MU-MIMO and CSI Feedback Performance of NR/LTE

The Re1-15 physical layer air-interface specifications for NR (5G) and LTE (4G) has been released by 3GPP in September 2018. This paper aims to study two principal gain mechanisms introduced in NR (compared to LTE) targeting spectral efficiency improvements -(i) high order MU-MIMO with up to 12 layer spatially multiplexed transmission and (ii) higher resolution of CSI feedback with Type II feedback mechanism. In order to perform a fair comparison between LTE and NR, we maintain the same overhead, sub-carrier spacing and antenna configurations among other parameters. Although it is known that NR can provide benefits over LTE in terms of deployment flexibility, spectral efficiency and latency using mechanisms like multiple numerology support, lower control channel and reference signal overhead or lower guard bands we believe that the gain mechanisms (i) and (ii) are fundamental and a quantification of this aspect is attractive to operators and vendors who may consider NR or LTE deployments in sub-6 GHz bands.

I. Introduction

The first NR (New Radio) physical layer specifications is available in the form of an initial release as of September 2018 (Re1–15) [1] – [5] . We note that the first LTE release (Re1–8, 2008) was designed primarily as a spectrally efficient broadband OFDM based air-interface targeting cellular connectivity and spectrum. The target for LTE Re1–8 MIMO technology was conventional base-station antenna arrays (2Tx to 4Tx). Over the next decade till Re1–15 (2018), LTE has evolved to support other services, lower latency requirements and advanced antenna arrays (up to 32Tx). It is thus timely to discuss the benefit of NR over LTE in sub-6 GHz spectrum, specifically considering the same release (Re1–15) [6] – [9] . We note, however, that NR supports operation in both sub-6 GHz spectrum (450 MHz–6 GHz) and millimeter-wave spectrum (24.25–52.6 GHz) [5] . It is known that NR provides more flexibility for network deployments by supporting multiple subcarrier spacings and multiple services can be easily multiplexed in the same carrier. It allows a lower overhead (control channels and reference signals), lower latency and better spectrum utilization (smaller guard bands). Although in terms of MIMO technology directly impacting physical layer spectral efficiency (SE) it is largely similar to LTE, two areas have been enhanced that can provide improved SE-(i) higher order MU-MIMO where up to 12 layer transmission is supported in NR compared to 4 layers in LTE, (ii) higher CSI accuracy where Type II CSI feedback is defined with higher spatial resolution in NR compared to Advanced CSI in LTE [10] . In this paper, we provide analysis and system simulation results to show the benefit of these gain mechanisms. The main contributions of this paper may be listed as follows: •

We motivate and describe the Re1–15 specification features that enable high order MU-MIMO and high-resolution CSI feedback in LTE and NR. We also provide practical algorithms for scheduling, demodulation and CSI feedback that can be utilized to leverage these features.

•

We show that with full-buffer traffic, cell-average and cell-edge SE gains due to higher order MU-MIMO can be 20%-40% with 32 Tx antenna configuration ( Figure 3 , Figure 4 ). Cell-average and cell-edge SE gains due to higher CSI feedback accuracy can be 5%-10% with 32 Tx antenna configuration ( Figure 5 , Figure 6 ).

•

We show that the gains due to higher CSI accuracy(Type II) are not observed as UE speed is increased to 30 km/h ( Figure 7 , Figure 8 ). We also observe that outdoor to indoor propagation environments benefit more from Type II feedback ( Figure 7 , Figure 8 ).

•

We observe that the SE gains are quite reduced with reduced number of Tx antennas (8Tx compared to 32 Tx). The cell-average and cell-edge SE gains due to higher order MU-MIMO is less than 5% ( Figure 9 , Figure 10 ). The cell-average and cell-edge SE gains due to higher CSI feedback accuracy is less than 6% ( Figure 11 , Figure 12 ).