17 research outputs found
Symbol-level and Multicast Precoding for Multiuser Multiantenna Downlink: A State-of-the-art, Classification and Challenges
Precoding has been conventionally considered as an effective means of mitigating or exploiting the interference in the multiantenna downlink channel, where multiple users are simultaneously served with independent information over the same channel resources. The early works in this area were focused on transmitting an individual information stream to each user by constructing weighted linear combinations of symbol blocks (codewords). However, more recent works have moved beyond this traditional view by: i) transmitting distinct data streams to groups of users and ii) applying precoding on a symbol-per-symbol basis. In this context, the current survey presents a unified view and classification of precoding techniques with respect to two main axes: i) the switching rate of the precoding weights, leading to the classes of block-level and symbol-level precoding, ii) the number of users that each stream is addressed to, hence unicast, multicast, and broadcast precoding. Furthermore, the classified techniques are compared through representative numerical results to demonstrate their relative performance and uncover fundamental insights. Finally, a list of open theoretical problems and practical challenges are presented to inspire further research in this area
A Low-Complexity Design for Rate-Splitting Multiple Access in Overloaded MIMO Networks
Rate-Splitting Multiple Access (RSMA) is a robust multiple access scheme for
multi-antenna wireless networks. In this work, we study the performance of RSMA
in downlink overloaded networks, where the number of transmit antennas is
smaller than the number of users. We provide analysis and closed-form solutions
for optimal power and rate allocations that maximize max-min fairness when
low-complexity precoding schemes are employed. The derived closed-form
solutions are used to propose a low-complexity RSMA system design for precoder
selection and resource allocation for arbitrary number of users and antennas
under perfect Channel State Information at the Transmitter (CSIT). We compare
the performance of the proposed design with benchmark designs based on Space
Division Multiple Access (SDMA) to show that the proposed low-complexity RSMA
design achieves a significantly higher performance gain in overloaded networks
Rate-Splitting Multiple Access: Finite Constellations, Receiver Design, and SIC-free Implementation
Rate-Splitting Multiple Access (RSMA) has emerged as a novel multiple access
technique that enlarges the achievable rate region of Multiple-Input
Multiple-Output (MIMO) broadcast channels with linear precoding. In this work,
we jointly address three practical but fundamental questions: (1) How to
exploit the benefit of RSMA under finite constellations? (2) What are the
potential and promising ways to implement RSMA receivers? (3) Can RSMA still
retain its superiority in the absence of successive interference cancellers
(SIC)? To address these concerns, we first propose low-complexity precoder
designs taking finite constellations into account and show that the potential
of RSMA is better achieved with such designs than those assuming Gaussian
signalling. We then consider some practical receiver designs that can be
applied to RSMA. We notice that these receiver designs follow one of two
principles: (1) SIC: cancelling upper layer signals before decoding the lower
layer and (2) non-SIC: treating upper layer signals as noise when decoding the
lower layer. In light of this, we propose to alter the precoder design
according to the receiver category. Through link-level simulations, the
effectiveness of the proposed precoder and receiver designs are verified. More
importantly, we show that it is possible to preserve the superiority of RSMA
over Spatial Domain Multiple Access (SDMA), including SDMA with advanced
receivers, even without SIC at the receivers. Those results therefore open the
door to competitive implementable RSMA strategies for 6G and beyond
communications.Comment: Submitted to IEEE for publicatio
A Tutorial on Interference Exploitation via Symbol-Level Precoding: Overview, State-of-the-Art and Future Directions
IEEE Interference is traditionally viewed as a performance limiting factor in wireless communication systems, which is to be minimized or mitigated. Nevertheless, a recent line of work has shown that by manipulating the interfering signals such that they add up constructively at the receiver side, known interference can be made beneficial and further improve the system performance in a variety of wireless scenarios, achieved by symbol-level precoding (SLP). This paper aims to provide a tutorial on interference exploitation techniques from the perspective of precoding design in a multi-antenna wireless communication system, by beginning with the classification of constructive interference (CI) and destructive interference (DI). The definition for CI is presented and the corresponding mathematical characterization is formulated for popular modulation types, based on which optimization-based precoding techniques are discussed. In addition, the extension of CI precoding to other application scenarios as well as for hardware efficiency is also described. Proof-of-concept testbeds are demonstrated for the potential practical implementation of CI precoding, and finally a list of open problems and practical challenges are presented to inspire and motivate further research directions in this area