Abstract
Non-orthogonal multiple access (NOMA) aided cell-free massive multiple-input multiple-output (CF mMIMO) systems have gained great attention over the past few years to be employed in the sixth generation of the wireless network. With distributed antennas over a large area, CF mMIMO systems have much better cell coverage and higher achievable sum-rate compared to the conventional cellular and small-cell networks. This paper will propose a novel cooperative NOMA-aided CF mMIMO system. The proposed scheme will increase the achievable sum-rate and improve the system reliability by creating additional data streams between users within the same NOMA cluster. Also, we will suggest an adaptive switching algorithm between non-cooperative NOMA, cooperative NOMA, and orthogonal multiple access (OMA) modes in order to maximize the power efficiency and achievable sum-rate. We will derive closed-form expressions for the achievable downlink data rates of the mentioned modes. Lastly, we will offer a special user clustering algorithm that needs no complex optimization and ensures the existence of reliable cooperative links between the users. The simulation results will prove the superiority of the proposed scheme over the conventional NOMA-based and OMA-based CF mMIMO systems.
Highlights
T HE fifth generation of the wireless network (5G) was full of novel and enabling technologies, like the use of millimeter-wave bands and massive multiple-input multiple-output technique
Cell-free massive MIMO systems are one of the most prominent candidates to be employed in the sixth generation of the wireless network
CF massive multiple-input multiple-output (mMIMO)-Cooperative Non-orthogonal multiple access (NOMA) with the PUG CF mMIMO-Cooperative NOMA with the SOUG
Summary
T HE fifth generation of the wireless network (5G) was full of novel and enabling technologies, like the use of millimeter-wave bands and massive multiple-input multiple-output (mMIMO) technique. [20] proposed NOMA-aided CF mMIMO system in which users within the same NOMA cluster employ the same orthogonal pilot sequence. Due to the errors like intra-cluster interference, pilot contamination, and imperfect SIC in the NOMA-based approach, OMA-aided CF mMIMO has better performance than the NOMA-aided one when there are only a few connected users. [24] studied the achievable rate of NOMA-aided CF mMIMO underlaid below a primary mMIMO network and proposed a sub-optimal user-clustering method. The addition of cooperative links will increase the achievable sum-rate and improve the system reliability Some of those retransmissions are unnecessary because the weaker user might have an acceptable direct link connection. We will derive closed-form expressions for the achievable downlink data rates of non-cooperative NOMAaided, cooperative NOMA-aided, and OMA-aided CF mMIMO systems. A.bT indicates the inner product of vectors a and b with n an equal length of n, which defines as aibi
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