Rate splitting (RS) is a potentially powerful and flexible technique for multi-antenna downlink transmission. In this paper, we address several technical challenges towards its practical implementation for beyond 5G systems. To this end, we focus on a single-cell system with a multi-antenna base station (BS) and $K$ single-antenna receivers. We consider RS in its most general form with $2^{K}-1$ streams, and joint decoding to fully exploit the potential of RS. First, we investigate the achievable rates under joint decoding and formulate the precoder design problems to maximize a general utility function, or to minimize the transmit power under pre-defined rate targets. Building upon the concave-convex procedure (CCCP), we propose precoder design algorithms for an arbitrary number of users. Our proposed algorithms approximate the intractable non-convex problems with a number of successively refined convex problems, and provably converge to stationary points of the original problems. Then, to reduce the decoding complexity, we consider the optimization of the precoder and the decoding order under successive decoding. Further, we propose a stream selection algorithm to reduce the number of precoded signals. With a reduced number of streams and successive decoding at the receivers, our proposed algorithm can even be implemented when the number of users is relatively large, whereas the complexity was previously considered as prohibitively high in the same setting. Finally, we propose a simple adaptation of our algorithms to account for the imperfection of the channel state information at the transmitter. Numerical results demonstrate that the general RS scheme provides a substantial performance gain as compared to state-of-the-art linear precoding schemes, especially with a moderately large number of users.

中文翻译：

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多天线下行链路的速率拆分：预编码器设计和实际实现

速率拆分 (RS) 是一种用于多天线下行链路传输的潜在强大且灵活的技术。在本文中，我们解决了超越 5G 系统实际实施的几个技术挑战。为此，我们专注于具有多天线基站 (BS) 和 $K$ 单天线接收器的单小区系统。我们考虑具有 $2^{K}-1$ 流的最一般形式的 RS，以及联合解码以充分利用 RS 的潜力。首先，我们研究了联合解码下可实现的速率，并制定了预编码器设计问题，以最大化通用效用函数，或在预定义的速率目标下最小化发射功率。基于凹凸过程（CCCP），我们为任意数量的用户提出了预编码器设计算法。我们提出的算法通过一些连续细化的凸问题来逼近棘手的非凸问题，并且可以证明收敛到原始问题的驻点。然后，为了降低解码复杂度，我们考虑了预编码器和连续解码下的解码顺序的优化。此外，我们提出了一种流选择算法来减少预编码信号的数量。通过减少接收器的流数量和连续解码，我们提出的算法甚至可以在用户数量相对较大的情况下实现，而以前在相同的设置下复杂度被认为高得令人望而却步。最后，我们建议对我们的算法进行简单的调整，以解决发射机信道状态信息的不完善问题。