Rate Splitting for Multi-Antenna Downlink: Precoder Design and Practical Implementation

Abstract

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.

Space-Time Rate Splitting for the MISO BC With Magnitude CSIT

“…Next, we sketch the structure of the STRS scheme for K users. In the K-user setting, public messages addressed to different groups of users need to be devised along the line of [29]- [31]. For the three-user case, [30] proves that a hierarchy of public messages is needed to achieve the GDoF in the MISO BC with finite precision CSIT.…”

“…For the three-user case, [30] proves that a hierarchy of public messages is needed to achieve the GDoF in the MISO BC with finite precision CSIT. In [29] and [31] a multi-layer RS scheme is presented, which requires a number of successive interference cancellation stages at the RXs equal to the number of layers of common messages. Each user message is partitioned into several independent submessages, which are to be decoded by different subsets of users which include the intended destination of the original message.…”

“…More in general, a message m k is split into different submessages m Ki,c , which are sub-messages to be decoded by the users which are contained in the set K i . The cardinality of S k = i K i is the number of all possible combinations of the users which include user k. A detailed analysis of the grouping for RS schemes is presented in [31]. The number of SIC stages at the RXs follow from the depth of the grouping subsets, i.e., the number of subsets that a given user belongs to.…”

“…The number of SIC stages at the RXs follow from the depth of the grouping subsets, i.e., the number of subsets that a given user belongs to. The contribution of some common streams is expected to be marginal, so they can be removed without much impact [31]. As a result, two-layer grouping is quite a common strategy, with a common message addressed to all users, followed by different sub-messages intended for a second layer of G disjoint subsets K g , g = 1, .…”

Rate Splitting Multiple Access in C-RAN

“…Several works have recently studied RS techniques in multiple input single output broadcast channel (MISO-BC) [31]- [40] as a promising transmission strategy candidate to manage the interference in B5G networks. The key benefit of RS in multi-antenna BC is the information-theoretical optimality in the sense of achieving the optimal degrees of freedom (DoF) region of the MISO BC with imperfect CSIT [41].…”