This letter presents a new downlink cooperative non-orthogonal multiple access (NOMA) transmission scheme to serve three users within only two time slots. The scheme involves a base station (BS), two direct-link users and one indirect-link user. The BS sends superposed signals to the two direct-link users which decode and forward the signals to the indirect-link user in an alternating fashion. Closed-form expression is derived for the sum-rate of the proposed scheme and a simple expression of sum-rate is also derived in the high signal-to-noise ratio (SNR) region. In the presence of strong inter-user interference (IUI), the proposed scheme can be applied by having each direct-link user to detect the IUI first and then cancel the IUI to decode the signals destined for the indirect-link user and itself. Simulation results show that the scheme with configurable decoding orders at the users is able to achieve a higher sum-rate than existing orthogonal multiple access (OMA) based alternatives.

中文翻译：

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三用户协同NOMA传输

这封信提出了一种新的下行协作非正交多址 (NOMA) 传输方案，以在仅两个时隙内为三个用户提供服务。该方案涉及一个基站（BS）、两个直接链路用户和一个间接链路用户。BS 向两个直接链路用户发送叠加信号，这两个直接链路用户以交替的方式解码并转发信号到间接链路用户。推导出所提出方案的总和率的闭式表达式，并且还导出了高信噪比（SNR）区域中的总和率的简单表达式。在存在强用户间干扰(IUI)的情况下，可以通过让每个直接链接用户首先检测IUI然后取消IUI以解码发往间接链接用户和自身的信号来应用所​​提出的方案。