Is NOMA Efficient in Multi-Antenna Networks? A Critical Look at Next Generation Multiple Access Techniques

Abstract

In the past few years, a large body of literature has been created on downlink Non-Orthogonal Multiple Access (NOMA), employing superposition coding and Successive Interference Cancellation (SIC), in multi-antenna wireless networks. Furthermore, the benefits of NOMA over Orthogonal Multiple Access (OMA) have been highlighted. In this paper, we take a critical and fresh look at the downlink Next Generation Multiple Access (NGMA) literature. Instead of contrasting NOMA with OMA, we contrast NOMA with two other multiple access baselines. The first is conventional Multi-User Linear Precoding (MU-LP), as used in Space-Division Multiple Access (SDMA) and multi-user Multiple-Input Multiple-Output (MIMO) in 4G and 5G. The second, called Rate-Splitting Multiple Access (RSMA), is based on multi-antenna Rate-Splitting (RS). It is also a non-orthogonal transmission strategy relying on SIC developed in the past few years in parallel and independently from NOMA. We show that there is some confusion about the benefits of NOMA, and we dispel the associated misconceptions. First, we highlight why NOMA is inefficient in multi-antenna settings based on basic multiplexing gain analysis. We stress that the issue lies in how the NOMA literature, originally developed for single-antenna setups, has been hastily applied to multi-antenna setups, resulting in a misuse of spatial dimensions and therefore loss in multiplexing gains and rate. Second, we show that NOMA incurs a severe multiplexing gain loss despite an increased receiver complexity due to an inefficient use of SIC receivers. Third, we emphasize that much of the merits of NOMA are due to the constant comparison to OMA instead of comparing it to MU-LP and RS baselines. We then expose the pivotal design constraint that multi-antenna NOMA requires one user to fully decode the messages of the other users. This design constraint is responsible for the multiplexing gain erosion, rate and spectral efficiency loss, ineffectiveness to serve a large number of users, and inefficient use of SIC receivers in multi-antenna settings. Our analysis and simulation results confirm that NOMA should not be applied blindly to multi-antenna settings, highlight the scenarios where MU-LP outperforms NOMA and vice versa, and demonstrate the inefficiency, performance loss, and complexity disadvantages of NOMA compared to RSMA. The first takeaway message is that, while NOMA is suited for single-antenna settings (as originally intended), it is not efficient in most multi-antenna deployments. The second takeaway message is that another non-orthogonal transmission framework, based on RSMA, exists which fully exploits the multiplexing gain and the benefits of SIC to boost the rate and the number of users to serve in multi-antenna settings and outperforms both NOMA and MU-LP. Indeed, RSMA achieves higher multiplexing gains and rates, serves a larger number of users, is more robust to user deployments, network loads and inaccurate channel state information and has a lower receiver complexity than NOMA. Consequen...

Rate-Splitting Multiple Access: Fundamentals, Survey, and Future Research Trends

“…-First, multi-antenna NOMA can suffer from a loss of DoF due to the inefficient use of SIC. As per Table II in [13], for a MISO BC employing one M -antenna transmitter having perfect CSIT and simultaneously serving K single-antenna users, the sum-DoF achieved by multi-antenna NOMA with G = 1 is only 1, which is equivalent to the DoF of OMA/single user-MISO transmission. In comparison, SDMA achieves the optimal sum-DoF of min{M, K} in the same setting without using any SIC at the users.…”

On the Physical Layer Security of the Cooperative Rate-Splitting-Aided Downlink in UAV Networks

“…Interestingly, upon increasing the transmission power the algorithm's convergence rate decreases. This arises from the fact that increasing the transmission power results in higher information leakage to Eve and thus more iterations are required to meet the security constraints of (44).…”

Performance Analysis of Rate Splitting in Massive MIMO Systems with Low Resolution ADCs/DACs

“…where R k,RZF is the rate for the non-RS scheme where the BS uses RZF precoder and zero power is allocated to the common message in (9). The power allocation factor for the private messages , t, is derived as…”

“…Therefore, the RS strategy is a robust interference management technique outperforming NOMA with a lower computational complexity in various network settings [6,7]. The RS scheme also improves the achievable degrees-of-freedom (DoF) compared to that of NOMA [9]. Rate-splitting was also shown to enhance the EE of DL MISO transmission [10,11].…”