本論文提出兩個應用於非正交多重接取毫微米波系統的多使用者混和波束成形技術設計方案，目標將使用者之加權傳輸速率最大化。第一個「迫零混和波束成形」方案中，對應到每個群集的射頻波束成形被選擇來增強不同使用者間有效通道的正交性。基頻波束成形則是根據迫零波束成形來設計，用來消除使用者間的干擾。我們提出了一個貪婪式演算法，迭代地決定每個群集內使用者的腳色與用來設計迫零波束成形的通道，再根據上述方法來更新射頻與基頻波束成形，使系統達到更高的加權傳輸速率總和。第二個「加權均方誤差和最小化」方案中，混和波束成形設計是透過兩個階段來完成。第一階段中，藉由加入輔助的變數，加權傳輸速率最大化問題可以等效轉換為加權均方誤差和最小化問題，透過解這個最佳化問題，能達到加權速率總和最大化之全數位多使用者波束成形會先被得到。第二階段裡，考量系統射頻鏈數量的限制，使用正交匹配追蹤演算法將上階段得到之全數位多使用者賦形拆解為射頻和基頻波束成形。為優化加權傳輸速率最大化的結果，固定射頻波束成形，再次採用第一階段中同樣的最佳化程序來優化基頻波束成形。我們提出一個簡易的演算法來決定每個群集內使用者的腳色，目的同樣是為了使系統的加權傳輸速率最大化，並藉由模擬結果來展現我們提出的兩個方案的效果。

This work proposes a multiuser hybrid beamforming scheme for non-orthogonal multiple access (NOMA) in millimeter wave (mmWave) systems to reach the goal of weighted sum rate maximization. While NOMA may be effective in terms of enhancing user fairness, hybrid beamforming is necessary to reduce the transceiver cost as the system moves towards higher frequency. The main objective is to design hybrid beamformers to solve a series of weighted sum rate maximization problem. Two criteria are considered to derive the hybrid beamformer, namely, zero-forcing (ZF) and weighted sum mean square error (WSMSE) minimization criteria. In the ZF scheme, each analog RF beamformer for each cluster is chosen by enhancing the orthogonality of different users' effective channel. The baseband digital beamformers are then chosen based on the zero-forcing criterion to eliminating the inter-user interference among users. We iteratively exchange the used channel in each cluster to design ZF beamformer and update RF beamformer in the same way of above methods to achieve higher weighted sum rate. In the second scheme, by adding auxiliary parameters, the weighted sum rate maximization is transformed into the weighted sum MSE minimization problem where the local optimal point can be efficiently obtained by iterative optimization algorithm. An user role decision algorithm is proposed to decide the role of strong and weak users to improve the weighted sum rate. Simulation results are provided to demonstrate the effectiveness of the proposed schemes.

[1] Y. Saito, Y. Kishiyama, A. Benjebbour, T. Nakamura, A. Li, and K. Higuchi, ''Nonorthogonal multiple access (NOMA) for future radio access,'' in Proc. IEEE VTC Spring, pp. 1-5, June 2013.

[2] B. Kim, S. Lim, H. Kim, S. Suh, J. Kwun, S. Choi, C. Lee, S. Lee, and D. Hong, ''Nonorthogonal multiple access in a downlink multiuser beamforming system,'' in Proc. IEEE Mil. Commun. Conf., pp. 1278-1283, Nov. 2012.

[3] X. Sun, D. Duran-Herrmann, Z. Zhong, and Y. Yang, ''Non-orthogonal multiple access with weighted sum-rate optimization for downlink broadcast channel,'' in Proc. IEEE Mil. Commun. Conf., pp. 1176-1181, Oct. 2015.

[4] J. Umehara, Y. Kishiyama, and K. Higuchi, ''Enhancing user fairness in non-orthogonal access with successive interference cancellation for cellular downlink,'' in Proc. of IIEEE
International Conference on Communication Systems (ICCS), pp. 324-328, Nov. 2012.

[5] S. Timotheou and I. Krikidis, ''Fairness for non-orthogonal multiple access in 5G systems,'' IEEE Signal Process. Lett., vol. 22, pp. 1647-1651, Oct. 2015.

[6] Z. Ding, P. Fan, and V. Poor, ''Impact of user pairing on 5G non-orthogonal multiple access downlink transmissions,'' IEEE Trans. Veh. Commun., vol. 65, pp. 6010-6023, Sep. 2015.

[7] F. Liu, P. Mahonen, and M. Petrova, ''Proportional fairness-based user pairing and power allocation for non-orthogonal multiple access,'' in Proc. of IEEE Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC),
pp. 1127-1131, Sept. 2015.

[8] Z. Pi and F. Khan, ''An introduction to millimeter-wave mobile broadband systems,'' IEEE Commun. Mag., vol. 49, pp. 101-107, Jun. 2011.

[9] J. Nsenga, A. Bourdoux, and F. Horlin, ''Mixed analog/digital beamforming for 60 GHz MIMO frequency selective channels,'' in Proc. of IEEE International Conference on Communications (ICC), pp. 1-6, May. 2010.

[10] G. Caire and S. S. (Shitz), ''On the achievable throughput of a multiantenna gaussian broadcast channel,'' IEEE Trans. Inf. Theory, vol. 49, pp. 1691-1706, Jul. 2003.

[11] S. S. Christensen, R. Agarwal, E. D. Carvalho, and J. M. Cioffi, ''Weighted sum-rate maximization using weighted MMSE for MIMO-BC beamforming design,'' IEEE Trans. Wireless Commun., vol. 7, pp. 4792-4799, Dec. 2008.

[12] Q. Shi, M. Razaviyayn, and Z.-Q. Luo, ''An iteratively weighted MMSE approach distributed sum-utility maximization for a MIMO interfering broadcast channel,'' IEEE Trans. Signal Process., vol. 59, pp. 4331-4340, Sep. 2011.

[13] O. E. Ayach, S. Rajagopal, S. Abu-Surra, Z. Pi, and R. W. Heath, ''Spatially sparse precoding in millimeter wave MIMO system,'' IEEE Trans. Wireless Commun., vol. 13, pp. 1499-1513, March 2014.

[14] L. B. H. D. H. N. Nguyen and T. Le-Ngoc, ''Hybrid MMSE precoding for mmWave multiuser MIMO systems,'' in Proc. of IEEE International Conference on Communications (ICC), pp. 1-6, May 2016.

[15] J. Geng, W. Xiang, Z. Wei, N. Li, and D. Yang, ''Multi-user hybrid analogue/digital beamforming for relatively large-scale antenna systems,'' vol. 8, pp. 3038-3049, Nov. 2014.

[16] M. Kim and Y. H. Lee, ''MSE based hybrid RF/baseband processing for millimeter-wave communication systems in MIMO interference channels,'' IEEE Trans. Veh. Technol.,
vol. 64, pp. 2714-2720, Aug. 2014.

[17] F. Sohrabi and W. Yu, ''Hybrid digital and analog beamforming design for large-scale antenna arrays,'' IEEE J. Sel. Topics Signal Process., vol. 10, pp. 501-513, April 2016.

[18] A. Alkhateeb, G. Leus, and R. W. Heath, ''Limited feedback hybrid precoding multiuser millimeter wave systems,'' IEEE Trans. Wireless Commun., vol. 14, pp. 6481-6494, Nov. 2015.

[19] J. Geng, Z. Wei, X. Wang, W. Xiang, and D. Yang, ''Multiuser hybrid analog/digital beamforming for relatively large-scale antenna arrays,'' in 2013 IEEE Globecom Work-shops (GC Wkshps), pp. 123-128, Dec 2013.

[20] A. Alkhateeb, O. E. Ayach, G. Leus, and R. W. Heath, ''Channel estimation and hybrid precoding for millimeter cellular systems,'' IEEE Signal Process. Lett., vol. 8, pp. 831-846, Dec. 2014.

[21] T. E. Bogale and L. B. Le, ''Beamforming for multiuser massive MIMO systems: digital
versus hybrid analog-digital,'' in Proc. of Global Communications Conference (Globecom), pp. 4066-4071, Dec. 2014.

[22] Z. Xu, S. Han, Z. Pan, and C. L. I, ''Alternating beamforming methods for hybrid analog and digital MIMO transmission,'' in Proc. of IEEE International Conference on Communications (ICC), pp. 1595-1600, Jun. 2015.

[23] O. E. Ayach, R. W. Heath, S. Abu-Surra, S. Rajagopal, and Z. Pi, ''Low complexity precoding for large millimeter wave MIMO systems,'' in Proc. of IEEE International Conference on Communications (ICC), pp. 3724-3729, Jun. 2012.

[24] N. Otoa, Y. Kishiyama, and K. Higuchi, ''Performance of non-orthogonal access with SIC in cellular downlink using proportional fair-based resource allocation,'' in Proc. of IEEE International Symposium on Wireless Communications Systems (ISWCS), pp. 476-480,
Aug. 2012.

[25] J. A. Tropp and S. J. Wright, ''Computational methods for sparse solution of linear inverse problems,'' in Proc. IEEE, vol. 98, pp. 948-958, Jun. 2010.