standard for local and metropolitan area networks part 16: Air interface for _xed and mobile broadband wireless access systems amendment 2: Physical and medium access control layers for combined _xed and mobile operation in licensed bands and corrigendum 1,” IEEE Std 802.16e-2005 and IEEE Std 802.16-2004/Cor 1-2005 (Amendment and Corrigendum to IEEE Std 802.16-2004), pp. 1-822, 2006.
[22] Mobile Multimedia Broadcasting (China) Part 1: Framing Structure, Channel Coding and Modulation for Broadcasting Channel. released by SARFT, China, 2006.