Rate splitting for MIMO wireless networks: a promising PHY-layer strategy for LTE evolution

Abstract

MIMO processing plays a central part towards the recent increase in spectral and energy efficiencies of wireless networks. MIMO has grown beyond the original point-to-point channel and nowadays refers to a diverse range of centralized and distributed deployments. The fundamental bottleneck towards enormous spectral and energy efficiency benefits in multiuser MIMO networks lies in a huge demand for accurate channel state information at the transmitter (CSIT). This has become increasingly difficult to satisfy due to the increasing number of antennas and access points in next generation wireless networks relying on dense heterogeneous networks and transmitters equipped with a large number of antennas. CSIT inaccuracy results in a multi-user interference problem that is the primary bottleneck of MIMO wireless networks. Looking backward, the problem has been to strive to apply techniques designed for perfect CSIT to scenarios with imperfect CSIT. In this paper, we depart from this conventional approach and introduce the readers to a promising strategy based on rate-splitting. Rate-splitting relies on the transmission of common and private messages and is shown to provide significant benefits in terms of spectral and energy efficiencies, reliability and CSI feedback overhead reduction over conventional strategies used in LTE-A and exclusively relying on private message transmissions. Open problems, impact on standard specifications and operational challenges are also discussed. IntroductionPromising approaches for 5G consist in densifying the network by adding more antennas in a distributed or co-localized manner. A distributed deployment leads to dense homogeneous/heterogeneous networks where the widely recognized bottleneck is interference. Interference management relying on multi-point cooperation have drawn a lot of attention in industry (i.e. CoMP in LTE-A [1]) and academia. Co-localized deployment leads to massive MIMO (i.e. FD-MIMO in LTE-A).Although appealing in their concept, those aforementioned MIMO techniques are hampered by several practical factors. Among these, the acquisition of accurate CSI knowledge at the transmitter (CSIT) is the major challenge. The availability of accurate CSIT is crucial for Downlink (DL) multi-user MIMO wireless networks. The beamforming and interference nulling performance heavily depends on the channel estimation accuracy. Unfortunately, pilot reuse tends to impair channel estimation in TDD and a significant feedback overhead is required to guarantee sufficient feedback accuracy in FDD due to the large number of antennas. Delay and inaccurate calibrations of the RF chains also contribute to making the CSIT inaccurate. CSIT inaccuracy results in a multi-user interference and link adaptation problem that is the primary bottleneck of MIMO wireless networks, as highlighted e.g. in [2] for for CoMP.Looking backward, the problem has been to strive to apply techniques designed for perfect CSIT to scenarios with imperfect CSIT. Following the same path will only increase the ...

Rate-Splitting Unifying SDMA, OMA, NOMA, and Multicasting in MISO Broadcast Channel: A Simple Two-User Rate Analysis

“…SUM-RATE ANALYSIS Our objective is to derive tractable and insightful sumrate expressions to illustrate the flexibility of RS in unifying SDMA, OMA, NOMA, and multicasting. To that end, we do not optimize the precoding directions jointly with the power allocation as in [3], [7] but rather fix the precoding directions using ZF for the private streams, and adjust the power allocation among all the streams 1…”

“…Linearly precoded Rate-Splitting (RS) with Successive Interference Cancellation (SIC) receivers has recently appeared as a powerful non-orthogonal transmission and robust interference management strategy for multi-antenna wireless networks [1]. Though originally introduced for the two-user Single-Input Single-Output Interference Channel (IC) in [2], RS has become an underpinning communication-theoretic strategy to tackle modern interference-related problems and has recently been successfully investigated in several Multiple-Input Single-Output (MISO) Broadcast Channel (BC) settings, namely, unicast-only transmission with perfect Channel State Information at the Transmitter (CSIT) [3], [4] and imperfect CSIT [5]- [13], (multigroup) multicast-only transmission [14], as well as superimposed unicast and multicast transmission [15].…”

Linearly Precoded Rate-Splitting Techniques with Block Diagonalization for Multiuser MIMO Systems

“…The system employs the RS scheme, which splits the messages into a common part and a private part [18], [9]. Since we focus on the sum rate analysis, it suffices to consider that only one stream is split.…”

Sum-Rate Maximization of Uplink Rate Splitting Multiple Access (RSMA) Communication

“…In consequence, rate splitting multiple access (RSMA) has been recently proposed as an effective approach to provide more general and robust transmission framework compared to non-orthogonal multiple access (NOMA) [3]- [8] and space-division multiple access (SDMA). However, implementing RSMA in wireless networks also faces several challenges [9] such as decoding order design and resource management for message transmission.…”