Downlink MIMO-NOMA With and Without CSI: A Short Survey and Comparison

Abstract

Non-orthogonal multiple access (NOMA) concatenated with multiple-input multiple-output (MIMO) or with massive MIMO, has been under scrutiny for both broadband and machinetype communications (MTC), even though it has not been adopted in the latest 5G standard (3GPP Release 16), being left for beyond 5G. This paper dwells on the problems causing such cautiousness, and surveys different NOMA proposals for the downlink in cellcentered systems. Because acquiring channel state information at the transmitter (CSIT) may be hard, open-loop operation is an option. However, when users clustering is possible, due to some common statistical CSI, closed-loop operation should be exploited. The paper numerically compares these two operating modes. The users are clustered in beams and then successive interference cancellation (SIC) separates the power-domain NOMA (PD-NOMA) signals at the terminals. In the precoded closedloop system, the Karhunen-Loève channel decomposition is used assuming that users within a cluster share the same slowly changing spatial correlation matrix. For a comparable number of antennas the two options perform similarly, however, while in the open-loop downlink the number of antennas at the BS is limited in practice, this restriction is waived in the precoded systems, with massive MIMO allowing for a larger number of clusters.

Comments on SIC design for SISO NOMA Systems over Doubly Selective Channels.

“…For application in SISO channels, two possible approaches have been proposed for multi-user interferences mitigation through SIC, namely a Chaos-based filtering approach and an approach based on the m sequential hypotheses testing. Both of them show several attractive features for application over Doubly Selective channels such as high fidelity for multiuser decoding, possible robustness and reduced decision time, which allows to recommend them not only for Doubly Selective channels, but also for the "conventional" channels as well, besides the large number of already reported algorithms for them (see [1]- [12] and references therein).…”

“…(see details at [1], [3], etc.). A brief overview of the works considered as most important [1][2][3][4][5][6][7][8][9][10][11][12] is presented in the following. As it can be inferred from [4]- [5], the SIC decoding order based on the CSI turns critical and can have important influence on the system's performance.…”

“…However, authors in [8] do not work with high-mobility scenarios that entail Doubly Selective Channels. Notice, that almost every proposed SIC technique in the literature strongly depends on the CSI [12]. However, the usage of the Channel State Information (CSI) or its modifications [1], [2], [6], [10] is hardly possible, due to the Doubly Selective Channels.…”

“…Nevertheless, 5G and beyond networks have proposed also MIMO techniques in order to increase capacity. Furthermore, with the employment of Massive MIMO techniques the usage of CSI becomes challenging since the number of users goes large [6]- [12]. In [6] a blind belief propagation (BP) detection was proposed for noncoherent NOMA with massive MIMO, where the transmitter of each user first performs differential modulation on PSK symbols, and then spreads its symbols using low-density spreading (LDS); the receiver of the base station (BS) employs differential demodulation and then detects all users' symbols using a blind BP detection without knowledge of CSI.…”

Massive Wireless Energy Transfer With Statistical CSI Beamforming

“…Therefore, Λ l is only going to have r l non-zero diagonal elements. When the channel correlation is known one can take advantage of the KL channel representation, which has been applied in the context of NOMA [27], [58], MIMO spacial division multiplexing [56], rate-splitting in MU-MIMO systems [59], and also as an efficient way of generating fading samples with a given correlation matrix [60], [61, Sec. 2.2].…”